WAEC SSCE - Biology - 1992

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The first scientist to describe the cell was Robert Hooke. In 1665, he examined a thin slice of cork under a microscope and observed small, box-like structures that he called "cells." Hooke's discovery was significant because it provided the first evidence that living organisms are made up of individual units called cells. However, Hooke did not realize the full importance of his discovery at the time, and it was not until several decades later that other scientists, such as Matthias Schleiden and Theodor Schwann, developed the cell theory, which established the cell as the basic unit of life.

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The underlying principle in the adoption of biological control of pests is the predator-prey relationship in the ecological community. Biological control involves the use of living organisms, such as predators or parasites, to control pests and reduce their damage to crops. This method is based on the understanding that there is a natural balance in ecosystems, and that predators can keep the population of pests in check. By introducing natural enemies of pests, such as predators or parasites, the natural balance is restored, and the need for chemical pesticides is reduced. Therefore, the predator-prey relationship is the key principle behind the adoption of biological control of pests.

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The use of predators or parasites to control pests in the farm is known as biological control. This method involves introducing natural enemies, such as predatory insects or parasitic wasps, to control pest populations. The predators or parasites feed on the pests, reducing their numbers without harming the crops. Biological control is considered a more sustainable and environmentally friendly approach to pest management compared to chemical control, which involves the use of pesticides that can be harmful to non-target organisms and the environment.

Maelezo ya Majibu

Maelezo ya Majibu

The blood vessels that carry oxygenated blood into the heart are called the pulmonary veins. They are responsible for transporting blood from the lungs, where oxygen is added to the blood, to the left atrium of the heart. The left atrium then pumps the oxygenated blood into the left ventricle, which in turn pumps it out through the aorta to supply oxygen and nutrients to the rest of the body. The other blood vessels listed, such as the pulmonary artery and the venae cavae, carry deoxygenated blood from various parts of the body to the heart for oxygenation.

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The exchange of gases in multicellular organisms, such as humans, is crucial for respiration. This process occurs in the respiratory organs, such as lungs, where oxygen is absorbed and carbon dioxide is released. To ensure successful gas exchange, there must be a concentration gradient across the respiratory surface, meaning that there is a difference in the concentration of gases on either side of the respiratory surface, allowing for diffusion to occur. The respiratory surface must also be thin to facilitate diffusion of gases across it. Additionally, there must be a large surface area of the respiratory organ to maximize the exchange of gases. The last transportation of absorbed gases, such as oxygen, to the rest of the body is also important for successful gas exchange. However, the presence of a small, dry surface area of the respiratory organ would not ensure successful gas exchange, as it would not provide enough surface area for gas exchange, and the dry surface would not allow for efficient diffusion of gases. Therefore, this option is the one that does not ensure successful gas exchange in multicellular organisms.

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The groups of sensory cells found on the upper surface of the tongue are called "taste buds." Taste buds are small structures that are responsible for detecting different tastes, such as sweet, sour, salty, bitter, and umami. They are located on the tongue, as well as on the roof of the mouth and in the throat. When food or drink enters the mouth, the taste buds send signals to the brain, which interprets the taste. Taste buds also play a role in detecting potential toxins, which can help protect us from ingesting harmful substances.

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A climax community is characterized by a stable composition of plant and animal species. This means that over time, the community has reached a balance where the different species are able to coexist and maintain a relatively constant population size. The community has reached a state of equilibrium where the different species have found their niche and are able to survive and reproduce. Unlike earlier stages of succession, where there are rapid changes in the composition of species, a climax community has reached a state of balance where the number of species remains relatively constant. Although there may be some gradual changes in animal population, the overall structure of the community remains stable.

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The mineral salt that is a trace element is Zinc. Trace elements are minerals that are needed by the body in small amounts to carry out essential functions. Zinc is an important trace element that plays a vital role in many biological processes, such as immune function, wound healing, and protein synthesis. Carbon, hydrogen, and calcium are not trace elements because they are needed in much larger amounts by the body and are not considered to be trace elements. Carbon and hydrogen are the building blocks of all living things, while calcium is an essential mineral that is required for strong bones and teeth, among other functions. Potassium is not a trace element either, but it is an essential mineral that helps to regulate fluid balance in the body and supports proper muscle and nerve function.

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Identical twins inherit their genes from the same ovum (or egg) and the same sperm. This means that a single fertilized egg splits into two embryos, which develop into two babies with the same genetic information. So, both twins have the same DNA and are almost identical in terms of physical appearance and traits.

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Maelezo ya Majibu

The property of clay soil that prevents it from supporting thick vegetation is its tendency of becoming waterlogged. Clay soil has small spaces between its particles that can trap water, leading to poor drainage and a lack of oxygen in the soil. This makes it difficult for plants to grow deep roots and absorb the nutrients they need to thrive. As a result, clay soil can be challenging to cultivate, and often requires amendments like sand or organic matter to improve its structure and drainage.

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The option that does not illustrate adaptation to the environment is "Development of big muscles by a weightlifter." Adaptation refers to a trait or characteristic that helps an organism survive and reproduce in its environment. The other options listed all describe physical features that have evolved over time to help organisms survive in their environments. Color changes by a chameleon help it blend into its surroundings and avoid predators or catch prey. The streamlined shape of fishes helps them move through water more efficiently and catch prey. Light bones in birds help them fly more easily. Possession of fins by fishes helps them swim and maneuver in water. On the other hand, the development of big muscles by a weightlifter is a conscious choice that has nothing to do with survival in the natural environment. While weightlifting may help an individual become stronger, it is not a biological adaptation that has evolved over time in response to environmental pressures.

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Carbon dioxide is not given out during the process of photosynthesis. Photosynthesis is the process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of glucose. During photosynthesis, carbon dioxide is taken in by plants, and using the energy from sunlight, it is converted into glucose and oxygen. Therefore, carbon dioxide is not given out during photosynthesis. On the other hand, carbon dioxide is given out during respiration in plants, decay of organisms, burning of organic matter, and burning of fuels. Respiration is the process by which plants and animals convert glucose and oxygen into energy, releasing carbon dioxide and water as waste products. Decay of organisms involves the breakdown of organic matter by microorganisms, which releases carbon dioxide as a byproduct. Burning of organic matter and fuels involves the combustion of carbon-containing materials, which releases carbon dioxide into the atmosphere.

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The agency that is not responsible for conservation in Nigeria is the Nigerian Red Cross Society. This is because the Nigerian Red Cross Society is a humanitarian organization that focuses on providing relief and assistance to people affected by emergencies, disasters, and conflicts. On the other hand, the Forestry Department, Nigerian Conservation Society, Games Reserve Authority, and Ministry of Agriculture and Natural Resources are all directly involved in the conservation of Nigeria's natural resources, such as forests, wildlife, and agricultural lands. They are responsible for ensuring that these resources are sustainably managed and protected for future generations.

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The exchange of gases in multicellular organisms is crucial for their survival. This process occurs between the respiratory surface and the surrounding environment. To ensure successful gas exchange, several conditions need to be met. Firstly, there must be a concentration gradient across the respiratory surface. This means that there should be a difference in the concentration of gases between the organism's respiratory surface and the surrounding environment. This difference drives the movement of gases across the respiratory surface, allowing for gas exchange to occur. Secondly, the respiratory surface must be thin. A thin respiratory surface allows for gases to diffuse quickly between the organism and its surroundings. Thirdly, the absorbed gases need to be transported to where they are needed in the body. This is usually achieved through the circulatory system, which transports gases such as oxygen and carbon dioxide to and from the respiratory surface. Fourthly, the respiratory organ must have a large surface area to facilitate the exchange of gases. A larger surface area means that there is more space for gas exchange to occur, which increases the efficiency of the process. Finally, the respiratory organ must have a moist surface area. This is because gases diffuse more easily through moist surfaces than dry ones. Therefore, the presence of a small, dry surface area of the respiratory organ would not ensure successful exchange of gases in multicellular organisms.

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Isotonic sugar solution cannot control the growth of harmful microorganisms. Isotonic means that the concentration of solutes in the solution is the same as that of the cells in the environment. Therefore, isotonic sugar solutions do not create an environment that is unfavorable for the growth of microorganisms. In fact, isotonic sugar solutions can actually promote the growth of some microorganisms that can metabolize the sugar. On the other hand, antibiotics, hypertonic salt solution, disinfectants, and antiseptics can control the growth of harmful microorganisms. Antibiotics are substances that can kill or inhibit the growth of bacteria, while hypertonic salt solution, disinfectants, and antiseptics work by disrupting the cell membrane or other structures of the microorganisms, leading to their death or inhibition of growth.

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One of the major criticisms against Mendelian laws is that they do not recognize that one trait is often controlled by many pairs of genes. In other words, Mendelian laws assume that a single gene is responsible for determining a particular trait, when in fact, many traits are determined by multiple genes working together. This concept is known as polygenic inheritance and it plays an important role in determining many traits such as height, skin color, and intelligence. Therefore, the Mendelian laws provide an oversimplified view of genetic inheritance and do not fully account for the complexity of genetic traits.

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One of the factors that must be considered for safe blood transfusion is the compatibility of the rhesus factors between the donor and the recipient. The rhesus factor is a protein that is present on the surface of red blood cells, and it can be either positive or negative. If a person with a positive rhesus factor receives blood from a person with a negative rhesus factor, their immune system may see the red blood cells as foreign and attack them, leading to potentially life-threatening complications. Therefore, it is important to match the rhesus factors between the donor and the recipient before a blood transfusion to ensure the safety and effectiveness of the procedure.

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The adaptation that is not related to the desert environment is "Broad leaves for storage". In the desert, water is scarce, so plants and animals need to adapt to conserve water. Well-developed taproot systems help plants reach deep water sources, while small leaves with thick epidermis help reduce water loss through transpiration. Stems with spike-like leaves also help plants reduce water loss by reducing surface area exposed to sunlight. Some desert animals excrete metabolic waste in the form of uric acid instead of urea because it requires less water to excrete. This helps them conserve water in their bodies. However, broad leaves are not an adaptation that helps plants or animals survive in the desert. In fact, broad leaves would be disadvantageous in the desert environment because they have a large surface area, which means more water loss through transpiration. Therefore, broad leaves are not an adaptation to the desert environment.

Maelezo ya Majibu

The adaptation that is not related to the desert environment is "broad leaves for storage". Plants in the desert have adapted to conserve water and minimize moisture loss. They have small leaves with thick epidermis to reduce water loss through transpiration. Well-developed taproot systems help plants reach deep water sources. Stems with spike-like leaves protect the plant from predators while also reducing water loss. Regarding animals, some desert animals have adapted to conserve water by excreting metabolic waste in the form of uric acid, which is a highly concentrated form of waste that requires less water to eliminate. However, broad leaves are not an adaptation for desert plants because they lose more water through transpiration, which is not beneficial in the arid desert environment.

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The structure that functions as an excretory system in flatworms is the flame cell. Flame cells are specialized cells that are found in the body of flatworms and are responsible for removing excess water and waste materials from the body. These cells have hair-like structures called cilia that beat in a coordinated manner, creating a current that moves waste materials out of the body. The waste materials are then expelled through pores in the body wall. Therefore, the correct answer is flame cell.

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When pure strains interbreed, the offspring produced are called hybrids. A pure strain is a group of organisms that have the same genetic makeup or genotype. When two different pure strains interbreed, they produce offspring that have a mixture of the genetic traits from both parents. These offspring are called hybrids because they have a combination of traits that are not found in either pure strain. The traits that are expressed in the hybrids are called the phenotype, while the genetic makeup or combination of alleles that determine those traits are called the genotype. Dominance refers to the relationship between two alleles of the same gene, where one allele is dominant and masks the expression of the other recessive allele.

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The organ in Euglena that is sensitive to light is called the "eye spot." This is a small, oval-shaped structure located near the base of the flagellum. The eye spot contains a pigment called "stigma" which is sensitive to light. This helps Euglena to detect changes in light intensity and to move towards or away from light sources, allowing it to carry out photosynthesis efficiently.

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Maelezo ya Majibu

Both typical animal cells and typical plant cells have a cell membrane. The cell membrane is a thin, flexible layer that surrounds the cell and acts as a protective barrier. It controls what enters and exits the cell, allowing nutrients to enter and waste to leave. While there are some differences in the structure and function of cell membranes in animal and plant cells, the presence of this organelle is a shared similarity between the two cell types.

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The organism that reduces nitrates in the soil to gaseous nitrogen is the denitrifying bacterium. These bacteria convert nitrates into nitrogen gas, which is released into the atmosphere. This process is called denitrification, and it is an essential part of the nitrogen cycle. Without denitrifying bacteria, nitrates would accumulate in the soil, which could lead to problems like eutrophication and decreased soil fertility. In summary, denitrifying bacteria play a crucial role in maintaining a healthy soil environment by converting excess nitrates into nitrogen gas.

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Maelezo ya Majibu

The swarming behavior at the beginning of the rainy season is shown by winged termites as a part of their courtship behavior. During this time, male and female winged termites leave their nests and fly together in large numbers to find a mate and start a new colony. This behavior is important for the survival of the species, as it allows for genetic diversity and the creation of new colonies.

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Lamarck's theory of evolution can be summarized by the statement: "the changing environment imposes structural, physiological, and behavioral changes in organisms." This means that Lamarck believed that organisms could adapt to their environment by developing new traits and characteristics in response to the challenges posed by their surroundings. According to Lamarck, these changes could be passed down to future generations, resulting in the evolution of new species over time. This is in contrast to the theory of natural selection proposed by Charles Darwin, which posits that the fittest individuals are more likely to survive and reproduce, passing on their advantageous traits to their offspring.

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The chromosome is typically found inside the nucleus of a eukaryotic cell. The nucleus is a membrane-bound organelle that contains the cell's genetic material, including the chromosomes. The other parts of the cell mentioned in the options, such as the Golgi body, cytoplasm, cell membrane, and cell wall, do not typically contain chromosomes. The Golgi body is involved in processing and transporting proteins, the cytoplasm is the fluid-like substance that fills the cell, the cell membrane is a thin layer that surrounds the cell and controls what enters and leaves it, and the cell wall is a rigid layer found in some cells that provides structural support. So, the answer is: Nucleus.

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The characteristic of "increase in primary production" is not directly related to overcrowding in plant and animal communities. Overcrowding occurs when the population of a species exceeds the available resources, such as space and food, leading to increased competition within the population. This competition can lead to negative effects on the health and survival of individuals within the population. While an increase in primary production may provide more resources for a population, it is not necessarily indicative of overcrowding, as other factors such as environmental conditions and resource allocation can also affect primary production.

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Maelezo ya Majibu

Among the options given, the medium of transportation of nutrients with unicellular organisms is protoplasm. Protoplasm is a jelly-like substance that makes up the entire cell of unicellular organisms. It contains all the organelles and structures necessary for the cell to carry out its functions, including the transportation of nutrients. In unicellular organisms, nutrients enter the cell through the cell membrane and are transported throughout the cell via the protoplasm. The protoplasm also helps to eliminate waste products from the cell. Blood, serum, plasma, and lymph are all fluids found in multicellular organisms that are used for transportation of nutrients and waste products, but they are not relevant for unicellular organisms as they do not have specialized circulatory systems.

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The disease caused by deficiency of insulin in the body is diabetes mellitus. Insulin is a hormone produced by the pancreas that helps regulate the amount of sugar (glucose) in the blood. In people with diabetes, the body either doesn't produce enough insulin or can't effectively use the insulin it produces. Without enough insulin, the body can't properly regulate the amount of glucose in the blood, which can lead to high blood sugar levels. Over time, high blood sugar levels can cause damage to various organs and tissues in the body, leading to a range of health problems. Symptoms of diabetes can include increased thirst and urination, fatigue, blurry vision, and slow-healing wounds. Treatment typically involves managing blood sugar levels through diet, exercise, and medication, including insulin injections for people with type 1 diabetes who can't produce enough insulin on their own.

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The organ that is associated with deamination of proteins is the liver. Proteins are made up of building blocks called amino acids. When proteins are broken down in the body, they are typically first broken into their constituent amino acids. The liver then plays a key role in removing the amino group (which contains nitrogen) from the amino acids, in a process called deamination. Deamination is important because the body needs to get rid of excess nitrogen that can be harmful if it builds up. The liver converts the nitrogen into a compound called urea, which can then be safely excreted by the kidneys in urine. So, in summary, the liver is the organ responsible for deaminating amino acids and converting the resulting nitrogen into urea, which is excreted by the kidneys.

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The largest game reserve in Nigeria is Yankari Game Reserve, which is located in Bauchi State. It covers an area of about 2,244 square kilometers and is home to a variety of wildlife such as elephants, lions, baboons, and antelopes. It also has natural springs and waterfalls, making it a popular tourist destination.

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Maelezo ya Majibu

The tropical rainforest is a type of forest characterized by several features. Among these features, there is an abundant water supply, loose and moist soil, short trees growing beneath tall trees, and the presence of many animals. However, one of these options does not belong, which is "scanty trees with small leaves." In fact, the opposite is true for tropical rainforests; they are known for having a dense canopy of tall trees with large leaves that block most of the sunlight from reaching the forest floor. As a result, only a small amount of light filters through to the ground, which limits the growth of understory plants and makes it difficult for small trees with small leaves to thrive. Instead, the forest floor is usually covered in a layer of decomposing plant material, which helps to enrich the soil and support the growth of larger trees.

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The changing of colour by a chameleon to match its surroundings is an example of protective coloration. Protective coloration is a type of camouflage where an animal's coloration helps it blend in with its environment to avoid predators or prey. In the case of the chameleon, its ability to change its skin color allows it to blend in with its surroundings and avoid being seen by predators or prey.

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The term pyramid of numbers refers to the graphical representation of the number of individuals present at each trophic level in a food chain or web. This means that it shows the number of organisms starting from the primary producers at the bottom to the top predators at the top of the pyramid. In a simple way, it shows the progressive decrease in the number of individuals from lower to higher trophic levels, as energy is transferred from one level to the next. Therefore, best explains the term pyramid of numbers.

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Ultrafiltration in the kidney takes place in Bowman's capsule. Bowman's capsule is a cup-shaped structure at the beginning of each nephron in the kidney. Blood flows into the nephron through the afferent arteriole and into the glomerulus, a cluster of tiny blood vessels. The high pressure in the glomerulus forces small molecules, such as water, ions, and waste products, out of the blood and into Bowman's capsule. This process is called ultrafiltration, and it is the first step in urine formation. The filtered fluid then flows through the rest of the nephron, where important substances are reabsorbed back into the bloodstream and the final urine is produced.

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The respiratory organ found in the cockroach is the trachea. The trachea is a system of tubes that carry air directly to the cells of the cockroach's body. These tubes are connected to small openings called spiracles, which are located along the cockroach's abdomen. When the cockroach breathes, it opens and closes these spiracles to regulate the flow of air in and out of its body. The tracheal system is very efficient and allows the cockroach to obtain the oxygen it needs for survival.

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The function of chromosomes is to transmit hereditary traits from one generation to the next. Chromosomes are made up of DNA, which carries the genetic information that determines the traits of an individual. When an organism reproduces, its chromosomes are passed down to its offspring, ensuring that the traits are inherited. Protein synthesis, enzyme manufacture, energy production, and excretion are not functions of chromosomes, but rather, functions of other cellular components such as ribosomes, mitochondria, and other organelles.

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The correct statement is that there is an excess of potassium ions inside the axon and an excess of sodium ions outside. When an axon is at rest, the concentration of sodium ions is higher outside the cell membrane, while the concentration of potassium ions is higher inside the cell membrane. This difference in ion concentration generates an electrochemical gradient across the membrane, which creates an electrical potential difference across the axon. This potential difference is called the resting membrane potential, and it is negative inside the axon compared to the outside. So, is partially correct, but it is the result of the difference in ion concentrations mentioned above. is incorrect because there is actually an excess of sodium ions on the outer side of the axon, not the inner side. is also incorrect because chloride ions do not play a significant role in generating the resting membrane potential, although they can influence it during action potentials. is also incorrect because calcium ions are not the primary ions involved in the resting membrane potential.

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The relationship between a tapeworm and an infected mammal is parasitism. Parasitism is a type of relationship in which one organism (the parasite) benefits at the expense of the other organism (the host). In this case, the tapeworm lives in the intestine of the mammal, feeds on its nutrients, and can cause harm to the host by interfering with digestion or causing other health issues. Therefore, the tapeworm benefits while the infected mammal is harmed.

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The division of labor in social insects is an example of behavioral adaptation. Social insects, such as ants, bees, and termites, have evolved to work together in organized colonies, where each individual performs specific tasks according to their abilities and needs of the colony. This division of labor is not predetermined by the insect's physical characteristics, but rather determined by their behavior and interactions with other members of the colony. This behavioral adaptation allows for the efficient functioning and survival of the colony as a whole.

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(a) Female reproductive system of a named mammal (woman/human, diagram essential)

The labelled diagram (front view) should show and label:

• Two ovaries: almond-shaped organs that produce the ova (eggs) and secrete the hormones oestrogen and progesterone.
• Two oviducts (fallopian tubes): tubes with funnel-shaped, fringed openings (fimbriae) that receive the ovum and carry it towards the uterus; fertilisation takes place here.
• Uterus (womb): a muscular, pear-shaped organ with a soft inner lining (endometrium) in which the fertilised egg implants and the foetus develops.
• Cervix: the narrow neck of the uterus that opens into the vagina.
• Vagina: the muscular tube that receives the penis during mating and serves as the birth canal.

In brief, the ovaries release an ovum at ovulation; it passes into the oviduct where, if sperm are present, fertilisation occurs; the resulting zygote travels to the uterus, implants in its lining and develops into a foetus until birth.

(b) Three differences between reproduction in mammals and in amphibians

(c) Structure and function of the male reproductive organ of a flowering plant (the stamen)

The male reproductive organ is the stamen (androecium). Each stamen is made up of two parts:

• The anther: a two-lobed structure containing four pollen sacs. Inside these sacs the pollen grains are formed. The anther produces and stores the pollen grains, which carry the male gametes, and splits open (dehisces) at maturity to release them for pollination.
• The filament: the stalk that bears the anther. It holds up and supports the anther in a suitable position to expose the pollen for dispersal, and supplies it with water and nutrients through its vascular tissue.

Each mature pollen grain contains a generative nucleus (which divides to form two male gametes) and a tube nucleus (which controls the growth of the pollen tube during fertilisation). Thus the stamen's function is to produce the male gametes and make them available for pollination and fertilisation.

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(a)(1) Voluntary action - involves the response to a stimulus that is controlled at will by the brain. in this action, an impulse is carried from a sensory neurone via the spinal cord to the brain which interprets and sends the message back through the motor neurone to effector organ. The action is not automatic. e.g. running, writing, jumping, laughing, reading, smiling.

(II) Involuntary action or reflex action-It is a rapid or automatic response to a stimulus. It does not involve the brain for initiation. Therefore, the action is not under the control of the will, e.g. sneezing, coughing, knee jerk, salivation, secretion of grand, blinking

(b)(i) Functions of insulin:
(1) Assists in oxidation of glucose to produce energy
(2) regulates the amount or level of glucose in the blood
(3) assists in protein synthesis in some cells

(ii) Functions of auxin
(1) Promotes lateral bud development
(2) promotes reproduction, flowering and fruit formation in plants
(3) promotes the production of adventitious roots. 

(c) Experiment to show that plant roots respond positively to gravity:

Aim - To demonstrate that plant roots respond positively to gravity.
Apparatus: A bean seedling, Wet blotting paper, damp saw dust, cupboard.
Procedure: Put some dust on a wet blotting paper and place the bean seedling on the saw dust horizontally. Carry the set up and place it inside a dark cupboard to eliminate light for few days.
Observation: When observed after few days, one will discover that the root has curved and growing downwards while the shoot grow upward. Conclusion: Since the root has grown downwards and the shoot upwards, we can say that roots respond to gravity, showing positive geotropism while the shoot shows negative geotropism. 

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(a) Principal sense organs in mammals and their functions

Sense organ	Function
Eye	Sight – detects light, colour, shape, size and distance of objects.
Ear	Hearing (detects sound) and balance / sense of body position (equilibrium).
Nose	Smell (olfaction) – detects chemicals (odours) in the air.
Tongue	Taste – the taste buds detect dissolved chemicals (sweet, sour, salt, bitter).
Skin	Touch, pressure, pain, and temperature (heat and cold).

(b)(i) The defect in which only distant objects are seen clearly

This defect is long sight (hypermetropia / far-sightedness). The person can see distant objects clearly but near objects appear blurred. It is caused by the eyeball being too short from front to back (or by the lens being too weak/flat and the ciliary muscles having lost their elasticity), so that light rays from a near object are brought to a focus behind the retina instead of exactly on it.

(b)(ii) Correction

Long sight is corrected by wearing spectacles fitted with a convex (converging) lens. The convex lens converges (bends inward) the diverging rays from a near object before they enter the eye, shortening the effective focal length so that the eye then brings them to a focus exactly on the retina, and near objects are seen clearly. The two diagrams below show the uncorrected eye (rays focusing behind the retina) and the corrected eye (a convex lens placed in front bringing the rays to a focus on the retina).

(c) Vertical section through the mammalian skin

The skin has two main layers: an outer epidermis (with a dead, cornified surface layer, a granular layer, and a living Malpighian / germinative layer that makes new cells and the pigment melanin) and, below it, the thicker dermis. The dermis contains the hair growing from its hair follicle, the erector (hair) muscle, the sebaceous (oil) gland, the coiled sweat gland opening at a sweat pore, sensory nerve endings, and blood capillaries. Beneath the dermis lies the subcutaneous fat (adipose) layer which stores fat and insulates the body. A large labelled vertical section is shown below.

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(a) Six ways government contributes towards self-sufficiency in food production

1. Granting agricultural loans and credit facilities to farmers at low interest.
2. Subsidising farm inputs such as fertilizers, improved seeds, agro-chemicals and machinery.
3. Providing agricultural research and extension services to teach farmers modern methods.
4. Constructing irrigation schemes, dams and boreholes so that farming can continue in the dry season.
5. Guaranteeing fair, stable prices for produce and establishing marketing boards to buy surplus crops.
6. Providing storage, processing and preservation facilities (silos, cold rooms) to reduce post-harvest losses.

Other acceptable points: building access roads to farms, mechanisation programmes, and training of farmers.

(b) Five methods of conserving soil fertility

1. Crop rotation, especially including leguminous crops that fix nitrogen.
2. Application of organic manure and inorganic fertilizers to replace lost nutrients.
3. Bush fallowing, resting the land to allow natural nutrient recovery.
4. Mulching and cover cropping to protect the soil and add humus.
5. Preventing erosion by contour ploughing, terracing and avoiding bush burning.

(c) How three named agricultural practices destroy the balance of an ecosystem

(i) Bush burning: it kills soil micro-organisms, insects and small animals, destroys vegetation that provides food and shelter, and exposes the soil to erosion. Food chains are broken and biodiversity is reduced.

(ii) Use of pesticides and herbicides: these chemicals kill non-target useful organisms such as bees, earthworms and predators. They accumulate along the food chain (bioaccumulation), poisoning higher consumers and upsetting predator-prey relationships.

(iii) Deforestation / clearing land for monoculture: removing natural forest destroys the habitats of many species, lowers biodiversity, alters the water cycle and local climate, and encourages soil erosion, all of which disturb the natural balance.