WAEC SSCE - Biology - 1993

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A climax community refers to the final stage of succession, where a community of plants and animals have reached a stable state of balance with the environment. The correct option is: "a stable composition of plant and animal species." A climax community is characterized by a diverse range of plant and animal species that have adapted to the prevailing climatic and soil conditions. The community has a complex food web and nutrient cycling processes. In a climax community, the species composition remains relatively stable over time and the community can persist for hundreds or thousands of years without significant disturbance. In summary, a climax community is an equilibrium state of a stable and diverse community of plants and animals that have evolved over time to create a self-sustaining and resilient ecosystem.

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The human eye is a complex organ that works by refracting and focusing light onto the retina at the back of the eye. The light ray entering the eye goes through the following route: The correct option is: "II, I, III, IV". - The cornea (II) is the transparent outer layer of the eye that first refracts and bends the light rays as they enter the eye. - The iris (I) controls the amount of light entering the eye by adjusting the size of the pupil. The pupil dilates in dim light and constricts in bright light. - The lens (III) located behind the iris, further refracts and focuses the light rays onto the retina. - The retina (IV) at the back of the eye, converts the light energy into neural signals that are sent to the brain through the optic nerve. Therefore, the correct sequence of the route taken by light as it enters the eye is II (cornea), I (iris), III (lens), and IV (retina).

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The chromosome in a cell is found in the nucleus. The nucleus is a membrane-bound organelle in eukaryotic cells that contains most of the genetic material in the form of chromosomes. Chromosomes are long, coiled-up strands of DNA that contain genes, which provide the instructions for making proteins and other important molecules needed for the cell's function. The other options listed do not typically contain chromosomes in eukaryotic cells. The Golgi body is responsible for modifying and sorting proteins, the cytoplasm is the fluid that fills the cell, the cell membrane is the outer boundary of the cell that regulates the flow of materials in and out of the cell, and the cell wall is a rigid structure found in plant cells that provides support and protection.

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The offspring produced when pure strains interbreed is described as a hybrid. When two pure strains with different traits are crossbred, their offspring will inherit a mix of the traits from both parents. These offspring are known as hybrids. Hybridization is commonly used in agriculture to create new varieties of plants or animals with desired traits, such as increased yield, disease resistance, or improved quality. Hybrids can also occur naturally, such as in the case of mules, which are the offspring of a male donkey and a female horse.

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Zinc is a trace element among the listed mineral salts. A trace element is a mineral that is required by the body in very small amounts, typically less than 100 milligrams per day. Zinc is an essential mineral that plays a crucial role in many bodily processes, including immune function, wound healing, and DNA synthesis. Although it is only required in small amounts, it is still an important mineral that the body needs to function properly. The other minerals listed, such as carbon, hydrogen, potassium, and calcium, are also important for various bodily processes, but they are not classified as trace elements.

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One of the factors that must be considered for safe blood transfusion is the rhesus factor of the donor and the recipient. The rhesus factor is a protein found on the surface of red blood cells, and individuals can either be Rh-positive (have the protein) or Rh-negative (do not have the protein). If a Rh-negative person receives blood from a Rh-positive donor, their immune system can recognize the Rh protein as foreign and mount an immune response, which can result in hemolysis (destruction of red blood cells) and potentially life-threatening complications. Therefore, it is important to ensure that the donor and the recipient have compatible rhesus factors to prevent any adverse reactions during blood transfusion.

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Sea water taken in by a living organism can be recycled into the atmosphere through four of the processes listed: transpiration, excretion, respiration, and decay. Transpiration is the process by which water evaporates from the surface of plants into the atmosphere, so it doesn't apply to all living organisms. Digestion involves breaking down food into nutrients that the organism can use for energy and growth, and it doesn't involve water recycling. Excretion is the process by which waste products are removed from the body, including water, so it can be recycled. Respiration is the process of taking in oxygen and releasing carbon dioxide, which can also involve water vapor released into the atmosphere. Decay is the breakdown of dead organic matter into simpler compounds, including water, which can be released back into the environment. Therefore, the process that doesn't allow for recycling of sea water back into the atmosphere is digestion.

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The function of the chromosome is the transmission of hereditary traits. Chromosomes are thread-like structures that carry genetic information in the form of genes, which are made up of DNA. During cell division, chromosomes are replicated and passed on to daughter cells, ensuring that genetic information is transmitted from one generation to the next. The transmission of hereditary traits is essential for the continuity of life and the survival of species. Therefore, is the correct answer. Options (b), (c), (d), and (e) are not functions of the chromosome.

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The part labelled "l" is most likely the female reproductive organ of a flower, known as the pistil. The function of the pistil is to receive pollen grains and facilitate fertilization, resulting in the formation of seeds. The uppermost part of the pistil is called the stigma, which is where the pollen grains land and begin to germinate. The stigma leads down into the style, which is a long tube-like structure that connects the stigma to the ovary. The ovary contains the ovules, which eventually develop into seeds after fertilization. So, the main function of the part labelled "l" is to receive pollen grains and facilitate fertilization, ultimately leading to the production of seeds.

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The exchange of gases in multicellular organisms is a vital process to obtain oxygen and get rid of carbon dioxide. For successful gas exchange, there are certain conditions that need to be met. The first condition is a concentration gradient across the respiratory surface. This means that there must be a difference in the concentration of gases between the internal and external environments. Oxygen should be more concentrated outside while carbon dioxide should be more concentrated inside. This difference in concentration allows for the diffusion of gases across the respiratory surface. The second condition is the presence of a thin membrane as the respiratory surface. The respiratory surface should be thin to allow for the easy diffusion of gases across it. This ensures that gases can quickly move in and out of the body. The third condition is the last transportation of absorbed gases. After gases have diffused across the respiratory surface, they should be transported to the cells that need them. This transportation is done through the circulatory system, which moves gases from the respiratory surface to the cells and back again. The fourth condition is the presence of a large surface area of the respiratory organ. The larger the surface area, the more space there is for gas exchange to occur. This is important for animals that require a lot of oxygen, like birds or mammals. The fifth condition is the absence of a small, dry surface area of the respiratory organ. This option is incorrect because a dry surface area would make it difficult for gases to diffuse across it. The respiratory surface should be moist to allow for the easy diffusion of gases. In summary, successful exchange of gases in multicellular organisms requires a concentration gradient, a thin membrane, transportation of absorbed gases, a large surface area of the respiratory organ, and a moist respiratory surface.

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The water pollutant that may contain organisms that cause dysentery is sewage. Sewage is the wastewater that contains human and animal feces, as well as urine, so it may contain pathogenic microorganisms such as bacteria, viruses, and parasites that can cause diseases like dysentery. Dysentery is a type of gastrointestinal infection that results in severe diarrhea with mucus and blood in the stool. It is typically caused by the bacteria Shigella or Entamoeba histolytica, which are commonly found in human feces. The other water pollutants listed in the options, such as pesticides, fertilizers, industrial wastes, and crude oil, may contain harmful chemicals that can contaminate water and affect aquatic life or human health, but they do not necessarily contain microorganisms that cause dysentery.

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Lamarck’s evolution theory could be summarised by the statement that the changing environment imposes structural physiological and behavioural changes in organisms. According to Lamarck, organisms adapt to their environment through the use or disuse of organs, and these acquired characteristics can be passed on to their offspring. For example, if an organism repeatedly uses a certain organ, that organ will become more developed and passed on to its offspring. Lamarck believed that these adaptations over time led to the evolution of new species. However, this theory has been largely disproven by modern science, as genetic changes and natural selection are now understood to be the primary drivers of evolution.

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The largest game reserve in Nigeria is the Yankari game reserve located in Bauchi state. The Yankari game reserve covers an area of about 2,244 square kilometers and is home to a variety of wildlife species such as elephants, baboons, waterbucks, and antelopes. The reserve also has several natural warm springs and caves that are popular tourist attractions. Therefore, is the correct answer. Options (a), (b), (d), and (e) are also game reserves in Nigeria, but they are not the largest.

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The groups of sensory cells found on the upper surface of the tongue are called taste buds. Taste buds are clusters of specialized cells responsible for detecting different tastes, including sweet, sour, bitter, salty, and umami (savory). When food particles come into contact with these cells, they send signals to the brain to interpret the taste. Taste buds are located not only on the tongue but also in the throat and roof of the mouth.

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Pyramid of numbers is a graphical representation of the number of organisms present at different levels in a food chain. It shows the relationship between the number of organisms at each trophic level, starting from the producers at the base and moving up to the top predators. The correct option that explains the term pyramid of numbers is: "Progressive decrease in the number of individuals from lower to higher trophic level." This means that the number of organisms decreases as we move from the lower to the higher trophic level. For example, there are more plants than herbivores, and more herbivores than carnivores. This results in a pyramid shape when represented graphically. Therefore, the pyramid of numbers provides a simple and clear way to understand the structure of an ecosystem and the flow of energy through different levels of the food chain.

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The agencies responsible for conservation in Nigeria are the Forest Department, Nigerian Conservation Society, Games Reserve Authority, and Ministry of Agriculture and Natural Resources. The Nigerian Red Cross Society is not responsible for conservation in Nigeria. The Nigerian Red Cross Society is a humanitarian organization that provides assistance during emergencies and disasters, such as war, floods, and epidemics. In summary, while the Forest Department, Nigerian Conservation Society, Games Reserve Authority, and Ministry of Agriculture and Natural Resources are all involved in conservation efforts in Nigeria, the Nigerian Red Cross Society is not.

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The organ in Euglena that is sensitive to light is the eye spot. The eye spot, also known as the stigma, is a small, pigmented organelle located near the base of the flagellum. It contains a photoreceptor pigment that enables the organism to sense the direction of light and adjust its movement accordingly.

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The part labelled VII is the diaphragm. The diaphragm is a sheet of muscle that separates the thoracic and abdominal cavities. It plays an important role in breathing by contracting and relaxing to change the volume of the thoracic cavity, causing air to move in or out of the lungs.

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Maelezo ya Majibu

The process whereby microorganisms can convert atmospheric nitrogen into nitrogenous compound is known as "nitrogen fixation". This process involves the conversion of atmospheric nitrogen gas (N2) into a form that can be used by plants and other organisms, such as ammonia (NH3) or nitrate (NO3-). Some microorganisms, such as certain bacteria, are able to perform this conversion through biological processes, such as nitrogenase activity. Nitrogen fixation is an important part of the global nitrogen cycle, and it plays a critical role in the growth and productivity of plants and other organisms.

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The changing of color by a chameleon to that of the environment is an example of protective coloration. Protective coloration is a form of camouflage where an animal blends into its environment to avoid being seen by predators or prey. The chameleon's ability to change color allows it to match the color and pattern of its surroundings, making it less visible to predators and making it easier to capture prey. The chameleon's skin contains cells called chromatophores that contain pigments, which expand or contract to change the color of the skin. This adaptation allows the chameleon to adjust its color rapidly and precisely to match its surroundings. The other options listed are not related to the chameleon's ability to change color for protective purposes, but rather to other behaviors or adaptations, such as adaptive radiation, courtship display, display of body color, or territorial behavior.

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A marine habitat can be best described as a large body of water with a high concentration of salt. This type of habitat is characterized by its saltwater environment, which is home to a diverse array of plant and animal species. The high concentration of salt in the water is what differentiates marine habitats from other types of aquatic environments, such as freshwater habitats. Marine habitats can be found in various locations, such as oceans, seas, and coral reefs. They are essential for sustaining the life of many marine organisms, including fish, sharks, whales, and sea turtles, among others.

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The presence of a cell membrane is a similarity between a typical animal cell and a typical plant cell. A cell membrane, also known as the plasma membrane, is a thin layer that encloses the cell, separating the internal environment from the external environment. Both animal and plant cells have a cell membrane that controls the movement of substances in and out of the cell, maintains the cell shape and structure, and protects the cell from external factors.

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The structure that functions as an excretory system found in flatworms is the flame cell. Flame cells are specialized cells that are lined with cilia, and they help to remove waste materials and excess water from the body of the flatworm. As fluid is moved through the flame cells, the cilia create a flickering effect that looks like a flame, hence the name "flame cell." The waste materials are then transported out of the body through a system of tubules and ducts. Other organisms such as planarians also have flame cells as their excretory system.

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Maelezo ya Majibu

The first scientist to describe the cell was Robert Hooke. In 1665, he observed cork cells using a microscope and described them as "a great many little Boxes, or Cells." This was the first time cells had been observed and named. While other scientists, such as Anton van Leeuwenhoek and Matthias Schleiden, also made significant contributions to our understanding of cells, Robert Hooke is credited with being the first to use the term "cell" and to recognize that cells were the basic unit of life.

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Carbondioxide is given out in almost all natural and human-made processes that involve the combustion of organic matter or respiration. However, there is one process in which carbondioxide is not given out, and that is during photosynthesis. Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, which is stored in the form of glucose. During this process, carbon dioxide is absorbed by the leaves of the plant, along with water and sunlight, to produce glucose and oxygen. The oxygen is then released into the atmosphere, while the glucose is used as food by the plant. In summary, carbondioxide is not given out during photosynthesis, but rather it is absorbed by the plant to produce glucose and oxygen.

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The use of predators or parasites to control pests in the farm is known as biological control. This involves introducing natural enemies of the pest to the farm to control their populations, rather than using chemical pesticides or other methods. The natural enemies can be predators, parasites, or pathogens that attack the pests and help to reduce their numbers. Biological control is an environmentally friendly and sustainable way to control pests, as it does not harm the ecosystem or affect other beneficial organisms.

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Ultrafiltration in the kidney takes place in the Bowman's capsule. The Bowman's capsule is a cup-shaped structure at the beginning of the nephron in the kidney. It contains a cluster of blood vessels called the glomerulus, which is surrounded by a space called the Bowman's space. When blood enters the glomerulus, it is filtered under pressure and the filtrate (which contains water, ions, glucose, and waste products) enters the Bowman's space. This process is known as ultrafiltration and is the first step in the formation of urine. The filtrate then moves through the rest of the nephron, where it is modified and eventually excreted from the body as urine.

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The respiratory organ found in a cockroach is the trachea. The trachea is a network of tubes that is responsible for carrying air to and from the body tissues of the cockroach. These tubes are made up of chitin and are connected to the outside environment through openings called spiracles. The spiracles allow air to enter and exit the tracheal system. Inside the tracheae, gases are exchanged between the air and the body tissues of the cockroach through diffusion, which is facilitated by a system of smaller tubes called tracheoles. This allows the cockroach to obtain the oxygen it needs for respiration and to release carbon dioxide as a waste product.

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Maelezo ya Majibu

The adaptation of plants or animals to desert environment that is not on the list is broad leaves for storage. In the desert, water is a scarce resource, and many plants and animals have developed adaptations to conserve water. For example, plants may have well-developed tap root systems to reach deep underground water sources or small leaves with thick epidermis to reduce water loss through transpiration. Some animals, such as camels, have adapted to store water in their bodies and produce metabolic waste in the form of uric acid, which does not require as much water as urea. However, broad leaves are not an adaptation commonly found in desert environments, as they are more likely to increase water loss through transpiration rather than conserve it.

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One of the major criticisms against Mendelian laws is that they do not recognize that one trait is often controlled by many pairs of genes. This means that a single trait can be influenced by multiple genes, rather than just one pair of genes as suggested by Mendelian laws. This is known as polygenic inheritance, and it can result in a wide range of possible phenotypes. In some cases, the expression of a trait may also be influenced by environmental factors, further complicating the relationship between genotype and phenotype.

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The medium of transportation of nutrients within unicellular organisms is protoplasm. Protoplasm is the living substance that forms the contents of a cell, composed of a mixture of organic and inorganic compounds, including water, enzymes, and nutrients. Within unicellular organisms, nutrients are transported through the protoplasm, which carries out all of the cell's metabolic functions. Blood, serum, plasma, and lymph are components of multicellular organisms and are not relevant to the transportation of nutrients within unicellular organisms.

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Diabetes mellitus is caused by a deficiency of insulin in the body. Insulin is a hormone produced by the pancreas that helps regulate the levels of glucose in the blood. In people with diabetes, the pancreas either produces little or no insulin, or the body's cells do not respond properly to the insulin produced. This leads to high levels of glucose in the blood, which can cause a range of health problems over time if left untreated.

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Identical twins inherit their genes from the same ovum and the same sperm. Identical twins are formed when a single fertilized egg (zygote) splits into two identical embryos. Therefore, they have the same genetic makeup, as they come from the same fertilized egg. Identical twins are also referred to as monozygotic twins. The genetic similarity between identical twins makes them very useful in studies of genetics and heredity.

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When an axon is at rest, the concentration of ions on either side of the membrane are different. The correct statement about the concentrations of ions on either side of the membrane is that there is an excess of potassium ions inside the axon and an excess of sodium ions outside. This is because the axon membrane is selectively permeable, meaning it allows certain ions to pass through while preventing others from doing so. The axon actively pumps sodium ions out and potassium ions in, which creates an imbalance in ion concentrations. This difference in ion concentrations generates a resting potential, with the inside of the axon negatively charged relative to the outside.

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The underlying principle in the adoption of biological control of pests is the predator-prey relationship in ecological community. Biological control involves the use of natural enemies such as predators, parasites, and pathogens to manage pest populations. These natural enemies attack and feed on the pest species, ultimately reducing their population to an acceptable level. The success of biological control depends on understanding the complex interactions between the target pest, the natural enemy, and the environment. By utilizing the natural enemies of pests, biological control can effectively manage pest populations without the use of harmful chemicals, which can negatively impact the environment and human health.

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The blood vessel that carries oxygenated blood into the heart is the pulmonary vein. The pulmonary vein is responsible for transporting blood that has picked up oxygen in the lungs and returning it to the left atrium of the heart. The left atrium then pumps the oxygen-rich blood into the left ventricle, which contracts and sends the blood out to the body through the aorta. The other blood vessels listed in the options either carry deoxygenated blood (such as the pulmonary artery) or return blood to the heart that has already been circulated through the body (such as the anterior and posterior vena cava).

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Clay soil has small particles that are closely packed together, which creates a high density and low porosity soil. This means that the soil particles have a high attraction to each other, and they hold onto water tightly, making it difficult for water and air to move through the soil. This can lead to waterlogging, a condition where water accumulates on the soil surface, making it difficult for plant roots to breathe and grow. The property of clay soil that prevents it from supporting thick vegetation is its tendency of becoming waterlogged.

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The position of the ovary in relation to the receptacle is a way of describing the structure of a flower. - Superior ovary: When the ovary is located above the attachment point of the other floral parts, it is called a superior ovary. - Inferior ovary: When the ovary is located below the attachment point of the other floral parts, it is called an inferior ovary. - Semi-inferior ovary: When the ovary is partially below and partially above the attachment point of the other floral parts, it is called a semi-inferior ovary. Therefore, the correct answer to the question is: The position of the ovary in relation to the receptacle is referred to as "superior" when the ovary is located above the attachment point of the other floral parts.

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In order to provide oxygen to the body and remove carbon dioxide, exchange of gases occurs in the respiratory system of animals. In birds, exchange of gases takes place in their unique respiratory system which includes air sacs. These air sacs are connected to the lungs and allow the air to flow in one direction, providing a continuous supply of fresh oxygen. The exchange of gases, i.e., the uptake of oxygen and release of carbon dioxide, primarily occurs in these air sacs, which are contained in option IV.

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The organ that is associated with deamination of proteins is the liver. Deamination is the process of removing the amino group (-NH2) from amino acids, which is the first step in the breakdown of proteins. The liver contains a group of enzymes called transaminases, which are responsible for the deamination of amino acids. Once the amino group is removed, it is converted to ammonia and then to urea, which is eliminated from the body in urine. Therefore, the liver plays a crucial role in protein metabolism and waste elimination.

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The organism that reduces nitrates in the soil to gaseous nitrogen is a denitrifying bacterium. Denitrifying bacteria are microorganisms that can use nitrates as a source of oxygen for respiration, converting them into nitrogen gas (N2) which is then released into the atmosphere. This process is called denitrification and it plays an important role in the nitrogen cycle by returning nitrogen gas to the atmosphere and reducing the amount of available nitrogen in the soil. The other options are not involved in the process of denitrification. Euglena and protozoans are single-celled eukaryotic organisms that are not capable of denitrification. Parasitic molds are fungi that feed on living organisms, and nitrifying bacteria are involved in the conversion of ammonia to nitrate during nitrification, which is the opposite of denitrification.

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Isotonic sugar solution cannot control the growth of harmful microorganisms. Isotonic sugar solution has the same concentration of solutes as the microorganisms and as a result, it does not create an environment that is unfavorable for their growth. The other options listed have properties that can control the growth of harmful microorganisms. For example, antibiotics target specific types of bacteria, hypertonic salt solution creates a high concentration of salt that causes the microorganisms to lose water and disinfectants and antiseptics kill or inhibit the growth of microorganisms.

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Maelezo ya Majibu

The division of labor in social insects, such as ants and bees, is an example of behavioral adaptation. Behavioral adaptation refers to a change in the behavior of an organism that allows it to better survive and reproduce in its environment. Social insects divide labor among members of the colony based on their specialized abilities and roles, such as foraging for food, caring for the young, or defending the nest. This specialization of labor allows the colony to be more efficient and successful in meeting the needs of the group as a whole.

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(a) Characteristics of the Northern Guinea Savanna

1. The vegetation is made up of tall grasses with scattered, short, fire- and drought-resistant trees.
2. The trees are shorter and more widely spaced than in the southern (Guinea) savanna, and many have thick, fire-resistant bark and small or thorny leaves.
3. It has a longer dry season and lower total rainfall (about 1000 to 1200 mm a year) than the forest and southern savanna.
4. The grasses dry up and are often burnt during the dry season, and shed leaves reduce water loss.
5. Common plants include shea butter, locust bean and acacia trees, and common animals include antelopes, grasscutters and grasshoppers.

(Any well-explained points describing the tall-grass-with-scattered-trees structure, seasonal drought, bush fires and drought adaptations are credited.)

(b) Estimating the density of a plant species using a quadrat

1. Select the area to be studied and lay out a quadrat (a square frame of known area, e.g. 1 m by 1 m = 1 m²).
2. Throw or place the quadrat at random within the study area to avoid bias.
3. Count the number of individuals of the chosen plant species that fall inside the quadrat.
4. Repeat the sampling several times at different random positions and record each count.
5. Find the average (mean) number of the plant per quadrat.
6. Calculate the density using \[ \text{Density} = \dfrac{\text{total number of the plant counted}}{\text{total area of all the quadrats}} \] which gives the number of individuals per unit area (e.g. per m²). Multiplying by the whole area estimates the total population.

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None

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(a)(i) Two diseases each caused by bacteria

Plants: bacterial wilt of tomato; bacterial soft rot of yam/cassava (bacterial leaf blight of rice is also acceptable).

Animals (including man): tuberculosis; cholera (others: tetanus, gonorrhoea, anthrax, typhoid).

(a)(ii) Three ways in which bacteria are useful

1. Decay bacteria (decomposers) break down dead organisms and recycle nutrients in the soil.
2. Nitrogen-fixing and nitrifying bacteria enrich the soil with nitrogenous compounds for plant growth.
3. Bacteria are used in industry to make food and drugs, e.g. yoghurt and cheese production, and antibiotics; some also aid digestion in the gut.

(b) Three methods of controlling mosquitoes and the reason for each

(c) Experiment to demonstrate the presence of bacteria under the finger nails

1. Prepare two sterile Petri dishes containing sterile nutrient agar (A and B).
2. Dish A: with an unwashed finger, scrape the material under the finger nails and gently streak it across the agar surface, then cover.
3. Dish B: leave untouched (or streak with a finger washed thoroughly with soap and disinfectant) to act as the control.
4. Incubate both dishes at about 37 °C for 2 to 3 days.
5. Observation: numerous colonies of bacteria grow on dish A, while the control B remains clear or nearly clear.
6. Conclusion: the growth of colonies on A shows that bacteria were present under the finger nails.

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(a) Process of inhalation (breathing in) in man

1. The external intercostal muscles contract, pulling the ribs upwards and outwards.
2. The diaphragm muscles contract, so the diaphragm flattens and moves downwards.
3. These two movements increase the volume of the thoracic cavity (chest).
4. The increased volume lowers the air pressure inside the lungs below atmospheric pressure.
5. Because outside air is now at higher pressure, air rushes in through the nose and trachea into the lungs, and the lungs inflate.

(b) Gaseous exchange in the shoot system of flowering plants

In the shoot, gaseous exchange takes place mainly through the stomata of the leaves (and through lenticels in woody stems). Gases diffuse in and out through these openings and through the air spaces of the spongy mesophyll. By day, when photosynthesis is faster than respiration, carbon dioxide diffuses in and oxygen diffuses out. At night, when only respiration occurs, oxygen diffuses in and carbon dioxide diffuses out. The exchange is by simple diffusion along concentration gradients across the moist cell surfaces.

(c) Experiment to demonstrate the effect of distilled water on mammalian red blood cells

1. Obtain a fresh sample of blood and place a drop in a test tube.
2. Add a quantity of distilled water to the blood and mix gently.
3. Place a drop on a slide and examine under a microscope; also observe the colour of the mixture.
4. Observation: the red cells swell, become spherical and burst; the mixture turns clear red because the haemoglobin is released.
5. Explanation / conclusion: distilled water is hypotonic to the cell contents, so water enters the cells by osmosis until they burst. This bursting is called haemolysis.