WAEC SSCE - Biology - 2012

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The pulmonary circulation involves the movement of blood to and from the lungs. It is a part of the circulatory system that specifically deals with the transportation of deoxygenated blood from the heart to the lungs, where the blood is oxygenated, and then returned to the heart. This process is facilitated by the pulmonary arteries and veins, which carry the blood to and from the lungs, respectively. The oxygenated blood is then circulated to the rest of the body through the systemic circulation, while the deoxygenated blood returns to the lungs via the pulmonary circulation.

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Magnesium ions must be present in the soil for the growth of a healthy green plant. Magnesium is an essential mineral nutrient for plants and is involved in many important plant functions, such as photosynthesis, enzyme activation, and the production of chlorophyll. Chlorophyll is the pigment responsible for the green color of leaves and is necessary for photosynthesis. Without magnesium, plants cannot produce enough chlorophyll, which can result in yellowing leaves, stunted growth, and poor yields. While the other options listed are important for plant growth, magnesium ions are particularly crucial for a healthy green plant.

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The epidermis of a leaf is considered an example of a tissue because the cells all possess a similar structure and function. The epidermal cells are tightly packed and flattened to form a continuous layer that covers the entire surface of the leaf. They are covered by a waxy cuticle, which helps to prevent water loss through the leaf surface. The main function of the epidermis is to protect the leaf from external damage and regulate the exchange of gases, such as carbon dioxide and oxygen, between the leaf and the environment. Therefore, the epidermis of a leaf is considered a tissue because it is composed of a group of similar cells that perform a specific function.

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The main raw materials required for photosynthesis are carbon dioxide and water. During photosynthesis, plants use energy from sunlight to convert carbon dioxide from the air and water from the soil into glucose (a type of sugar) and oxygen. This process occurs in the chloroplasts of plant cells, which contain the pigment chlorophyll that absorbs light energy. The oxygen produced during photosynthesis is released into the atmosphere as a byproduct, while the glucose serves as the plant's source of energy for growth and development.

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The disadvantage of complexity of organisation in the higher organisms is that it leads to a slower rate of diffusion of oxygen to individual cells. In higher organisms, such as animals and plants, there is an intricate network of tissues and organs that work together to perform vital functions. This complexity requires a vast amount of energy to maintain and a greater reliance on oxygen for cellular respiration. However, as organisms get bigger and more complex, the distance between the surface of the organism and the innermost cells increases, resulting in a slower rate of diffusion of oxygen to individual cells. This can limit the size of organisms and their ability to survive in low-oxygen environments.

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Water is the food substance that will have little or no calorific value. This is because water does not contain any significant amount of energy or calories that can be metabolized by the body to provide energy. While proteins, carbohydrates, and lipids all contain calories that can be used as a source of energy, water is primarily used by the body for hydration and other physiological functions but not for providing energy. Therefore, consuming water will not contribute significantly to the caloric intake of an individual.

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The high concentration of lactic acid found in the muscle cell fluid of an athlete immediately after a 10 m race is likely caused by anaerobic respiration that occurred during the race. Anaerobic respiration is a process of cellular respiration that takes place in the absence of oxygen, producing lactic acid as a byproduct. During intense exercise, such as a sprint, the body may not be able to supply enough oxygen to the muscles, forcing them to switch to anaerobic respiration. This produces lactic acid which can accumulate in the muscles and cause fatigue and discomfort. Therefore, the most likely cause of the high concentration of lactic acid found in the athlete's muscle cell fluid is due to the anaerobic respiration of the muscles during the race.

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Accessory pigments play a major role in photosynthesis by trapping light energy that chlorophyll cannot absorb. Chlorophyll is the primary pigment that absorbs light energy, but it can only absorb certain wavelengths of light. Accessory pigments such as carotenoids and phycobilins are able to absorb additional wavelengths of light that chlorophyll cannot, and transfer the energy to chlorophyll. This increases the efficiency of photosynthesis by allowing more light energy to be absorbed and converted into chemical energy. Therefore, the correct option is "trap light energy that chlorophyll cannot absorb".

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Excess carbohydrates can be stored in the muscles in the form of glycogen. Glycogen is a complex carbohydrate made up of many glucose molecules linked together in a branched chain. When the body has more glucose than it needs for energy, it stores the excess glucose as glycogen in the liver and muscles. The glycogen in the muscles can be broken down into glucose when the body needs energy during exercise or other activities.

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Impulses are conducted towards the nerve cell through the dendrons. Dendrons are the branch-like structures that extend from the cell body of a neuron and receive impulses from other neurons or sensory receptors. These impulses then travel through the dendrites and into the cell body before being transmitted along the axon to other neurons or effectors such as muscles or glands. The myelin sheath is a fatty layer that covers some axons and helps to speed up the transmission of impulses, while the synaptic nodes are small gaps between neurons where neurotransmitters are released to allow for communication between neurons.

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The area around the earth where life activities go on is referred to as the "biosphere". The biosphere is a combination of all living organisms, their habitats, and the ecological processes that sustain life. It includes the atmosphere (air), lithosphere (land), and hydrosphere (water), where living organisms interact with each other and the physical environment. The biosphere is essential for the survival and existence of all life on earth, and it plays a vital role in regulating the planet's climate, air quality, and water cycles.

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Plants with breathing roots are characteristic features of vegetation in saltwater swamp. In saltwater swamps, the soil is often waterlogged with high salt content, and oxygen levels can be low. Therefore, plants in these regions have evolved special adaptations to survive in such conditions, such as breathing roots, which are roots that grow above the water surface to allow the exchange of gases between the atmosphere and the root system. This adaptation helps the plants to obtain oxygen from the air and remove excess carbon dioxide and other gases from the roots. Examples of plants with breathing roots found in saltwater swamps include mangroves and some species of palms.

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The science of life is referred to as biology. Biology is the study of living organisms, including their structure, function, growth, evolution, and distribution. It encompasses a wide range of topics, from the smallest cells to the largest ecosystems, and provides us with knowledge about the diversity of life on earth, the interrelationships between organisms, and their interaction with the environment. Biology is an important field of study because it helps us to better understand the world around us and to develop new technologies and medical treatments based on our knowledge of living organisms.

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A sudden change in a gene or chromosome is called a "mutation." Mutations are changes in the DNA sequence that can result in altered or new characteristics in an organism. They can occur spontaneously or be induced by external factors such as radiation or chemicals. Mutations can have different effects, ranging from neutral or harmful to beneficial, depending on the location and nature of the change. The occurrence of mutations is the basis of genetic diversity, which is essential for evolution and adaptation of species to changing environments.

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The test that would be used to confirm the nature of the gas given off in the experimental setup is to pass the gas through lime water. This is because lime water (Calcium hydroxide solution) reacts with carbon dioxide gas, which is one of the gases produced during yeast fermentation, to form a milky precipitate of calcium carbonate. If the gas given off by the yeast is carbon dioxide, then the lime water will turn milky when passed through it, confirming the nature of the gas.

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Thunder and lightning are natural phenomena that occur during thunderstorms. Thunderstorms usually bring rain, and rainwater is essential for crop growth. When lightning occurs, it splits nitrogen molecules in the air which then combine with oxygen to form nitrates. These nitrates dissolve in the rainwater and are deposited in the soil as it rains. Nitrates are an essential nutrient for plants and help them to grow better. Therefore, the benefit of thunder and lightning to farmers is that they add nitrates to the soil, which can lead to increased crop yields.

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Squirrels can easily kill some trees because they gnaw the bark and destroy it. The bark is essential to the tree's survival because it contains the phloem and xylem, which transport food and water throughout the tree. Without bark, the tree cannot transport these essential materials, and it will eventually die. Squirrels gnaw the bark for various reasons, such as to obtain food or create a nest, but the result is often fatal for the tree.

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Label IV, the kidney, contains the highest amount of nitrogenous waste. The kidneys are responsible for filtering and removing waste products such as urea and uric acid from the blood. These nitrogenous wastes are produced as a result of protein metabolism in the body, and the kidneys work to remove them from the blood and excrete them in the urine. Therefore, since the kidneys are responsible for the removal of nitrogenous waste, they contain the highest amount of it among the labelled parts in the diagram.

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The classes of fingerprints in man are: whorl, arch, loop, and compound. Whorls are spiral patterns with at least two deltas (triangular ridges) and a ridge tracing that makes a complete circuit; arches have a ridgeline that enters from one side of the finger, makes a rise in the center, and exits from the opposite side; loops have ridges that enter from one side, curve around or upward, and exit from the same side they entered; and compound fingerprints have two or more patterns with no single pattern present in 50% or more of the fingerprint. These classes are used in forensic analysis and identification, as each individual has a unique set of fingerprints.

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The correct evolutionary trend in plants is Algae → liverworts → mosses → ferns. This is because algae, liverworts, mosses, and ferns represent the different stages in the evolution of plants. Algae are simple aquatic plants that lack specialized tissues for conducting water and nutrients. Liverworts and mosses evolved from algae and have specialized structures to absorb water and nutrients from their environment. Ferns evolved from mosses and have vascular tissue for conducting water and nutrients throughout the plant. Therefore, the correct sequence of the evolutionary trend is Algae → liverworts → mosses → ferns.

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The correct answer is "homologous chromosomes separate". Meiosis is a type of cell division that occurs in sexually reproducing organisms, and it involves two rounds of cell division. The first phase of meiosis is called meiosis I, and it involves the separation of homologous chromosomes. In the second phase, meiosis II, the sister chromatids of each chromosome separate, resulting in four haploid daughter cells. Therefore, the process that occurs in the second phase of meiosis is the separation of sister chromatids, and this is what leads to the formation of four haploid daughter cells.

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The structure in a flower that develops into the seed is the ovule. The ovule is located inside the ovary of the flower and contains the female gamete. After pollination and fertilization, the ovule develops into a seed which contains an embryo that will develop into a new plant. The integument is the protective covering of the ovule and becomes the seed coat after fertilization. The testa is the protective outer covering of the seed.

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The function of the convex lens in correcting eye defects is to converge light rays from far objects to focus their image on the retina. In people with myopia (short-sightedness), the image is focused in front of the retina, resulting in a blurred image. Placing a convex lens in front of the eye causes light rays to converge more, allowing the image to be focused on the retina. This corrects the vision so that distant objects appear clear.

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When fertilization occurs, the nucleus of the sperm fuses with the nucleus of the egg cell. This results in the formation of a zygote, which contains the genetic material from both parents. The tail of the sperm and its other components are left outside the egg cell and do not fuse with it. The fusion of the sperm and egg nuclei triggers a series of events that lead to the development of a new organism.

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In this scenario, both hybrid plants have a recessive factor for green flower color, which means they both have the genotype "yy". When these two plants are crossed, the Punnett square for the F1 generation will result in all offspring having the genotype "Yy" (yellow flower color). However, since the "Y" allele is dominant, they will all exhibit the yellow flower color phenotype. If two of these F1 offspring are crossed, the Punnett square for the F2 generation will result in a ratio of 3 yellow-flowered plants to 1 green-flowered plant. This is because both parents have a genotype of "Yy", meaning they each have a 50% chance of passing on the dominant "Y" allele or the recessive "y" allele to their offspring. The only way for a plant to have green flowers is to inherit two copies of the recessive "y" allele, which occurs with a probability of 1 in 4 or 25%. Therefore, the correct ratio is 1 in 4. So, the correct answer is "1 in 4".

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The best title for the above diagram is "Terrestrial food chain". This is because the diagram shows the flow of energy from producers (plants) to primary consumers (herbivores), to secondary consumers (carnivores), and so on, within a land-based ecosystem. The arrows in the diagram indicate the direction of energy transfer, and the labels on each organism show their position in the food chain. The term "food web" typically refers to a more complex diagram that includes multiple interconnected food chains, whereas this diagram only shows a single, linear food chain.

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The answer is bacteria. Bacteria are living organisms and belong to the biotic components of the ecosystem. Biotic components of the ecosystem include all living things such as plants, animals, fungi, and microorganisms. Abiotic components, on the other hand, include all non-living things in the ecosystem such as water, air, soil, rocks, sunlight, wind, and topography. Therefore, is the correct answer as it is a living organism and not an abiotic component of the ecosystem.

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The use of water closet and septic tanks is preferred to the use of bucket and pit latrines because contaminated faeces are completely kept away from flies. Water closets and septic tanks are designed in such a way that the human waste is channeled away from the house and stored in an underground chamber called a septic tank. The septic tank is designed to digest and decompose the waste and discharge water into the soil. This system ensures that the waste is completely kept away from flies and other insects, preventing the spread of diseases. Conversely, bucket and pit latrines are not completely sealed, and the waste can attract flies, which can spread disease. Therefore, the use of water closet and septic tanks is a more effective way to prevent the spread of disease.

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Among the options given, the organism at the organ level of organization of life is "Ginger". Ginger is a plant that exhibits organ level of organization. It has different organs such as roots, stem, leaves, and flowers that have specialized functions. The roots of ginger are used for storage of food and absorption of water and nutrients from the soil, the stem provides support for the leaves and flowers, the leaves are responsible for photosynthesis, and the flowers are involved in reproduction. On the other hand, Euglena and Spirogyra are unicellular organisms that exhibit cellular level of organization, and Tapeworm is a multicellular organism that exhibits tissue level of organization.

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Brightly colored petals in some plants serve as an adaptive coloration for pollination. Pollination is the process by which pollen grains are transferred from the male part of a plant to the female part of a plant, leading to fertilization and seed production. Brightly colored petals help to attract pollinators, such as bees, butterflies, and hummingbirds, which are attracted to the bright colors and sweet nectar of the flowers. The pollinators then transfer the pollen from one flower to another, ensuring cross-fertilization and genetic diversity. Therefore, the brightly colored petals are an adaptation of the plant to increase the chances of successful pollination and reproduction.

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Bees are important to farmers because they pollinate flowers. When bees fly from flower to flower collecting nectar, they transfer pollen from the male part of the flower to the female part, which fertilizes the plant and allows it to produce fruit or seeds. This is crucial for the reproduction of many crops, including fruits, vegetables, and nuts. Without bees, the yields of these crops would be significantly reduced, and in some cases, the plants would not be able to produce fruit or seeds at all. While bees do produce honey, this is not their primary role in agriculture.

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The purpose of proteins in the diet of a mammal is to promote growth and repair cells. Proteins are essential macronutrients made up of amino acids, which are the building blocks of cells. When we eat proteins, our digestive system breaks them down into individual amino acids which can be used by our bodies to build and repair tissues such as muscles, skin, and organs. Amino acids are also involved in the production of enzymes and hormones that help to regulate various body functions. Additionally, some amino acids can be used as a source of energy when needed.

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The element that is most likely to be deficient in the mother's diet during pregnancy and cause weak and crooked bones in the child is calcium and phosphorus. Calcium is an important element for bone formation and its deficiency can lead to a condition called rickets which results in weak and deformed bones. Phosphorus is also necessary for proper bone growth and development. Deficiency in both these elements during pregnancy can result in skeletal abnormalities in the developing fetus. Therefore, the most appropriate answer would be (c) Calcium and phosphorus.

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The correct answer is Lipase. Bile is produced by the liver and stored in the gallbladder. It helps in emulsifying fats into smaller droplets, which increases the surface area for lipase to act upon. Lipase is the enzyme responsible for the breakdown of fats into fatty acids and glycerol. Without sufficient bile to emulsify fats, the lipase enzyme will have a reduced surface area to act upon, thus leading to decreased digestion and absorption of fats. Therefore, if the liver fails to produce bile, the activity of lipase will be greatly affected.

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Walking is not an example of a simple reflex action. Simple reflex actions are those that occur automatically and involuntarily in response to a specific stimulus, without involving conscious thought. Examples of simple reflex actions include swallowing, sneezing, and blinking. However, walking is a complex action that involves the coordinated movement of muscles and the involvement of various parts of the brain, and is not a simple reflex action.

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Sickle cell anemia is the disease that is transmittable through genes. It is a genetic disorder caused by a mutation in the hemoglobin gene. People with sickle cell anemia inherit two copies of the mutated gene, one from each parent, and this causes the hemoglobin in their red blood cells to be abnormal. The abnormal hemoglobin causes the red blood cells to take a crescent or sickle shape, which can block blood flow, leading to pain, organ damage, and an increased risk of infections.

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The most appropriate instrument for comparing air pressure at different places in the habitat is a barometer. A barometer is an instrument used to measure atmospheric pressure, and it can help in identifying high-pressure and low-pressure areas in the habitat. By comparing the readings of the barometer at different places in the habitat, we can determine which areas have higher or lower air pressure, which can have implications for weather patterns and other environmental conditions. The other instruments listed (hygrometer, photometer, and air pressure disc) measure humidity, light intensity, and air pressure changes, respectively, but they are not specifically designed to measure atmospheric pressure at different locations.

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The primary cause of species endangerment is disappearing habitats. This means that the natural habitats of many species are being destroyed or altered, which leads to a reduction in their population and can ultimately lead to their extinction. Human activities such as deforestation, urbanization, and pollution are the major drivers of habitat loss. As habitats disappear, species lose access to food, water, and shelter, making it difficult for them to survive. Therefore, conservation efforts are essential to protect habitats and prevent the extinction of species.

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If two plants with red flowers were backcrossed, it means that one parent is homozygous dominant for red flowers (RR) and the other parent is heterozygous for red flowers (Rr). Therefore, the possible genotypes of the offspring are: - 50% chance of getting Rr (heterozygous for red flowers) - 50% chance of getting rr (homozygous recessive for white flowers) If the plants are heterozygous for red flowers (Rr), the phenotype (physical appearance) ratio of their offspring will be 3:1 (red:white). This means that 75% of the offspring will have red flowers and 25% will have white flowers. Therefore, the answer is "75% red and 25% white" which is the option that indicates heterozygosity.

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As shown in the illustration, part II, the vas deferens, can be cut to ensure permanent sterility. This is because the vas deferens is a tube that carries sperm from the testes to the urethra during ejaculation. By cutting the vas deferens, the sperm cannot be released during sexual intercourse, thus resulting in permanent sterility.

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The interdependence of cells refers to the mutual reliance of cells on each other for survival and functioning as a single organism. Among the given options, Volvox is a multicellular organism where interdependence of cells is maintained. In Volvox, each cell has a specific role to play, and they work together to perform various functions like movement, feeding, and reproduction. The cells are interconnected by cytoplasmic strands that allow them to communicate and share resources. The interdependence of cells ensures the proper functioning of the organism as a whole.

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The zone with very few organisms is zone IV, which is also known as the abyssal zone. This zone is characterized by very high pressure, very low temperatures, and complete darkness. The extreme conditions in this zone make it very difficult for organisms to survive, and as a result, only a few organisms are adapted to live in this zone. The organisms that can survive in the abyssal zone are often specially adapted to the extreme conditions, such as high pressure and low temperatures. They also have specialized feeding and reproductive mechanisms to cope with the lack of sunlight and low levels of organic matter.

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(a) Four factors that affect the diffusion of substances

• Concentration gradient - the steeper the difference in concentration between two regions, the faster the rate of diffusion.
• Temperature - a rise in temperature increases the kinetic energy of the particles, so they move and diffuse faster.
• Surface area - the larger the surface area over which diffusion occurs, the faster the rate.
• Distance / thickness of the membrane - the shorter the distance (thinner the membrane), the faster the diffusion (the size and molecular weight of the diffusing particles also matter: small, light particles diffuse faster).

(b) Explanation of terms

• (i) Active transport - The movement of substances across a cell membrane against a concentration gradient (from a region of lower to higher concentration) using energy released from respiration (ATP).
• (ii) Transpiration - The loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves.

(c) Four ways plants reduce a high rate of transpiration

• Reducing the number of stomata, and having sunken stomata in pits.
• Having a thick waxy cuticle on the leaf surface.
• Reducing the leaf surface area or reducing leaves to spines/scales.
• Closing the stomata (especially during the hottest part of the day) and shedding leaves in the dry season; presence of hairs on the leaf surface also helps.

(d) Features of red blood cells and how they adapt the cell to its functions

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(a)(i) Epigeal germination

Epigeal germination is a type of seed germination in which the cotyledons are carried above the soil surface. This is brought about by rapid elongation of the hypocotyl (the part of the axis below the cotyledons), which arches and pulls the cotyledons out of the soil. The cotyledons then turn green, spread out and function as the first leaves (foliage leaves) carrying out photosynthesis until the true leaves develop. Examples include cowpea, groundnut and castor oil.

(a)(ii) Three differences between epigeal and hypogeal germination

(Example of hypogeal: maize, oil palm.)

(b)(i) Seed dormancy

Seed dormancy is a resting condition in which a viable (living) seed fails to germinate even when the external conditions (water, oxygen and suitable temperature) are favourable, until certain internal or external requirements are met.

(b)(ii) Three ways of breaking seed dormancy

• Scarifying or cracking the hard seed coat (mechanically or by heat/fire) to allow water and oxygen in.
• Soaking the seeds in water or in dilute acid to soften or wear away the seed coat.
• Exposing the seeds to a period of low temperature (chilling) or removing germination inhibitors, or treating with growth hormones such as gibberellin.

(c) Six advantages of using contraceptives in human populations

• They help to control and reduce the rate of population growth (check overpopulation).
• They enable proper child-spacing, improving the health of the mother.
• They reduce maternal and infant mortality.
• They allow parents to have only the number of children they can adequately care for, improving family welfare.
• They help to prevent unwanted pregnancies.
• Some (for example condoms) help to prevent the spread of sexually transmitted infections such as HIV/AIDS.

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(a)(i) What is a gene?

A gene is a unit of heredity located at a specific position (locus) on a chromosome, made up of a segment of DNA, which carries the coded information for a particular protein and is responsible for the transmission of a particular trait or character from parents to offspring.

(a)(ii) Difference between genotype and phenotype

Genotype	Phenotype
The genetic make-up (the actual combination of alleles/genes) of an organism, e.g. \(SS\), \(Ss\) or \(ss\).	The observable, physical or biochemical characteristics of an organism, e.g. smooth seed coat or rough seed coat.
It is inherited and remains constant, independent of the environment.	It is determined jointly by the genotype and the environment.

(b) Explanation of terms

(i) Hybrid: An offspring produced from a cross between two genetically different parents that possess contrasting forms of a character; such an offspring is heterozygous, e.g. \(Ss\).

(ii) Pure breeding: A condition in which an organism is homozygous for a trait (e.g. \(SS\) or \(ss\)) so that, when self-pollinated or crossed with an identical type, it produces offspring showing only that same trait generation after generation.

(iii) Nucleotide: The basic structural unit (building block) of a nucleic acid, each one consisting of a pentose sugar (deoxyribose in DNA or ribose in RNA), a phosphate group and a nitrogenous base (adenine, guanine, cytosine, and thymine or uracil).

(c) Genetic cross

Let the allele for smooth seed coat be \(S\) (dominant) and the allele for rough seed coat be \(s\) (recessive).

The homozygous rough pea has the genotype \(ss\).

The smooth-seeded plant had rough-coated parents. Since a rough parent can only supply a recessive \(s\) allele, this smooth plant must have received one \(s\) allele; because it is smooth it must also carry one dominant \(S\) allele. It is therefore heterozygous, genotype \(Ss\). (This is a test cross.)

Genetic diagram of the cross

Results:

Genotypic ratio of offspring \(=\;1\;Ss\;:\;1\;ss\).

Phenotypic ratio of offspring \(=\;1\) smooth seed coat \(:\;1\) rough seed coat, i.e. 50% smooth-seeded and 50% rough-seeded plants.

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(a) Explanation of terms

• (i) Disease - A disease is any disorder or departure from the normal state of health of the body, in which the normal functioning of a part or the whole of the body is disturbed, usually accompanied by characteristic signs and symptoms.
• (ii) Symptoms of disease - Symptoms are the observable or felt signs and abnormal changes in the body that indicate the presence of a disease (for example fever, headache, weakness, vomiting or rashes).

(b)(i) Two physical and two chemical barriers

Physical barriers: the skin (intact epidermis) which keeps out pathogens; hairs and mucus / cilia lining the respiratory tract that trap microbes.

Chemical barriers: hydrochloric acid in the stomach which kills swallowed bacteria; lysozyme enzyme in tears, saliva and sweat which destroys bacteria (sebum on the skin is also chemical).

(b)(ii) How vaccination protects the body from infectious diseases

A vaccine contains dead, weakened (attenuated) or modified pathogens, or their harmless toxins (antigens). When introduced into the body, these antigens stimulate the white blood cells (lymphocytes) to produce specific antibodies against that pathogen without causing the disease. The body also produces memory cells. If the real pathogen later enters the body, the memory cells enable the body to produce the correct antibodies rapidly and in large amounts, destroying the pathogen before it can cause disease. This gives the body active, long-lasting immunity.

(c) Difference between an antibody and an antigen

An antigen is a foreign substance (often a protein on the surface of a pathogen) that, when it enters the body, stimulates the production of antibodies. An antibody is a protein produced by the white blood cells (lymphocytes) in response to an antigen, which combines with and neutralises or destroys that specific antigen.

(d) Causative agents

• (i) Malaria - a protozoan, Plasmodium.
• (ii) Cholera - a bacterium, Vibrio cholerae.
• (iii) AIDS - a virus, the Human Immunodeficiency Virus (HIV).

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(a) Role of the stomach in digestion

The stomach is a muscular, J-shaped organ that stores food temporarily and continues digestion. Its walls contain gastric glands that secrete gastric juice. When food arrives, the muscular walls churn and mix it with the gastric juice to form a semi-liquid mass called chyme. The gastric juice contains:

• Hydrochloric acid, which provides the acidic medium needed for the enzymes to act, kills bacteria in the food, and activates pepsin.
• Pepsin, an enzyme that begins the digestion of proteins, breaking them down into peptones and polypeptides.
• Rennin (in young mammals), which curdles the soluble milk protein caseinogen into insoluble casein so it can be acted upon by pepsin.

The stomach also absorbs a little water, alcohol and simple substances, and releases the chyme gradually into the duodenum. Mucus secreted by the wall protects the stomach lining from being digested by its own acid and enzyme.

(b)(i) Three parts of plants in which food can be stored, with examples

(Leaves, as in onion bulb scales, are another storage part.)

(c) Regulation of blood sugar level in mammals

The level of glucose in the blood is kept fairly constant (about 90 mg per 100 cm³) by two hormones from the islets of Langerhans in the pancreas, working with the liver.

• When blood sugar rises (for example after a meal), the pancreas secretes insulin. Insulin causes the liver and muscles to convert the excess glucose into glycogen for storage, and increases the oxidation of glucose by cells. This lowers the blood sugar back to normal.
• When blood sugar falls (for example during fasting or exercise), the pancreas secretes glucagon. Glucagon causes the liver to convert stored glycogen back into glucose which is released into the blood, raising the level to normal. (The hormone adrenaline also raises blood sugar in the same way.)

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(a) Explanation of terms

• (i) Conservation - Conservation is the careful management, protection and wise use of natural resources (such as forests, wildlife, water and soil) so that they are not exhausted or destroyed, and are preserved for the benefit of present and future generations.
• (ii) Endangered species - An endangered species is a species of organism whose population has fallen so low that it is in danger of becoming extinct (dying out completely) unless steps are taken to protect it.

(b)(i) Five reasons why conservation of forests is important

• Forests provide timber, wood, fuel, fruits, herbs and other useful products.
• They serve as a habitat and source of food for wildlife, preserving biodiversity.
• They prevent soil erosion and help to maintain soil fertility.
• They help to regulate climate and the water cycle, and reduce flooding.
• They purify the air by absorbing carbon dioxide and releasing oxygen (and serve as sites for recreation, tourism and research).

(b)(ii) Four ways forest reserves can be conserved

• Controlled (selective) felling of trees and enforcing laws against indiscriminate cutting.
• Reafforestation - replanting trees to replace those that are cut.
• Preventing and controlling bush burning.
• Creating forest reserves and game reserves, and educating the public on the value of forests.

(c)(i) Importance of decomposers in the ecosystem

Decomposers break down the dead bodies and waste products of plants and animals, releasing simple inorganic nutrients (such as nitrates, carbon dioxide and mineral salts) back into the soil and air. This recycles nutrients so that they can be re-used by green plants, and keeps the environment clean by removing dead organic matter.

(c)(ii) One plant and one animal decomposer

• Plant decomposer: a fungus such as Mucor (mushroom / mould), or bacteria.
• Animal decomposer: earthworm (or termite).