WAEC SSCE - Biology - 2008

Maelezo ya Majibu

Overfishing is a major problem that can lead to the depletion of fish populations in a lake. To avoid overfishing, several measures can be taken. Limiting fishing periods, regulating the size of nets, and fish breeding exercise are all helpful measures to avoid overfishing. However, fishing throughout the year is not a helpful measure, as it can lead to the continuous depletion of fish populations without giving them a chance to recover. Therefore, the answer is "fishing throughout the year."

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The part of the brain that controls and coordinates body posture is the cerebellum. The cerebellum is located at the base of the brain and is responsible for coordinating voluntary movements such as posture, balance, and coordination. It receives information from the sensory systems, the spinal cord, and other parts of the brain, and uses this information to regulate and coordinate movements. Damage to the cerebellum can result in problems with movement and balance, as well as other cognitive and emotional deficits.

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Energy is not recycled in an ecosystem. Energy flows through an ecosystem and is transferred from one organism to another as they eat and are eaten, but it is not recycled. In contrast, carbon, water, and nitrogen are cycled through an ecosystem. Carbon is released through respiration and decomposition, and is taken up by producers during photosynthesis. Water is constantly cycled through the process of evaporation, condensation, and precipitation. Nitrogen is converted between different forms by various organisms, such as bacteria, and is also cycled through the ecosystem.

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Light is likely to affect the distribution of plants in a pond. This is because plants require sunlight for photosynthesis, which is their primary source of energy. The amount of light available in different parts of the pond will determine where plants can grow, and therefore, their distribution in the pond. Plants that require a lot of sunlight will grow in areas of the pond where light is abundant, while those that require less sunlight will grow in areas with less light. The other physical factors listed may also affect plant growth in a pond, but they are less likely to directly impact plant distribution.

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Maelezo ya Majibu

Pond weeds normally lack thick layers of cuticle because there is no danger of becoming desiccated. Cuticle is a waxy layer covering the surface of leaves and other parts of plants to prevent excessive water loss. Pond weeds are submerged in water, so they are not exposed to air and do not need to conserve water like land plants. Therefore, they do not need to develop a thick cuticle layer.

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The presence of glucose in the urine of a patient is an indication of malfunctioning of the pancreas. The pancreas is an organ in the digestive system that produces insulin, a hormone that regulates the level of glucose in the blood. When the pancreas is not functioning properly, it may not produce enough insulin or the body may not respond to the insulin produced, leading to high levels of glucose in the blood. The excess glucose may then be excreted in the urine, a condition known as glycosuria. Therefore, the presence of glucose in the urine is a sign of possible diabetes or other pancreatic disorders.

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The order of evolution trend in plants is Thallophyta, Bryophyta, Pteridophyta, and Spermatophyta. Thallophyta is a group of plants that lack true roots, stems, and leaves, and they include organisms such as algae and fungi. Bryophyta are non-vascular plants that have true roots, stems, and leaves, but they lack a specialized transport system for water and nutrients. Pteridophyta are vascular plants that have specialized tissues for the transport of water and nutrients, and they include ferns and horsetails. Spermatophyta are seed-bearing plants that have evolved further to produce seeds as a means of reproduction. They include gymnosperms and angiosperms. Therefore, the correct order of evolution trend in plants is Thallophyta, Bryophyta, Pteridophyta, and Spermatophyta.

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Filaments of Spirogyra placed in a beaker of water in a dark cupboard died because the filaments could not photosynthesize. Photosynthesis is the process by which green plants, including Spirogyra, convert light energy into chemical energy in the form of glucose. This process requires light, which was absent in the dark cupboard. Without light, the Spirogyra filaments could not produce the food they need to survive, and they eventually died. Conjugation, temperature, and flooding would not have directly caused the death of the Spirogyra filaments in this scenario.

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Maelezo ya Majibu

The correct statement about the transfer of energy in the ecosystem is "Energy is lost at each trophic level". As energy is transferred from one trophic level to the next, some energy is lost in the form of heat and metabolic processes. Therefore, the amount of energy available to support organisms decreases as we move up the trophic levels. This is why there are fewer organisms at the top trophic levels, as there is not enough energy available to support a large population. In addition, since energy is lost at each trophic level, it is not possible to support more organisms at higher trophic levels, contrary to the second statement. The first statement is also incorrect, as energy does not increase at higher trophic levels.

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Many diseases caused by bacteria can be treated and cured by using antibiotics. Antibiotics are drugs that are specifically designed to target and kill bacteria or inhibit their growth. They work by interfering with bacterial processes or structures, such as cell wall synthesis or protein synthesis, that are essential for their survival. This ultimately leads to the death of the bacteria and the resolution of the infection. However, it is important to note that antibiotics are not effective against all types of bacteria, and their overuse or misuse can contribute to the development of antibiotic-resistant bacteria. Therefore, antibiotics should be used only when prescribed by a healthcare professional and taken as directed.

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Maelezo ya Majibu

The activity that constitutes biological control is Tilapia feeding on the larvae and pupae of mosquito. Biological control is a method of managing pests, diseases or weeds using natural enemies, such as predators, parasites, and pathogens. In this case, Tilapia is a natural predator of mosquito larvae and pupae. By feeding on the mosquito larvae and pupae, Tilapia helps to control the mosquito population and reduce the risk of mosquito-borne diseases like malaria, dengue fever, and Zika virus. This method is environmentally friendly and sustainable compared to the use of chemical pesticides, which can have harmful effects on non-target organisms and the ecosystem. The other activities mentioned do not fall under biological control as they do not involve the use of natural enemies to manage pests.

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The process by which the eyes can properly focus images of objects of various distances on the retina is called accommodation. This is achieved by the lens changing its shape to adjust the focal length, allowing light to be properly focused on the retina. When an object is far away, the lens becomes flatter, while it becomes rounder when the object is closer. This change in shape of the lens is controlled by the ciliary muscles, which contract or relax depending on the distance of the object being viewed. Therefore, accommodation is the process by which the eyes can adjust the focus of the lens to provide clear vision at different distances.

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The passage of nerve impulse across a synapse is facilitated by the secretion of acetylcholine. Acetylcholine is a neurotransmitter, a chemical messenger that helps to transmit signals across nerve cells or synapses. When an electrical signal, or nerve impulse, reaches the end of a nerve cell, it triggers the release of acetylcholine from vesicles into the synapse. The acetylcholine then binds to receptors on the adjacent nerve cell, causing an electrical signal to be generated in the receiving cell. This process allows nerve impulses to be transmitted from one cell to the next, enabling communication between different parts of the nervous system.

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The part of the young root that pushes its way through the soil is the root cap. The root cap is a protective structure located at the tip of the root that covers and protects the growing cells behind it. As the root grows and elongates, the root cap cells are constantly being pushed forward and eventually sloughed off, leaving a slimy lubricant that helps the root move through the soil. The root cap also senses gravity and helps to direct the growth of the root downward. Therefore, the correct answer is root cap.

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In the binomial system of naming organisms, the second name is called the specific name. The binomial system of naming organisms involves giving each organism a two-part scientific name: the genus name and the specific name. The specific name refers to the unique species within the genus. For example, in the scientific name for humans, Homo sapiens, "sapiens" is the specific name, which distinguishes humans from other species within the Homo genus.

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The process of regulating the amount of water and solutes in the body fluids is called osmoregulation. Osmoregulation is a vital biological process that helps maintain the proper balance of water and dissolved substances, or solutes, in the body's fluids. This process is necessary because the body needs to maintain a stable internal environment, even when external conditions, such as water availability, change. Osmoregulation involves the control of water and solutes across cell membranes, as well as the balance of water and solutes in the blood and other body fluids. Osmosis and diffusion are related processes but do not refer specifically to the regulation of body fluids, while homeostasis is a broader term that refers to the maintenance of a stable internal environment in the body.

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The wilting of the plant is due to the fact that the salt solution has caused water loss from the plant cell, leading to the loss of turgor. Normally, water moves from a region of high concentration to a region of low concentration, which is the plant cell. However, the concentrated salt solution has a higher concentration than the plant cell, causing water to move out of the cell, resulting in the loss of turgor pressure. When the plant loses turgor, the leaves and stem begin to wilt. The high concentration of salt in the soil can also lead to the death of the root hairs, which are responsible for absorbing water and minerals from the soil.

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The impedance (arrangement or structure) of the concentration of chloroplast in the palisade layer of the leaf is for maximum exposure to light. The palisade layer of the leaf contains the highest concentration of chloroplasts, which are responsible for photosynthesis. Chloroplasts contain chlorophyll, a pigment that absorbs light energy and converts it into chemical energy that can be used by the plant. The arrangement of chloroplasts in the palisade layer is optimized for maximum exposure to light. The chloroplasts are located near the upper surface of the leaf, where they can receive the most sunlight. Additionally, the elongated shape of the palisade cells allows them to be packed closely together, increasing the surface area for absorption of light. During photosynthesis, the chloroplasts use the energy from sunlight to convert carbon dioxide and water into glucose and oxygen. The glucose is used by the plant as a source of energy, while the oxygen is released into the atmosphere. In summary, the impedance of the concentration of chloroplast in the palisade layer of the leaf is optimized for maximum exposure to light. The arrangement of chloroplasts in the palisade layer is designed to absorb as much sunlight as possible for photosynthesis, which converts carbon dioxide and water into glucose and oxygen.

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The statement "Ability to taste the chemical is hot inheritable" is not correct. Phenylthiocarbamide (PTC) is a chemical compound that some people perceive as intensely bitter, while others do not taste it at all. The ability to taste PTC is determined by genetics, and the trait for PTC tasting is inherited. However, it is not a dominant trait, meaning that someone may inherit the gene for tasting PTC from one parent, but not be able to taste it because they also inherited a non-tasting gene from the other parent. Therefore, the correct statement is that the ability to taste the chemical is inheritable, but it is not a dominant trait, so some people may not be able to taste it even if they inherit the gene for tasting it.

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Maelezo ya Majibu

The resemblance of a palatable insect to a poisonous insect is an example of mimicry. Mimicry is a type of defense mechanism in which an organism mimics the appearance or behavior of another organism, usually one that is more dangerous or less palatable, to deter predators from attacking. In this case, the palatable insect has evolved to resemble the poisonous insect, which serves as a warning signal to predators that it is not a good choice for food. This way, the palatable insect avoids being eaten, while the predator avoids the risk of poisoning.

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The process that is not an example of a reflex action is reading. A reflex action is an automatic and rapid response to a stimulus that does not involve conscious thought. Blinking of the eye, beating of the heart, and sneezing are all examples of reflex actions because they occur automatically in response to certain stimuli without the need for conscious thought. However, reading is a voluntary and conscious process that involves the interpretation of symbols and the comprehension of their meaning, and it does not involve an automatic response to a stimulus.

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The carpals do not have a protective function. Carpals are a group of small bones found in the wrist joint. They are involved in wrist movement and provide attachment points for muscles that control finger movement. Unlike the other options listed, the carpals do not serve a primarily protective function. The ribs protect the thoracic cavity and its contents, including the heart and lungs. The skull protects the brain and the sense organs, while the pelvis protects the reproductive organs and the bladder.

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A community is best described as populations of different species living together and interacting with each other in one habitat. It refers to all the different living organisms that inhabit a particular area and how they interact with each other and with the non-living parts of the environment. These interactions can include competition for resources such as food and shelter, as well as cooperation in the form of mutualism or symbiosis. A community can be made up of different populations of animals, plants, fungi, and microorganisms that all play a role in shaping the ecosystem they inhabit.

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Color blindness is commonly observed in human males and is sex-linked. Color blindness is a genetic disorder that affects how people see color. It is caused by an abnormality in the genes that encode for the photopigments in the cone cells of the retina. Because these genes are located on the X chromosome, which is one of the two sex chromosomes, the disorder is sex-linked. Males only have one X chromosome, so if they inherit a color blindness gene on that chromosome, they will have the disorder. Females, on the other hand, have two X chromosomes, so they are less likely to have the disorder, as they need to inherit the gene on both X chromosomes to be affected.

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The effective cells involved in sickle-cell anemia are erythrocytes, which are also known as red blood cells. In sickle-cell anemia, the shape of the erythrocytes becomes distorted, and they become sickle-shaped instead of their normal round shape. This sickle shape makes them less flexible, and they tend to clump together and block blood vessels, leading to various complications. The sickle shape is caused by a mutation in the hemoglobin gene, which affects the ability of the red blood cells to carry oxygen efficiently.

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The speed of the flow of water in a river is faster in the middle than along the banks due to the resistance offered by the wall of the banks. As water flows downstream, it encounters friction and resistance from the riverbed and banks. The friction between the water and the banks causes the velocity of the water to decrease, resulting in slower flow along the banks. In contrast, the middle of the river has less friction and resistance, allowing water to flow more freely and at a faster rate. Therefore, the speed of the flow of water in a river is faster in the middle than along the banks due to the difference in resistance between the banks and the open water.

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Parthenocarpy is the process by which flowers produce fruit without the process of fertilization. This means that the fruit is produced without the involvement of male gametes or pollination. The fruit produced by parthenocarpy is usually seedless or contains very few undeveloped seeds. This process is used in some agricultural practices to produce fruits that are of higher quality, have longer shelf lives, and are more visually appealing to consumers. Examples of parthenocarpic fruits include bananas, seedless grapes, and seedless watermelons.

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Anemometer is used to measure wind speed. Anemometer is an instrument that has cups mounted on a rotating shaft. As the wind blows, the cups rotate, and the speed of the wind is measured by how fast the cups rotate. Therefore, anemometer is the correct answer.

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The accepted concepts for the theory of Natural Selection do not include the use and disuse of body parts. The theory of Natural Selection, proposed by Charles Darwin, explains how evolution occurs in populations of living organisms. It includes three main concepts: the tendency for organisms to overproduce, the struggle for existence, and survival of the fittest. The tendency for organisms to overproduce means that more offspring are produced than can survive to adulthood. The struggle for existence refers to the competition for resources such as food, water, and shelter. Survival of the fittest means that the individuals with the most favorable traits for their environment are more likely to survive and reproduce, passing on those traits to their offspring. The use and disuse of body parts was a concept proposed by Jean-Baptiste Lamarck, but it was later shown to be incorrect and not supported by scientific evidence.

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The character that is not an example of discontinuous variation is skin color. Discontinuous variation refers to traits that fall into distinct categories with no intermediate values, like sex type or ability to roll tongue. On the other hand, skin color is an example of continuous variation, where there is a range of values and no distinct categories. Fingerprint is also an example of continuous variation.

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Maelezo ya Majibu

Heights of a class of school children may give an approximate normal distribution curve when the variation is plotted. This is because height is a continuous trait that is influenced by multiple genes and environmental factors, leading to a wide range of possible values. When plotted on a graph, the distribution of heights typically follows a bell-shaped curve with most individuals falling close to the average height and fewer individuals on either end of the spectrum (very tall or very short). This is known as a normal distribution curve, and it is commonly seen in traits that are influenced by many different factors, leading to a wide range of possible values. Blood groups, ear shapes, and sex distribution, on the other hand, are discrete traits with only a few possible values, and therefore would not typically follow a normal distribution curve when plotted.

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The correct answer is "insulin". Insulin is a hormone found in animals and is involved in regulating blood sugar levels. The other options, auxin, cytokinin, and gibberellin, are all plant hormones involved in various processes such as cell elongation, cell division, and seed germination.

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The animal that was analyzed to have small pieces of bone, seeds, roughages, and insects in its gut content is most likely to be an omnivore. An omnivore is an animal that consumes both plants and animals. The presence of small pieces of bone suggests that the animal consumes meat, while the presence of seeds and roughages suggests that it consumes plants. The presence of insects in its gut content further supports the idea that it is an omnivore since many omnivorous animals consume insects as well.

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The structure labeled "I" refers to the head of the humerus bone in the upper arm. In the scapula, which is a bone in the shoulder blade, the structure that receives the head of the humerus is called the glenoid cavity. The head of the humerus fits into the glenoid cavity, creating a ball and socket joint. This joint allows for a wide range of movements in the arm, including rotation, flexion, and extension.

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Adding more water to the soil would revive the plant. When a plant is dehydrated, it can wilt and become unable to perform vital functions such as photosynthesis. Adding water to the soil will replenish the water supply for the plant and allow it to recover. Covering the plant with dark paper to prevent further water loss may help to slow down dehydration, but it will not provide the necessary water for the plant's recovery. Leaving the plant to adapt to the condition may cause further damage or death. Exposing the plant to higher concentrations of oxygen may not help revive it, as oxygen is not the limiting factor in plant survival.

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The sudden change in a gene structure or chromosome number in an organism that is likely to cause an inheritable change in the phenotype is known as mutation. A mutation is a permanent alteration in the DNA sequence that makes up a gene. This can occur spontaneously or as a result of exposure to mutagens such as radiation, chemicals or viruses. Mutations can affect the phenotype of an organism by changing its physical, biochemical or behavioral characteristics. Some mutations can be beneficial, neutral, or harmful to an organism's survival and reproductive success, depending on the environmental conditions. Mutations can also be passed down to offspring through the genetic material, making them inheritable.

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Phenotype refers to the observable characteristics of an individual that result from the interaction between their genetic makeup (genotype) and environmental factors. This includes physical traits like eye color and height, as well as behavioral and physiological traits. The phenotype can be influenced by a variety of environmental factors such as nutrition, exposure to toxins, and lifestyle choices. For example, an individual's height is influenced by their genes, but can also be affected by factors such as diet and exercise.

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Cabbage gives the least amount of calories among the listed food substances. This is because cabbage is a low-calorie vegetable with a high water and fiber content and a low carbohydrate and fat content. It contains approximately 25 calories per 100 grams, while rice contains about 130 calories per 100 grams, groundnuts contain about 567 calories per 100 grams, and egg yolk contains about 322 calories per 100 grams. Therefore, if you are looking to reduce your calorie intake, cabbage is a good food choice to consider.

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Maelezo ya Majibu

A vaccine is introduced into a person's body to stimulate the body to produce antibodies. Vaccines contain weakened or dead pathogens that cannot cause disease, but can still trigger an immune response in the body. This means that when the person is exposed to the actual pathogen, their immune system will recognize it and quickly produce the necessary antibodies to fight the infection. By stimulating the body to produce antibodies in advance, vaccines can help prevent or reduce the severity of certain diseases.

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The occurrence that is not a feature of meiosis is the formation of two diploid cells. Meiosis is a type of cell division that occurs in sexually reproducing organisms to produce gametes, such as sperm and eggs. Meiosis involves two successive nuclear divisions, resulting in the formation of four haploid cells, each containing half the number of chromosomes as the original parent cell. During meiosis, homologous chromosomes pair up during prophase I and exchange genetic material in a process called crossing over. This results in genetic variation among the haploid cells produced. Therefore, the correct answer is the formation of two diploid cells, which is not a feature of meiosis.

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The chemical composition of the genetic material of all living organisms consists of Deoxyribonucleic acid (DNA). DNA is a complex molecule that contains the genetic instructions used in the development and function of all living organisms. The DNA molecule is made up of four types of nucleotides: adenine, guanine, cytosine, and thymine. These nucleotides are arranged in a specific sequence, which determines the genetic code. The sequence of nucleotides in DNA is responsible for the diversity of life on Earth, as it determines the characteristics and traits of living organisms. DNA is found in the nucleus of cells and is organized into structures called chromosomes. Humans have 23 pairs of chromosomes, while other organisms may have more or fewer chromosomes. Each chromosome contains many genes, which are specific segments of DNA that code for a particular trait or characteristic. In summary, the genetic material of all living organisms consists of Deoxyribonucleic acid (DNA), a complex molecule made up of four types of nucleotides arranged in a specific sequence. DNA is responsible for the diversity of life on Earth, and determines the genetic code, characteristics, and traits of living organisms.

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Conservation can be promoted by encouraging tree planting. Trees are important for the environment as they provide oxygen, absorb carbon dioxide, prevent soil erosion, and provide habitats for wildlife. Tree planting programs are a way to combat deforestation and help to restore degraded ecosystems. Encouraging tree planting can also have economic benefits. Trees can be harvested for timber and other forest products, which can provide a source of income for local communities. In addition, trees can be used for agroforestry, where crops are grown in the shade of trees, which can help to increase crop yields and improve soil fertility. By promoting tree planting, we can help to mitigate the negative impacts of deforestation, which can lead to loss of biodiversity, soil erosion, and climate change. Tree planting is a simple and effective way to promote conservation and sustainable land use practices. In summary, conservation can be promoted by encouraging tree planting. Trees provide numerous environmental and economic benefits, and by planting more trees, we can help to mitigate the negative impacts of deforestation and promote sustainable land use practices.

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Primary growth in plants is brought about by the activity of the meristem. Meristem is a specialized tissue in plants that contains undifferentiated cells that can divide and differentiate into various plant cell types. These cells are responsible for the growth in length of roots and shoots in a process known as primary growth. The meristem is located at the tips of the roots and shoots and is responsible for the formation of new tissues that elongate and expand the plant body. As the meristem cells divide, some become part of the root cap, which protects the root as it grows through the soil, while others become part of the apical meristem, which gives rise to the primary tissues of the root and shoot. Therefore, meristem is responsible for the continuous growth of the plant throughout its life cycle.

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(a) Explanation of terms

• (i) Population - A population is the total number of organisms of the same species living together in a given habitat at a particular time, and which can interbreed. Example: all the tilapia fish in one pond.
• (ii) Competition - Competition is the struggle among organisms for the same limited resources such as food, water, light, space and mates. It may be intraspecific (between members of the same species) or interspecific (between members of different species).
• (iii) Succession - Succession is the gradual and orderly replacement of one community of organisms by another in a given area over a period of time, until a stable community (the climax community) is finally established.

(b)(i) Three sources of noise pollution

• Vehicles and heavy traffic (cars, lorries, motorcycles) and aircraft engines.
• Industrial machinery in factories.
• Loud music from public address systems, loudspeakers and entertainment houses (also generators and construction equipment).

(b)(ii) Two ways of reducing noise pollution

• Fitting machines, engines and vehicles with silencers or mufflers, and servicing them regularly.
• Enforcing laws that ban or limit loud noise, and planting trees or building walls that act as sound barriers around noisy areas.

(c)(i) Three effects of smoke on humans

• It irritates the respiratory tract and causes coughing and breathing difficulty (bronchitis, asthma).
• It reduces the oxygen-carrying capacity of the blood (carbon monoxide in smoke combines with haemoglobin).
• It can cause lung cancer and other lung diseases, and irritates the eyes.

(c)(ii) Three effects of sewage on humans

• It contaminates drinking water and spreads water-borne diseases such as cholera, typhoid and dysentery.
• It provides a breeding ground for disease vectors such as houseflies and mosquitoes.
• It produces offensive smells and can cause food poisoning and infection by parasitic worms.

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(a) Main stages of mitotic division

Mitosis is a type of nuclear division that produces two daughter cells each having the same number of chromosomes as the parent cell. Before it begins (interphase) the chromosomes replicate so that each consists of two chromatids joined at a centromere. Mitosis then proceeds through four main stages:

1. Prophase - The chromosomes shorten, thicken and become visible, each seen as two chromatids joined at a centromere. The centrioles move to opposite poles and spindle fibres begin to form. Towards the end, the nuclear membrane and nucleolus break down and disappear.
2. Metaphase - The spindle is fully formed. The chromosomes line up singly along the equator (metaphase plate) of the spindle, each attached to a spindle fibre by its centromere.
3. Anaphase - The centromere of each chromosome splits, and the two chromatids (now called daughter chromosomes) are pulled apart to opposite poles of the spindle by the shortening spindle fibres.
4. Telophase - The daughter chromosomes reach the poles, lengthen and become indistinct. A new nuclear membrane forms round each group and the nucleolus reappears, giving two daughter nuclei. This is followed by cytokinesis, the division of the cytoplasm into two daughter cells.

(b) Four ways in which mitosis is important to living organisms

• It brings about growth by increasing the number of body (somatic) cells.
• It is used in the repair of worn-out tissues and healing of wounds by replacing damaged or dead cells.
• It is the basis of asexual reproduction in many organisms (for example binary fission, budding and vegetative propagation).
• It ensures genetic stability, because each daughter cell receives exactly the same number and type of chromosomes as the parent cell, maintaining the chromosome number of the species.

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(a) Hormones

Hormones are chemical substances (organic compounds) produced in very small quantities by the endocrine (ductless) glands of the body, and carried by the bloodstream to specific target organs where they regulate and coordinate the body's activities, growth and development.

(b) Four differences between nervous and hormonal coordination

(c)(i) Three effects of over-secretion of thyroxin

• Raised metabolic rate leading to loss of body weight despite increased appetite.
• Rapid or irregular heartbeat, restlessness, nervousness and excitability.
• Protrusion of the eyeballs and enlargement of the thyroid gland (exophthalmic goitre / Grave's disease).

(c)(ii) Three effects of under-secretion of thyroxin

• Lowered metabolic rate leading to gain in body weight and sluggishness.
• In children, retarded physical and mental growth (cretinism).
• In adults, swelling of the tissues, dry skin and mental dullness (myxoedema); enlargement of the thyroid (simple goitre) where iodine is lacking.

(d) Diagram of the reflex arc

A labelled diagram, 10 - 12 cm long, should show a spinal-cord reflex arc with these labelled parts arranged in order: receptor in the skin, sensory (afferent) neurone with its cell body in the dorsal root ganglion, dorsal root, the grey matter and white matter of the spinal cord with a relay (intermediate) neurone, synapses, motor (efferent) neurone leaving by the ventral root, and the effector (muscle). Arrows should show the direction of the impulse from receptor to effector.

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(a) One function of each structure

• (i) Arteries: carry blood at high pressure away from the heart to the body organs.
• (ii) Capillaries: allow exchange of materials (oxygen, food, carbon dioxide, wastes) between the blood and the body cells.
• (iii) Veins: carry blood at low pressure back to the heart (their valves prevent backflow).
• (iv) Xylem: conducts water and dissolved mineral salts from the roots up to the leaves (and gives support).
• (v) Phloem: conducts (translocates) manufactured food such as sucrose from the leaves to other parts of the plant.

(b) Changes in the composition of blood as it passes through the

• (i) Lungs: blood gains oxygen and loses carbon dioxide (becomes oxygenated).
• (ii) Liver: excess glucose is removed and stored as glycogen so blood sugar is regulated; amino acids are deaminated so urea is added; toxins are removed; blood may gain heat.
• (iii) Kidneys: urea, excess salts and water are removed, so the blood leaving is purified of nitrogenous waste.
• (iv) Small intestine: blood gains digested food such as glucose, amino acids, vitamins and mineral salts (it becomes rich in nutrients after a meal).

(c) Cellular components of blood

1. Red blood cells (erythrocytes)
2. White blood cells (leucocytes)
3. Platelets (thrombocytes)

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(a) Modes of nutrition in plants

• (i) Autotrophic nutrition - This is a mode of nutrition in which an organism manufactures its own organic food (such as glucose) from simple inorganic raw materials. In green plants this is done by photosynthesis, using carbon dioxide and water in the presence of sunlight and chlorophyll.
• (ii) Chemosynthetic nutrition - This is a form of autotrophic nutrition in which an organism synthesises its own food from inorganic materials using energy obtained from chemical reactions (the oxidation of inorganic substances) rather than from sunlight.
• (iii) Carnivorous (insectivorous) nutrition - This is a mode of nutrition in certain green plants that trap and digest small animals, especially insects, to obtain nitrogenous nutrients. Such plants photosynthesise but grow in nitrogen-poor soils, so they supplement their nitrogen supply by digesting the trapped animals.

(b) One example of a plant for each mode

• Autotrophic: maize (or any green plant such as hibiscus).
• Chemosynthetic: nitrifying bacteria such as Nitrosomonas (or Nitrobacter).
• Carnivorous: pitcher plant (Nepenthes) or sundew (Drosera); Venus fly trap.

(c)(i) Three ways nitrogen is added to the soil for plant use

• Nitrogen fixation - free-living and symbiotic nitrogen-fixing bacteria (for example Rhizobium in the root nodules of legumes) convert atmospheric nitrogen into nitrogenous compounds.
• Decomposition - decomposers (bacteria and fungi) break down dead plants, animals and their waste, releasing ammonium compounds which are nitrified to nitrates.
• Lightning and rainfall - electrical discharge during thunderstorms combines atmospheric nitrogen and oxygen into oxides that dissolve in rain and reach the soil as nitrates (application of nitrogenous fertilisers and manure is another way).

(c)(ii) Two ways nitrogen is lost from the soil

• Denitrification - denitrifying bacteria convert soil nitrates back into free nitrogen gas which escapes into the atmosphere.
• Leaching - soluble nitrates are washed down beyond the reach of roots by percolating rain water (uptake by plants and removal at harvest, and loss by soil erosion, are others).

Maelezo ya Majibu

(a) Transpiration

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves, but also through the cuticle and lenticels. The evaporation of water from the moist surfaces of the mesophyll cells into the air spaces, followed by diffusion of the vapour out through the stomata, creates a suction pull that draws water up the plant.

(b)(i) Two types of transpiration

• Stomatal transpiration - loss of water vapour through the stomata (the most important, accounting for the greatest proportion).
• Cuticular transpiration - loss of water vapour directly through the cuticle of the leaf and stem surface.

(Lenticular transpiration, through the lenticels of woody stems, is a third minor type.)

(b)(ii) Two factors affecting the rate of transpiration

• Temperature - a rise in temperature increases evaporation and so raises the rate.
• Humidity - high atmospheric humidity lowers the rate because the vapour-pressure gradient between the leaf and the air is reduced.

(Others include light intensity, wind or air movement, and available soil water.)

(c) Mechanism of opening and closing of the stomata

Each stoma is bordered by two guard cells whose inner walls (next to the pore) are thick and inelastic while the outer walls are thin and elastic. Opening and closing depend on changes in the turgor of these guard cells, controlled by movement of ions and sugars.

Opening (usually in daylight): In light, guard cells photosynthesise and potassium ions (K⁺) are actively pumped into the guard cells. This lowers the osmotic potential of the guard cells, so water enters them by osmosis from the neighbouring epidermal cells. The guard cells become turgid; because the thin outer wall stretches more than the thick inner wall, the guard cells curve outward and the pore between them opens.

Closing (usually at night or when water is scarce): K⁺ ions move out of the guard cells, raising their osmotic potential, so water leaves them by osmosis. The guard cells become flaccid, lose their curvature, straighten and come together, closing the pore.

(d)(i) Two similarities between transpiration and sweating

• Both involve the loss of water from the body of the organism to the environment.
• Both bring about cooling of the organism by the evaporation of water.

(d)(ii) Two differences between transpiration and sweating