Biology Essays: Form 1 and 2

Explain the various ways in which a typical cell is adapted to its functions

• Has a cell membrane; with pores; that regulates substances entering and leaving the cell;
• Cytoplasm; contain sugars and salts; for maintaining its osmotic pressure; also has a liquid medium; for all biochemical reactions;
• Nucleus; contain chromosomes having hereditary material; and controls all the activities of the cell;
• Ribosomes; are sites for protein synthesis;
• Golgi bodies/apparatus; for secretion of hormones and enzymes; formation of lysosomes;
• Lysosomes; contain lytic enzymes for breaking down worn-out organelles; secretory vesicles; formed from golgi apparatus for secreting substances; smooth endoplasmic reticulum; synthesizes and transports lipids;
• Rough endoplasmic reticulum; transport proteins;
• Nucleolus; Manufacture of ribosomes;
• Mitochondria; form sites for energy production;
• Centrioles; formation of cilia and flagella; forms spindle fibres used in cell division;
• Sap vacuoles; store salts and other dissolved substances; controls osmotic pressure and turgidity of cells;
• Food vacuoles involved in digestion of engulfed food; chloroplasts; form sites for photosynthesis in plant cells;

Describe the structural adaptations of the leaf to the process of photosynthesis

• The cuticle is transparent to allow light to pass through to reach photosynthetic tissues.
• Has a broad flat lamina to provide a large surface area for trapping optimum light for photosynthesis and allows maximum gaseous exchange;
• The leaf epidermis is thin to reduce the distance across which diffusion of carbon (IV) oxide gas to palisade cells and oxygen gas from palisade cells takes place;
• Has numerous stomata that allow easy diffusion of gases into and out of the palisade tissue.
• The leaf cuticle and epidermis are transparent to allow easy penetration of light to the photosynthetic tissue;
• The palisade cells are numerous, elongated and contain numerous chloroplasts to trap optimum light for photosynthesis;
• The palisade tissue is just beneath the upper epidermis exposing them to trap optimum light for photosynthesis;
• The leaf has numerous leaf veins consisting of xylem for conducting water and dissolved mineral salts from the soil to the photosynthetic tissue, and phloem tissue for translocation of manufactured food from the leaf to storage organs and other parts of the plant;
• Numerous and large air spaces in the spongy mesophyll layer for optimum gaseous exchange with the photosynthetic tissue;
• Leaf mosaic arrangement minimizes overshadowing and overlapping exposing all leaves to light for photosynthesis;
• The prominent midrib and leaf veins reduce the chances of rolling of leaves maintaining a large surface area for trapping optimum light for photosynthesis;

Discuss the light and dark stages of photosynthesis

Light stage

• It occurs in the grana/granum of the chloroplast; the chlorophyll in the chloroplast traps; light energy; used to split water molecules into oxygen molecules and hydrogen ions; in the process called photolysis.
• The hydrogen ions produced enter the dark stage, while oxygen is released to  the atmosphere/re-utilized by the plant in respiration; some of the light energy absorbed by chlorophyll molecule is used in the formation of Adenosine triphosphate (ATP), used later in the dark stage;

Dark stage

• It involves the combination of carbon (IV) oxide and hydrogen ions; in a series of enzymes catalyzed; reactions to form simple sugar; (e.g. sucrose) in the process called carbon (IV) oxide fixation; the process required energy which is provided by ATP; (formed during the light stage); some of the sugars formed are directly utilized; by the plant cells while the rest are converted to starch; for storage.
• Amino acids; and fatty acids; are also formed during this stage of photosynthesis;

Explain why the following conditions are necessary for photosynthesis

• Light: Used to break down water molecules (photolysis) into hydrogen ions and oxygen molecules. Energy in form of ATP is also formed. The hydrogen ions and energy formed are used in the dark stage.
• Carbon (IV) oxide: Combines with Hydrogen ions in the dark stage to form glucose/amino acids/lipids, a reaction called Carbon (IV) oxide fixation. High CO₂ concentration increases the rate of photosynthesis while low CO₂ concentration decrease the rate of photosynthesis.
• Water: It is split to yield hydrogen ions, oxygen, and energy. Forms a medium for biochemical reactions. Also acts as a solvent.
• Chlorophyll: The green pigment that traps light energy used for photolysis.
• Optimum temperature: Temperature affects the action of enzymes involved in photosynthesis;

Explain factors that affect enzyme-controlled reactions.

• Temperature: Enzymes are sensitive to changes in temperature. Enzymes work best within an optimum range of temperature. A high temperature above optimum denatures enzymes hence the reaction slows down and eventually stops; Whereas at a low temperature below optimum, the enzymes become inactive hence reducing the reaction rate.
• pH: Enzymes work best within an optimum pH range. When pH is altered above or below the optimum range, enzyme action decreases because the enzymes get denatured. Different enzymes have different optimum pH ranges.
• Substrate concentration: With a constant enzyme concentration, the enzyme action increases with increasing substrate concentration until a certain level when all active sites of enzymes are occupied and the rate of reaction then levels off.
• Enzyme concentration: With all factors held constant and substrate concentration high, the rate of reaction increases with an increase in enzyme concentration i.e. the rate of enzyme reaction is proportional to enzyme concentration.
• Enzyme co-factors and co-enzymes: Enzyme co-factors are non-proteinous substances that activate enzymes. Examples include Zinc, Manganese, Copper and Iron. Co-enzymes are organic non-protein molecules that work in association with particular enzymes. Many co-enzymes are derived from vitamins.
• Enzyme inhibitors: They reduce the rate of enzyme action or completely stop the reaction.

Describe the dentition of carnivorous mammals and their adaptation to the mode of feeding

• Have sharp pointed incisors; for tearing and stripping flesh from bones
• Have canines that are long/curved/pointed; for piercing, grasping, and holding the prey;
• Have carnassial teeth (upper fourth premolar and lower first molar) with smooth sides; sharp edges to shear and slice flesh from bones and crush the bones; 
• Have powerful jaw bones with powerful muscles attached to prevent dislocation of jaws for cutting and shearing of flesh;
• Have sharp and curved claws; for holding and grasping prey;

Describe the process through which a piece of ugali undergoes in man from the time of ingestion up to the time of absorption.

• The ugali is ingested into the mouth; The ugali is then masticated/chewed by the teeth to increase large surface area for the action of salivary amylase/ptyalin; The food mixes with saliva produced by salivary glands; Saliva contains mucus, water and enzyme ptyalin/salivary amylase.  Water softens the food, Mucus lubricates the food; ptyalin speeds up the conversion of starch to maltose; ugali is made into boluses in the mouth; and then swallowed; The bolus moves along the oesophagus and peristalsis/by contraction and relaxation of circular and longitudinal muscles into the stomach; The digestion of starch stops in the stomach due to the presence of hydrochloric acid that denatures ptyalin/salivary amylase;
• The food moves into the duodenum by peristalsis in form of acidic chyme; where it mixes with the bile from the liver and pancreatic juice from the pancreas; bile salts being alkaline neutralize the acidic chyme, and provides an optimum pH/alkaline medium for the enzymes to act on carbohydrates; Pancreatic juice contains three enzymes out of the which enzyme pancreatic amylase speeds conversion of the remaining starch to maltose; When food reaches the ileum; it mixes with intestinal juice/succus entericus which contains several enzymes.  Maltase – speeds up the conversion of maltose to glucose; Lactase speeds up the conversion of lactose to glucose and galactose; Sucrase speeds up the conversion of sucrose into fructose and glucose;
• Absorption – glucose, which is  the end product of all carbohydrates diffuses through the epithelium of villi and capillary walls and  enters into the bloodstream, and is carried to the liver via hepatic portal veins;
• Assimilation – in the liver excess glucose is converted into glycogen and stored; The rest of the glucose is carried by the blood tissues which is oxidized during tissue respiration to release energy; √

Describe the digestion of proteins and carbohydrates along the human gut

• The meal is ingested into the mouth. It is then chewed/masticated by teeth; into smaller particles; and then mixed with saliva from the salivary gland; Saliva contains water, mucus and salivary amylase/ptyalin. Salivary amylase catalyses the breakdown of starch into maltose; the food is then rolled by the tongue into boluses, and then swallowed; The boluses move by peristalsis down the oesophagus into the stomach; Constant contractions and relaxations of the stomach walls mix the food to form chyme; The presence of food in the stomach further stimulates the production of gastric juice; which contain hydrochloric acid, pepsinogen, and rennin; Digestion of starch stops in the stomach due to presence of hydrochloric acid that denatures salivary amylase/ptyalin;
• Pepsinogen is activated to pepsin; by hydrochloric acid. Pepsin catalyze the breakdown of proteins into peptides; Rennin coagulates caseinogen (soluble milk protein) into casein (insoluble); it is then pushed into the duodenum through the pyloric sphincter; In the duodenum, bile juice and pancreatic juice are secreted.  The pancreatic juice contains trypsinogen (the inactive form of trypsin); which digests the remaining proteins into peptides; Also contains pancreatic amylase; that digests the remaining starch to maltose;
• In the ileum, intestinal juice/succus entericus is secreted; it contains peptidase enzymes; which break down peptides into amino acids; it also contains polypeptidase enzyme; which breaks down polypeptides to amino acids; Also contains maltase that digests maltose to glucose; lactase that digests lactose to glucose and galactose; sucrase that digests sucrose to glucose and fructose; completing the process of digestion of proteins and carbohydrates;

Discuss the adaptations of the ileum to its functions

• It is long to provide a large surface area for digestion and absorption of food.
• It is narrow so as to bring digested food into close contact with the walls of the ileum for easier absorption of food.
• It is highly coiled to slow down the movement of food and thus allow more time for digestion and absorption.
• Has numerous villi and microvilli to increase surface area for digestion and absorption of food.
• Has thin epithelium to shorten diffusion distance for faster absorption of food.
• Has highly vascularized villi (with a dense network of blood capillaries) to maintain a steep concentration gradient for faster diffusion of digested food
• Presence of lacteals in the villi for absorption of fatty acids and glycerol.
• More muscular walls to allow peristalsis;

Biology Essays: Transport in Plants and Animals

Explain the adaptations of the root hairs to their function

• Are elongated to increase the surface area for the absorption of water and mineral salts.
• Have thin cell walls to reduce the distance through which water and mineral salts will be absorbed.
• Has dense cytoplasm with numerous mitochondria to produce more energy for active transport.
• Have a cell sap that is hypertonic to create a steep concentration gradient for faster absorption of water and nutrients;

Discuss the adaptations of xylem to its function

• Lignified walls to prevent collapsing during the transport of water.
• Narrow lumen to facilitate capillarity
• Lack cross walls/end walls for continuous flow of water.
• Side walls are pitted to allow lateral movement of water and mineral salts.

Describe ways in which the phloem is adapted to their function

• Has companion cell with numerous mitochondria for the production of large amounts of energy for translocation;
• Has plasmodesmata that allow communication between companion cells and sieve elements.
• Sieve tubes have perforated sieve plates to allow the movement of materials from one sieve element to the next.
• Has cytoplasmic filaments whose contraction brings about translocation.
• The sieve elements lack protoplasm to allow the movement of manufactured food without resistance.

Describe how water moves from the soil to the leaves in a tree.

• Water exists as a thin film in the soil between soil particles. The cell sap is hypertonic to the surrounding solution in the soil; Thus drawing water molecules across the cell wall and membrane into the root hair cells; by osmosis; water drawn into the root hair cell dilutes the cell sap/makes it hypotonic to the adjacent cell; water moves into the cortex cells by osmosis; across the endodermis; into the xylem vessels of the root; They conduct the water up into the xylem of the stem; into the xylem of leaves.
• Water rises up the stem (in the xylem) by root pressure, capillarity; cohesion and adhesive forces; water moves as a continuous an uninterrupted water column in the xylem (vessel) up the tree to the leaves.
• As water vaporizes from the spongy mesophyll cells; their cells sap becomes more concentrated(hypertonic) than adjacent, water flows into the cells from other surrounding cells; which in turn takes in water from xylem vessels within the leaf veins. This creates a pull/suction force/ transpiration pull that pulls a stream of water from the xylem vessel in the stem and roots; the transpiration pull maintains a continuous uninterrupted column of water from the roots into the leaves called the transpiration stream;

Explain the environmental factors that increase the rate of transpiration

• Light intensity; Increase in light intensity increase the rate of photosynthesis in guard cells; more stomata open; more water lost; increase increases the internal temperature of the leaf; air in the sub-stomatal air chamber warms making water in the spaces to vapourize; more water lost;
• Amount of water available in soil; with more water available in soil; mesophyll cells become saturated; giving more water to the intercellular spaces; guard cells become turgid; stomata open, rate of transpiration increases;
• Temperature; Higher temperatures provide latent heat of vapourisation. The latent heat increases the rate at which water evaporates. Hence, transpiration rate increases with an increase in temperature.
• Air  currents/windy condition; Air sweeps a layer of water vapour outside the leaf; creating diffusion gradient; more water vapour lost.
• Relative humidity; when atmosphere is dry; there is more water in the intercellular spaces; increasing diffusion gradient; more water is therefore lost;
• Atmospheric pressure; The lower the atmospheric pressure the higher the rate of evaporation hence the rate of transpiration is also high.

Explain the structural factors that increase the rate of transpiration

• Some plants with a thick waxy cuticle that is waterproof to lower the rate of transpiration.
• Small leaves, which are needle-like in shape, hence reduced surface area exposed, lowering the rate of transpiration. Broader leaves have a large surface area exposed hence a higher transpiration rate.
• Rolled leaves have reduced surface area exposed for transpiration.
• Rolled leaves also trap moisture reducing saturation deficit hence minimizing transpiration rate.
• Plants with many stomata experience higher transpiration rates compared to those with fewer stomata.
• Plants with more stomata on the upper side than on the lower side experience higher rates of transpiration.
• Some have sunken stomata with sunken pits that trap moisture; reducing saturation deficit and hence minimizing transpiration rate.
• Some have hairy stomata with hairs that trap moisture; reducing saturation deficit hence minimizing transpiration rate.
• Leaves with many and large air spaces will lose more water than a leaf with fewer and smaller air spaces. This is because large air spaces have mesophyll cells from which water evaporates into air spaces. This increases the rate of transpiration.
• Some plants have hairy leaves, hairs trap water vapour hence reducing the saturation deficit hence reduce rate of transpiration.

Describe the structural adaptations of the mammalian heart to its function

• It has cardiac muscles which are myogenic and hence contract without nervous stimulation to pump blood;
• It is supplied by the vagus and sympathetic nerves; which controls the rate of heartbeat; (depending on the body’s physiological requirements)
• It has tricuspid valves and bicuspid valves; (between the atrium and ventricles) which prevent backflow of the blood into the right and left ventricles respectively.
• The presence of valve tendons attached to the walls of ventricles and to the atrium; to prevent atria-ventricular valves from turning inside out due to changes in the pressure in the ventricles;
• The heart is supplied by the coronary artery; which supplies food and oxygen to the cardiac muscle for their pumping action; the coronary vein in the heart removes metabolic wastes;
• The heart is enclosed by the pericardium membrane; which secretes pericardium fluid that lubricates it (reducing friction on the walls as it pumps);
• The heart is divided into two by the atrioventricular septum; which prevents the mixing of oxygenated blood and deoxygenated blood;
• The left ventricle has a thick muscular wall to pump blood at higher pressure to the distant body issues;
• The outer part of the pericardium is covered with a fatty layer; which acts as a shock absorber to keep the heart in position.
• The Sino Atrial Node (SAN), the pacemaker region which initiates the wave of contraction leading to contraction and relaxation of muscles;
• The atrioventricular node (AVN) in the heart spreads out waves of contraction throughout the heart creating the heart beat;

Describe double circulation in mammals

• Deoxygenated blood from body tissues (except lungs) enters the heart via the right auricles through the vena cava; it flows to the right ventricle via the tricuspid valve. The right ventricle contracts pumping blood via the semi-lunar valves through the pulmonary artery to the lungs for oxygenation. The oxygenated blood from the lungs flows through the pulmonary vein to the left auricle via the bicuspid valve to the left ventricle. The left ventricle contracts pumping blood via the semilunar valves through the aorta to the rest of the body tissues.

Describe the process of blood clotting

• When a blood vessel is cut/injured, the platelets/ thrombocytes at the damaged tissue/wound are exposed to the air; they release thrombokinase/thromboplastin; an enzyme that activates the conversion of prothrombin; to thrombin; in the presence of calcium ions; vitamin K; is needed for the formation of prothrombin; Thrombin converts (soluble blood protein) fibrinogen; into (the fibrous form) fibrin; which forms a meshwork across the wound; The clot so formed prevents excessive bleeding; and entry of pathogens/micro-organisms;

Biology Essays: Gaseous Exchange

Describe the mechanism of opening and closing of the stomata using the photosynthetic theory

Opening

• During the day in presence of light, chloroplasts in the cytoplasm of the guard cells photosynthesize and form glucose. The glucose formed in the cytoplasm of the guard cells is osmotically active and thus increases the osmotic pressure in the cytoplasm of the guard cells. The guard cells through osmosis then draw water from the adjacent epidermal cells and become turgid. The walls of the guard cells due to their differences in thickness stretch unevenly causing the stoma to open.

Closing

• At night, there is no light and no photosynthesis takes place. The glucose in the cytoplasm of the guard cells is then converted to starch, which is osmotically inactive. This decreases the osmotic pressure in the cytoplasm of guard cells. The guard cells then lose water to the adjacent epidermal cells through osmosis and become flaccid. The inner and outer walls relax thus closing the stoma.

Describe the mechanism of opening and closing of the stomata using the starch-sugar interconversion theory

Opening

• During the day, in presence of light photosynthesis occurs and uses up the carbon (IV) oxide in the cytoplasm thus raising the pH (raises alkalinity). Alkalinity activates enzymes that catalyze the hydrolysis of starch, which is osmotically inactive, to glucose, which is osmotically active. The guard cells then draw water from the adjacent epidermal cells through osmosis and become turgid. The thicker inner walls of the guard cells are pulled outwards and the stoma opens.

Closing

• At night, photosynthesis does not take place. Respiration occurs producing carbon (IV) oxide that lowers the pH (increases acidity). Increased acidity inactivates the enzymes that catalyse the conversion of starch to glucose. The osmotic pressure of the guard cells reduces compared to that of the adjacent epidermal cells. Guard cells then lose water through osmosis to the adjacent epidermal cells and become flaccid. The inner thicker walls relax, straighten up and the stoma closes.

Describe how the mammalian gaseous exchange system adapted to its function

• The nasal cavity has secretory cells that produce mucus that together with hairs/cilia traps and propel dust/microbes to the pharynx to be breathed out;
• The nasal cavity is supplied with blood capillaries that warm the air for faster flow;  
• Epiglottis covers the trachea during swallowing to prevent entry of food and water in the trachea;
• The trachea and bronchi have cartilage rings to keep the passages open/prevent them from collapsing;
• Trachea and bronchi are lined with mucus membrane with cilia to propel dust/microbes;
• Trachea and bronchi are richly supplied with blood vessels to warm the air for faster flow;
• Lungs have numerous alveoli to increase surface area for gaseous exchange;
• Lungs have alveoli have a thin epithelium to reduce the diffusion distance;
• Lungs have alveoli are moist to allow diffusion of gases in solution form;
• Lungs are spongy because of many air sacs that contain a large amount of air;
• Lungs are supplied with many blood vessels for the transportation of gases;
• Lungs are supplied with a network system of trachea, bronchi and bronchioles to provide an efficient system/large surface area for gaseous exchange;
• Lungs are enclosed in a pleural membrane which secrete pleural fluid that protect the lung surface; lubricate the chest cavity; allowing smooth movement of lungs as they change volumes;
• Ribs protect the lungs and have intercostal muscles that contract and relax to allow for inhalation and exhalation;
• Has diaphragm muscles whose contraction and relaxation leads to inhalation and exhalation respectively;

Describe how the lungs adapted to their function

• Lungs have numerous alveoli that provide a large surface area for efficient gaseous exchange;
• Has alveoli with thin epithelium to shorten diffusion distance for rapid exchange of gases;
• It has alveoli that are highly vascularised to efficiently transport respiratory gases to and from the body tissues respectively;
• Lungs are covered with pleural membrane which is air-tight thus changes in pressure within the lungs can occur without external interference;
• They have alveoli with moist lining to dissolve diffusing respiratory gases.

Describe the mechanism of gaseous exchange in man

Inhalation

• The external intercostal muscles contract while the internal intercostal muscles relax. The ribs and rib cage move upwards and outwards as a whole. The diaphragm muscle contracts and the diaphragm flattens. The volume of the thoracic cavity increase as pressure decrease compared to the atmospheric pressure. Air rich in oxygen enters the lungs through the nostrils, trachea, bronchi, and bronchioles into the numerous alveoli.

The Process of Gaseous Exchange

• Oxygen in the air within the alveolus dissolves in the moist film lining the inside of the alveolus wall. Oxygen which is in high concentration diffuses through the thin wall of the alveolus and the thin-walled blood capillaries into the blood within the capillaries. At the same time, Carbon (IV) oxide which is high in concentration in blood diffuses from the blood into the alveoli.

 Exhalation

• The external intercostal muscles relax while the internal intercostal muscles contract. The ribs and the rib cage moves downwards and inwards as a whole. The diaphragm muscles relax and the diaphragm become dome-shaped. Volume of the thoracic cavity decrease as pressure increase compared to the atmospheric pressure. The air in the lungs is forced out through the bronchioles, bronchi, trachea and out through nostrils. 

Describe the different ways in which carbon (IV) oxide is transported by blood.

• Carbon (IV) oxide diffuses out of the tissues into the red blood cells where it reacts with water in the presence of carbonic anhydrase enzyme to produce carbonic acid. The acid dissociates into hydrogen and hydrogen carbonate ions   
• The hydrogen carbonate ions then diffuse out of the red blood cells into the plasma; where it further dissociates to produce carbon (IV) oxide on reaching the alveolar cavities of the lungs and diffuses into the alveoli.
• Some carbon (IV) oxide combines with the amine group in the  haemoglobin  molecule  forming carbaminohaemoglobin; which  dissociates  in  the lungs  producing  carbon (IV)  oxide.
• Some carbon (IV)  oxide dissolve in blood plasma forming carbonic acid which later dissociate to carbon (IV) oxide on reaching the lungs;

Describe how the respiratory surfaces in mammals adapted to their functions

• Large/numerous to provide a large surface area; across which large amounts of gases diffuse;
• Moist surfaces to dissolve respiratory gases; so as to diffuse.  
• Made of a thin epithelium to reduce diffusion distance;
• Highly vascularized to carry away respiratory gases thus creating a steep diffusion gradient.

Biology Essays: Excretion and Homeostasis

Explain why plants lack elaborate excretory organs like those found in animals

• Plants wastes accumulate slowly due to low metabolic rate;
• Plants produce less toxic wastes;
• Some excretory products are recycled/re-used by plants in other processes e.g. CO₂ in photosynthesis while O₂ in respiration. 
• Plant tissues are tolerant to toxic wastes;
• Plant wastes are stored in temporary structures which fall off – deposition e.g. leaves

Describe any excretory products in plants and give economic uses.

Products	Uses
Nicotine;	Insecticide;
Caffeine;	Mild stimulant;
Papain;	Meat tenderizer;
Tannin;	Leather tanning;
Khat/miraa;	Stimulant;
Cannabis;	Pain killer;
Cocaine;	Anesthesia/painkiller/stimulant;
Colchicine;	Used in cancer therapy and genetics to induce polyploidy;
Morphine;	Manufacture of drugs/painkillers
Quinine;	Treatment of malaria
Latex;	Manufacture of tyre/rubber products;

Describe how human skin is adapted to its functions

• Has a cornified layer that is made up of dead cells that are tough/keratinized to protect against mechanical damage, desiccation, and infections;
• Has a granular layer consisting of living cells that gives rise to the cornified layer.
• The Malpighian layer has cells containing melanin to protect the skin against ultraviolet rays.  
• The Malpighian layer also has actively dividing cells that give rise to new epidermal cells;
• Has sweat glands that produce sweat to enhance thermoregulation.
• Has erector pili muscles that contract and relax when cold and hot respectively to make the hair erect or lie on the skin to enhance insulation or heat loss from the skin respectively.
• Has nerve endings that are sensitive to stimuli like pain, touch, and heat
• Has subcutaneous fat layer/adipose tissue  that insulates the body ;
• Has blood vessels (arteries)_ that supply nutrients and oxygen, and (veins) that remove metabolic wastes;
• The blood vessels/arterioles vasodilate when temperatures are high, to lose heat by conduction, radiation, or convection. The arterioles vasoconstrict when temperatures are low to reduce heat loss.
• Has a sebaceous gland which secretes sebum that is antiseptic. Sebum is also hence makes the skin supple;

Describe the functions of the mammalian skin in organisms.

• Protection – It protects the underlying tissues against mechanical injury, ultraviolet rays and entry of pathogens;
• Excretion – Act as an excretory organ, it enables organisms to eliminate excess water, ions and traces of urea;
• Sensitivity – Act as a sensory organ, it enables the organisms to be aware of deviations in pressure, touch and temperature from the external environment;
• Thermoregulation – It is a thermoregulatory organ as it enables the body to lose excess heat to lower its temperature back to the norm or may enable the organisms to store their heat if the temperature is lower and hence raise it back to the norm;
• Osmoregulation sweating enables the body fluids to get rid of excess water or excess ions;
• pH balance – Enhances the regulation of the pH of body fluids by enabling the body to get rid of either hydrogen ions or bi-carbonate ions;

Describe the role of the skin in excretion and homeostasis

• Sweat glands excrete sweat that contains urea, excess water and salts; hence maintaining salt and water balance in the blood.
• Evaporation of sweat cools the body due to the loss of latent heat of vaporization; when the body temperature rises above normal, blood vessels in the skin vasodilate allowing more blood to flow near the skin surface; thus heat is lost to the environment by radiation/convection/conduction.
• The erector pili muscle relaxes hence hair lies on the skin in a hot environment reducing insulation; hence heat is lost from the body by radiation/convection/conduction to the environment.

Explain the role of the skin in thermoregulation.

(i) Hair shaft;

• During cold conditions – the Erector pili muscle contracts raising the hair shaft/hair erect. The hair traps air which insulates the body.
• During hot conditions – the erector pili muscles relax hence the hair shaft lies on the skin. Little or no air is trapped. Hence heat is lost through convection /conduction /radiation;

(ii) Blood vessels

• During cold conditions – -the blood vessels vasoconstrict hence less blood flows near the skin surface to minimize heat loss through conduction/convection/ radiation;
• During hot conditions – the blood vessels vasodilate. Hence more blood flows near the skin surface leading to heat loss through conduction /convection/ radiation;

(iii) Sweat gland

• During hot conditions – sweating occurs and as the sweat evaporates heat is lost in form of the latent heat of vaporization cooling the body;
• During cold conditions – sweating reduces; hence heat is retained in the body.

(iv) Subcutaneous fatty layer /Adipose tissue

• Insulation of the body;

Describe the process of urine formation in the mammalian kidneys

• The afferent arterioles which branches from the renal artery, supply blood to the glomerulus. The afferent arteriole has a wider lumen than the efferent arteriole which takes away blood from the glomerulus. The differences in the diameter of the afferent and the efferent arterioles cause high pressure leading to ultrafiltration of blood. The walls of the blood capillaries are one cell thick hence glucose, amino acids, vitamins, water, salts and urea filter into Bowman’s capsule, to form glomerular filtrate. Blood cells and plasma proteins are too large to pass through the capillary wall and hence remain in blood capillaries. Glucose, amino acids, vitamins, water and sodium chloride are selectively reabsorbed back into the bloodstream at the proximal convoluted tubule. The tubule is highly coiled to increase surface area for selective reabsorption of the substances. The cells lining the proximal convoluted tubule have numerous mitochondria to provide energy for the active reabsorption of substances. The glomerular filtrate flows into the descending and the ascending loop of Henle, blood in the capillaries and the glomerular filtrate in the loop of Henle move in opposite directions/counter-current flow to create a steep concentration gradient that leads to maximum reabsorption. Sodium chloride is actively absorbed from the ascending loop of Henle in the blood capillaries under the influence of the aldosterone hormone. In the distal convoluted tubule, more water is reabsorbed by osmosis under the influence of antidiuretic hormone. The glomerular filtrate flows into the collecting tubule now referred to as urine. From the collecting duct, the urine passes through the pyramid, pelvis and ureter into the bladder where it is stored for some time. The sphincter on the urethra relaxes to allow urine to be released from the body.

Explain the adaptations of Kidney tubules to their function 

(a) Proximal Convoluted Tubule

• Cells lining the tubules have numerous mitochondria to produce energy for the active reabsorption of materials back into the bloodstream.
• Cells lining the tubules have numerous microvilli to increase the surface area for reabsorption.
• It is highly coiled to reduce the speed of the glomerular filtrate thus giving it more time for efficient reabsorption.
• It is highly vascularized to transport reabsorbed substances.
• Have thin-walled blood capillaries to reduce diffusion distance.

(b) Loop of Henle

• Cells lining the tubules have numerous microvilli to increase the surface area for reabsorption.
• Long to increase the surface area for reabsorption.
• Cells lining the tubules have numerous mitochondria to produce more energy for active reabsorption of materials back to the bloodstream.
• It is U-shaped to allow a counter-current flow system hence creating steep concentration gradient for rapid reabsorption.
• It is highly vascularized to transport reabsorbed salts;
• Have thin-epithelium blood capillaries to reduce diffusion distance.

(c) Distal Convoluted Tubule

• Cells lining the tubules have microvilli to increase the surface area for reabsorption.
• It is highly coiled to reduce the speed of the glomerular filtrate thus giving it more time for efficient reabsorption.
• It is highly vascularized to transport reabsorbed substances;
• Have thin-walled blood capillaries to reduce diffusion distance;

Trace the shortest route for a molecule of urea from the site of manufacture until it reaches the urinary bladder

• Urea is formed in the liver; it leaves via the hepatic vein into the vena cava; into the right ventricle; into the heart; into the pulmonary artery; lungs; pulmonary vein; into the left atrium; then the left ventricle of the heart; aorta; renal artery; kidneys arterioles; into the glomeruli capillaries; into the Bowman’s capsule; then the nephron; into the collecting duct; out of the kidney via ureter; into the bladder;

Explain how the osmotic pressure in the human blood is maintained at a normal level

• When the osmotic pressure of blood increases above the normal level ( osmoreceptors in the) hypothalamus is stimulated to send an impulse to the pituitary gland; The pituitary gland is stimulated to produce more antidiuretic hormone (ADH); Which make kidney tubules more permeable to water; and more water is reabsorbed into the blood lowering the osmotic pressure to normal;
• When osmotic pressure of blood decreases; (osmoreceptors in) hypothalamus and pituitary gland are less stimulated; less ADH  is produced ;which make the kidney tubules less permeable to water ;less water is reabsorbed into the blood raising the osmotic pressure to normal;

Outline and explain the various homeostatic functions of the liver in mammals

• Deamination– gets rid of excess amino acids in the body as the body is not able to store them; Excess amino acids are broken down into amino group and carboxyl group. The amino group combines with Hydrogen ion to form ammonia. Ammonia enters the Ornithine cycle where it combines with Carbon (IV) oxide to form urea. Urea is excreted in urine through the kidney.
• Detoxification– is the process where toxic compounds are converted to less toxic compounds. Toxicity is caused by medication, drugs and microorganisms. Detoxification prevents the accumulation of toxins in body cells which could lead to death or malfunctioning of body cells;
• Thermoregulation– many metabolic activities take place in the liver releasing heat energy that is distributed by blood to other parts of the body. When the blood vessels dilate during hot conditions when temperatures are low; the blood vessels constrict under the influence of the endocrine and nervous systems hence less blood is stored in the liver; which contributes to thermoregulation.
• Blood sugar regulation– liver cells convert excess glucose into glycogen and fats under the influence of insulin hormone; The stored glycogen is converted into glucose when the glucose level is low by liver cells under the influence of glucagon hormone;
• Regulation of plasma proteins – plasma proteins such as prothrombin and fibrinogen are manufactured in the liver using the amino acids found in the liver; they play a major role in blood clotting that prevent excessive blood loss and infection at the injured area;

Explain the role of the following hormones during homeostasis

• Antidiuretic hormone (ADH)

Secreted by the posterior lobe of the pituitary gland in response to an increase in the osmotic pressure of blood (detected by the hypothalamus). The hormone stimulates the distal convoluted tubule and collecting ducts to increase their permeability to water. This increases the reabsorption of water into the bloodstream. Hence concentrated and less urine is excreted. When the osmotic pressure decreases, less or no hormone is produced hence the tubules become impermeable to water. Less is water is reabsorbed into the bloodstream. Hence more dilute urine is excreted.

• Insulin

Secreted by the pancreas in response to a rise in blood sugar level. It stimulates liver cells to convert the excess glucose into glycogen and fats for storage in the liver and muscle cells; increases glucose metabolism to yield energy, water and carbon (IV) oxide.

• Glucagon

Secreted by the pancreas in response to a decline in blood glucose level. It stimulates liver cells to convert the stored glycogen and fats back to glucose. Inhibit glucose metabolism. The glucose formed is released to the bloodstream causing a rise of glucose level to normal.

BIOLOGY ESSAYS FORM 3 AND 4

Biology Essays: Ecology

Explain how abiotic factors affect plants

• Wind

Windy conditions increase transpiration rate;

Wind disperses fruits/seeds/spores;

It is also an agent of pollination;

• Temperature;

Change in temperature affect rate of biochemical reactions/metabolic/ photosynthesis/enzymatic reactions; rise in temperature rises transpiration.

• Light;

Green plants need light for photosynthesis;

Some plants need it for flowering;

Some seeds (like lattice) require it for germination;

• Humidity

When humidity is low, transpiration rate rises;

• PH

Each plant requires specific PH to grow well;  Acidic or alkalinity or neutral

• Salinity

Plants with salt tolerant tissues (e.g. mangrove) grow in saline area; plants in estuaries adjust to salt fluctuations;

• Topography

North falling slopes in temperate lands have more plants than south facing slopes;

Windward side plants have stunted and distorted growth; leeward side plants are stunted/wind ward normal growth;

• Rainfall/water

Few plants in dry areas/where rainfall is less;

Water for germination;

Water as a raw material for photosynthesis;

Water as solvent for mineral salts;

Provides turgidity;

Water for dispersal;

A medium of transport of plant nutrients;

• Mineral salts

Plants thrive (grow well in soils with mineral salts)

Plants living in soil with deficiency of particular element have special methods of obtaining it. Legumes obtain nitrogen by nitrogen fixation /carnivorous plants/insectivorous plants, carnivorous trees obtain their nutrients from mycorrhizal association;

Explain how abiotic factors affects population growth of organism.

Biotic factors are all the living organisms in an environment and their effects on other living organism;

• Competition

Organisms compete for food, mates, shelter and water. Competition between individuals of the same species is called intraspecific competition; while competition that occurs between individual of different species is called interspecific competition. During competition the less successful one dies or migrates.

• Predation

It is feeding relationship in which the predator feeds on the prey. When prey population increases, the predator population also increases. When predator population increases, prey population decreases.

• Parasitism

It is association in which an organism lives in/on another living organism. The parasite causes diseases and weakness to the host and may kill the host. The host provide food and shelter for the parasite.

• Saprophytism

It is a type of nutrition in which an organism obtain nutrients from dead decaying matter. Saprophyte releases digestive enzymes which digest dead matter into nutrients that plants can absorb and utilize thus increasing their populations. Some saprophytes aid in recycling elements such as nitrogen.

• Symbiosis

It is a mutual relationship between two organisms where each organism benefit from each other. Example;

Lichen is a relationship between blue-green algae and fungus.

Symbiotic bacteria (Rhizobium) in the root nodules of leguminous plants. Rhizobium bacteria fixes nitrogen to the plant while the plant provides food and shelter to the bacteria.

Describe the Nitrogen cycle

Nitrogen Cycle; It is the cycling of nitrogen and its compounds in nature.

Involve 3 main processes; nitrogen fixation, nitrification and denitrification. Free atmospheric nitrogen is converted into nitrates through a process known as nitrogen fixation.

• Nitrogen fixation

Nitogen fixation is the conversion of the free atmospheric nitogen into forms that are utilisable by the plant. It occurs in 2 forms; biological and non-biological fixation.

Biological fixation of nitrogen is done by nitrogen-fixing bacteria; which are either symbiotic or free-living;

Symbiotic bacteria are of the genus Rhizobium and are found in the root nodules of the leguminous plants. The bacteria convert free atmospheric nitrogen into ammonia that is used directly by the leguminous plants to form nitrogen containing organic compounds (amino acids, nucleic acids, proteins).

Nitrogen fixation is also done by free-living bacteria e.g. Clostridiumand some algae e.g.  Anabaena, Chlorella and Nostoc.

Non-biological nitrogen fixation is achieved by lightning. During thunderstorms, the lightning energy combines atmospheric nitrogen with oxygen to form nitrous acid and nitric acid.

These are then chemically converted into nitrates. Plants absorb nitrates from the soil and then assimilate them into plant proteins. Thus animals obtain the nitrogen in form of proteins directly or indirectly from plants. The proteins are digested into amino acids and assimilated into animal proteins.

When organisms die (or their waste and droppings), saprophytic bacteria and fungi break down the protein material in their bodies into ammonia. Ammonia is eventually converted into nitrates in a process called nitrification.

• Nitrification

It is the conversion of ammonia to nitrates.

Occurs in 2 steps;

• Conversion of ammonia to nitrites by nitrifying bacteria of genus Nitrosomonas and Nitrococcus.
• Conversion of nitrites to nitrates by Nitrobacter.

Nitrification enriches the soil with nitrates.

• Denitrification

Denitrifying bacteria (e.g. Pseudomonas denitrificans and Theobacillus denitrificans ) reduce nitrate to nitrites, ammonia and even nitrogen gas. They utilise the oxygen released in the process for their respiration. Denitrification deprives the soil of fixed nitrogen gas. But it helps release free nitrogen into the air,  making it available for cycling.

Describe the adaptation of floating water lily leaf to its photosynthetic function.

• Broad flat lamina to provide large surface for carbon (IV) oxide and sun light absorption;
• Thin leaf to allow light/ carbon (iv) oxide to pass through a shortdistance;
• Presence of numerous stomata; on the upper epidermis, ensuring efficient diffusion of carbon (IV) oxide;
• Transparent cuticle; to allow penetration of light to palisade cells / photosynthetic cell;
• Presence of numerous aerenchyma cells; for buoyancy to reach light and carbon (IV) oxide;
• Has extensive network of veins; Conducting water and mineral salt; the photosynthetic cells;
• Veins contain xylem for transport of water and mineral salts; and phloem for translocation of product of photosynthesis;

Explain how halophytes are adapted to their habitat. 

• They have root hair cells with highly concentrated sap vacuole to increase uptake of water by osmosis
• Some plants have salt secreting glands that secrete excess salts
• Some have pneumatophores /breathing rootswhich emerge above water to obtain atmospheric oxygen
• Some that grow in mudflats have buttress roots for support and anchorage;
• Fruits have tissues with large air spaces for gaseous exchange and buoyancy;
• Submerged halophytes have sensitive chloroplast capable of photosynthesizing in low light intensities;

Discuss air pollution under the following headings:

Causes, sources and effects in the environment

• Sulphur (IV) oxide; produced from processing industries;

Cause respiratory disease and affects gaseous exchange.

It combines with moisture to form acid rain which falls and destroy plants leaves reducing the surface area for photosynthesis. Acid rain also corrodes metallic roof tops.

•  Aerosols (pesticides, fungicides and herbicides which are sprayed to control pests, fungi and weeds);

Contain heavy metals that affect respiratory system of animals by causing irritation and poisoning.

• Chlorofluorocarbons (CFCs); produced from refrigeration and air condition units;

Destroy ozone layer thus increase filtration of radioactive rays from the sun which cause genetic mutation and sunburns or skin cancer.

• Smoke and fumes; produced in areas with heavy industries or motor vehicles that burn fuels.

Smoke contains Carbon (II) oxide which combine with haemoglobin and reduce its ability to carry oxygen, cause suffocation and finally death.

Fumes also settle on plant leaf surface hence reducing rate of photosynthesis;

Excessive CO₂ in smoke cause greenhouse effect resulting into over-heating of the lower horizon of the atmosphere inhabited by living organisms beyond the optimum levels for enzymatic reaction.

• Noise; produced from heavy machines, vehicles and aeroplane.

Destroy eardrum thus impair hearing 

• Dust; produced in quarries, dusty roads, cement and limestone producing industries;

Settle on plant leaves and block leaf surface area exposed for trapping of light and CO₂ trapping, lowering the rate photosynthesis;

• Radioactive emissions; from mines, atomic bombs, nuclear reactants;

Cause mutations (change genetic make-up).

Control of Air Pollution

• Using lead-free petroleum products and paints.
• Banning discharge of toxic gases from industries and vehicles and smoking in public places.
• Recycling CFCs in refrigerators.
• Using absorbers and filters of toxic fumes in vehicles and industries.
• Biological control of weeds, pests and diseases.
• Encouraging the use of renewable sources of energy e.g. wind and solar energy.
• Using catalytic converters to reduce the release of oxides of nitrogen and Sulphur. Improving engine designs so that CO is converted to CO₂.
• Educating people on the need for sustainable environmental management.
• The government should be a signatory to global treaties on environmental conservation.

Discuss water pollution under the following heading:

Causes, Sources and Effects of water pollution

• Industrial effluents
• Contains heavy metals such as lead and mercury. These kill aquatic life and pollute domestic water.
• Contains wastes of organic compounds, nitrogen and phosphorus. May lead to eutrophication (massive growth of aquatic algae). Eutrophication leads to reduced oxygen and reduced light in the water. This causes the suffocation of organisms and reduced photosynthesis.
• Hot effluents reduce dissolved gases in water and kill some organisms.
• Domestic waste and raw sewage

Involves disposal of waste water with detergents, garbage, faeces and nitrogenous wastes into water masses.

• It gives rise to disease epidemics such as cholera, amoebic dysentery and typhoid.
• The pollutants also kill aquatic life such as fish as there is a shortage of oxygen in the water.
• Oil spills

Oil spillage occurs in the oceans from oil tankers and offshore oil refineries.

• The oil layer reduces oxygen supply in the water which leads to suffocation and death of aquatic plants.
• The oil coats photosynthetic algae and kills them.
• The oil layer leads to reduced light penetration into the water which affects photosynthesis.
• Agricultural chemicals

Include fertilizers, pesticides and herbicides.

They contain heavy metals like lead and mercury which are toxic.

They reach water through seepage especially when it rains.

• They affect respiratory systems of aquatic organisms. Lead to death of organisms when they get to high concentrations.
• Nitrates and phosphates cause eutrophication in water bodies.
• Lead

Comes from pipes and tanks in domestic water supply systems.

• It affects the physiological functioning of body organs.
• It damages the nervous, resulting in mental problems.
• Soil erosion
• Causes siltation in water bodies, making water unfit for domestic use,
• It clogs respiratory surfaces of aquatic organism like gills in fish and stomata in plants, thus interfering with gaseous exchange.

Control of Water Pollution

• Treating sewage and industrial waste.
• Using biodegradable organic fertilizers.
• Replacing metal pipes with plastic pipes.
• Practicing good farming methods like contour farming to reduce soil erosion
• Establishing laws against oil spillage by tankers and imposing penalties.
• Educating the public on correct use of agricultural chemicals.

Describe the adaptations of Schistosoma spp to their parasitic mode of life

• Has two hosts (snail and humans) thus enhancing their survival.
• It can tolerate low oxygen concentration in the host’s tissues.
• Has suckers for attachment to avoid being dislodged from the host’s body.
• It lays many eggs to increase its survival chances.
• Adult worms secrete chemicals (defense mechanism) which protect them from the hosts.
• Reproduces by many larval forms (cercaria and miracidia) in snails, to increase chances of transmission and survival;
• Have glands that secrete lytic enzymes that soften the tissues of human/snail to allow for penetration.
• Exist as separate sexes with the male carrying the female to ensure that eggs produced by females are fertilized before being shed into the blood stream;

Outline the measures that can be employed to prevent and control the spread of Schistosoma spp

• Proper disposal of faeces and urine.
• Treat or boil drinking water.
• Wear protective garments when swimming, bathing and walking in water.
• Kill the snails using molluscicides.
• Drainage of stagnant water.
• Provide medical treatment to infected persons.
• Personal hygiene

Describe the adaptations of the Xerophytes to their environment

• Some have folded leaves to reduce surface area for transpiration;
• Some have needle-like leaves to reduce surface area for transpiration;
• Some have leaves modified to spines or scales to reduce surface area for transpiration;
• Some have fewer leaves to reduce surface area for transpiration;
• Some (deciduous) shed leaves during drought to reduce surface area for water loss.
• Others have hairy leaves; the hairs trap moisture reducing saturation deficit to reduce water loss through transpiration.
• Some have few stomata on the upper surface of the leaf.
• Some have reversed stomatal rhythm (close stomata during the day and open at night) to reduce water loss.
• Small sized stomata; usually located on the lower epidermis; to reduce transpiration.
• Some have sunken stomata; that trap moisture in the sunken pits; lowering saturation deficit; hence reduced transpiration rate.
• Some have thick waxy cuticle to reduce transpiration e.g. sisal.
• Some have long tap root system to absorb water from lower layers of the soil.
• Others have superficial roots that grow horizontally to absorb surface water after light showers.
• Some have succulent stems and leaves with large parenchyma cells to store water e.g. cactus, Bryophyllum
• Some have a very short life cycle to evade drought, hence survive as seeds or as underground perennating organs e.g. corms and bulbs.

Describe the adaptations of the Mesophytes to their environment

• Trees grow tall to compete for light. There is little undergrowth.
• Climbers such as lianas support themselves on stems of large trees in order to reach light.
• Have numerous leaves with numerous chloroplasts to increase the surface area for transpiration and photosynthesis.
• Show leaf mosaic arrangement to minimize overlapping of leaves for maximum exposure to sunlight for photosynthesis.
• Have broad leaves with thin cuticles and numerous stomata on both sides of the leaves to increase photosynthesis and transpiration.
• Some have buttress roots for extra support.
• Some in drier areas have more stomata on the lower leaf surface than on the upper surface to reduce the rate of transpiration.
• Some have waxy and glossy surfaces to reflect light rays and allow rain water to drop off easily.
• Epiphytes (mosses and orchids) grow on branches of taller trees.

Describe the adaptations of the Hydrophytes to their environment

• Emergent hydrophytes (such as water lily and water hyacinth) have broad leaves with many stomata on the upper surface to increase transpiration.
• Submergent hydrophytes (such as Ceratophyllum plants) have highly dissected leaves to increase surface area for photosynthesis and gaseous exchange.
• Their leaves have numerous and sensitive chloroplasts that photosynthesize under low light intensity.
• Have aerenchyma (large air-filled tissues) for buoyancy and gaseous exchange.

Aerenchyma tissue is also tolerant to ethanol produced during anaerobic respiration.

• Their flowers are raised above the water to allow pollination.
• Floating hydrophytes have long fibrous roots to absorb mineral salts.
• Some have poorly developed roots to reduce absorption of water e.g. Salvinia
• Others have poorly developed vascular bundles because they absorb water by diffusion e.g. Salvinia
• Some have hairy and waxy leaves, hence waterproof to keep water away from the leaf surface.
• Submerged hydrophytes (e.g. Elodea) have no stomata on their leaves. Oxygen produced by photosynthesis is stored in their aerenchyma tissues and used for respiration.

Describe the adaptations of the Halophytes to their environment

• Have pneumatophores (aerial roots) for gaseous exchange.
• Have lenticels for gaseous exchange.
• They have root cells have a very concentrated sap vacuole  to increase uptake of water by osmosis
• Some plants have salt secreting glands that secrete excess salts
• Some that grow in mudflats have buttress roots for support and anchorage
• Fruits have aerenchyma tissue for air storage that makes them buoyant
• Submerged halophytes have sensitive chloroplast capable of photosynthesizing in low light intensities.

Explain the importance of protecting the forest ecosystem with reference to the following;

• climate change
• Promote (regular) rainfall/ precipitation/ prevent desertification;
• Act as wind breakers;
• Keep earth temperatures cool/ reduce global warming;
• Keeps biogeochemical cycles going e g hydrological, carbon, nitrogen, phosphorous, sulphur cycles;
  • Biodiversity
• Conserve diverse flora fauna;
• Conserve genetic variety;
• Prevent extinction of rare species;
• Source of research/ employment;
• Aesthetic attracting tourism in foreign exchange;
• Have impact on culture/ religion/ politics;
• Food and shelter for other organisms and man;
• Source of oxygen;
  • biotechnology
• Manufacture of medicines/directly used as medicinal
• Source of food/ food products;
• Provide fuel (when regulated);
• Provide paper and related by-products (when regulated);
• Provide timber (when regulated);
• Products used in other industries e  tannin, wax, rubber, oil, honey;

• water conservation
• Increased ground water/ high water tables;
• Adds into rivers/ lakes/ permanency in  existing water bodies/ reservoirs; ,
• Water towers/ water catchment;

• pollution
• Minimize soil pollution ensuring cover against surface run-off/ wind erosion/ denudation
• Trees/ vegetation clean the soil surface by absorbing nutrients from decomposed matter e g sewage;
• Large scale clean-up of polluted air/ dust;
• Muffle noise pollution;

Biology Essays: Reproduction in Plants and Animals

Describe the mode of reproduction in a named fungus

Rhizopus/mucor/mould reproduce asexually; by sporulation; Spores develop from a single cell in the sporangium; which bursts on maturity releasing spores which are dispersed by air currents/wind; germinating to form new generation/form mycelium (if it lands on a suitable medium)

OR

Yeast reproduce asexually; by budding; parent cell form an outgrowth/projection/bud; followed by division of nucleus into two; One of nuclei moves into the new bud; which grow and develops into a new cell;

Discuss adaptations of fruits and seeds to their various methods of dispersal

Wind Dispersal

• Censor mechanism; the seed capsule is in a way that when it opens , seeds are easily blown away by wind;
• Have wing-like/feather-like/hair-like structures for floating in air;
• Have fine hair to increase surface area for floating in air;
• Small and light so that they are easily carried away by wind;
• Are loosely attached on the stalks so that they can easily be broken and carried away by wind;
• Have parachute like-structure for buoyancy and to be blown easily by wind;  

Water Dispersal

• Have air spaces/spongy/fibrous; thus light and able to float on water easily;
• Pericarp/endocarp/testa is waterproof to prevent entry of water;
• Have outer covering that easily absorb water, swell and burst open to release seeds; eg water lily
• The seeds can remain viable while in water and only germinate while on a suitable medium;

Animal Dispersal

• Large and conspicuous to be seen easily;
• Fleshy succulent to be eaten;
• Brightly coloured when ripe to attract animals;
• Seeds have hard seed coats hence pass through the digestive system undigested;
• Some have hooks for clinging onto the animal’s body; eg black jack
• Some have sticky hairs that attach on the animal’s body.

Self-explosive mechanism

• Have sutures /lines of weaknesses along which pods splits open to release seeds;
• Censor mechanism; in which pods/capsules splits violently to release seeds into the air;

Discuss adaptations of flowers to wind and insect pollination

• Insect Pollinated (Entomophilous) Flowers
• Some are scented to attract insects;
• Have sticky stigma for pollen grains to attach on;
• Brightly coloured petals to attract insects;
• Presence of nectar to attract insects;
• Have nectarines to secrete nectar; Have nectar guides to guide the insect to the nectarines;
• Stigma located inside the flower to increase chances of contact by insects;
• Anthers located inside the flower to increase chances of contact by insects;
• Sticky pollen grains which stick on the body of insect.
• Large and conspicuous flowers easily seen to attract insects;
• Anthers firmly attached to filament for insect to brush against them;
• Have landing plat form to ensure contact with anther and stigma;
• mimicry to attract (male insect);

• Wind Pollinated (Anemophilous) flowers
• Stigma hang outside the flowers to increase chances of pollination,
• The style is long to expose stigma;
• The filament is long to expose anther;
• Stigma is hairy/feathery to increase surface area over which pollen grains land/trapped;
• Pollen grains are smooth/light/small to be easily carried by wind;
• Large amounts of pollen grains;
• Pollen grains may have structures which contain air to increase structures which contain air to increase buoyancy;
• Flowers have long stalks holding them out in the wind.

Describe the process of double fertilization in flowering plants

• Pollen grains land onto the stigma and adhere to it as a result of the stigma cells secreting a sticky substance. It absorbs nutrients; and germinates forming a pollen tube; the pollen tube grows down the style to the ovary; deriving nourishment from surrounding tissue.
• The pollen tube has tube nucleus at the tip; and generative nucleus immediately behind it; As the pollen tube grows downwards into the ovary the generative nucleus divides mitotically, to give rise to two male nuclei; which represent the male gametes; the pollen tube penetrate the ovule/embryo sac through micropyle.
• After the pollen tube enters the embryo sac the tube nucleus breakdown/ disintegrates; leaving a clear passage for the entry of the male nuclei. The two male nuclei then enter into the embryo sac; where one male nucleus fuses with the egg cell nucleus; to form a diploid zygote; which develop into an embryo.
• The other male nuclei fuse with the two polar nuclei; to form a triploid nucleus/ primary endosperm nucleus; which becomes the endosperm. This is called double fertilization;

Discuss the various events that occur in a flower after fertilization

• Integuments develops into the seed testa.
• The ovary wall forms fruit wall;
• The ovule(s) develops into seed(s);
• The corolla dries and withers away;
• The  calyx may persist or dries and wither away;
• Stigma and style shrivels, dries and wither away;
• The androecium (male part) shrivels dries and withers away;
• Triploid nucleus develop into primary endosperm of the seed;
• The zygote formed develops into embryo;

Describe what happens when the pollen tube reaches the embryo sac during double fertilization in flowering plants.

• Tip of the pollen tube burst open; one of the nuclei fuses with the egg cell nucleus; to form a diploid zygote; while the remaining male nucleus fuses with the polar nuclei; to form a triploid endosperm nucleus;

Describe how the male reproductive system is adapted to its function.

• Consists of two testes suspended outside the abdominal cavity in a scrotal sac/scrotum for protection;
• The testes are suspended outside the abdominal cavity to provide a relatively lower temperature suitable for sperm production;
• Testes have seminiferous tubules that are highly coiled and have actively dividing cells that produce sperms;
• Seminiferous tubules have interstitial cells that secrete androgens (testosterone) that promote development of secondary sexual characteristics;
• Seminiferous tubules join together to form epididymis that enhance temporary storage of sperms;
• The sperm duct/vas deferens which has thick muscular walls that contract to propel sperms to the urethra;
• Seminal vesicle produce an alkaline fluid containing nutrients for the spermatozoa/sperm cells
• Prostate gland that secretes an alkaline fluid that neutralizes the acidic vaginal fluids  and also activates the sperms;
• Cowper’s gland which secretes an alkaline fluid which neutralizes the acidity caused by urine along the urethra;
• The urethra is a long tube running the length of the penis, used for conduction and expulsion of urine and passage of sperms during copulation;
• The urethra follows the penis that projects from the body at the lower abdomen;
• The penis consists of a retractable skin called prepuce/foreskin that covers a swollen/bulbous end region of the penis known as the glans;
• The penis is made up of spongy erectile tissue consisting of numerous small blood spaces, muscles and blood vessels; the spongy tissue gets filled with blood making the penis become erect during sexual stimulation excitement and activity enabling the penis to penetrate the vagina during coitus/copulation/sexual intercourse, in order to deposit sperms in the vagina of female;

Describe how the various structures of the human female reproductive system are adapted to their function

Ovaries

• Have several Graafian follicles that develop and burst open to release/produce mature ova
• Secretes sex hormone (Oestrogen) which initiate/control development of secondary sexual characteristics
• Produce hormones Oestrogen and progesterone which prepare the uterus for implantation and subsequent nourishment of the embryo

Oviduct (fallopian tube)

• Are thin narrow and tubular to increase flowing speed of semen containing sperms
• Are funnel shaped on the end next to ovary which enables them to receive the ovum
• Their lining contains cilia which propel the ovum towards the uterus
• Has peristaltic muscles that enable movement of zygote/ovum to the uterus for  implantation
• Is fairly long to increase surface area for fertilization

Uterus         

• Is muscular for protection of developing embryo
• Has elastic wall that allows growth and development of foetus/embryo
• Has a highly vascularised endometrium that provides nutrients to developing embryo

Cervix

• Has valves that close the lower end of the uterus to ensure continued pregnancy during gestation period
• is capable of dilating
• Has narrow entrance/neck-like entrance to uterus that enables quick swimming of sperms to uterus
• Has suction mechanism that draws up/pulls sperms into uterus
• Has a “W” shape that fits well with the glands of the penis to ensure sperms are deposited at the right point

Vagina

• It is elastic and muscular to enable good accommodation or penetration of the penis thus proper deposition of sperms and for easy parturition
• Allows menstrual flow
• Has sensitive labial walls which secrete/produce lubricating substances that ensure/enable/facilitate good coition
• Capable of considerable enlargement, due to elastic muscles, to accommodate baby during parturition

Clitoris

• Has sensitive cells for orgasm

Describe the menstrual cycle

• Just after menstruation, the anterior lobe of pituitary gland starts secreting FSH.
• FSH cause the development of the Graafian follicles in the ovary.
• FSH also stimulate the ovarian tissues to secrete oestrogen.
• Oestrogen cause the repair and healing of the endometrium which is destroyed during menstruation.
• At certain level, oestrogen stimulates the anterior lobe of the pituitary gland to produce LH.
• LH stimulates maturation of the Graafian follicle causing ovulation.
• After releasing the ovum, the Graafian follicle changes into a yellow body called corpus luteum.
• LH then stimulates the corpus luteum to secrete a hormone called progesterone.
• Progesterone stimulates the thickening and increased blood supply to the endometrium.
• This prepares the uterus for implantation of the blastocyst.
• If fertilisation takes place, the level of progesterone increases and thus inhibits FSH from stimulating the maturation of another Graafian follicle.
• If fertilization does not occur, the corpus luteum disintegrates and no more progesterone is produced. Hence the endometrium sloughs off and menstruation occurs.
• This completes the cycle.
• Menstrual cycle lasts for about 28 days in human beings.

Describe the role of hormones in the human menstrual cycle

Anterior lobe of pituitary glands secretes follicle stimulating hormones (FSH). 

Follicle Stimulating Hormone

• Causes primary follicle to develop into Graafian follicle in the ovary.
• It also stimulates tissues of ovary to secrete Oestrogen.

Oestrogen

• Causes repair/healing of uterine wall after menstruation;
• Stimulates anterior lobe of pituitary to produce LH.

Luteinising Hormone

• Causes ovulation.
• It also causes remains of the burst Graafian follicle to change into corpus luteum
• Stimulates corpus luteum to secrete progesterone.

Progesterone

• Causes proliferation of uterine wall in preparation for implantation.
• Inhibits the production of FSH by anterior lobe of pituitary gland thus no more follicles develops and oestrogen production reduces.
• In the next two weeks, progesterone level rises and inhibits production of LH.

If fertilization fails, then the corpus luteum disintegrates. This leads to decline in the level of progesterone hence the endometrium sloughs off and menstruation sets in. the anterior lobe of pituitary gland starts secreting FSH again.

List down the role of the following hormones during pregnancy

Oestrogen

• Stimulates the development of duct system of breasts.
• Inhibits FSH release.
• Inhibits release of prolactin and hence inhibits lactation.
• Stimulates growth of uterus (particularly muscles).
• Increases sensitivity of myometrium to oxytocin.

Progesterone

• Stimulates development of milk glands in breast ready for lactation.
• Inhibits FSH release.
• Inhibits prolactin release and therefore inhibits lactation.
• Inhibits contraction of myometrium.
• Maintains lining of uterus.

Describe the hormonal control of the production of gametes and the development of secondary sexual characteristics in human.

(a) IN FEMALES,

FSH

• Promotes development of primary follicle into Graafian follicle.
• Stimulates secretion of oestrogen by the tissues of the ovary.

Oestrogen

• stimulates pituitary gland to secrete LH
• Cause development of secondary sexual characteristics.

LH

• Stimulates maturation of the Graafian follicle causing ovulation
• Stimulates release of progesterone by.

Progesterone

• Sustains pregnancy by inhibiting release of prolactin and oxytocin during pregnancy.

Prolactin

• Stimulates milk formation by mammary glands.

Oxytocin

• Causes parturition and milk ejection from alveoli into the reservoirs from where it can be sucked by the baby.

(b) IN MALES

• FSH– stimulates synthesis of and maturation of sperms.
• LH – stimulates production of androgens (testosterone)
• Testosterone – stimulates development of secondary sexual characteristics.

Describe the process of fertilization in female mammal

• Fertilization is the process of fusion of the nuclei of the male and female gametes to form a diploid zygote.
• It occurs in the upper part of the oviduct after copulation. The sperms are drawn up by suction through the cervix into the uterus from where they swim up to the oviduct using their tails. Although a single ejaculation contains millions of sperms, only one fertilize the ovum.
• The sperm disperses the follicle cells surrounding the egg. Once it comes into contact with the egg, the acrosome bursts open and releases lytic enzymes which dissolve the egg membranes. The acrosome then turns inside-out forming a fine filament that is used to penetrate the egg. Hence the head of the sperm enters the ovum but the tail is left outside the ovum. The vitelline membrane undergoes a change which stops any other sperm from entering the ovum. Once in the cytoplasm the head bursts to release the male nucleus which then fuses with the female nucleus to form a diploid zygote.

Describe the role of pituitary gland in female reproductive system.

Produces:

• FSH – which stimulates the development of Graafian follicle; Also stimulates the corpus luteum to secret progesterone;
• LH- Stimulates maturation of gratin follicle and ovulation. Stimulates the corpus luteum to secret progesterone;
• Oxytocin – stimulates the construction of the   where wall to bring about parturition. Stimulates production of milk of the mammary gland (milk let down)

Biology Essays: Growth and Development

Discuss the various phases of growth.

Lag phase/ slow growth;

• Growth is slow; due to 
• No. of cells dividing are few;
• Cells not adjusted to environmental factors;

Exponential phase/Lag phase;

• Growth rapid;
• Increase in number of cells dividing;
• Cells adjusted to new environment;
• Food/other factors (to be mentioned) not limiting; hence no completion for resources;
• Rate of cell increase is higher that rate of cell death;’

Decelerating phase;

• Slow growth;
• Most cells fully differentiated;
• Fewer cells still dividing;
• Environmental factors/must mention examples e.g. shortage of oxygen, nutrients;
• Space limited due to high number of cells;
• Accumulation of metabolic wastes;

Plateau phase;      

• Growth has ceased;
• Cell division equals cell death;
• Cells and tissues fully differentiated; therefore no increase in number of cells

Explain the conditions necessary for germination in seeds.

• Water (moisture);
• Activates germination enzymes;/ breaks seed dormancy
• Provides medium for enzymes to act;
• Is a medium for transportation of dissolved food;
• Softens seeds coat, which burst open to allow emergence of radicle and plumule;
• Hydrolysis of food during germination;

• Oxygen   
• Oxidation of food during respiration to provide energy for germination;/cell division and formation of new tissues;

• Optimum temperature
• Suitable for action of germination enzymes which hydrolyse stored food;
• Low temperature below optimum inactivates germination enzymes slowing down germination rate;
• High temperature above 400c denatures germination enzymes stopping germination;

• Enzymes
• Breaks down food by oxidation
• Food stored are in insoluble form e.g. carbohydrates fats and proteins and hence must be acted upon by the enzymes ;

Example 

Viability;

• Refers to percentage change that a seed will germinate and when planted;
• Only seeds with live and healthy embryos will germinate and grow;
• Seeds stored for a long time lose their viability;

Hormones

• These stimulate certain metabolic pathways in the germination process;

Describe the role of growth hormones in plant growth and development

(i) Auxins/Indole Acetic acid

• Promote fruit formation/parthenocarpy;
• Promote cell division;
• Influences cell elongation/tropic responses;
• Promote formation of abscission layers/bring about fruit fall/leaf fall;
• Cause apical dominance/inhibit growth of lateral branches/buds;
• Promote growth of adventitious roots;

(ii) Ethylene

• Induces stem thickening;
• Inhibit stem elongation;
• Induces ripening of fruits;
• Promotes germination of certain seeds;
• Promotes development of abscission layer leading in fruit/leaf fall;
• Florigens
• Promote flowering

(iii) Cytokinins

• Promote root formation;
• Stabilizes proteins and chlorophyll;
• Promote flowering in some plant species;
• Breaks seed dormancy in some plant species;
• In low concentration it promotes leaf aging;

(iv) Gibberellins

• Promotes parthenocarpy;
• Initiate formation of IAA;
• Promote ripening of fruits after fertilization;
• Inhibit growth of adventitious roots;
• Slow down leaf abscission;
• Activates hydrolytic enzymes during seed germination;
• Promote cell division/cell elongation in dwarf varieties;

(v) Abscisic Acid

• Causes seed dormancy/bud dormancy;
• Causes abscission of leaves/fruit fall;
• Inhibit seed germination;
• In high concentration it leads to closure of stomata;

Describe secondary thickening in flowering plants 

• Secondary thickening is facilitated by meristematic cells; called cambium; located between phloem and xylem in vascular bundles of the plant; the cambium divides radially; to form secondary phloem outside and secondary xylem inside; the ring cells forms intravascular cambium/cambium between vascular bundles divides  to form secondary parenchyma; thereby increasing the growth of modularly rays; much more xylem is formed than phloem; thus pushing phloem and cambium ring outwards; the rate of secondary growth is dependent on seasons/rains; resulting in annual/rings; cork cambium divides to form new cork/bark tissue ; to accommodate increased growth) on the outside and secondary cortex on the inside;

Describe one method which can be used to measure the average growth rate of a single leaf on a plant

Identify a young germinating seedling; mark its radical with Indian ink or permanent ink at intervals e.g. of 2mm; Leave it to grow for some time e.g. 24 hrs or overnight; Measure the distances between successive ink marks; and record; calculate the growth rate as;

Biology Essays: Genetics

Distinguish between mutations, mutants and mutagens. Give two causes of mutations. Describe the causes and effects of chromosomal mutations

Distinguish between mutations, mutants and mutagens.

• Mutations – are sudden, spontaneous and permanent changes in an individual’s genetic make-up;
• Mutants – are individuals who develop and exhibit unusual characteristics that were not previously present in the population due to mutations;
• Mutagens – are factors in the environment that cause mutations;

Give two causes of mutations

• Radiations like gamma  rays and ultra-violet rays
• Chemical substances like heavy metals such as mercury, lead and asbestos
• Sudden extreme temperatures

Describe the causes and effects of chromosomal mutations

• Deletion – refers to the absence of a portion of a chromosome; It results from breakage and falling off a portion of a chromosome leading to loss of a group of genes that may have a disastrous effect on the development of an organism;
• Inversion – refers to the reversal of normal sequence of genes in portion of a chromosome. Occurs when the middle portion of a chromosome breaks and turns/rotates through 180⁰ and joins again. This does not change the genetic constitution of an organism; but may bring into close proximity genes whose combined effects produce beneficial or disadvantageous effects to an organism
• Translocation – attachment of a portion of a chromosome to a non-homologous chromosome. This may lead to serious consequences depending on what genes are missing;
• Duplication – a set of genes is represented twice in a chromosome;

A section of chromatid formed during cell division may replicate further to form an extra piece which may attach onto same or another chromatid; resulting to traits controlled some genes being excessively expressed;

• Non-disjunction – is the failure of a pair of homologous chromosomes to separate during the first stage of meiosis, resulting in one of the daughter cells formed after cell division having two of one kind of a chromosome; while the other cell having less or none. Syndromes result from these aberration like; Down’s syndrome, Turner’s syndrome and Klinefelter’s syndrome.
• Polyploidy – it is the presence of more than two sets of chromosomes in a cell.

Occurs due to failure of a cell to divide after the first stage of meiosis or after chromosomes have replicated in mitosis. Common in plants. Cause beneficial traits like; resistance to pests and diseases; improved yields, early maturity and resistance to drought;

Explain the different forms of chromosomal mutation

Chromosomal mutation change involves number or structure chromosome i.e.   

• Deletion; Part of chromosome break away and does not rejoin to the original chromosome. Leads to loss of some genes;
• Duplication; chromosome replicate itself either in whole as a portion of itself. This causes extra chromosomes i.e. polyploidy;
• Translocation; A part of the chromosome detaches itself from one chromosome and attaches to another non – homologous chromosome;
• Inversion; a part of chromosome gets detached, rotates at 180⁰ then rejoins to the original chromosome;
• Non-disjunction; this is failure to segregate in a pair of homologous chromosome during meiosis; leading to some cells having extra set of chromosome and others without chromosome;       

Biology Essays: Evolution

Describe the various evidences of organic evolution.

1. Fossil records
2. Comparative anatomy
3. Comparative embryology
4. Geographical distribution
5. Cell biology
6. Comparative serology
7. Fossil record: Fossils are remains of organisms that existed long time ago that have been preserved in plant resins and sedimentary rocks. These remains include bones, artifacts, and teeth cellulose cell walls in plants. Fossils are important because:
   1. They show an evidence of the type of organisms that existed longtime ago.
   1. They show gradual change from one form to another over long period of time.
   1. They show evolutionary relationship between organisms.
   1. Fossils have been used to construct evolutionary history of organism like human beings, horse

• Comparative anatomy: It involves comparison of internal structures of various organisms. Those organisms with similar structures are believed to share common origin. The structures can be grouped into:
  • Homologous structures: These are structures with common embryonic origin but modified within the course of evolution to perform different functions due to adaption of the organisms to different habitats leading to divergent evolution or adaptive radiation. Examples; Pentadactyl limbs in vertebrates and Beaks in birds
  • Analogous structures: These are structures with different embryonic origin but have been modified within the course of evolution to perform similar functions due to adaptation and exploitation of the same ecological niche. This type of evolution is called convergent evolution. Examples; wings in birds and wings in insects. Those of birds originate from endoskeleton while those of insects from exoskeleton. Eye in human being and that of octopus.
  • Vestigial organs: Structures reduced in size and have become functionless in course of evolution but where functional in ancestral forms. e.g. Reduced wings in kiwi and appendix, coccyx in human  

• Geographical distribution of living organisms: It is believed that long time ago, all the continents formed one large land mass (single land mass). The land mass later broke and the pieces moved away from one another forming present day continents in a process called continental drift. Closely related organisms were separated and isolated from one another leading to evolution and subsequent different species. Different species of organisms inhabit different continents. Thus although certain climatic conditions are similar in the same latitudes of different continents, their flora and fauna are not identical. Examples;
  • Monkeys with long tails live in amazon forest in South America while in Africa forests there are short tails monkeys
  • The ilamas of the amazon forest and modern camels of Africa and Asia are thought to have a common ancestry in North America but migrated and became isolated and evolved into different species. Australia drifted before evolution of placental mammals hence presence of marsupials
  • Isolated oceanic islands evolved their own characteristic plants and animals eg Galapagos islands has both giant iguama lizards which are terrestrial and marine

• Comparative embryology: Embryology is the study of formation and development of an embryo. Embryos of different  vertebrates group have been found to have similar morphological features during their early stages of development. This suggests a common ancestry. Recapitulation theory states that the closer the resemblance between embryos in early stage closer the phylogenetic relationship of the organism

• Comparative serology: Analysis of blood proteins and the antigens show phylogenetic relationship. Species more phylogenetically related contain more similar blood proteins. The amount of immunological reaction that form a precipitate can be used to show relationship between organisms. The greater the amount of precipitate the closer the phylogenetic relationship between the animal.

• Cell biology; Cells of all organisms show basic similarities in their structure and function. All these cells contain cell membranes, ribosomes, golgi bodies, mitochondria. They also have some biological chemicals in common eg ATP and DNA. This strongly indicates that all cell types have a common ancestral origin. There are also some differences between plant cells and animal cells. Example;
  • Plant cells contain cellulose cell wall, cell sap chloroplast, starch, chlorophyll. All these features are absent in animal cells.
  • In animals, blood pigments are of universal occurrence e.g. haemoglobin widely in vertebrates; These differences in plants and animals indicate that from the common ancestral stock, plants and animals separated and evolved along different lines

How does natural selection bring about adaptation of species to its environment?

• Organisms in the same environment are always competing for resources such as food, mates, shelter etc, as well as enduring the harshness of the environment; This phenomenon is described as a struggle for existence. Those organisms that are best adapted, survive to reproductive maturity and give rise to off springs of the next generation. The less-adapted die young, hence survival of the fittest.  If the favourable characteristics possessed by the ‘fittest’ organisms are genetic, they are passed onto the offspring. This leads to a natural occurrence of variation within a species; If these variations are genetic, they are transmitted to successive generations of offspring Consequently, there is a gradual change in the characteristics of the species making it better adapted to its environment.
• Accumulation of small variations over a long period of time lead to the emergence of new forms of life i.e. new species. If suited and well adapted to the new environment, these new forms reproduce successfully and pass on their characteristics. If not suited, these new forms are eliminated by nature leaving mutant forms, which are better adapted to the environment. Through this process nature selects those organisms with better adaptations while ensuring the elimination of those not able to adapt to the changing environment; Thus the changing environment (nature) forces and organism (a species) to adapt or otherwise be eliminated;

Discuss three examples of natural selection in action

• Industrial melanism
• In Europe there are two forms of peppered moths, white and black. Before industrialization the tree trunks were white hence the white peppered moths were camouflaged. The black varieties were easily noticed and fed upon by predatory birds. The white form therefore reproduced and increased in number.
• During industrialization the smoke from industries coated tree trunks black. The black variety became camouflaged reproduced and increased in population. The white variety were easily noticed and fed upon by predators. They reduced in population;

• Resistance against drugs and antibiotics
• When microorganism are continually exposed to a certain drug. Their cells synthesise specific proteins which counter the drug. This ability to synthesise the protein is passed onto the offspring.

• Resistance to pesticides
• When insects are continually exposed to a particular pesticide synthesise a specific protein which make them resistant to the pesticide. This is transmitted to their offspring.

Discuss Lamarck’s and Darwin’s theories of evolution

• Lamarck’s theory
• States that when environment demands the need or use of a particular structure in the body; the body develops in response; e.g.  Giraffes used to have short necks, when all the grass was exhausted, they started stretching their necks in search of leaves on trees. Therefore they developed long necks which then were inherited by their offspring. However when a structure is not continually used, it reduces in size and becomes functionless. This theory fails to explain how environmentally acquired characteristics become inherited.

• Darwin’s theory
• Suggests that in nature there occur struggle for existence due to environmental pressure. There exists variations; favourable (advantageous) and unfavourable (disadvantageous) variations. Only those individuals with favourable (advantageous) traits survive to maturity, reproduce and pass on the favourable (disadvantageous) traits to their offspring. Individuals with unfavourable traits fail to compete, do not reproduce and die before maturity, they become extinct.
• Nature selects individuals that are well adapted hence survival of the fittest.

Biology Essays: Reception, Response And Coordination

Describe different ways in which plants respond to different stimuli.

• Phototropism; causes plant shoots to grow by bending towards the direction of light source due to the migration of auxins to the non-exposed side where they cause faster growth;
• Geotropism; causes auxins to be highly concentrated on the lower surface of a root where they inhibit growth leading to a bend curvature towards gravity due to faster growth on the upper surface;
• Thigmotropism; enables the plants with weak stems to respond to contact by coiling around the supporting surface due to migration of auxins to the outer surface of the plant at the point of contact;
• Hydrotropism enables plant roots to grow towards the sources/direction of water in the soil;
• Thermotropism enables some plants to orient their growth towards the direction of higher temperatures;
• Chemotropism enables plant roots to grow by bending towards the direction of higher nutrients (chemicals) in the soil; this makes roots to absorb nutrients from the soil.    

Describe the adaptations of the nervous system to its functions

• The central nervous system consists of the brain and the spinal cord and nerve fibres that serve the sensory organs and the effector organs and glands. The brain is a collection of millions of neurons that form the biggest ganglion.
• The brain is highly convoluted to provide a large surface area for impulse reception, processing and transmission. The brain and the spinal cord are protected by the meninges.
• The brain and the spinal cord have spaces/canals and ventricles filled with cerebrospinal fluid that nourish the brain cells and remove metabolic wastes.
• The brain has centres for storage, retrieval and processing of impulses.
• The cerebrum processes and stores information.
• The cerebellum sends impulses to joints and muscles to correct balance.
• The medulla oblongata sends impulses to the cardiovascular and breathing system to regulate them;
•  The brain has the hypothalamus that sends secretes a neurosecretion to influence a pituitary gland that secretes hormones involved in reproduction and homeostasis.
• The hypothalamus detects changes in temperature and osmotic pressure and sends impulses to relevant effector organs for their regulation;
• The thalamus receives most of impulses and channels them to the relevant area of brain regions;
• The brain and spinal cord have grey matter that enable rapid processing /transmission of impulses.
• The nervous system has neurons that transmit impulses rapidly to and from the CNS
• Neurons have an electrochemical gradient that allow for generation of electrical impulses.
• Neurons have numerous mitochondria to produce more energy by sodium pump for polarization and depolarization during impulse transmission and refractory periods.
• The spinal cord has no integration functions hence its suited for reflex functions, needed in emergencies;
• The spinal cord is long and connects nerve fibres of the peripheral nerves with the brain for storage of information;
• The spinal cord has a dorsal root for sensory neurons and a ventral root for motor neurons.

Using a relevant example in each case, describe simple and conditional reflex actions.

Simple reflex action

• Example: withdrawal of a finger from a sharp/hot object; It is automatic response to a specific stimulus. When the finger touches a sharp/hot object, pain receptors or thermoreceptors in the skin are stimulated and trigger off a nerve impulse. The nerve impulse is transmitted via the sensory neuron to the grey matter of the spinal cord/CNS /brain. The impulse is then transmitted via a synapse to the relay neuron and then through another synapse to the motor neuron. The impulse is then transmitted to the effector muscles in the hand. The effector muscles/biceps contract and the finger is withdrawn from the sharp/hot object.

Outline the activities that occur in the body when one touches a hot object

• When the hot object is touched, the pain receptors in the skin of the finger are stimulated; nerve impulses are initiated and transmitted through the sensory neurone to the grey matter of the spinal cord to the brain for interpretation. The impulses are then transmitted through the relay neurone via a synapse. The impulses from the relay neurone are then transmitted via the motor neurone through another synapse to the effector which are the biceps muscles of the upper arm making the muscles to contract; straightening the arm and the arm is withdrawn from the hot object;

Describe how parts of human eye are adapted to their functions.

• The sclera/sclerotic layer is fibrous to maintain the shape of the eyeball;
• The cornea is curved to refract light onto the retina; It is transparent to allow light to enter the eye;
• The lens is biconvex to refract light onto the retina; It is transparent to allow light into the eye;
• The choroid is heavily pigmented to absorb stray light; It is also highly vasularised to nourish the eye;
• The aqueous humour which is transparent to allow light to pass; It is also rich in nutrients and oxygen to nourish the eye tissues;
• The vitreous humour is viscous to maintain the shape of the eyeball; It is also viscous to refract light; Transparent to allow light pass through;
• Ciliary body has suspensory ligaments which hold the lens in position;
• Has ciliary muscles that contract and relax to alter the curvature of the lens (during accommodation);
• Iris has an aperture called pupil which allows light enter the eye; Iris is also pigmented/has melanin to absorb light; give the eye its colour;
• Retina has photoreceptors / light-sensitive cells which perceive light of different intensities
• Retina has fovea centralis which is highly concentrated with cones to perceive light of high intensity;
• Rods are located in the peripheries of the retina to perceive light of low intensity;
• Has optic nerves which transmit light impulses to the brain;

Describe how accommodation in the human eye is brought about when focusing on a near object.

• Light rays from near object are more divergent and need to bend more. Ciliary muscles contract, suspensory ligaments lengthen, the lens become thicker/more convers/increases in curvature/reduces focal length. Light from the object is refracted more in order to be focused sharply/form an image on the retina.

Discuss the adaptations of the mammalian ear to its function

• Pinna; funnel shaped to collect sound waves;
• External auditory meatus; tube to allow passage of sound waves; its cells secrete wax to trap dust;
• Tympanic membrane/Eardrum; thin tough membrane that converts sound waves into vibrations;
• Ear ossicles (malleus, incus and stapes) – bones that amplify vibrations.
• Eustachian tube; equalizes pressure between middle ear and outer ear;
• Oval window; thin membrane that transmits vibrations from middle ear to inner ear;
• Cochlea; spirally shaped/highly coiled tube that has sensory cells to detect sound vibrations;
• Semi-circular canals; arranged at right angles to detect change in body position;
• Utriculus / sacculus; gelatinous cupula with otoliths structures with sensory cells; to detect movement in body;
• Auditory nerve; transmit impulse to brain;

Biology Essays: Support and Movement in Plants and Animals

Describe how the various supportive tissues in plants adapt them to their habitats

• Sclerenchyma tissue consists of long, slender cells with tapering ends, with walls thickened with lignin to provide support and protection of more delicate tissues;
• Xylem vessels longitudinally elongated cells; with perforated end walls; with heavily lignified walls to increase rigidity and strength to the plant;
• Tracheids are made up of long tapering dead cells; cell walls are highly lignified to offer extra support;
• Collenchyma tissues are longitudinally elongated living cells; located beneath the epidermis and mid rib of leaf veins. Thickened at the corners by cellulose and pectin compounds to provide support in leaves, herbaceous plants and young woody plants;
• Parenchyma tissue consists of large spherical shaped cells; with thin cellulose cell walls forming the bulk of cortex and pith of most plants; become tightly packed and rigid when turgid  to attain and maintain an erect posture of plants; main support structures in herbaceous stems;

A dicotyledonous stem offer support to the plant. Give three necessities for this support.

• To expose the leaves to sunlight for photosynthesis;
• Expose flowers to agents of pollination;
• Expose fruits and seeds to agents of dispersal;

Describe the adaptation of the stem of a dicotyledonous plant to its function.

• The stem has several strengthening tissue; that provide support i.e. collenchyma and sclerenchyma;
• These tissue; are strengthened by lignin;
• Xylem tissue; made up of xylem vessels and tracheids. The xylem tissues have thickened walls by lignin; to prevent walls from collapsing during transpiration;
• Xylem vessels are narrower; to facilitate upward movement of water by capillarity;
• Xylem vessels have bordered pits; lateral movement of water and mineral salt;
• Phloem tissue contain contractile cytoplasmic strands; to push organic food substance from one sieve tube to the next;
• Phloem tissue contain plasmodesmata; that joins companion cells to sieve elements; allowing for passage of protein and ATP to be used in translocation of substances;
• Cambium tissue for secondary growth within the vascular bundles;
• Parenchyma tissue /cells stores water and food hence support through turgidly;
• Suberin in the stem prevents excess loss of water and entry to pathogens;
• Lenticels that facilitate gaseous exchange;
• Some stems have parenchyma cells with chlorophyll for photosynthesis;

Describe the exoskeleton and its functions in insects.

• The exoskeleton is made of chitin, chitin is not evenly distributed/is thin and flexible at joints for movement. Exoskeleton is secreted by epidermal cells, when still soft it allows for growth of the insect. When (in contact with the air) it hardens limiting growth, it is shed regularly thus regulating growth.
• It also supports internal structures, because it is hard, it protects internal organs from mechanical damage.
• It is waterproof preventing water loss/desiccation of the insect.
• It also provides a surface for attachment of muscles.
• It is light/modified to wings for flight.
• It can be modified to form jaws/mouth for biting/piercing/grinding,
• It is pigmented for camouflage, can be transparent in some places allowing entry of light into the eyes for camouflage in water.

Describe how a finned fish such as Tilapia moves in water.

• Movement of fish in water is by swimming. It involves forward movement and control of the body position in water.
• Fish has streamlined body to reduce resistance during movement.
• Fish body is covered by mucus to reduce resistance during movement.
• Fish has backward-overlapping scales to reduce resistance during movement. Propulsion is caused by the tail.
• The tail is long (almost half the large of the body of the fish) to enable it create enough force (to enable the fish push forward).  Propulsion is achieved when the tail pushes sideways against water.
• Sideways movement is brought about by muscles arranged in segmented blocks/ myotomes on both sides of vertebrate column to swing sideways; when the muscle blocks on the right relax and those on the left contract; the body bends to the left side. When the muscle of the left relax and those on the right contract; the body bends to the right side; the fish uses its fins to control the position of body in water.
• During forward movement paired fin (pectoral & pelvic fins). Lie flat on the body surface to reduce resistance. To change direction the fish uses the paired fins. Paired fins also are used by fish to change its level in water / control pitching. The fish spreads out the pectoral & pelvic fins at 90⁰ to the body; to enable it brake. Fish can also use the swim bladder to change its level in water. When the bladder fills up with air the fish become less dense/ lighter making it to rise in water; when the air leaves the bladder the fish becomes more dense/ heavier; making it to sink deeper in the water.
• Water currents may cause sideways swaying of the body of the fish/ yawing.
• The dorsal and ventral fins (also) prevent rolling and yawing;

Describe how the Bony fish is adapted for movement and living in water.

• Streamlined body to reduce water resistance;
• Scales overlap backwards to reduce water resistance;
• Gills for gaseous exchange;
• Lateral line for detecting vibration in water;
• Muscle blocks (myotomes), on either side of the vertebral column, that contact and relax antagonistically to cause movement;
• Broad tail fin, that lashes from side to side thus propelling the fish forward;
• Presence of glands on the skin that secrete mucus over the scales to reduce friction during movement;
• Have large mouth for taking large quantities of water from which oxygen is extracted;
• Has inflexible head to ensure forward thrust;
• Has swim bladder which stores air to enhance buoyancy;
• Has paired fins (pelvic and pectoral) to control pitching; for braking; steering and balancing.
• Has unpaired fins; Caudal fin propel and steer fish while in motion; Dorsal and anal fins prevent the fish from rolling and maintains balance.
• Silvery white colour on the ventral side and dark colour on the dorsal side to camouflage;

Explain the adaptations of thoracic, cervical and lumbar vertebrae to their functions.

Cervical vertebra

• Presence of vertebraterial canals for passage of arteries
• Has wide neural canal for passage of spinal cord
• Branched transverse processes for attachment of neck muscles

Atlas

• Has a wide neural canal to accommodate the large spinal cord at the neck region;
• Has large/broad wing-like cervical ribs to increase the surface area for attachment of the neck muscles;
• Has facets on the anterior side for articulation with the occipital condyles of the skull to allow nodding (up and down movement) of the head;
• Has posterior facets for articulation with the anterior facets of the axis forming a joint that allows sideways movement of the head;

Axis

• Has a broad centrum that projects to form the odontoid process for articulation with the neural canal of the atlas; a joint that allows turning of the head.
• Has a large and broad neural spine and flat cervical ribs to increase the surface area for attachment of neck muscles.
• Axis has a wide centrum that projects/odontoid process which articulates with the atlas

Thoracic vertebrae

• Long neural spine to increase surface area for attachment of back muscles
• Tuberculum facet which articulates with the tuberculum of rib and the capitular demifacets on the centrum articulates with the capitulum of rib
• Short transverse processes which provides an additional surface area for muscles attachment
• Pre- zygapophyses and post-zygapophyses for articulation with adjacent vertebrae.

Lumbar vertebrae

• Wide / broad neural spine to increase surface area for attachment of the abdominal muscles
• long transverse processes which provides an additional surface area for muscles attachment
• Broad centrum to offer support /resists compression by spreading the force towards the sacral vertebrae
• Anapophyses/Metapophyses/Hypapophyses provides additional surfaces for muscles attachment;
• Pre-zygapophyses and post-zygapophyses provides articulation between vertebrae

Sacral vertebra

• Has large and broad centrum to offer support.
• Has large with wing-like transverse processes to provide additional surfaces for muscles attachment;
• Fused to form a rigid structure that make the sacrum strong and firm to bear body weight

Discuss the structure and functions of the various muscle tissues found in humans

• Smooth muscle (lines cavities and organs)

Consists of spindle-shaped cells made up of long filaments or myofibrils.

The cells lack cross striations and sarcolemma;

They are uninucleated;

They contract and fatigue slowly to bring contraction and relaxation of the walls of blood vessels and gut

• Skeletal muscles

Made up of long cylindrical cells with long myofibrils running parallel to each other.

Cells have cross striations;

Are multinucleated

They form bundles of long fibres attached to bones by tendons;

They contract and fatigue rapidly to bring about movement of bones;

They contain contractile protein myosin and actin

• Cardiac muscles (heart muscles)

Are made up of short cylindrical cells with parallel myofibrils

The ends of each cell thickened into intercalated discs that connect adjacent cells;

Myofibrils have cross striations

Each cell uninucleated;

The myofibrils contract without fatigue;

Describe how the bones of pectoral and pelvic girdles are adapted to their functions.

• Pectoral girdle
• Has a broad scapula to provide large surface area for attachment of pectoral muscles
• Scapula has cartilage on broad end to reduce friction;
• Scapula has a spine on the dorsal side to increase surface area for muscle attachment;
• Scapula has a projection, metacromion, for muscle attachment;
• Scapula has a projection, acromion, for attachment of clavicle;
• Scapula has glenoid cavity for articulation with humerus;
• Scapula has clavicle with facets for articulation with acromion and sternum and for limiting the movement;
• Pelvic girdle
  • It is broad to increase surface area for attachment of muscles of thigh and sacrum;
  • Has acetabulum cavity for articulation with femur;
  • Has a flexible cartilage, pubis symphysis, which permit widening of pelvis during birth in women;