WAEC SSCE - Biology - 1991

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The theory of natural selection is one of the most important concepts in the study of evolution. It explains how species change and adapt over time in response to their environment. The theory of natural selection was formulated by Charles Darwin. In the mid-19th century, he traveled around the world on the HMS Beagle, studying plants, animals, and fossils. Based on his observations, he developed the theory of natural selection, which explains how species evolve over time through a process of natural selection. According to the theory of natural selection, individuals within a species exhibit variation, and some of these variations are more favorable than others in a given environment. Those individuals with more favorable traits are more likely to survive and reproduce, passing on those favorable traits to their offspring. Over time, these favorable traits become more common in the population, leading to the evolution of new species. Therefore, Charles Darwin is the scientist who formulated the theory of natural selection in evolution.

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Seminal vesicles are not part of the female reproductive system. Seminal vesicles are part of the male reproductive system and they produce and store seminal fluid which mixes with sperm to form semen. In females, the ovaries produce and release eggs, the uterus is where the fetus grows during pregnancy, the fallopian tubes transport the eggs from the ovaries to the uterus, and the vagina is the opening through which sexual intercourse occurs and through which a baby is delivered during childbirth.

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The structure that adapts the aorta to the high pressure of blood from the heart is thick elastic walls. The aorta is the main artery that carries blood away from the heart, and the blood it carries is under high pressure. If the walls of the aorta were thin, they would not be able to withstand this pressure and could easily rupture, leading to serious health problems. Similarly, if the walls were too stiff, they would not be able to expand and contract to accommodate the flow of blood, which would also be problematic. Thick elastic walls, however, are able to expand and contract as needed, while also providing the necessary strength and durability to withstand the pressure of the blood. The elasticity of the walls allows them to stretch when blood is pumped into the aorta and then contract again to push the blood forward, ensuring a steady and consistent flow of blood throughout the body.

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Puddles, ponds, rivers, lakes, seas, and oceans are grouped together as aquatic habitats. Aquatic habitats refer to any body of water that supports a diverse array of plant and animal life, including both freshwater and saltwater environments. They are distinguished from terrestrial habitats, which refer to habitats on land, such as forests, grasslands, and deserts. Aquatic habitats can be further classified as either marine or freshwater habitats. Marine habitats refer to habitats in the ocean or sea, while freshwater habitats refer to habitats in rivers, lakes, ponds, and other bodies of freshwater. Aquatic habitats are incredibly important to the health of our planet and support a wide range of species, from tiny plankton to massive whales. They provide a range of ecological services, such as water filtration, carbon sequestration, and nutrient cycling. However, aquatic habitats are also under threat from a variety of human activities, including pollution, overfishing, and climate change.

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When a germinating seed is attached horizontally on a revolving klinostat, it will experience a continuous change in orientation. This causes the gravitational stimulus to be distributed evenly throughout the seedling, and as a result, it will not show any curvature. Therefore, the correct answer is: "plumule and radicle will not show any curvature." In other words, the revolving klinostat negates the gravity's effect on the seedling and makes the seedling's growth response uniform in all directions, resulting in the absence of any curvature in the plumule (the embryonic shoot) or the radicle (the embryonic root).

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Kidney stones are solid particles that form in the kidneys due to the accumulation of certain substances such as calcium, oxalate, and uric acid. When these particles become too large, they can block the tubules in the kidney, which are responsible for filtering waste products from the blood and producing urine. This blockage can cause severe pain, discomfort, and can lead to complications such as infection or damage to the kidney. Therefore, kidney stones result in the blockage of the tubules by solid particles.

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The part of the alimentary system of a bird where grinding of maize occurs is the gizzard. The gizzard is a muscular part of the bird's stomach that contains small stones and pebbles. When a bird ingests food, the food is stored in the crop, which is a pouch-like structure located at the base of the bird's neck. From there, it moves to the stomach, where digestive enzymes begin to break it down. The food then passes into the gizzard, where the stones and pebbles help to grind it into smaller pieces. The gizzard's strong muscular walls then contract and help to further break down the food into a more digestible form. This process is important for birds that consume hard seeds, like maize, which are difficult to digest without the aid of grinding. Therefore, the gizzard is a critical part of a bird's digestive system that helps them extract nutrients from their food.

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The substance from the mother's blood that diffuse through the placenta into the blood vessels of the fetus are glucose and oxygen. The placenta is an organ that forms in the uterus during pregnancy and serves as a connection between the mother and the developing fetus. It allows for the exchange of nutrients, gases, and waste products between the mother's blood and the fetus's blood. Glucose is a type of sugar that is essential for the growth and development of the fetus. It is the main source of energy for the developing fetus's cells. Oxygen is also essential for the fetus's growth and development as it is required for cellular respiration, which is the process that generates energy from glucose. Therefore, during pregnancy, glucose and oxygen diffuse from the mother's blood, through the placenta, and into the blood vessels of the fetus, where they are used for growth and development. Carbon dioxide, which is a waste product of cellular respiration, also diffuses from the fetus's blood, through the placenta, and into the mother's blood for elimination.

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The female part of a flower consists of three structures: the stigma, style and ovary. The stigma is the sticky surface located at the top of the style, which is a long, slender tube-like structure. The ovary is located at the base of the style and contains one or more ovules. Together, the stigma, style, and ovary make up the female reproductive organ of the flower, called the pistil. When pollen grains land on the stigma, they begin to grow down through the style towards the ovary where they fertilize the ovules, leading to the formation of seeds.

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The human skin is considered a sense organ because it has nerve endings that can detect different sensations, such as touch, pressure, temperature, and pain. These nerve endings send signals to the brain, which allows us to feel and respond to our environment. Although the skin also serves other important functions, such as regulating body temperature and protecting the body from the outside world, its ability to sense different stimuli is what classifies it as a sense organ.

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The number of individuals in a habitat in relation to the unit space available to each organism is referred to as density. Density refers to the number of individuals of a particular species living in a given area or habitat, relative to the amount of space available to each organism. For example, a dense population of birds in a forest would have a high number of individuals per square kilometer of forest area, while a less dense population of birds in the same forest would have a lower number of individuals per square kilometer. The concept of density is important in ecology because it helps us understand how populations of different species interact with each other and with their environment. For example, if a population of prey animals has a high density, it may put pressure on the availability of food resources, leading to increased competition and changes in the behavior or distribution of the animals. Similarly, a high density of predators in an area may put pressure on the populations of their prey, leading to changes in the abundance and distribution of different species in the ecosystem.

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The primary producer in a food chain is green plants. This is because primary producers are the organisms that produce their own food through photosynthesis, using energy from the sun, water, and carbon dioxide. Green plants are able to produce their own food in this way and form the base of many food chains. Herbivores, carnivores, and parasites are all organisms that consume other organisms as food. Saprophytes are organisms that feed on dead and decaying organic matter. While they play an important role in the ecosystem by breaking down organic matter and recycling nutrients, they are not considered primary producers because they do not produce their own food. In summary, the primary producer in a food chain is the organism that produces its own food through photosynthesis, and in most cases, this is green plants. They form the base of many food chains and are essential for supporting life in an ecosystem.

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Food for animals is produced during the process of photosynthesis. Photosynthesis is a process by which green plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into glucose (a type of sugar) and oxygen. During photosynthesis, the energy from the sun is used to split water molecules into hydrogen and oxygen. The hydrogen is combined with carbon dioxide to form glucose, which the plant uses as food. The oxygen is released into the air as a byproduct. Animals are not capable of carrying out photosynthesis, so they rely on plants (or other animals that have eaten plants) for their food. Herbivorous animals eat plants directly, while carnivorous animals eat other animals that have eaten plants. In this way, the energy captured by plants during photosynthesis is transferred up the food chain to support the growth and development of other organisms. In summary, food for animals is produced during the process of photosynthesis, which is carried out by green plants, algae, and some bacteria. During photosynthesis, these organisms convert sunlight, carbon dioxide, and water into glucose, which serves as a source of energy for animals that consume them.

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The organelle which eliminates excess water from the body of protozoa is the contractile vacuole. Contractile vacuoles are small, membrane-bound organelles that occur in freshwater protozoa. They are responsible for removing excess water from the cell by contracting and expelling it out of the cell through a small pore. This process helps to regulate the water balance of the cell and prevent it from bursting due to osmotic pressure. The plasma membrane is a semi-permeable membrane that encloses the cell and regulates the movement of substances in and out of the cell. The cell wall is a rigid structure that occurs in plant cells and provides support and protection to the cell. The nucleus is a membrane-bound organelle that contains the genetic material of the cell and controls its functions. Therefore, the correct answer is that the contractile vacuole eliminates water from the body of protozoa.

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The Savanna zones of Nigeria have a limited amount of rainfall, which means that there is not enough water for many trees to grow. Also, the area is often exposed to grazing animals, which can prevent tree growth. Additionally, farmers in the area often engage in bush-burning, which further reduces the number of trees. Therefore, a fewer number of trees is a characteristic of the Savanna zones of Nigeria.

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The organism that can be transmitted by houseflies are Vibrio cholerae (causes cholera) and Poliomyelitis virus (causes polio). Houseflies can pick up disease-causing organisms like bacteria and viruses from various sources such as sewage, garbage, and feces, and then transmit them to humans through contact with food or surfaces. Therefore, the correct answer is option IV: II and IV only (Vibrio cholerae and Poliomyelitis virus).

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The bicuspid valve, also known as the mitral valve, is located between the left auricle and the left ventricle of the heart. It is one of the four valves in the heart that regulate blood flow through the chambers of the heart. The heart is divided into four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. The bicuspid valve is located between the left atrium and the left ventricle, separating the two chambers. The valve consists of two flaps, or cusps, that open and close in response to changes in pressure within the heart. When the left ventricle contracts, the pressure within the chamber increases, causing the bicuspid valve to close and preventing blood from flowing back into the left atrium. When the left ventricle relaxes, the pressure within the chamber decreases, causing the bicuspid valve to open and allowing blood to flow from the left atrium into the left ventricle. In summary, the bicuspid valve is located between the left auricle and the left ventricle of the heart, and plays an important role in regulating blood flow through the heart.

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The phenomenon whereby some organisms with certain features get established in an environment is known as natural selection. Natural selection is the process by which organisms with certain advantageous traits are more likely to survive and reproduce, passing on these traits to their offspring. This results in the gradual evolution of a species, as the advantageous traits become more prevalent in the population over time. It is an important mechanism of evolution, driving the diversity of life on Earth.

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Infective hepatitis is a disease of the liver. Hepatitis is a term used to describe inflammation of the liver, which can be caused by a variety of factors, including viruses, alcohol, and drugs. Infective hepatitis, specifically, is caused by a viral infection of the liver, and it can lead to a range of symptoms, including fatigue, fever, nausea, abdominal pain, and jaundice (yellowing of the skin and eyes). The liver is a vital organ that plays a number of important roles in the body, including filtering toxins from the blood, producing bile to aid in digestion, and storing and releasing glucose for energy. When the liver becomes inflamed, its ability to carry out these functions is compromised, which can lead to a variety of health problems. Overall, infective hepatitis is a serious condition that can cause significant damage to the liver and other organs, and it requires prompt medical attention and treatment to manage its symptoms and prevent complications.

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Predators are beneficial in pest control because they feed on the pests that cause damage to crops and other plants. For example, certain insects and spiders feed on aphids and other crop-damaging pests. By eating these pests, predators help to control their populations and prevent them from causing further damage to crops. This is a natural form of pest control that has been happening for thousands of years. Without predators, pest populations can grow out of control, leading to crop failure and other negative effects. Additionally, predators only eat a small proportion of the pest population, allowing the remaining pests to continue to reproduce and maintain a healthy population size. It's important to note that predators are not harmful to the environment or other animals. They are a natural part of the ecosystem and play a vital role in keeping pest populations in check. Farmers and gardeners can encourage predator populations by providing habitat and food sources for them, rather than relying on harmful pesticides and other chemical treatments.

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The function of controlling growth rate in animals is not a function of the skeleton. The skeleton is a framework of bones that provides support for the body, protects delicate internal organs, maintains the shape of the body, and provides attachment points for muscles. The skeleton plays a crucial role in allowing animals to move, as it provides the structure and support necessary for muscles to generate movement. In addition, the skeleton protects vital organs such as the brain, heart, and lungs from injury. The skeleton also helps to maintain the shape of the body, contributing to overall body form and structure. This is particularly important in animals that rely on external features, such as horns or antlers, for communication and display. Finally, the skeleton provides attachment points for muscles, allowing animals to perform a wide range of movements and activities. Without the skeleton, muscles would have no structure or support, and movement would be severely limited. In summary, while the skeleton performs several essential functions in the body, controlling growth rate is not one of them.

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The sweat gland is directly involved in excretion in the mammalian skin. Sweat glands are tiny, coiled glands located in the dermis layer of the skin that produce sweat, which is mostly composed of water, salt, and urea. Sweat glands are an important part of the excretory system, as they help to regulate body temperature by releasing sweat onto the skin's surface, where it evaporates and cools the body. In addition to regulating body temperature, sweat glands also help to remove waste products from the body. Urea, a nitrogen-containing compound that is a byproduct of protein metabolism, is excreted through sweat. This helps to maintain the balance of electrolytes and other substances in the body. Sebaceous glands, on the other hand, produce an oily substance called sebum that helps to lubricate and protect the skin and hair. The horny layer is the outermost layer of the epidermis that helps to protect the skin from damage. Hair follicles are structures that grow hair and are located in the dermis layer of the skin. The Malpighian layer is a layer of cells located between the dermis and epidermis that is responsible for the production of new skin cells.

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Muscles are attached to bones by means of tendons. Tendons are strong, flexible cords of connective tissue that attach muscles to bones. They are made up of collagen fibers, which are incredibly strong and provide the necessary support and stability for the muscles to move the bones. When a muscle contracts, it pulls on the tendon, which in turn pulls on the bone, causing it to move. This allows us to perform a wide range of movements, from simple actions like bending our fingers to more complex movements like running, jumping, or dancing. Overall, tendons are essential for our ability to move our bodies and carry out a variety of activities, and they play a crucial role in connecting muscles to bones to make this movement possible.

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The process of anaerobic respiration of yeast in a sugar solution is known as fermentation. During fermentation, yeast breaks down glucose in the absence of oxygen to produce energy in the form of ATP, as well as alcohol (ethanol) and carbon dioxide. This process is widely used in the food and beverage industry to produce beer, wine, and bread, among other products. The yeast consumes the sugars in the solution and converts them into alcohol and carbon dioxide through the process of fermentation. Therefore, the correct answer is fermentation.

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The scaly skin is a protective adaptive feature of the Agama lizard to terrestrial habitat. The skin of the Agama lizard is covered in small, overlapping scales that provide a barrier between the lizard's body and the external environment. This scaly skin helps to protect the lizard from physical damage, such as cuts and abrasions, and also helps to prevent moisture loss. In addition, the scaly skin of the Agama lizard also serves as a protective mechanism against predators. The scales provide an added layer of protection against predators' bites or claws, and can also make it difficult for predators to get a good grip on the lizard, allowing it to escape. While the other structures listed (claws, gular fold, nuchal crest, and tail) are also important adaptive features of the Agama lizard, they are not specifically related to its adaptation to terrestrial habitats. Claws, for example, are used for climbing and grasping prey, while the gular fold is used for communication and display. The nuchal crest and tail are also important structures, but they serve different functions, such as providing added stability and balance during movement or aiding in communication and defense.

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Natality is a factor that affects population density, as it is a measure of the birth rate in a population. Natality refers to the number of young ones produced by a population over a given period of time. Therefore, the correct statement is that natality involves the number of all young ones produced in the population. This is because natality is a measure of the rate at which new individuals are added to the population, which is directly affected by the number of offspring produced by the population. An increase in natality will lead to an increase in population density, as there will be more individuals added to the population. The number of mature females in the population and the number of females in the population can also affect natality, as they are the ones who are capable of reproducing. However, the most direct measure of natality is the number of young ones produced in the population.

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The unit of transmission of hereditary traits in living organisms is genes. Genes are specific sequences of DNA that are located on chromosomes in the nucleus of a cell. They contain the instructions for the development and functioning of an organism, including physical traits such as eye color and height, as well as physiological traits like metabolism and immunity. During reproduction, genes are passed from parents to offspring through gametes, which are specialized cells that fuse during fertilization to form a new individual. Each gene has a specific location on a chromosome, and different versions of the same gene, called alleles, can exist at that location. Therefore, genes are the fundamental units of heredity that determine the traits and characteristics of all living organisms.

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Diffusion is the movement of molecules from an area of high concentration to an area of low concentration, due to random thermal motion. Therefore, the statement that is correct about diffusion is "molecules move from a region of higher concentration to a region of lower concentration." This is because diffusion is a natural process that occurs when there is a concentration gradient, and molecules will move from an area where there are more molecules to an area where there are fewer molecules until equilibrium is reached. Diffusion can occur in any type of solution, whether it involves water or not. It does not require a differentially permeable membrane to occur, but a membrane can affect the rate at which diffusion occurs. Furthermore, diffusion does not involve when a solute can no longer dissolve in a solvent of only solute molecules into the Bowman's capsule, or when a solute can no longer dissolve in a solvent, as this is a different process called precipitation.

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The part of the brain responsible for the control of voluntary movement and interpretation of sensation is the cerebrum. The cerebrum is the largest part of the brain and is located at the top of the brainstem. The cerebrum is responsible for a wide range of functions, including voluntary movement, sensation, perception, and learning. It is divided into two hemispheres, the left and the right, which are connected by a bundle of nerve fibers called the corpus callosum. The outer layer of the cerebrum is called the cerebral cortex and is responsible for the interpretation of sensation and the initiation of voluntary movement. Different areas of the cerebral cortex are responsible for processing different types of sensory information, such as touch, vision, and hearing. Similarly, different areas of the cerebral cortex are responsible for the initiation of movement in different parts of the body. In summary, the cerebrum is the part of the brain responsible for the control of voluntary movement and the interpretation of sensation. It is the largest part of the brain and is divided into two hemispheres that work together to process sensory information and initiate movement.

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Homologous chromosomes refer to two chromosomes that are similar in size, shape, and genetic content, but come from different parents. Each parent contributes one set of chromosomes to their offspring, meaning that each individual has two copies of each chromosome (excluding the sex chromosomes in mammals). These two copies are not identical, but rather are homologous pairs, with one chromosome inherited from the mother and one from the father. Homologous chromosomes play an important role in sexual reproduction, as they allow for the exchange of genetic material between chromosomes during meiosis, which is the process by which sex cells (gametes) are formed. This exchange of genetic material, known as crossing over, helps to increase genetic diversity within a population. In summary, homologous chromosomes are pairs of chromosomes that are similar in size, shape, and genetic content, but come from different parents.

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Cambium is not found in the stem and root of monocotyledons. Cambium is a type of tissue found in the stems and roots of dicotyledonous plants, which are plants with two cotyledons or seed leaves. Cambium is responsible for the growth in diameter of the plant stem or root, which allows for the formation of new xylem and phloem tissues. In contrast, monocotyledonous plants, which are plants with only one cotyledon or seed leaf, do not have cambium tissue in their stems and roots. Instead, monocots have a different type of tissue called a ground tissue, which includes the pith and cortex. The pith is the central portion of the stem, while the cortex is the outer layer of the stem. Monocots also have a type of tissue called the pericycle, which is responsible for the formation of lateral roots. The xylem tissue, which is responsible for transporting water and nutrients from the roots to the rest of the plant, is also found in the stems and roots of monocots, but it is arranged differently than in dicots. The xylem tissue in monocots is scattered throughout the stem or root, rather than forming a distinct ring as it does in dicots.

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In the desert, the resource that is most limited and competed for by organisms is water. This is because deserts are characterized by low and irregular rainfall, and water is crucial for survival. Organisms in the desert have evolved various adaptations to conserve water, such as having a thick waxy layer on leaves, storing water in their tissues, and being active at night when temperatures are cooler and moisture levels are higher. Without enough water, desert organisms would not be able to survive for long periods and would struggle to carry out essential functions like photosynthesis and regulating body temperature. While other resources such as light, temperature, oxygen, and carbon dioxide are also important, water is the most critical limiting factor in the desert ecosystem.

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The mechanism of gaseous exchange in living organisms is essentially by diffusion. This is the process by which gases move from an area of high concentration to an area of low concentration. In the case of respiration, oxygen moves from the air outside the body into the lungs, where it diffuses into the bloodstream and is transported to cells throughout the body. At the same time, carbon dioxide, a waste product of metabolism, diffuses out of cells and into the bloodstream, which then carries it back to the lungs to be exhaled. Breathing, which involves inhalation and exhalation, is the process by which air is moved in and out of the lungs to facilitate gas exchange by diffusion.

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Warming soup regularly as a food preservation practice is essentially done to kill bacteria that can cause the soup to spoil. When soup is heated to a high temperature, it destroys any harmful bacteria that may be present, making it safe to eat for a longer period of time. This is because bacteria thrive in warm, moist environments, and by heating the soup, you eliminate these conditions and reduce the risk of spoilage. Warming soup regularly does not help to break down cellulose, remove excess water, or retain taste.

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Thyroxin is a hormone secreted by the thyroid gland and plays a crucial role in regulating the metabolism of the body. Over-secretion of thyroxin, a condition known as hyperthyroidism, can cause several health problems. One of the most common symptoms of hyperthyroidism is weight loss or thinness in the body, even with an increased appetite. This is because the excess thyroxin in the body increases the rate of metabolism, leading to increased energy consumption and weight loss. Other symptoms of hyperthyroidism may include an increased heart rate, sweating, irritability, and nervousness. Therefore, the correct option from the given choices is "thinness in body."

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The statements explain the idea of survival of the fittest, which is a key concept in the theory of evolution by natural selection. According to this theory, individuals within a population show natural variation in traits such as size, color, and behavior. In a given environment, certain traits may be more advantageous than others, allowing individuals with those traits to survive and reproduce more effectively. Over time, the frequency of these advantageous traits increases within the population, leading to the evolution of new species. The statements also refute the idea of acquired characteristics, which was proposed by Jean Lamarck. Lamarck believed that organisms could acquire new traits during their lifetime, and that these traits could be passed on to their offspring. However, the theory of evolution by natural selection suggests that traits are inherited from parents and are subject to random variation through mutation and recombination. Furthermore, the statements do not imply that organisms modify their environment in order to survive. Rather, they suggest that the environment acts as a selective pressure on organisms, determining which traits are most advantageous for survival and reproduction. In summary, the statements support the idea of survival of the fittest, which is a key concept in the theory of evolution by natural selection. They also refute the idea of acquired characteristics, which was proposed by Jean Lamarck, and do not imply that organisms modify their environment in order to survive.

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The part of the ear that is responsible for the maintenance of balance is the semicircular canals. The semicircular canals are three fluid-filled structures located in the inner ear that are oriented at right angles to each other. They are responsible for detecting rotational movements of the head, which are important for maintaining balance and equilibrium. When the head moves, the fluid in the semicircular canals moves as well, which triggers hair cells to send signals to the brain. These signals are then interpreted by the brain to determine the position and movement of the head in space. Therefore, the semicircular canals are essential for maintaining balance and spatial orientation.

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The pituitary gland is regarded as the master gland because its secretions control the functions of many other endocrine glands in the body. The pituitary gland is a small, pea-sized gland located at the base of the brain. It is often referred to as the "master gland" because it secretes hormones that control the functions of many other endocrine glands throughout the body, including the thyroid gland, adrenal glands, and ovaries or testes. These hormones regulate a wide range of physiological processes, including growth and development, metabolism, reproduction, and stress response. While the pituitary gland may not be the largest endocrine gland in the body, it is one of the most important in terms of its control over other glands. Its secretion of hormones is tightly regulated by the hypothalamus, a region of the brain that helps to maintain homeostasis in the body by monitoring and responding to changes in internal and external environments.

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Encouraging poaching in game reserves is not a way of conserving natural resources. Poaching involves the illegal hunting, killing, or capturing of wild animals, and it is a major threat to the conservation of natural resources, particularly wildlife. Poaching can lead to a decline in animal populations, which can disrupt ecosystems and have negative impacts on local communities that rely on these resources for their livelihoods. Therefore, encouraging poaching in game reserves is not a conservation practice but rather a destructive and illegal activity that undermines conservation efforts. In contrast, the other options listed - controlling farming practices, establishing forest reserves, establishing parks and gardens, and protecting endangered species - are all ways of conserving natural resources and preserving biodiversity for future generations.

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The upper part of the trachea in mammals is called the "larynx." It's a small structure located at the top of the trachea, where it meets the back of the throat. The larynx helps to protect the airway during swallowing by closing off the opening to the trachea with a flap called the epiglottis. It also plays an important role in producing sound, as it contains the vocal cords.

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Waterlogged topsoil has a negative effect on the roots of plants because it restricts the supply of air to the roots. When the soil is saturated with water, it fills the small air spaces in the soil, preventing oxygen from reaching the roots. Roots require oxygen for respiration, which is the process that provides energy for growth and other metabolic activities. Without oxygen, the roots cannot function properly, and their growth and nutrient absorption capacity are severely affected. Therefore, the correct option is that the roots will be starved of air.

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The transverse section of a young monocotyledonous stem typically contains parenchyma, phloem, xylem, and sclerenchyma tissues. However, cambium is not seen in the transverse section of a young monocotyledonous stem. This is because monocots lack secondary growth, which is the result of cell division in the cambium. The cambium is a layer of cells that is responsible for the formation of secondary xylem (wood) and secondary phloem in dicotyledonous plants. Since monocots lack secondary growth, they also lack the cambium layer. Therefore, the correct answer is that cambium is not seen in the transverse section of a young monocotyledonous stem.

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The pulmonary vein is a blood vessel that is responsible for carrying blood to and from the lungs. Contrary to its name, the pulmonary vein carries oxygenated blood from the lungs to the left auricle of the heart. This is because the blood that flows through the lungs is deoxygenated, and it is the process of gas exchange in the lungs that oxygenates the blood. After the deoxygenated blood from the body enters the right atrium, it is then pumped into the right ventricle. From there, it is pumped into the lungs via the pulmonary artery, where it is oxygenated. The oxygenated blood then returns to the heart via the pulmonary vein, where it enters the left atrium. Therefore, the correct statement is that the pulmonary vein carries oxygenated blood from the lungs to the left auricle of the heart.

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In testing for a reducing sugar, the food substance is usually warmed with Benedict's solution. Benedict's solution is a chemical reagent that contains copper sulfate, sodium carbonate, and sodium citrate. When Benedict's solution is heated in the presence of a reducing sugar, such as glucose or fructose, it undergoes a chemical reaction that causes the copper ions in the solution to be reduced to copper oxide, which gives a brick-red precipitate. This color change indicates the presence of a reducing sugar in the food substance being tested. Therefore, by heating the food substance with Benedict's solution, we can detect the presence of a reducing sugar in the food.

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(a)(i) The cell theory

The cell theory states that:

• All living organisms are made up of one or more cells, and of the products of cells.
• The cell is the basic structural and functional unit of life (the smallest unit that can carry out life processes).
• All cells arise from pre-existing cells by cell division.

(a)(ii) Two scientists who contributed to the cell theory

Robert Hooke and Matthias Jakob Schleiden. (Theodor Schwann and Rudolf Virchow also made important contributions.)

(b) Large labelled diagram of a typical plant cell

The diagram shows the rigid cellulose cell wall forming the outer boundary, the cell (plasma) membrane lining it, the cytoplasm in which the organelles are suspended, the nucleus with its nuclear membrane and nucleolus, the large central vacuole bounded by the tonoplast, several green chloroplasts, and the mitochondria.

(c) Three ways a typical plant cell differs from a typical animal cell

(d) Differences between Hydra and mammals

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(a)(i) Five excretory organs found in living organisms, with (ii) an example organism for each

(The leaf/stoma of green plants and the general body surface are also acceptable examples.)

(b)(i) Two ways the excretory organ of the earthworm is similar to that of mammals

• Both use tubular structures (nephridia in earthworm, nephrons in the mammalian kidney) that filter body fluids/blood.
• Both remove nitrogenous waste and excess water and reabsorb useful substances, producing urine.

(b)(ii) Three differences between them

(c) Mechanism of opening and closing of the stomata

• Opening (in light/day): In the presence of light the guard cells photosynthesise and actively pump in potassium ions; sugar/solute concentration in the guard cells rises, lowering their water potential. Water then moves into the guard cells by osmosis, making them turgid. Because the inner wall of each guard cell is thicker and less elastic than the outer wall, the turgid guard cells curve apart, opening the stoma.
• Closing (at night/water shortage): Potassium ions and sugars leave the guard cells, raising their water potential; water passes out of the guard cells by osmosis and they become flaccid. The guard cells straighten and come together, closing the stoma. This also happens when the plant is short of water, to reduce transpiration.

Détails de la réponse

(a) Labelled diagram of the internal structure of the mammalian ear

(b) Mechanism of hearing in mammals

1. The pinna collects sound waves and directs them through the auditory canal (auditory meatus) to the tympanic membrane or ear drum.
2. The sound waves make the ear drum vibrate.
3. These vibrations are transmitted through, and amplified by, the three ossicles: malleus, incus and stapes.
4. The stapes transmits the vibrations to the oval window. This causes the fluid in the cochlea to vibrate.
5. The vibrations stimulate the sensory hair cells in the organ of Corti in the cochlea, producing nerve impulses.
6. The auditory nerve carries the impulses to the brain, where they are interpreted as sound.

(c) Care of the ear

1. Keep the outer ear clean, but do not insert pins, sticks or other sharp objects into the ear canal.
2. Avoid excessive loud noise and consult a doctor promptly if there is ear pain, discharge or infection.

Détails de la réponse

(a)(i) Three components of an ecosystem

• Producers (green plants/autotrophs).
• Consumers (herbivores, carnivores, omnivores).
• Decomposers (bacteria and fungi).

(The non-living abiotic component of water, air, soil and light is also part of an ecosystem.)

(a)(ii) Interaction among the components

The producers trap solar energy and manufacture food by photosynthesis. The consumers feed on the producers (and on other consumers), so energy and materials are transferred along food chains and food webs. When producers and consumers die, the decomposers break down their bodies and wastes, releasing mineral nutrients back into the soil and air, which the producers reuse. In this way energy flows through the system and nutrients are recycled among the components.

(b) Three major steps in studying plants and animals in a community

1. Sampling / collection of organisms using suitable methods (quadrat for plants and sedentary animals; pitfall traps, nets or sweep nets for mobile animals).
2. Identification and counting of the organisms collected, and estimation of their abundance/distribution.
3. Recording and analysis of the data (frequency, density, and the physical/abiotic factors measured) to describe the community.

(c) Importance of each factor and its effect on population growth

• (i) Competition: Members compete for limited resources (food, space, light, mates). Intense competition reduces the survival and reproductive rate of the weaker individuals, so it checks and limits population growth.
• (ii) Pathogens: Disease-causing organisms infect members of the population. Outbreaks of disease increase the death rate, thereby reducing the size and slowing the growth of the population.
• (iii) Water: Water is essential for all life processes. An adequate supply promotes survival, reproduction and rapid growth of the population, while a shortage of water raises the death rate and reduces population growth.