WAEC SSCE - Biology - 1994

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The theoretical probability that a normal male child will be born is 1/2 or 50%. This means that for every two children born, one is expected to be male and the other is expected to be female. This probability is based on the fact that human beings have 23 pairs of chromosomes, with one pair determining the sex of the child. Females have two X chromosomes, while males have one X and one Y chromosome. During fertilization, the sperm from the father can either carry an X or a Y chromosome, while the egg from the mother always carries an X chromosome. The chance of the sperm carrying an X or a Y chromosome is roughly equal, which means that there is an equal chance of conceiving a male or female child. Therefore, the theoretical probability of a normal male child being born is 1/2 or 50%.

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Muscles are attached to bones by means of tendons. Tendons are tough, fibrous connective tissues that connect muscles to bones, allowing for the transmission of the force generated by the contraction of the muscle to the bone, resulting in movement of the body part. They are composed mainly of collagen fibers and can withstand tension and stress, which is necessary for the movement and stability of the skeletal system.

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The structural feature that adapts Drosera (Sundew) to its carnivorous mode of nutrition is the club-shaped glandular hairs on its leaves. These glandular hairs have a sticky secretion that traps insects and other small organisms, providing the plant with the necessary nutrients it cannot get from the soil. The broad leaves with long spines and wide stomata under the leaves are not directly involved in the carnivorous mode of nutrition, while the long root with glandular hairs and prominent lenticel on the stem serve other purposes, such as anchoring the plant and allowing gas exchange, respectively.

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A cellulose cell wall is present in Spirogyra. Spirogyra is a filamentous green algae which possesses a cell wall made up of cellulose. Cellulose is a complex carbohydrate that provides structural support to the cells of plants and many algae. Amoeba and paramecium are unicellular organisms that do not possess a cellulose cell wall. Mammalian white blood corpuscles are cells of the immune system which also do not possess a cellulose cell wall. Parasitic worms also do not have cellulose cell walls.

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The gradual, cumulative, adaptive and heritable changes in an organism over a long period of time resulting in emergence of new species is known as organic evolution. Organic evolution is the process by which species of organisms change over time through the gradual accumulation of beneficial adaptations. This process occurs through the mechanisms of natural selection, genetic drift, and gene flow, which act to alter the frequency and distribution of genetic traits within a population. Through organic evolution, new species can arise from common ancestors through the process of speciation, where isolated populations of organisms diverge genetically and eventually become distinct species. This process can occur through geographic isolation, ecological specialization, or reproductive isolation. The concept of organic evolution was first proposed by Charles Darwin in the mid-19th century, and has since become a fundamental principle in the field of biology. The study of evolution provides insights into the diversity of life on Earth and the complex relationships between different organisms and their environments. In summary, the gradual, cumulative, adaptive and heritable changes in an organism over a long period of time resulting in emergence of new species is known as organic evolution, which is a fundamental principle in the field of biology.

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Soil fertility is not a condition necessary for germination to occur in most seeds. Germination is the process by which a seed develops into a new plant. For germination to occur, most seeds require water to activate enzymes, air for respiration, and a suitable temperature to trigger metabolic processes. Soil fertility, although important for the growth of the new plant, is not a direct requirement for germination. Soil provides the nutrients necessary for the plant to grow after germination, but the seed can still germinate and begin to develop into a new plant without being in soil. Therefore, the correct option is "Soil fertility".

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The appendicular skeleton is composed of the bones that form the limbs and the girdles that connect the limbs to the axial skeleton. The axial skeleton includes the bones of the skull, vertebral column, and ribcage. Out of the options provided, the vertebral column is not part of the appendicular skeleton. The vertebral column is a part of the axial skeleton, and it forms the main axis of the body, providing support and protection for the spinal cord. The other options provided, including the clavicle, scapula, humerus, and femur, are all part of the appendicular skeleton. The clavicle and scapula form the pectoral girdle, while the humerus, radius, and ulna form the arm bones. The femur, tibia, fibula, and patella form the leg bones, and the pelvic girdle connects the legs to the axial skeleton.

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The structural features that help in controlling body temperature in animals include scales, skin, fur, capillaries, and feathers. Out of these, capillaries are not considered a structural feature that helps in controlling body temperature, as they are part of the circulatory system that facilitates the exchange of nutrients and gases between the blood and body tissues. Although capillaries play a crucial role in regulating body temperature by controlling blood flow to the skin's surface, they are not considered a structural feature as they are too small to be seen with the naked eye. Therefore, the correct answer is "Capillaries."

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The factor that does not control population growth is "Abundance of food." This is because an abundance of food can lead to increased reproduction and survival rates, which can in turn increase population growth. The other factors listed can all have an impact on population growth: food shortage can decrease reproduction and survival rates, emigration can decrease the size of the population, predation can decrease survival rates, and natural disasters can cause death and decrease survival rates.

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Spermatogenesis and Oogenesis are terms used to describe the process of gametogenesis. Gametogenesis is the process of producing gametes, which are the reproductive cells in sexually reproducing organisms. In males, the process of spermatogenesis produces sperm cells, while in females, the process of oogenesis produces egg cells. Both spermatogenesis and oogenesis involve meiosis, a type of cell division that produces haploid cells (cells with half the number of chromosomes as the parent cell) from diploid cells (cells with the full number of chromosomes).

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After fertilization, the ovary of a flower typically develops into the fruit. The fertilized ovules within the ovary will develop into seeds, which are contained within the fruit. The ovary walls thicken and enlarge as the seeds develop, and the fruit can take on a variety of shapes and sizes depending on the specific plant species. The fruit serves to protect and nourish the seeds, aiding in their dispersal and ensuring the survival and growth of the next generation of plants.

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The characteristics that enable an organism to survive in its environment are called adaptations. These adaptations can be morphological (related to the organism's physical structure), physiological (related to the organism's internal systems and processes), or behavioural (related to the organism's actions and interactions with its environment). Adaptations are the result of natural selection, which is the process by which organisms that are better suited to their environment are more likely to survive and reproduce. Over time, these advantageous traits become more common in a population, leading to the evolution of new species. Adapting to environmental changes is a key factor in an organism's ability to survive and thrive in its ecosystem.

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Mutation leads to evolution in that it gives rise to new individuals which differ considerably from their parents. Mutations are changes in the genetic material of an organism that occur randomly and can lead to new traits. If these new traits are advantageous in a particular environment, they can increase an organism's chances of survival and reproduction, leading to the evolution of a new species over time. Mutations do not always confer adaptability to the offspring, but they increase genetic diversity, which can provide the raw material for natural selection to act upon.

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In humans, the albino trait is recessive, which means that an individual must inherit two copies of the albino gene (one from each parent) to have albinism. Normal skin color is dominant, meaning that an individual only needs one copy of the normal skin color gene to express that trait. If both parents are heterozygous for albinism, meaning they carry one albino gene and one normal skin color gene, then each parent has a 50% chance of passing on the albino gene and a 50% chance of passing on the normal skin color gene to their child. Therefore, the probability of both parents passing on the albino gene to their child is (1/2) x (1/2) = 1/4 or 25%. This means that the probability of the heterozygous parents having an albino child is 1/4 or 25%.

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The structure used by fishes to detect the presence and movement of other animals by the vibration they produce is the lateral line. The lateral line is a sensory system present in most fish that runs along their sides from head to tail. It contains tiny sensory cells that can detect changes in water pressure and vibrations in the water. These vibrations could be produced by other fish, predators, prey or any other moving objects in the water. Fish use this sense to locate prey and navigate through their environment. The lateral line is crucial to the survival of many species of fish.

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The ability of the eye to alter the focal length of the lens, with regard to the distance of the object from the eye, is known as accommodation. This process enables us to focus on objects at different distances by changing the shape of the lens. When we look at objects that are far away, the ciliary muscles in the eye relax and the lens becomes flatter, allowing the light to be focused on the retina. Conversely, when we look at objects that are close, the ciliary muscles contract, causing the lens to become more curved and increase its refractive power, allowing us to see the object clearly. Accommodation is an important aspect of visual perception and is essential for everyday activities such as reading, driving, and using electronic devices.

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Continuous variation refers to the variation in which a trait can take any value within a range, with no distinct categories. Therefore, the characteristic that is not an example of continuous variation is the "ability to roll tongue," since this is a discrete trait that can be either present or absent, with no intermediate values. In contrast, height, skin color, finger length, and leaf size are all traits that can vary continuously within a population.

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The deficiency of calcium in a plant may result in stunted growth and poor root development. Calcium is an important nutrient for plant growth and development, and it plays a critical role in maintaining the structure and function of cell walls. Without sufficient calcium, plants may not be able to develop strong and healthy roots, which can lead to stunted growth and poor overall plant health. Additionally, calcium deficiency can also cause other symptoms such as leaf curling, necrosis, and tip burn.

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Homeostasis is the control of the water and salt requirements of the body in order to maintain a stable internal environment. It is the regulation and maintenance of a relatively constant internal environment within the body despite changes in external environmental conditions. Homeostasis is achieved through various mechanisms in the body such as regulation of body temperature, regulation of pH, and regulation of water and salt balance. The body's ability to maintain homeostasis is critical for survival, as many cellular and physiological processes depend on the maintenance of a stable internal environment.

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The process in which insects undergo conspicuous changes in form and appearance during development is known as metamorphosis. Metamorphosis is a biological process that occurs in some insects, including butterflies and beetles, where the insect undergoes a transformation in body structure and function. There are two types of metamorphosis: complete and incomplete. In complete metamorphosis, the insect undergoes a dramatic change in form, where it goes through four distinct stages: egg, larva, pupa, and adult. In incomplete metamorphosis, the insect goes through three stages: egg, nymph, and adult. These changes in form allow insects to adapt to different environments and ecological niches throughout their life cycle.

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The plant disease that is indicated when grains of cereals are covered with a mass of spores is called smut. Smut is a fungal disease that affects cereal crops like corn, wheat, and barley. It causes the formation of black or dark brown masses of spores on the grains, which can reduce crop yield and quality. These spores can easily spread to other plants, infecting them as well. Farmers can prevent smut by using resistant crop varieties, crop rotation, and fungicides.

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Sugar is not required for primary production to occur in plants. Primary production in plants refers to the process of converting light energy into chemical energy by the process of photosynthesis. During photosynthesis, plants use the energy from sunlight to convert carbon dioxide and water into glucose (a type of sugar) and oxygen. This process requires the presence of chlorophyll, a green pigment found in the chloroplasts of plant cells, which absorbs light energy from the sun. Water and carbon dioxide are also necessary for primary production to occur. Water is required to provide hydrogen ions for the synthesis of glucose, while carbon dioxide is a source of carbon that is used to build the glucose molecule. Solar energy is the ultimate source of energy for primary production in plants. Without sunlight, plants would not be able to carry out photosynthesis and produce the energy they need to survive. However, sugar is not a requirement for primary production to occur in plants. Glucose is produced as a result of photosynthesis, but it is not necessary for the process to occur. Rather, glucose is a byproduct of the process and is used by the plant as a source of energy for growth and other metabolic processes. Therefore, sugar is the option that is not required for primary production to occur in plants.

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The part of the ear which equalizes air pressure on either side of the eardrum is the Eustachian tube. The Eustachian tube connects the middle ear to the back of the throat, allowing air to flow in and out of the middle ear, which helps to equalize the pressure on either side of the eardrum. When there is a pressure difference, such as during changes in altitude, the Eustachian tube opens briefly to equalize the pressure and prevent discomfort or damage to the eardrum. In some cases, the Eustachian tube may become blocked, leading to ear pain or difficulty hearing, and this can be addressed through various treatments, such as nasal sprays or surgery.

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The correct answer is detoxification. Detoxification is the process by which harmful and poisonous substances are changed into harmless substances by the liver. These harmful substances can be toxins, drugs, alcohol, or waste products of metabolism. The liver breaks down these substances into harmless compounds that can be easily excreted from the body. This process is crucial to maintaining the body's health and well-being.

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The scientist who discovered the cell while examining a thin slice of cork under the microscope was Robert Hooke. In 1665, he observed the structure of a cork under a microscope and noticed tiny empty compartments which he called "cells" because they reminded him of the small rooms in a monastery. This discovery is considered a fundamental one in the field of biology and led to the development of the cell theory.

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Out of the options given, Spirogyra exists as a filament. A filament is a long chain of cells that are attached to each other end-to-end forming a thread-like structure. Spirogyra is a type of green algae that is commonly found in freshwater habitats. Its cells are arranged in long chains, forming filaments that can be seen with the naked eye. The other organisms listed are not filamentous, but exist as single cells or colonies.

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The sex of a human offspring is determined by the presence or absence of a Y chromosome. A normal male offspring has one X chromosome and one Y chromosome, which is represented as XY. Females have two X chromosomes, which is represented as XX. YY is not a possible combination for humans, and X and Y on their own do not determine a normal male offspring.

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The long and sharp claws in the front labeled ll on a bird indicate that it uses them to hold onto its prey. Birds of prey, such as eagles and hawks, have sharp talons for catching and holding onto their prey, usually small mammals or other birds. The talons are also used to help the bird climb and perch on branches or other surfaces. The shape and size of the talons vary depending on the species of bird and its feeding habits.

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All animals are dependent on green plants because plants manufacture food for animals. Plants are able to manufacture their own food through the process of photosynthesis, using energy from the sun, water, and carbon dioxide. This food is stored in the form of carbohydrates, which is then used by animals for their survival. Animals, in turn, are dependent on plants as a source of food, either directly by consuming them or indirectly by feeding on other animals that have eaten plants. Therefore, plants are the primary producers and form the basis of the food chain, making them essential for the survival of all animals.

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The kidney performs the same function as the contractile vacuole of a unicellular organism. Both the contractile vacuole and kidney are responsible for regulating the water content of the cell and the body, respectively. The contractile vacuole in unicellular organisms eliminates excess water from the cell, while the kidney in multicellular organisms eliminates excess water and other waste products from the body. This process helps to maintain the osmotic balance of the organism's internal environment.

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Desert encroachment can be caused by overgrazing. Overgrazing refers to the excessive grazing of livestock on a particular land area. When there are too many animals grazing in a particular area, they eat more plants than the ecosystem can replace, which leads to a reduction in vegetation cover. Without vegetation, the soil is exposed to harsh weather conditions like wind and sun, which makes it more susceptible to erosion. When soil erosion occurs, it can lead to the formation of desert-like conditions where once fertile land existed. On the other hand, afforestation, planting of trees, irrigation, and the establishment of game reserves can actually help to prevent desert encroachment by increasing vegetation cover and preventing soil erosion. Therefore, these practices are often used as strategies to combat desertification.

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Adult Schistosoma is found in the blood vessels of the bladder of humans. Schistosoma is a parasitic flatworm that causes a disease called schistosomiasis, which is transmitted by contact with contaminated water. When the larvae of the Schistosoma parasite penetrate the skin of a human host, they migrate to the veins in the abdominal area and then to the blood vessels of the bladder, where they develop into adult worms. These adult worms lay their eggs, which are passed out of the body in the urine, leading to a host of symptoms associated with schistosomiasis.

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The carrier of hereditary materials is a chromosome. Chromosomes are long, coiled-up molecules of DNA that carry the genes which contain the hereditary information that gets passed down from one generation to the next. They are found in the nucleus of eukaryotic cells and are visible under a microscope during cell division. The DNA in chromosomes is responsible for the traits that an organism inherits from its parents. Therefore, the correct option is "Chromosome".

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A food chain is a linear sequence of organisms that shows how energy is transferred from one organism to another as they consume each other. The correct order in a possible food chain is Diatom -> Crustacean -> Fish -> Man. Diatoms are microscopic algae that form the base of the food chain. Crustaceans such as zooplankton feed on diatoms, then fish feed on crustaceans, and finally, man feeds on fish. Therefore, the energy and nutrients flow from the primary producer, diatoms, to the top consumer, man, through the intermediate consumers, crustaceans and fish.

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The option that is not a pollution control measure is "Channelling sewage into streams". This is because sewage contains various pollutants such as pathogens, organic matter, and nutrients that can harm aquatic life and human health. Therefore, channelling sewage into streams will worsen the pollution and harm the ecosystem. Pollution control measures aim to reduce or prevent the release of pollutants into the environment to minimize their harmful effects. Penalizing those who dump refuse on water or land, controlling emissions of smoke from automobiles and industries, ensuring no spillage of oil, and establishing government agencies like Federal Environmental Protection Agency (FEPA) are all pollution control measures.

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The pair of structures that contribute to balance in mammals are the utriculus and sacculus. The utriculus and sacculus are two structures located in the inner ear, specifically in the vestibular system. These structures are responsible for detecting linear acceleration and head position, which is important for maintaining balance. The utriculus and sacculus contain specialized hair cells that are embedded in a gel-like substance. When the head moves, the gel-like substance moves as well, which bends the hair cells and creates an electrical signal. This electrical signal is then sent to the brain, which uses the information to help maintain balance. The other options listed (malleus and stapes, sacculus and cochlea, utriculus and pinna, and fenestra ovalis and sacculus) are all structures in the ear, but they do not play a direct role in balance. The malleus and stapes are part of the middle ear and are involved in hearing. The cochlea is also involved in hearing, while the pinna is the external part of the ear. The fenestra ovalis is a small opening in the inner ear that connects to the middle ear and is involved in transmitting sound waves.

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The correct answer is photosynthesis. Photosynthesis is the process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of organic compounds, mainly carbohydrates. This process occurs in the chloroplasts of plant cells and involves the use of pigments such as chlorophyll to absorb light energy, which is then used to power the conversion of carbon dioxide and water into glucose and oxygen. In photosynthesis, carbon dioxide is taken in through small openings in the leaves called stomata, while water is absorbed through the roots and transported to the leaves via the xylem. During the process, light energy is captured by chlorophyll and other pigments, and used to power the conversion of carbon dioxide and water into glucose and oxygen. The oxygen produced is then released into the atmosphere as a byproduct, while the glucose is used as an energy source for the plant.

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Reading a book is not an example of a simple reflex action. Simple reflex actions are automatic, involuntary and rapid responses of the body to a stimulus. They involve sensory neurons that detect the stimulus, and motor neurons that produce a quick response without involving the brain. Knee jerk, blinking of the eye, withdrawal of hand from hot object, and contraction of the iris are all examples of simple reflex actions. However, reading a book is a voluntary action that involves higher brain functions such as perception, comprehension, and memory. It is not a reflex action.

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Sunlight is not a conservable natural resource. This is because sunlight is a form of energy that is constantly being produced by the sun and cannot be depleted or conserved. While we can harness and use sunlight through solar energy technologies, the actual sunlight itself cannot be stored or saved for future use. On the other hand, water, forest, soil, and minerals are all natural resources that can be conserved through various management practices to ensure their sustainable use in the future.

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The option that does not imply an attempt at conservation is "Retraining from the use of the natural resource." This is because it suggests a complete cessation or abandonment of the use of a natural resource, rather than managing it in a sustainable way to conserve it for future generations. Conservation involves using natural resources in a way that maintains their availability and productivity over time, without causing harm to the environment or depleting them beyond their ability to replenish. Therefore, all the other options listed in the question imply an attempt at conservation by putting limits on the size of nets and vessels used in fishing, prohibiting hunting or fishing at certain seasons, and managing natural resources prudently to maintain their sustainability.

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The correct answer is "locomotion in organisms". The other options listed, such as the nitrogen cycle, carbon cycle, water cycle, and nutrient cycle, are all processes that demonstrate how materials and energy flow through an ecosystem, and how different organisms interact with each other and their environment. These cycles are constantly in motion and are essential for maintaining the balance and stability of an ecosystem. In contrast, locomotion in organisms refers to the movement of an organism from one place to another, which can certainly impact the ecosystem in various ways, but does not necessarily illustrate the dynamic nature of the ecosystem as a whole.

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The measure that does not ensure conservation is "enclosing fishes in ponds where they will not be stolen." While this action may protect a certain population of fish from being stolen, it does not contribute to the overall conservation of the species. Conservation involves preserving and protecting natural resources and ecosystems to maintain biodiversity and sustainability. Therefore, measures such as establishing reserves, promoting reforestation, and enacting laws that protect species and their habitats are more effective in achieving conservation goals.

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The least safe method of birth control among the given options is the withdrawal method. This method involves the male partner pulling out his penis before ejaculation, with the aim of preventing sperm from entering the vagina. However, this method is very unreliable because it's difficult to time and control ejaculation accurately, and even pre-ejaculate fluid can contain sperm. Therefore, it's possible for pregnancy to occur even if this method is used consistently and correctly. Additionally, the withdrawal method does not protect against sexually transmitted infections (STIs), so it's important to use another form of protection if STI prevention is also a concern.

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The correct answer is O. Blood types are classified based on the presence or absence of certain antigens on the surface of red blood cells. The two most important blood group systems are ABO and Rh. Blood group O does not have A or B antigens on the surface of its red blood cells. This means that blood group O can be safely transfused into individuals with any blood group (A, B, AB, or O) without causing an adverse immune reaction. Therefore, blood group O is often referred to as the universal donor. In contrast, individuals with blood group AB have both A and B antigens on the surface of their red blood cells, making them universal recipients. This is because they can safely receive blood from any blood group without the risk of an adverse immune reaction. Blood groups A and B are not considered universal donors because they contain antigens that can trigger an immune response in individuals with different blood groups.

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The branch of science that deals with resemblances, origin, and expression of biological variations is called genetics. Genetics is the study of genes, heredity, and genetic variation in living organisms. It encompasses many different fields, including molecular biology, biochemistry, and genomics, and is concerned with understanding how genetic information is passed from parents to offspring, how it is expressed in different organisms, and how it gives rise to variations in traits. Geneticists study the structure and function of genes, the mechanisms of inheritance, and how genetic variations can lead to diseases or affect the characteristics of an organism. They also investigate how genetic information can be used to improve agriculture, medicine, and biotechnology.

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(a) General arrangement of the viscera of a named mammal (the rabbit)

The viscera are the internal organs contained in the body cavity (coelom). In a mammal such as the rabbit the coelom is divided by a muscular sheet, the diaphragm, into a front thoracic cavity and a hind abdominal cavity.

Thoracic cavity (chest):

• The heart lies centrally, enclosed in the pericardium.
• A pair of lungs lies on either side of the heart.
• The trachea runs down and divides into two bronchi entering the lungs, and the oesophagus passes behind them through the diaphragm.

Abdominal cavity (belly):

• The stomach lies just behind the diaphragm on the left.
• The large liver lies behind the diaphragm, with the gall bladder associated with it.
• The coiled small intestine and the large intestine (with a large caecum in the rabbit) fill much of the cavity, held by the mesentery.
• The pancreas lies in the loop of the duodenum and the spleen near the stomach.
• A pair of kidneys lies against the dorsal body wall, leading by ureters to the bladder.
• The reproductive organs (ovaries and uterus in the female; testes in the male) lie in the lower part of the abdomen.

(b) Process of feeding

(i) Hydra: Hydra is carnivorous. When a small animal (e.g. a water flea) touches the tentacles surrounding its mouth, the stinging cells (cnidoblasts/nematocysts) discharge threads that paralyse and hold the prey. The tentacles then push the prey through the mouth into the enteron (gastrovascular cavity), where enzymes secreted by gland cells digest it (extracellular digestion); the products are then taken up by the endodermal cells and digested further inside them (intracellular digestion). Undigested remains are egested through the mouth.

(ii) Amoeba: Amoeba feeds by phagocytosis (holozoic nutrition). When it meets a food particle such as a smaller organism, it flows around it by pushing out pseudopodia which surround and enclose the food together with a little water, forming a food vacuole. Enzymes are secreted into the vacuole to digest the food; the soluble products are absorbed into the cytoplasm, and the undigested matter is left behind (egested) as the Amoeba moves on.

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(a)(i) Courtship behaviour

Courtship behaviour is a series of special activities or displays carried out by an animal (usually the male) to attract a mate of the same species, and to prepare both partners for mating. It includes displays such as special calls, songs, dances, release of scents (pheromones), colour displays, nest building and offering of food.

(a)(ii) How courtship aids reproduction

• It enables members of the same species to recognise and attract one another, preventing wasteful mating between different species.
• It helps each partner to identify the sex of the other, so that a male attracts a female and not another male.
• It ensures that the female is sexually receptive and ready to mate, so that mating and fertilization succeed.
• It synchronises the release of sperms and eggs, increasing the chance of fertilization.
• It reduces aggression between the sexes, so that mating can take place peacefully.
• It allows the female to select a strong, healthy mate, so that fit offspring are produced.

(b) Fertilization in flowering plants

Fertilization is the fusion of the male gamete with the female gamete to form a zygote. In flowering plants it follows pollination and occurs as follows:

1. After pollination, a pollen grain lands on the sticky stigma and absorbs sugary fluid.
2. The pollen grain germinates and grows a pollen tube which is guided by the tube nucleus down through the style towards the ovary.
3. The pollen tube secretes enzymes that digest the tissues of the style, making a path for its growth.
4. Inside the pollen tube the generative nucleus divides to form two male gametes (male nuclei).
5. The pollen tube enters the ovule through a small pore called the micropyle.
6. The tip of the tube bursts and releases the two male gametes into the embryo sac.
7. Double fertilization then occurs: one male gamete fuses with the egg cell (ovum) to form a diploid zygote, which develops into the embryo; the second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus, which develops into the endosperm (food store).
8. After fertilization the ovule develops into a seed and the ovary develops into a fruit.

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(a) Experiment to demonstrate the effect of auxin on the growth of a plant shoot

• Take several similar germinating oat (or maize) seedlings (coleoptiles) grown in the dark.
• From one seedling, cut off the tip of the coleoptile and place it on a small block of agar jelly for a few hours so that the auxin from the tip diffuses into the agar.
• Cut off the tip of a second (decapitated) seedling and place the auxin-containing agar block on one side of the cut stump. Set up a control with a plain agar block (no auxin).
• Keep all the seedlings in the dark (so light is not a variable) and observe over some hours.

Observation: the seedling with the auxin agar on one side bends away from that side (grows/curves), because the side receiving auxin grows faster and elongates more than the other side. The seedling with the plain agar block does not bend or grow.

Conclusion: auxin promotes the elongation (growth) of the cells of the shoot; unequal distribution of auxin causes unequal growth and hence bending. This confirms that auxin controls growth in the shoot.

(b) Three uses of auxin in agriculture

1. It is used as a rooting hormone to promote the growth of roots in stem cuttings.
2. It is used as a selective weed killer (in high concentration it kills broad-leaved weeds).
3. It is used to prevent premature dropping of fruits and to promote fruit development (including seedless fruits).

Also acceptable: to control ripening and to promote flowering.

(c) Mechanism of transmission of an impulse through a nerve fibre

A nerve impulse is transmitted as an electrochemical wave (action potential) along the fibre:

• In a resting nerve fibre, the outside of the membrane is positively charged and the inside negatively charged (the fibre is polarised), because sodium ions are kept outside and potassium ions inside.
• When the fibre is stimulated strongly enough (above threshold), its membrane at that point suddenly becomes permeable to sodium ions, which rush in and reverse the charge (the membrane becomes depolarised). This is the action potential.
• This local change stimulates the next region of the membrane, so the depolarisation, and with it the impulse, passes along the fibre from point to point.
• Behind the impulse the membrane quickly repolarises (potassium ions move out and the ions are pumped back to their resting positions), so the fibre is ready to carry another impulse.
• At the end of the fibre the impulse reaches a synapse, where a chemical neurotransmitter is released to carry the message across the gap to the next neurone.

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(a) Three major zones in the marine habitat with one characteristic feature each

The oceanic/pelagic zone (open surface waters) is also acceptable.

(b)(i) One plant and one animal example in the marine habitat

Plant: a brown seaweed (algae) such as Sargassum. Animal: a bony fish (e.g. herring).

(b)(ii) How each is adapted to the habitat

• Seaweed (plant): it has a holdfast that anchors it firmly to rocks against the pull of waves and currents; its body (thallus) is flexible/leathery so that it bends with the waves without breaking; and it has no true roots or rigid stem since water supports it.
• Fish (animal): it has a streamlined body that reduces resistance while swimming; gills for extracting dissolved oxygen from the water; fins for swimming, steering and balance; and its body fluids/kidneys are adapted to cope with the salinity of sea water.

(c) Three behavioural adaptations of animals to seasonal changes in a terrestrial habitat

1. Migration - moving to more favourable areas during unfavourable seasons.
2. Aestivation - becoming dormant during the hot, dry season.
3. Hibernation - becoming dormant/inactive during the cold season.

(d) Three characteristics each of the plant community of the tropical rain forest and the savanna

Tropical rain forest:

1. Tall trees forming several layers (storeys) with a closed canopy.
2. Trees are evergreen with broad leaves.
3. Presence of many climbers (lianas) and epiphytes; great variety (diversity) of species.

Savanna:

1. Dominated by tall grasses with scattered, short trees.
2. Trees are widely spaced, deciduous, with small leaves and often thorny.
3. Trees show drought adaptations (thick bark, deep roots); fewer species than the forest.