WAEC SSCE - Biology - 2000

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Parasitism is a type of relationship between two organisms in which one organism, called the parasite, benefits from the other organism, called the host, by obtaining nutrients, shelter, or other resources at the expense of the host. The host, in contrast, is harmed by the relationship and may suffer from reduced growth, reproduction, or survival. Therefore, in parasitism, one organism gains (parasite) while the other loses (host).

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Oxygen is not released during decay of living matter. During the process of decay, microorganisms break down complex organic compounds into simpler substances. In the absence of oxygen, this process is called anaerobic decay, and the end products include methane and carbon dioxide. In the presence of oxygen, the process is called aerobic decay, and the end products include carbon dioxide, water, and heat energy. However, oxygen itself is not released during the decay process.

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Plant cells mostly store food as starch. Starch is a complex carbohydrate made up of many glucose molecules linked together in a branched or unbranched chain. Starch is synthesized in the chloroplasts of photosynthetic cells and stored in specialized organelles called amyloplasts, which are found in roots, tubers, seeds, and other storage organs of the plant. When the plant needs energy, it breaks down the starch into glucose molecules that can be used for cellular respiration.

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The correct answer is "Bush burning" as it is a harmful practice that leads to the destruction of natural resources like forests and soil fertility. Establishment of national parks helps to conserve natural resources like wildlife and their habitats. Selective logging is a practice that involves carefully selecting trees to be cut down, leaving the rest to continue growing and maintaining the ecological balance. Building of dams can help in the conservation of water resources like rivers and lakes, by storing water for later use and controlling floods. On the other hand, bush burning is a harmful practice that leads to the destruction of natural resources like forests, which are home to many animal species and also provide ecological services such as oxygen production and carbon sequestration. It can also lead to soil erosion and loss of soil fertility. Therefore, it is not a means of conserving natural resources, but rather a practice that contributes to their destruction.

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The answer is biosphere. The biosphere refers to all the places on earth where living organisms can exist, which includes the atmosphere, lithosphere (the solid outermost shell of the earth), and hydrosphere (the total amount of water on earth). It is the zone of air, land, and water at the surface of the earth where living organisms are found. The biosphere is the sum of all the ecosystems and includes all the living organisms in the world, their interactions with each other, and with the non-living elements of the environment.

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Commensalism is a type of symbiotic relationship between two organisms where one organism benefits while the other organism is neither helped nor harmed. This means that the option that best describes commensalism is "one of the organisms gains while the other neither gains nor loses." In commensalism, the organism that benefits is called the commensal, while the organism that is neither helped nor harmed is called the host. The commensal benefits from the association by gaining resources such as shelter, food, or a place to live, while the host is not affected either positively or negatively. Therefore, commensalism is a relationship where one organism gains while the other is unaffected, making it different from other types of symbiotic relationships such as mutualism and parasitism where both organisms can either benefit or one organism gains while the other loses, respectively.

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Vitamin C is a water-soluble vitamin that is also known as ascorbic acid. Water-soluble vitamins are those that dissolve in water and are not stored in the body to a significant degree. This means that any excess amounts of vitamin C are excreted in the urine rather than stored for later use. Therefore, it is important to consume vitamin C regularly through dietary sources such as fruits and vegetables. Vitamin C has several important functions in the body, including acting as an antioxidant to protect cells from damage, supporting the immune system, and aiding in the production of collagen, which is important for healthy skin, bones, and connective tissue. A deficiency in vitamin C can lead to a condition known as scurvy, which causes fatigue, weakness, and skin and gum problems. Cabalamine, pyridoxine, and riboflavin are other B vitamins that are not related to vitamin C.

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Microorganisms have many useful effects on human activities, but one of the options listed is not an example of such an effect. Specifically, "decay of meat" is not a useful effect of microorganisms, as it can lead to spoilage and contamination of the meat. In contrast, the other options are all examples of beneficial effects of microorganisms. For example, microorganisms can be used to produce vaccines that help prevent the spread of disease, and they can also be involved in the curing of tobacco and the tanning of leather.

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Members of the vitamin B complex are water-soluble vitamins. Water-soluble vitamins are those vitamins that dissolve in water and are easily absorbed in the body. They are not stored in large amounts in the body, so excess amounts are eliminated through urine. The B complex vitamins include thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), biotin (B7), folate (B9), and cobalamin (B12). These vitamins play important roles in energy metabolism, nerve function, and DNA synthesis, among other functions. Because they are water-soluble, it is important to consume these vitamins regularly through a balanced diet that includes foods such as whole grains, nuts, seeds, legumes, and leafy green vegetables, as well as fortified cereals and animal products such as meat, fish, and dairy.

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The statement that is not correct about phosphorus is: "Helps to regulate the acid-base balance of the body". Phosphorus is an important mineral that is required for many functions in the body, including the formation of bones and teeth, cell growth and repair, and the production of DNA and RNA. It is also involved in regulating the metabolism of proteins, fats, and carbohydrates and is required for many chemical reactions in the body. However, phosphorus does not help to regulate the acid-base balance of the body. That is the role of other minerals such as sodium, potassium, and chloride.

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The term "contour" is not associated with fingerprints and classification. Fingerprint classification is a process used to identify individuals based on the unique characteristics of their fingerprints. These characteristics include patterns such as whorls, arches, and loops, as well as the number and location of ridge characteristics such as bifurcations and ending ridges. Whorls, arches, and loops are the three basic patterns used in fingerprint classification, and each pattern is further classified into subtypes based on the orientation and shape of the ridges. The term "contour" typically refers to the outline or shape of an object, and is not commonly used in the context of fingerprint classification.

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The level of organization refers to the degree of complexity in the structure of an organism. Among the options provided, the heart is the most complex in terms of its level of organization. The heart is a vital organ that pumps blood throughout the body to deliver oxygen and nutrients to cells and remove waste products. It consists of multiple types of tissues such as cardiac muscle, connective tissue, and blood vessels that work together in a coordinated manner to carry out its function. In addition, the heart is also regulated by complex systems such as the nervous system and hormonal system to ensure its proper functioning. In contrast, Euglena and Hydra are both single-celled organisms with simpler structures, and viruses are not even considered living organisms as they do not have cells and cannot carry out life processes on their own. Therefore, the heart, with its complex structure and regulatory mechanisms, has the most complex level of organization among the options provided.

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Amoeba has the largest surface area-to-volume ratio among the organisms listed. This is because amoebas are unicellular organisms with a relatively large surface area compared to their volume. Since their shape is irregular and constantly changing, they can increase their surface area by extending and retracting pseudopodia. This allows them to take in nutrients and oxygen from their environment and expel waste products more efficiently. In contrast, larger organisms such as man, earthworm, and grasshopper have much smaller surface area-to-volume ratios due to their larger sizes and more complex body structures. Therefore, the correct answer to the question is Amoeba.

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The non-renewable resource is mineral deposits. Mineral deposits refer to natural resources that are found in the earth's crust and have a finite supply that can't be replaced naturally within a short time frame. Once they are extracted and used up, they cannot be regenerated or renewed. Minerals such as coal, oil, and natural gas are examples of non-renewable resources.

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Vitamin C deficiency results in scurvy. Scurvy is a disease that can occur when a person does not consume enough vitamin C, which is also known as ascorbic acid. Vitamin C is important for the production of collagen, which is a protein that helps to hold cells together in the body. When a person does not get enough vitamin C, their body cannot produce enough collagen, which can lead to symptoms such as fatigue, weakness, joint pain, and bleeding gums. In severe cases, scurvy can also cause open sores on the skin and even death. To prevent scurvy, it is important to consume foods that are rich in vitamin C, such as citrus fruits, strawberries, kiwi, and broccoli.

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The statement that represents Mendel's second law of inheritance is "inherited independently of each other." Mendel's second law of inheritance, also known as the law of independent assortment, states that different traits, or factors, are inherited independently of each other. This means that the inheritance of one trait does not influence the inheritance of another trait. Mendel discovered this law by studying pea plants and observing that the inheritance of seed color was independent of the inheritance of seed texture. He found that the factors for these two traits were located on different chromosomes and inherited independently of each other. Therefore, the option "inherited independently of each other" represents Mendel's second law of inheritance, which states that traits are inherited independently and do not influence each other's inheritance. The other options listed do not represent Mendel's second law of inheritance.

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Relaxation of the erector muscles will cause a decrease in body temperature. These muscles are responsible for the raising of body hairs, which traps a layer of insulating air next to the skin. This layer of air helps to reduce heat loss from the body. When the erector muscles relax, the hairs lie flat, and the insulating layer is lost. This leads to an increase in heat loss from the skin to the environment, resulting in a decrease in body temperature.

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The statement is: "Each neurone consists of the following structures except?" The correct answer is "white matter". White matter is not a structure within a neuron. It refers to the tissue composed of nerve fibers (axons) in the central nervous system that are covered in myelin, which gives it a white appearance. A neuron, also known as a nerve cell, consists of three main structures: the cell body, dendrites, and axon. The cell body, or soma, contains the nucleus and other organelles necessary for the neuron to function. Dendrites are short, branching extensions that receive signals from other neurons and transmit them towards the cell body. The axon is a long extension that carries electrical impulses away from the cell body to other neurons or muscles. Therefore, the incorrect statement is "white matter".

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The presence of a large number of mitochondria in a cell indicates that the cell is very active. Mitochondria are organelles in cells responsible for producing energy through respiration. The more mitochondria a cell has, the more energy it can produce. Therefore, cells with high energy demands, such as muscle cells, have a higher number of mitochondria compared to cells with lower energy demands, such as skin cells. So, the presence of a large number of mitochondria in a cell is an indicator of its high activity level.

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The part of the eye that is sensitive to light is the retina. The retina is a thin layer of tissue at the back of the eye that contains photoreceptor cells called rods and cones. These cells are responsible for converting light into electrical signals that are transmitted to the brain through the optic nerve, where they are interpreted as visual images. The cornea, choroid layer, and optic nerves do not have the same sensitivity to light as the retina. The cornea is the clear outer covering of the eye that helps to focus incoming light, the choroid layer is a layer of tissue that provides oxygen and nutrients to the retina, and the optic nerves are responsible for transmitting visual information from the retina to the brain.

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Spirogyra is an organism that will not bring about decomposition. Decomposition is a natural process that breaks down dead organisms and organic waste into simpler substances, such as carbon dioxide and water, which can be used by other living organisms. Decomposition is mainly carried out by decomposers, such as bacteria and fungi, that break down the dead matter into smaller compounds. Earthworms, bacteria, and fungi (such as molds) are decomposers that play a significant role in decomposition. However, Spirogyra is a type of algae that does not participate in the process of decomposition. Algae like Spirogyra are producers that produce their own food through photosynthesis and are not involved in breaking down dead organic matter. Therefore, Spirogyra is an organism that will not bring about decomposition, unlike earthworms, bacteria, and molds.

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Ammonia is a highly toxic substance that needs to be eliminated from the body as soon as possible. In some organisms, ammonia is excreted directly as a waste product, while in others, it is converted into less toxic substances before excretion. Among the given options, Amoeba is an organism that excretes ammonia as a waste product. Amoeba is a unicellular organism that lives in water and has a simple structure. It does not have specialized excretory organs like kidneys, so it excretes metabolic waste directly through its cell membrane by diffusion. Since ammonia is highly soluble in water, it can diffuse out of the cell easily, making ammonia the primary excretory product of Amoeba. In contrast, humans, birds, and Spirogyra excrete urea or uric acid, which are less toxic and more water-conserving than ammonia.

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During blood transfusion, agglutination can occur if the donor blood group is not compatible with the recipient's blood group. This happens because of the presence of antigens on the surface of red blood cells that can trigger an immune response. Blood group O is considered the universal donor because it lacks the antigens A and B, which means it can be given to individuals with any blood group without causing an immune response. However, a patient with blood group O can only receive blood from a donor with blood group O, as they have antibodies against both antigens A and B, and receiving blood from other groups can cause an immune reaction and agglutination. Therefore, to prevent agglutination, a patient with blood group O should only receive blood from a donor with blood group O.

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The neurons that relay the message of effector organs are called efferent neurons. Efferent neurons are responsible for carrying signals from the central nervous system (brain and spinal cord) to the peripheral nervous system (muscles and glands). They are also referred to as motor neurons because they control muscle movement and other responses to external stimuli. These neurons transmit signals in one direction only, from the central nervous system to the peripheral nervous system, to produce a response or action.

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The nerves that arise from the brain are called cranial nerves. These are a set of twelve paired nerves that emerge directly from the brain and are responsible for transmitting information to and from various parts of the head and neck, including the eyes, ears, nose, tongue, and other areas. Each of the twelve cranial nerves has a specific function, such as controlling eye movements, facial expressions, hearing, and taste perception. They play a vital role in sensory and motor functions, allowing us to perceive the world around us and interact with it.

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The abiotic factor that determines the depth to which light penetrates in a pond is turbidity. Turbidity is the measurement of the amount of suspended particles in water, such as soil, silt, and plankton. These particles can absorb or scatter light, which reduces the amount of light that penetrates the water. The more suspended particles present in the water, the more turbid the water becomes, and the less light can penetrate the water. In a pond, the depth to which light can penetrate affects the growth of aquatic plants and algae. These photosynthetic organisms require sunlight to produce food, and the depth to which light can penetrate affects the amount of light available for photosynthesis. Therefore, the abiotic factor that determines the depth to which light penetrates in a pond is turbidity, which is the amount of suspended particles in water that absorb or scatter light. The other options listed do not determine the depth to which light penetrates in a pond.

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The concept of use and disuse of parts was postulated by Lamarck. This idea was a key part of Lamarck's theory of evolution, which proposed that organisms could pass on traits that were acquired during their lifetime to their offspring. Lamarck believed that if an organism used a particular body part frequently, it would become stronger and more developed, while disuse would cause the same part to weaken and eventually disappear over time. However, this theory was later disproven by the discovery of genetics and the mechanism of natural selection proposed by Charles Darwin.

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The adaptation of the earthworm to its habitat is best described by "Living in burrow during the day to prevent desiccation." Earthworms are found in soil, and they need to stay moist to survive because their skin must remain moist for the exchange of gases, like oxygen and carbon dioxide, to occur. They also need to keep their skin moist to prevent desiccation or drying out, which can be fatal. To avoid desiccation during the day, when the sun is shining and the temperature is high, earthworms burrow into the soil, creating a moist and humid environment for themselves. This way, they avoid exposure to the sun and can keep their skin moist, enabling them to breathe and stay hydrated. Therefore, living in burrows during the day to prevent desiccation is a crucial adaptation of earthworms to their habitat, which enables them to survive in soil environments. The other options listed do not describe adaptations that earthworms possess to survive in their habitat.

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The statement that is incorrect about diffusion is: "Molecules move from region of lower concentration to that of higher concentration." In diffusion, molecules move from regions of higher concentration to regions of lower concentration until they are evenly distributed. This movement occurs in gaseous, liquid, and solid media, and no membrane is required to effect the movement of molecules. There is no degradation of soluble particles in solvents in the process of diffusion.

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Spirogyra is a producer. Producers, also known as autotrophs, are organisms that can produce their own food through the process of photosynthesis. Spirogyra is a green algae that contains chloroplasts capable of photosynthesis. Therefore, it is a producer. Mucor, Rhizopus, and yeast are not producers as they are heterotrophic organisms and require external sources of food to survive.

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After fertilization, the ovule of a flower develops into a seed. The fertilized ovule undergoes changes and forms the embryo which later develops into a new plant. The ovary of the flower also enlarges and becomes the fruit that encloses the seed. Thus, the correct option is "becomes a seed."

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The increased heart rate of a boy who saw a python is due to the production of adrenaline. Adrenaline is a hormone released by the adrenal gland in response to stress, fear or danger. It prepares the body for a "fight or flight" response, which involves increased heart rate, blood pressure, and breathing rate, as well as the release of glucose from energy stores in the body. These physiological changes help the body to deal with a perceived threat or danger. In this case, the boy's body is preparing to either fight the python or flee from it, and the release of adrenaline is helping to facilitate this response.

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Heterotrophs are organisms that cannot produce their own food and rely on other organisms for their nutrition. They obtain food by consuming other organisms or organic matter. Among the options given, the group that consists of heterotrophs is "Man, Mucor and Mushroom". This is because "Man" refers to humans, which are heterotrophs, and both "Mucor" and "Mushroom" are fungi that are also heterotrophic. The other options include organisms that are either autotrophic (able to produce their own food) or have both autotrophic and heterotrophic modes of nutrition.

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The major reason why foods should be properly covered is to prevent micro-organisms in the air from contaminating the food. Micro-organisms such as bacteria, viruses, and fungi are present in the air and can settle on uncovered food, potentially leading to foodborne illnesses. By properly covering food, we can create a barrier that reduces the risk of these micro-organisms contaminating the food. This is particularly important for perishable foods like meat, dairy, and cooked dishes, which can spoil quickly if not stored properly. While it is true that covering hot foods can help them retain their temperature and prevent them from cooling off too quickly, this is not the primary reason why foods should be covered. Additionally, covering food does not necessarily maintain the taste, as certain flavors can be affected by exposure to air. Finally, covering food does not necessarily affect the evaporation of water, as moisture can escape even when covered depending on the type of covering used.

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The simplest form of nervous pathway which links receptors with effectors is known as the reflex arc. The reflex arc is responsible for reflex actions, which are automatic, involuntary responses to a particular stimulus. When a stimulus is detected by sensory receptors, the impulse travels along sensory nerves to the spinal cord, where it is processed by a small group of neurons called the reflex center. From there, the impulse travels along motor nerves to the effector muscles, which respond by contracting or relaxing. The reflex arc is a rapid and efficient way of responding to potentially harmful or dangerous stimuli without involving the conscious brain. Examples of reflex actions include pulling your hand away from a hot stove, blinking when an object comes close to your eye, or sneezing when dust irritates your nose.

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The two body layers of cells found in a hydra are the ectoderm and endoderm. The ectoderm is the outer layer of cells, while the endoderm is the inner layer of cells. The ectoderm is responsible for protecting the hydra from its environment, while the endoderm is responsible for digestion and nutrient absorption. These two layers work together to allow the hydra to survive and thrive in its aquatic habitat.

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The character which falls to express itself in the first filial generation but emerges in the second is a recessive trait. This means that the allele for the trait is present in the first generation but is masked by the dominant allele. However, in the second generation, the recessive allele can be expressed if both parents carry the recessive allele and pass it on to their offspring. The classic example of this is seen in Mendelian genetics with the inheritance of pea plant traits, where the wrinkled seed shape is a recessive trait.

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The group that consists of only micro-nutrients is "Manganese, molybdenum and zinc". Micro-nutrients are essential elements required by plants in small quantities for their proper growth and development. Manganese, molybdenum and zinc are all essential micro-nutrients that plants need for various functions such as enzyme activation, photosynthesis, and nitrogen fixation. The other options include at least one macro-nutrient, which is an essential element required in large quantities.

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Supporting tissues in plants are those tissues that provide mechanical support to the plant body. They include collenchyma, sclerenchyma, and xylem. Out of the options provided, the function of supporting tissues that is not correct is "secretory." Supporting tissues do not have a secretory function, as that role is typically carried out by specialized cells or tissues such as glands or trichomes.

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Trypanosomiasis, also known as African sleeping sickness, is a disease that is associated with the tsetse fly. The tsetse fly is a blood-sucking insect found in sub-Saharan Africa that can transmit the protozoan parasite Trypanosoma brucei through its bite. When the parasite enters the bloodstream, it causes a variety of symptoms, including fever, headache, joint pain, and fatigue. As the disease progresses, it can also affect the nervous system and lead to sleep disturbances, confusion, and ultimately coma if left untreated. Cockroaches, mosquitoes, and houseflies are not associated with trypanosomiasis, although they can transmit other diseases to humans.

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(a) Five methods of vegetative propagation with an example of each

Also acceptable: corm (cocoyam), runner/stolon (grass), budding and grafting (mango, citrus), and leaf propagation (Bryophyllum).

(b)(i) Five advantages of vegetative propagation

1. Offspring are genetically identical to the parent, so desirable qualities are retained.
2. Reproduction is faster; the plant matures and yields earlier than from seed.
3. Seedless plants and those that produce few or no viable seeds can still be propagated.
4. Only one parent is needed; no pollination or fertilization is required.
5. The new plant is well nourished from the parent's food store, so it establishes quickly.

(b)(ii) Five disadvantages of vegetative propagation

1. There is no variation, so the plants cannot readily adapt to changes in the environment.
2. Diseases present in the parent are passed on to all the offspring.
3. Overcrowding occurs because the new plants develop close to the parent.
4. The plants lack the dormancy and dispersal advantages of seeds.
5. The clones weaken over successive generations and may lose vigour.

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(a)(i) The four levels of organisation in living organisms

1. Cell
2. Tissue
3. Organ
4. System (organ system)

These build up in increasing complexity, from the cell as the smallest unit to systems which together form the whole organism.

(a)(ii) Explanation of two of the levels

Cell: The cell is the basic structural and functional unit of life. It is the smallest living part of an organism and can carry out all the processes of life such as respiration, feeding, excretion and reproduction. Examples include the amoeba (a single-celled organism) and individual cells such as red blood cells or muscle cells.

Tissue: A tissue is a group of similar cells that have the same origin and work together to perform a particular function. The cells are specialised for that function. Examples are muscle tissue (for contraction and movement), epithelial tissue (for covering and protection) in animals, and xylem or phloem tissue (for transport) in plants.

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(a) Osmosis

Osmosis is the movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a more concentrated solution) through a selectively permeable membrane.

(b) Experiment to show that xylem conducts water from the root to the shoot

• Uproot a young herbaceous plant such as balsam, with the roots intact.
• Wash the roots carefully to remove all soil particles.
• Place the roots of the plant in a beaker containing water coloured with red eosin dye.
• Expose the set-up to sunlight and allow it to stand for about one to six hours.
• Remove the plant and wash the roots thoroughly.
• Cut thin transverse sections of the root, stem and leaf.
• Observe the sections with a hand lens or microscope.
• Observation: Only the xylem vessels in the root, stem and leaf veins are stained red by the dye.
• Conclusion: Since the coloured water is found only in the xylem vessels, xylem tissue conducts water from the root to the shoot.

(c)(i) Red blood cell in a hypotonic solution

When a red blood cell is placed in a hypotonic or dilute solution, water enters the cell by osmosis through its selectively permeable cell membrane. The cell swells, becomes enlarged and eventually bursts. The bursting of red blood cells is called haemolysis.

(c)(ii) Three conditions that may cause haemolysis

1. Placing red blood cells in a hypotonic solution, such as distilled water.
2. Exposure to haemolytic toxins or poisons, such as some bacterial toxins or snake venom.
3. Transfusion of incompatible blood groups.
4. Some infections, for example malaria.

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(a) The water cycle

The water cycle is the continuous circulation of water between the earth's surface and the atmosphere. The main stages are:

1. Evaporation: heat from the sun evaporates water from seas, rivers, lakes and the soil into water vapour. Plants add water vapour by transpiration and animals by respiration and sweating.
2. Condensation: the rising water vapour cools at higher altitudes and condenses into tiny droplets that form clouds.
3. Precipitation: when the droplets become large and heavy they fall as rain, hail or snow.
4. Collection / run-off: the water flows over the ground as run-off into rivers, lakes and seas, or sinks into the soil as ground water, and the cycle repeats.

(An annotated diagram should show the sun, the sea, arrows for evaporation and transpiration rising to a cloud, condensation in the cloud, precipitation as rain falling, and run-off returning to the sea.)

(b) Importance of water to living organisms

1. It is the medium in which all metabolic reactions and enzyme actions take place inside cells.
2. It is a universal solvent for transporting dissolved food, minerals, gases and waste products.
3. It is a raw material for photosynthesis in green plants.
4. It helps to regulate body temperature (through sweating and transpiration).
5. It maintains turgidity in plant cells and gives them support.
6. It aids digestion, absorption and the removal of excretory waste, and forms a large part of protoplasm.

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(a) Five classes of food other than carbohydrates

1. Proteins
2. Fats and oils (lipids)
3. Vitamins
4. Mineral salts
5. Water

(Roughage/dietary fibre is also accepted as a class.)

(b) Why glucose rather than corn meal is taken by an athlete ready for a race

Glucose is a simple sugar (monosaccharide) that requires no digestion. It is absorbed directly and quickly through the wall of the small intestine into the blood, so it provides instant energy for the race. Corn meal, on the other hand, is a complex carbohydrate (starch) that must first be digested (broken down by enzymes into glucose) before it can be absorbed. This digestion takes time, so the energy from corn meal is released too slowly to be useful for an athlete about to run.

(c) Test for starch

1. Place a small quantity of the food sample on a white tile or in a test tube.
2. Add a few drops of iodine solution to the sample.
3. Observation: the iodine changes from brown/yellow to a blue-black colour.
4. Conclusion: the blue-black colour confirms that starch is present. (If the colour remains brown, starch is absent.)

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(a) What is a sense organ?

A sense organ is a specialised organ made up of a group of cells or tissues that contain receptors sensitive to a particular kind of stimulus (light, sound, heat, chemicals, pressure). The receptors detect the stimulus and convert it into nerve impulses which are transmitted along sensory nerves to the central nervous system for interpretation. Examples: the eye, ear, skin, nose and tongue.

(b)(i) Two main functions of the ear

• Hearing - the detection of sound.
• Balance (equilibrium) - the maintenance of body posture and balance.

(b)(ii) Mechanism of hearing

Sound waves in the air are collected and concentrated by the pinna and passed along the external auditory canal (meatus) to the ear drum (tympanic membrane), causing it to vibrate. These vibrations are transmitted across the middle ear and amplified by the three ear ossicles - the malleus, incus and stapes. The stapes presses on the oval window, setting up pressure waves in the fluid of the inner ear. The perilymph in the cochlea vibrates and this in turn makes the endolymph vibrate, stimulating the sensory hair cells of the organ of Corti on the basilar membrane. These cells generate nerve impulses which travel along the auditory nerve to the brain, where they are interpreted as sound.

(c) The eye defect of short-sight (myopia) and its correction

In short-sight the eyeball is too long from front to back (or the eye lens is too strongly converging). Light rays coming from a distant object are therefore brought to a focus in front of the retina; by the time the rays reach the retina they have spread out again, so a blurred image is formed and distant objects cannot be seen clearly. The defect is corrected by placing a concave (diverging) lens in front of the eye. This lens first diverges the incoming parallel rays, so that after passing through the eye lens they are brought to a focus exactly on the retina, forming a sharp image.