WAEC SSCE - Biology - 1995

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Hormones are transported around the mammalian body by the blood. Hormones are chemical messengers produced by endocrine glands and cells in various organs and tissues that travel through the bloodstream to target cells. The hormones bind to specific receptors on the surface or inside the cells and trigger various physiological responses, such as regulating metabolism, growth, and development. Hormones are typically released in response to specific stimuli, such as stress, hunger, or light, and their levels in the blood are tightly regulated by feedback mechanisms. Therefore, the blood plays a crucial role in distributing hormones throughout the body and allowing them to interact with their target cells.

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Drought is the ecological factor that causes food shortage. Drought is a prolonged period of abnormally low rainfall or lack of moisture that leads to a shortage of water for crops and livestock. This lack of water leads to poor crop yields and can even cause crop failure, leading to food shortages. In areas where agriculture is the main source of livelihood, drought can have devastating effects on the local economy and can lead to famine and malnutrition.

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The statement that is not correct about food chain is: "All organisms in a food chain are animals". This statement is not correct because a food chain may involve plants as well as animals. A food chain is a series of organisms that are linked by their feeding relationships. Typically, a food chain starts with a primary producer (usually a green plant) that is consumed by a herbivore (a primary consumer), which in turn is consumed by a carnivore (a secondary consumer), and so on. The animals in the chain are consumers, but not all organisms in the food chain are animals, as plants can also be part of the chain. Living things in an ecosystem are dependent on one another, and the food chain involves energy transfer from one organism to another.

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The factors density, mortality rate, and birth rate all affect population. Population refers to the number of individuals of a particular species living in a specific area or habitat. The population of a species is affected by various factors such as availability of resources, competition, predation, and environmental factors. Density refers to the number of individuals in a specific area, which can affect the availability of resources and competition for those resources. Mortality rate refers to the number of deaths in a population, which can reduce the population size. Birth rate refers to the number of births in a population, which can increase the population size. Therefore, all these factors affect the size and growth of a population.

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At the end of photosynthesis, green leaves are tested for starch rather than glucose because glucose is immediately converted into starch in the presence of light. In other words, plants use the energy produced by photosynthesis to make starch which can be stored for later use. Thus, it is easy to test for starch rather than glucose in leaves. The test for starch involves iodine solution which reacts with starch to produce a blue-black color, making it easy to identify the presence or absence of starch in the leaves.

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The correct statement is: "Triceps muscle originated at the scapula and head of the humerus." The triceps muscle is a large muscle located at the back of the upper arm and is responsible for extending the arm at the elbow joint. It has three heads or points of origin, one at the scapula bone and two at the humerus bone. The ulna bone forms the point of insertion for the triceps muscle. The biceps muscle, on the other hand, has its origin at the scapula bone and its insertion at the radius bone. It is responsible for flexing the arm at the elbow joint and rotating the forearm.

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The pair of tissues that is responsible for strengthening the plant is Collenchyma and Sclerenchyma. Collenchyma cells have thick cell walls and provide support to the plant while still allowing for growth. They are found in areas of the plant that are actively growing, such as in the stems and leaves. Sclerenchyma cells have even thicker cell walls and provide more rigid support to the plant. They are found in areas of the plant that are no longer actively growing, such as in the stems and roots. Together, Collenchyma and Sclerenchyma cells help to provide strength and support to the plant, allowing it to stand upright and withstand external forces.

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The correct route of transmission of impulses along a reflex arc is: "Receptor → sensory neurone → synapse → intermediate neurone → synapse → motor neurone → effector" A reflex arc is a neural pathway that controls reflex actions, and it involves a series of neurons that work together to produce a quick, automatic response to a stimulus. The pathway starts with a receptor, which detects the stimulus and sends a signal to a sensory neuron. The sensory neuron then carries the signal to the spinal cord or brainstem, where it synapses with an intermediate neuron. The intermediate neuron, in turn, synapses with a motor neuron, which carries the signal to the effector, which is the muscle or gland that produces the response. The correct option describes this pathway in the correct order: from the receptor, to the sensory neuron, to the intermediate neuron, to the motor neuron, and finally to the effector.

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Blood group is an example of discontinuous variation. Discontinuous variation is a variation in which individuals of a population can be divided into distinct groups based on certain traits. Blood groups are determined by the presence or absence of specific antigens on the surface of red blood cells, and there are only a limited number of blood types that exist in a population. Therefore, individuals can be grouped into distinct categories based on their blood group, making it an example of discontinuous variation. In contrast, traits such as height and foot length show continuous variation, as there is a range of values that individuals can exhibit within a population.

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Prevention of erosion is best carried out by contouring of sloping ground and afforestation. Sloping ground is prone to erosion due to water and wind action, and planting trees or other vegetation is an effective method of reducing erosion. The roots of the plants hold the soil together, preventing it from being washed away by water or blown away by wind. Additionally, contouring involves constructing ridges and furrows across the slope to help trap and slow down water, reducing its erosive power. These measures are effective in conserving soil and maintaining a healthy ecosystem. Other methods such as crop rotation, mulching, and strip cropping can also be used in combination with afforestation and contouring to further prevent erosion.

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The given diagram shows various shapes of bacteria. The staphylococci are spherical-shaped bacteria that occur in clusters, while the streptobacilli are rod-shaped bacteria that occur in chains. Therefore, staphylococci are represented by the cluster of spheres labeled IV and streptobacilli are represented by the chain of rods labeled VII. So, the answer is "IV and VII".

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The statement "the environment determines which organisms survive to reproduce" supports the idea of survival of the fittest. This means that organisms that are better adapted to their environment are more likely to survive and reproduce, passing on their advantageous traits to their offspring. The environment plays a crucial role in determining which organisms are better adapted, by selecting those that have traits that enable them to survive in their specific habitat, such as better camouflage or more efficient hunting abilities. The organisms that do not possess these advantageous traits are less likely to survive and reproduce. This idea was proposed by Charles Darwin as part of his theory of natural selection, which is a key principle of evolutionary biology. Therefore, the correct option is supports the idea of survival of the fittest.

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Heredity is defined as the transfer of characteristics or traits from parents to their offspring. This means that certain physical or genetic traits, such as eye color, hair color, or the predisposition to certain diseases, can be passed down from one generation to the next. It is a fundamental concept in biology and genetics, and helps to explain why offspring often resemble their parents.

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An individual with blood group AB is considered to be the universal recipient, which means they can receive blood from all blood groups, including A, B, AB, and O. This is because blood group AB has both A and B antigens on its red blood cells and does not produce antibodies against either antigen. Therefore, when an individual with blood group AB receives blood from any other blood group, their body does not recognize the foreign antigens as harmful and does not produce antibodies against them, resulting in no adverse reactions. Thus, the only option among the given choices that matches the blood group(s) that an individual with blood group AB can receive blood from is "A, B, AB, and O."

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An advantage of complexity in higher organisms is that cellular differentiation leads to internal structural specialization. This means that different cells and tissues within an organism can perform specific functions that are necessary for the organism's survival. For example, in humans, the heart is made up of specialized muscle cells that are responsible for pumping blood throughout the body, while the lungs are made up of specialized cells that are responsible for gas exchange. This specialization allows organisms to perform more complex tasks and functions than simpler organisms that lack such specialization. Therefore, complexity in higher organisms allows for greater adaptability and survival in changing environments.

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The correct option that characterizes a swamp is "The ground is flooded most of the time". A swamp is a type of wetland that is characterized by standing water, which floods the land most of the time. Swamps are usually found in areas where the water table is close to the surface, and the soil is poorly drained. Because of this constant flooding, the vegetation found in swamps has adapted to survive in wet conditions. The other options listed are not specific to the characteristics of a swamp.

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The one-celled and free-living organism is Chlamydomonas. Chlamydomonas is a unicellular green alga that lives in freshwater environments. It has a simple structure with a single nucleus and chloroplasts for photosynthesis. It uses its flagella to move around and can reproduce asexually by cell division. Unlike Hydra, which is a multicellular animal, Chlamydomonas is a single cell that can live and function independently without the need for other cells.

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The position occupied by an organism in a food chain is called its "trophic level". This means the organism's place in the food chain based on what it eats and what eats it. For example, plants are at the first trophic level because they make their own food through photosynthesis. Herbivores (plant-eaters) are at the second trophic level because they eat plants, and carnivores (meat-eaters) are at higher trophic levels because they eat other animals. Each trophic level represents a transfer of energy and nutrients from one organism to another, and the trophic level of an organism determines its role in the ecosystem.

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The ultra-filtration process at the Bowman's capsule is facilitated by the differential blood pressure between the glomerulus and the Bowman's capsule. The glomerulus is a network of tiny blood vessels in the kidney that filters waste products, while the Bowman's capsule is the structure that surrounds the glomerulus and collects the filtered fluid. The blood pressure in the glomerulus is higher than in the Bowman's capsule, which creates a pressure gradient that pushes fluid and small molecules, such as salts and sugars, through the walls of the glomerulus and into the Bowman's capsule. This process is called ultrafiltration and helps to remove waste products from the blood, which are eventually excreted as urine. Therefore, the differential blood pressure between the glomerulus and the Bowman's capsule is the main factor that facilitates the ultra-filtration process at the Bowman's capsule.

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The statement that is not correct is "Territorial defence encourages food scarcity in a habitat". Territorial defense actually discourages food scarcity because it prevents other organisms from occupying the territory and consuming the available resources. The other statements are true: shortage of food can be caused by various factors such as pests, diseases, and neglect of farming; shortage of food can lead to competition, emigration, death, and decline in the rate of reproduction; overcrowding can lead to scarcity of food; and dispersal of seeds and fruits prevents overcrowding by allowing new individuals to establish in other areas.

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In the anaerobic stage of respiration, also known as fermentation, the net ATP molecules produced per glucose molecule is two. During fermentation, glucose is broken down into pyruvate molecules, which are then converted into either lactic acid or alcohol and carbon dioxide, depending on the type of fermentation. This process generates a net of two ATP molecules per glucose molecule, which is much less efficient than aerobic respiration. The reason for this is that fermentation does not involve the electron transport chain, which is responsible for generating the majority of ATP molecules during aerobic respiration. Instead, fermentation relies on the partial breakdown of glucose to produce energy in the absence of oxygen. Therefore, the correct option is two.

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The group that carries out similar functions in living things is: - Sclerenchyma, cartilage and chitin. Sclerenchyma is a type of plant tissue that provides structural support to the plant. Cartilage is a type of connective tissue found in animals that provides support and flexibility to various body parts. Chitin is a polysaccharide found in the exoskeletons of arthropods, such as insects and crustaceans, and provides structural support and protection to the organism. All three tissues serve a structural function in their respective organisms, making them similar in function.

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The answer is Dermis. Dermis is not a tissue found in plants. It is actually a layer of skin in animals. The other options, Epidermis, Phloem, Xylem, and Mesophyll are all plant tissues. The epidermis is the outermost layer of cells that cover the entire plant body. Phloem and xylem are vascular tissues responsible for the transport of food and water respectively. Mesophyll is a photosynthetic tissue found in the leaves of plants.

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The characteristics described, very low annual rainfall, sparse vegetation, high day temperatures and cold nights are all features of the desert biome. Deserts are regions that receive very little rainfall, and as a result, they are often dry and barren with sparse vegetation. The high day temperatures and cold nights are due to the lack of moisture in the atmosphere, which makes it difficult for heat to be retained at night. Thus, the only option among the given choices that matches the characteristics of the biome described is "desert."

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The cell membrane is called semi-permeable because it selectively allows certain substances to pass through it while preventing others from entering or leaving the cell. The membrane is made up of a phospholipid bilayer, which acts as a barrier to large or charged molecules, like proteins and ions, while allowing smaller molecules, like oxygen and water, to pass through by diffusion or active transport. This process of selective permeability helps to maintain the internal environment of the cell and allows it to function properly. The membrane is also involved in various cellular processes, but it is not directly involved in energy production. The fatty acids in the membrane contribute to its structure, but they are not the only building blocks of the membrane.

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The practice that is aimed at soil conservation is asked. Therefore, we need to identify the option that is not aimed at soil conservation. The option that is not aimed at soil conservation is bush clearing. Bush clearing involves the removal of natural vegetation cover from the soil surface, which exposes the soil to erosion by wind and water. On the other hand, the other options - contour ridging, application of manures, strip cropping, and crop rotation - are all aimed at soil conservation. Contour ridging involves creating ridges along the contours of a hill, which slows down the flow of water and reduces soil erosion. Application of manures helps to improve soil fertility and structure, which helps to prevent soil erosion. Strip cropping involves planting crops in alternating strips, which reduces soil erosion by wind and water. Crop rotation helps to maintain soil fertility and reduce soil erosion by alternating crops that require different nutrients and have different root systems.

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The organism that has a symbiotic relationship with green algae is Hydra. Hydra is a freshwater organism that belongs to the phylum Cnidaria. It is a predatory animal that captures small aquatic animals with its tentacles. Hydra has a mutualistic symbiotic relationship with a species of green algae called Chlorella. Chlorella lives inside the cells of Hydra and provides it with food through photosynthesis. In return, Hydra provides Chlorella with a stable environment and protection from predation. The waste products of Chlorella also provide a source of nutrients for Hydra. This relationship between Hydra and Chlorella is an example of mutualism, a type of symbiosis in which both organisms benefit from the association.

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During photosynthesis, carbon dioxide enters the stomata of the plant through a process called diffusion. Diffusion is the movement of particles from an area of high concentration to an area of low concentration. In this case, the concentration of carbon dioxide is higher outside the plant than inside, so it diffuses into the plant through the stomata, which are small openings on the surface of leaves that allow gas exchange. No energy is required for this process, making it a passive transport mechanism. Therefore, diffusion is the correct answer among the given choices.

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Photosynthesis removes carbon from the atmosphere. During photosynthesis, plants use carbon dioxide from the air and convert it into organic matter, such as sugars and starches. This process releases oxygen into the atmosphere, and the organic matter produced by photosynthesis can be stored in the plant or used as food for other organisms. Therefore, photosynthesis is important for balancing the amount of carbon dioxide in the atmosphere and plays a crucial role in regulating Earth's climate.

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The term syncarpous is used to describe a flower when the carpels are many and fused. In botany, carpels are the female reproductive parts of a flower that contain the ovary, style, and stigma. A syncarpous flower is a type of flower in which multiple carpels have fused together, forming a single structure. This is in contrast to an apocarpous flower, where each carpel is separate and distinct. The term "syncarpous" comes from the Greek words "syn" meaning "together" and "karpos" meaning "fruit". This fusion of carpels in a syncarpous flower can have implications for pollination and fertilization, as well as fruit development.

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Germination is the series of changes that occur in a seed by which the embryo develops into a seedling. This process begins with the absorption of water by the seed, which triggers metabolic changes leading to the growth of the embryo. As the embryo grows, the seed coat splits, and the root emerges from the seed, followed by the shoot. During germination, stored food reserves in the seed are also broken down to provide energy for the developing seedling until it is able to photosynthesize on its own. Germination is a critical process in the life cycle of plants as it marks the transition from a dormant seed to an actively growing seedling.

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The reabsorption of metabolic products in the kidney tubules is made possible by the presence of a network of capillaries around the convoluted tubules. The capillaries around the tubules allow the metabolic products to diffuse from the tubules into the bloodstream, where they can be transported to other parts of the body for use or excretion. The convoluted nature of the tubules also increases the surface area for absorption, while the joining of the main collecting tubules to the kidney tubules allows for the proper regulation and elimination of excess metabolic waste.

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Amoeba is at the cellular level of organization. This means that it is made up of just one cell, which is capable of performing all the functions necessary for life, such as obtaining nutrients, producing energy, and reproducing. The cell of an amoeba is highly specialized and contains various structures, such as a nucleus, mitochondria, and a contractile vacuole. These structures work together to carry out the various functions required for the survival of the cell. Therefore, Amoeba is a unicellular organism and exists at the cellular level of organization.

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The conclusion that cannot be drawn from the graphs is "The quantity of ions absorbed is inversely proportional to the quantity of carbohydrate (sugar) synthesized." The reason is that the graphs do not provide any information about the relationship between ion uptake and carbohydrate synthesis. The graphs only show the effect of temperature, oxygen concentration, and respiratory substrates on ion uptake. Therefore, it is not possible to make any conclusions about the relationship between ion uptake and carbohydrate synthesis based on the information provided in the graphs.

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An association between two organisms in which both benefit is called "symbiosis". Symbiosis is a close and long-term biological interaction between two different species, where both species benefit from the relationship. Examples of symbiosis include lichens, which are a combination of algae and fungi, where the algae produce food for the fungi, and the fungi provide a protective environment for the algae. Another example is the relationship between nitrogen-fixing bacteria and legumes, where the bacteria provide nitrogen to the plants, and the plants provide a source of energy and nutrients for the bacteria. Symbiotic relationships can be mutualistic, where both organisms benefit, or they can be commensalistic, where one organism benefits and the other is neither harmed nor helped.

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Higher organisms use a transport system and not diffusion only to distribute nutrients because the ratio of their surface area to volume is small. Diffusion is the movement of particles from a region of high concentration to a region of low concentration, and it is an effective way to distribute nutrients in small organisms with a large surface area-to-volume ratio. However, larger organisms have a smaller surface area-to-volume ratio, which means that the diffusion of nutrients is slower and less efficient. Therefore, they need a specialized transport system, such as the circulatory system in animals or the vascular system in plants, to deliver nutrients and other essential substances to their cells. These transport systems have specialized structures, such as blood vessels or plant tissues, that can efficiently transport nutrients over longer distances and to specific locations within the organism. In conclusion, the small surface area-to-volume ratio of larger organisms is the main reason why they require a specialized transport system to distribute nutrients.

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Encouraging the use of charcoal for industrial and domestic purposes is not a useful measure for conserving forests. Charcoal production involves cutting down trees and burning them in the absence of oxygen, which releases carbon dioxide and other harmful gases into the atmosphere. This process contributes to deforestation and greenhouse gas emissions, which are major environmental issues. In contrast, the other measures listed are useful for conserving forests. Preventing and controlling accidental forest fires, combating and preventing plant diseases, reafforestation of depleted forests, and protecting young growing trees are all important strategies for maintaining the health and vitality of forest ecosystems. By reducing the loss of forest habitat and preserving biodiversity, these measures help to ensure the long-term survival of forest ecosystems and the many species that depend on them. Therefore, the correct option is encouraging the use of charcoal for industrial and domestic purposes.

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The statement "Water removed from oceans are permanently lost" is not applicable to the circulation of water in nature. Water is a renewable resource that is constantly being cycled through the environment. Water evaporates from the seas, rivers, lakes, and soil, forming water vapour that rises, cools, condenses and precipitates as rain and dew, which then flows back into the oceans and other bodies of water, completing the water cycle. Water is also absorbed by plants and animals from their external environment and is lost through transpiration and decay. However, the statement "Water removed from oceans are permanently lost" suggests that water that is removed from oceans does not return to the cycle, which is not true. Even water that is removed from oceans, such as through human activities like desalination, can be returned to the environment and participate in the water cycle.

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The most readily available natural resource to all organisms is air. Air is composed of various gases, including oxygen, nitrogen, and carbon dioxide, which are essential for the survival of living organisms. Oxygen is necessary for respiration, which provides energy to the cells of the body, while carbon dioxide is essential for photosynthesis, the process by which plants produce food. Nitrogen, although not directly used by most organisms, is essential for the formation of amino acids and nucleic acids, which are the building blocks of proteins and DNA, respectively. Therefore, air is the only option among the given choices that is most readily available to all organisms.

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The advantage of dispersal of fruits and seeds is that it allows for the even distribution of plants and the reduction of intraspecific competition. When fruits and seeds are dispersed away from the parent plant, they are able to grow and establish themselves in new locations where they are not competing with the parent plant and its offspring for resources like water, nutrients, and sunlight. This increases the chances of survival and growth of the dispersed plants, and also promotes genetic diversity in plant populations. Additionally, dispersal of fruits and seeds can also lead to a fair distribution of sources of food for animals, which can help to sustain wildlife populations.

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The condition that is not necessary for photosynthesis to occur in plants is the presence of xylem tissue used to conduct water. Photosynthesis is the process by which plants convert light energy into chemical energy, which is then stored in the form of glucose. This process occurs in the chloroplasts of plant cells, specifically in the chlorophyll pigments. In order for photosynthesis to occur, there must be light to provide energy for the process. Chlorophyll is also necessary as it traps the radiant energy from the light. Carbon dioxide is needed as a source of carbon, while water provides the hydrogen needed to make glucose. However, xylem tissue is not directly involved in the process of photosynthesis. Xylem tissue is responsible for transporting water from the roots to the leaves of the plant, where photosynthesis occurs. While water is an important ingredient for photosynthesis, the presence of xylem tissue is not required for photosynthesis to occur.

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Hereditary variation is best described as characters transmitted from generation to generation. Heredity refers to the passing on of traits or characteristics from parents to their offspring through genetic material (DNA). Hereditary variation refers to the differences in traits or characteristics that exist between individuals of the same species, and which are caused by variations in their genes. These variations may be due to mutations, which are changes in the DNA sequence, or recombination, which is the shuffling of genes that occurs during sexual reproduction. Hereditary variation is important because it provides the raw material for natural selection, which is the process by which organisms with advantageous traits are more likely to survive and reproduce. Therefore, hereditary variation is a key factor in the evolution of species over time.

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The characteristics of insect-pollinated flowers are brightly colored petals, scent, and nectar. Insects are attracted to these features as they are searching for food, and as they feed on the nectar, they pick up pollen from the flower. The pollen is then transferred to other flowers, allowing for cross-pollination. In contrast, dull colored flowers with no nectar are usually wind-pollinated, as they do not need to attract insects for pollination. The stigma in insect-pollinated flowers is not feathery, but rather smooth and designed to collect pollen from the insects. Thus, the only option among the given choices that matches the characteristics of insect-pollinated flowers is "Brightly colored petals, scent, and nectar."

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(a) Sensory cell

A sensory cell (receptor cell) is a specialised cell that is sensitive to a particular stimulus (such as light, sound, heat, touch or chemicals) and converts the stimulus into a nerve impulse which is sent to the central nervous system.

(b) Functions of parts of the eye

• (i) Lens: focuses (refracts) light rays onto the retina and, by changing its shape (accommodation), forms a clear image of both near and distant objects.
• (ii) Vitreous humour: the jelly-like fluid that fills the eyeball behind the lens; it keeps the eyeball firm and in shape and helps to refract light onto the retina.
• (iii) Optic nerve: carries nerve impulses from the retina to the brain, where the impulses are interpreted as sight.

(c)(i) Four structures that protect the eye

Eyelids, eyelashes, eyebrows, and tears (with the conjunctiva/bony eye socket also acceptable).

(c)(ii) How each performs its function

• Eyelids: close over the eye to shut out excess light and to prevent dust and foreign bodies from entering; blinking spreads tears over the eye.
• Eyelashes: hairs on the eyelids that trap dust and small particles, preventing them from reaching the surface of the eye.
• Eyebrows: hairs above the eye that prevent sweat and rain from running down into the eye.
• Tears (from the tear glands): wash away dust and contain an enzyme (lysozyme) that kills bacteria, keeping the eye clean and moist.

(d) Why a person is dazzled in bright sunlight

In a dark room the pupil is widely dilated (opened) to let in as much light as possible, and the light-sensitive pigment in the retina is fully built up. When the person is suddenly exposed to bright sunlight, a large amount of light floods into the eye through the still-wide pupil and over-stimulates the retina, so the person is dazzled and cannot see clearly. After a few seconds the circular muscles of the iris contract, the pupil becomes small (constricts) to reduce the amount of light entering, and the retina adjusts; normal vision then returns.

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(a) Labelled diagram of a vertical section of a mammalian tooth

(b) Roles in mammalian nutrition

1. Teeth: They cut, bite, tear and grind food during mastication. This breaks food into small particles, increases its surface area for enzyme action and makes swallowing easier.
2. Tongue: It moves food around the mouth during chewing, mixes it with saliva, moulds the food into a bolus and pushes the bolus into the pharynx for swallowing. It also bears taste buds for tasting food.
3. Saliva: It moistens and lubricates food, making chewing and swallowing easy. It acts as a solvent for soluble food substances and contains salivary amylase (ptyalin), which begins the digestion of cooked starch to maltose. It is slightly alkaline.
4. Stomach: It temporarily stores food and churns it, thereby mixing it with gastric juice. Gastric juice contains hydrochloric acid, which provides an acidic medium and kills many bacteria, and pepsin, which starts protein digestion. In young mammals, rennin coagulates milk protein.

(c) Test for protein: Biuret test

1. Prepare an aqueous suspension or solution of the food sample in a clean test tube.
2. Add about 2 cm³ of dilute sodium hydroxide solution and shake.
3. Add a few drops of dilute copper(II) sulphate solution and mix gently.

Observation: A violet or purple colour indicates the presence of protein. If protein is absent, the mixture remains blue.

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(a) What is a cell?

A cell is the basic structural and functional unit of life; it is the smallest unit of a living organism that can carry out all the processes of life. All living things are made up of one or more cells.

(b) Structure of DNA

DNA (deoxyribonucleic acid) is the hereditary material that carries genetic information. Its structure is:

• It is a very long molecule made up of two strands twisted round each other to form a double helix (like a twisted ladder).
• Each strand is a chain of units called nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
• There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C).
• The two strands are held together by hydrogen bonds between the bases. Bases pair specifically: adenine with thymine and guanine with cytosine (complementary base pairing).
• The sugar and phosphate groups form the sides (backbone) of the ladder, while the paired bases form the rungs.

(c) Process of mitosis

Mitosis is a type of cell division in which a parent cell divides to produce two daughter cells, each with the same number of chromosomes as the parent. It occurs in stages:

1. Interphase: the cell grows and the DNA (chromosomes) is copied (replicated).
2. Prophase: chromosomes shorten, thicken and become visible; each appears as two chromatids joined at the centromere; the nuclear membrane begins to break down and spindle fibres form.
3. Metaphase: chromosomes line up singly along the equator (middle) of the cell, attached to the spindle fibres.
4. Anaphase: the centromeres split and the two chromatids of each chromosome are pulled apart to opposite poles of the cell.
5. Telophase: the chromatids reach the poles; a new nuclear membrane forms around each group; the chromosomes uncoil.
6. Cytokinesis: the cytoplasm divides, forming two identical daughter cells.

(d) Importance of mitosis

• It brings about growth by increasing the number of cells.
• It repairs damaged tissues and replaces worn out or dead cells (healing of wounds).
• It is the basis of asexual reproduction in many organisms.
• It ensures that daughter cells receive an exact and constant number of chromosomes, keeping the species true to type.

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(a) Roles in the food chain

• (i) Green plants: they are the producers. Using chlorophyll they trap sunlight and manufacture their own food (glucose) from carbon dioxide and water by photosynthesis. They form the first trophic level and are the source of food and energy for all other organisms in the chain.
• (ii) Herbivores: they are the primary consumers. They feed directly on green plants and convert plant material into animal tissue, passing energy on to the next level.
• (iii) Carnivores: they are the secondary and tertiary consumers. They feed on herbivores (and on other carnivores), transferring energy along the food chain and helping to control the population of the animals they eat.
• (iv) Decomposers: bacteria and fungi that break down dead plants, dead animals and waste matter, releasing simple substances and mineral nutrients back into the soil and air, so that nutrients are recycled for use by green plants.

(b) The carbon cycle

The carbon cycle is the continuous circulation of carbon between the atmosphere, living organisms and the environment. A diagram should show carbon dioxide in the atmosphere at the centre with arrows into and out of it. The main processes are:

1. Photosynthesis: green plants remove carbon dioxide (CO₂) from the atmosphere and fix it into organic food (carbohydrates).
2. Feeding: carbon passes from plants to animals when herbivores eat plants and carnivores eat herbivores.
3. Respiration: plants and animals break down food and release CO₂ back into the atmosphere.
4. Death and decay: when plants and animals die, decomposers (bacteria and fungi) break down their bodies and release CO₂ during decomposition.
5. Combustion: burning of wood and fossil fuels (coal, petroleum, natural gas) releases CO₂ into the atmosphere.
6. Fossilisation: over millions of years the remains of dead organisms form fossil fuels which store carbon.

These processes keep the amount of carbon dioxide in nature roughly in balance.