WAEC SSCE - Biology - 2019

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The part labelled I in a cell is the centromere. The centromere is a specialized region on a chromosome where the two identical sister chromatids are joined together. During cell division, the spindle fibers attach to the centromere and pull the sister chromatids apart, ensuring that each new cell receives one complete set of chromosomes. Therefore, the centromere plays a crucial role in cell division and the distribution of genetic material to the daughter cells.

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Maelezo ya Majibu

Maelezo ya Majibu

The function of the part labelled II is that it controls many involuntary actions of the body. The part labelled II refers to the medulla oblongata, which is a part of the brainstem that connects the brain to the spinal cord. The medulla oblongata plays a crucial role in regulating various vital involuntary functions of the body, such as blood pressure, breathing, heart rate, and digestion. It also controls reflexes such as coughing, sneezing, and vomiting. Additionally, the medulla oblongata helps to regulate the body's internal environment by controlling the levels of certain chemicals in the blood, such as oxygen, carbon dioxide, and hydrogen ions. Therefore, the correct answer is that the medulla oblongata controls many involuntary actions of the body.

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Deficiency in insulin could lead to diabetes. Insulin is a hormone produced by the pancreas that helps regulate the amount of glucose (sugar) in the blood. In people with diabetes, the pancreas either does not produce enough insulin or the body cannot effectively use the insulin that is produced. When there is not enough insulin in the body, glucose builds up in the bloodstream instead of being absorbed by cells to provide energy. This can lead to high blood sugar levels, which can cause a range of health problems over time, including damage to the kidneys, nerves, eyes, and cardiovascular system. Therefore, insulin plays a crucial role in regulating blood sugar levels, and a deficiency in insulin can lead to diabetes.

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The factor "Mitosis" is not a source of variation in living organisms. Variation in living organisms refers to differences between individuals within a population. There are several sources of variation, including inheritance and the environment. Inheritance refers to the transfer of genetic information from parents to offspring through the genes they pass on. The environment also plays a role in determining the traits and characteristics of an organism, through factors such as diet, temperature, and exposure to toxins. Mitosis, on the other hand, is a type of cell division that results in the formation of two identical daughter cells, each with the same genetic information as the parent cell. Mitosis is not a source of variation because it simply results in the duplication of existing genetic information, rather than the introduction of new or different genetic information.

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The major mineral present in the shell of molluscs is Calcium. Molluscs are a group of organisms that includes snails, clams, and mussels. Their shells serve as a protective covering and are made of calcium carbonate, which is a mineral that is abundant in seawater. Calcium is essential for the formation and hardening of the shells and is continuously incorporated into the shells as the mollusc grows. This helps to maintain the strength and structural integrity of the shell and provides protection from predators, extreme temperatures, and other environmental factors.

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Maelezo ya Majibu

In the pyramid of energy illustrated above, the organism with the least amount of transferred energy is the bird. The pyramid of energy is a graphical representation of the flow of energy in an ecosystem. It shows the amount of energy that is transferred from one trophic level to another in the form of food. At each level, some energy is lost in the form of heat or used for metabolic processes, so the amount of energy available to the next level is always less than the previous level. In the given pyramid, the producers (Spirogyra) have the most energy, and energy decreases as we move up the pyramid through the different trophic levels. The tadpole and fish have more energy than the bird because they are at lower trophic levels and have more energy available to them from the producers. The bird, being at the highest trophic level, has the least amount of energy because it has to expend energy to hunt and consume prey, and much of the energy in the prey is lost as heat or used for metabolic processes before it reaches the bird. Therefore, the bird has the least amount of transferred energy in the given pyramid of energy.

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The role that is not performed by a government agency responsible for forest maintenance is regulating mining activities in the forest. A government agency responsible for forest maintenance is tasked with managing and protecting forest resources in a sustainable manner. This includes activities such as regulating the harvest of timber and poaching, creating and managing forest reserves, and tracking the movement of timber, wood, and wildlife. However, regulating mining activities in the forest is not typically a responsibility of a government agency responsible for forest maintenance. This is usually the responsibility of a separate agency, such as a department of mining or natural resources. Mining activities can have a significant impact on forest ecosystems, including deforestation, soil erosion, and water pollution. As a result, regulating mining activities in the forest requires specialized expertise and resources, which are often not within the purview of a government agency responsible for forest maintenance. Therefore, the role that is not typically performed by a government agency responsible for forest maintenance is regulating mining activities in the forest.

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Individuals with blood group AB can donate blood to individuals with blood group AB only. This is because blood group AB contains both A and B antigens on the surface of the red blood cells, and therefore does not produce antibodies against either A or B blood types. As a result, individuals with blood group AB can receive blood from any blood type that does not have antibodies against them, which is only the blood group AB. On the other hand, individuals with blood group O are known as universal donors because they do not have A or B antigens on their red blood cells, and therefore their blood can be donated to individuals with any blood type.

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An example of organ level of organization is the kidney. The organ level of organization is the level at which different tissues come together to form organs that perform specific functions in the body. The kidney is an organ that is made up of different types of tissues, including epithelial tissue, connective tissue, and muscle tissue. These tissues work together to perform the vital function of filtering waste products from the blood and regulating the body's fluid balance. The kidney contains functional units called nephrons that consist of various structures such as the glomerulus, proximal tubule, loop of Henle, and distal tubule, each with a specific function. Therefore, the kidney is an excellent example of the organ level of organization in the body.

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Maelezo ya Majibu

The rate of diffusion of molecules between two media will be higher if the difference in concentration of the two media is high. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration until they reach equilibrium. The rate of diffusion is directly proportional to the concentration gradient, which is the difference in concentration between the two regions. If the concentration gradient is high, that means there is a large difference in concentration between the two regions, which creates a steeper gradient for molecules to move down. As a result, the rate of diffusion will be faster because the molecules will move more rapidly to reach equilibrium. On the other hand, if the concentration gradient is low or negligible, that means there is little or no difference in concentration between the two regions, and the gradient for molecules to move down is less steep. Consequently, the rate of diffusion will be slower as the movement of molecules will be less rapid. If there is no difference in concentration between the two media, there will be no concentration gradient and no movement of molecules, and therefore, no diffusion.

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The main function of the amniotic fluid is for the protection of the foetus from shock. The amniotic fluid is a clear, colorless liquid that surrounds the foetus during pregnancy. It acts as a cushion to protect the foetus from any external pressure or shocks that might occur during pregnancy. The amniotic fluid also helps to regulate the temperature around the foetus and provides a medium for the foetus to move and develop. Additionally, the amniotic fluid also plays a role in the development of the foetus's respiratory and digestive systems. However, its primary function is to protect the foetus from any physical shocks or bumps that might occur inside the mother's womb. Therefore, the correct answer is that the main function of the amniotic fluid is for the protection of the foetus from shock.

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The cross between RrTt and rrtt will result in 50% of the offspring being tall with red fruits. The capital R and T represent dominant alleles for red colour and tallness, respectively, while the small r and t represent recessive alleles for white colour and shortness, respectively. When RrTt (heterozygous for both traits) is crossed with rrtt (homozygous recessive for both traits), the possible gametes for RrTt are RT, Rt, rT, and rt, while the gametes for rrtt are rt only. When the gametes combine, we get the following possible genotypes for the offspring: RT, Rt, rT, and rt. Of these, RT and rT will result in tall plants because they contain at least one dominant allele for tallness, while Rt and rt will result in short plants. Similarly, the genotypes RT and Rt will result in red fruits, while rT and rt will result in white fruits because red color is dominant over white. So, the possible genotypes for the offspring are: RT, Rt, rT, and rt, with the phenotypic ratio being 1:1:1:1 for tall with red fruits, tall with white fruits, short with red fruits, and short with white fruits, respectively. Therefore, 50% of the offspring will be tall with red fruits, which is the correct answer.

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The conversion of ammonium salts into nitrates is called nitrification. Nitrification is a process in the nitrogen cycle in which ammonia (NH3) is oxidized to nitrite (NO2-) and then to nitrate (NO3-) by bacteria. This process occurs in soil, water, and wastewater treatment plants, where bacteria play a crucial role in converting ammonium ions (NH4+) into nitrates (NO3-), which can be used by plants as a source of nitrogen. The first step in nitrification is the conversion of ammonium ions into nitrite ions by bacteria called Nitrosomonas. The second step is the conversion of nitrite ions into nitrate ions by bacteria called Nitrobacter. This process releases energy that the bacteria use for their metabolism. Nitrification is important because it provides a source of nitrogen for plants and other organisms that cannot fix nitrogen from the air. It also helps to reduce the amount of ammonium ions in the soil, which can be toxic to plants at high concentrations. The process of nitrification is also a key step in the removal of nitrogen from wastewater treatment plants before it is discharged into the environment.

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The biological importance of the dance by the worker honeybee is that it signals the availability of food. Honeybees are social insects that live in large colonies. They have a complex system of communication that allows them to share information about the location of food sources. When a worker honeybee finds a food source, such as a flower, it performs a dance to communicate the location of the food to other worker bees in the colony. The dance involves a series of movements that indicate the direction and distance of the food source relative to the colony. By performing this dance, the worker bee is able to alert other worker bees to the availability of food, which helps to ensure that the colony has access to a reliable source of nutrition. In summary, the biological importance of the dance by the worker honeybee is that it signals the availability of food, allowing other worker bees in the colony to be alerted to its presence and ensuring the colony has access to a reliable source of nutrition.

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Anaerobic respiration in the organism illustrated in the diagram above produces carbon dioxide and ethanol. Anaerobic respiration is a type of cellular respiration that occurs in the absence of oxygen. It is also known as fermentation and is an important metabolic pathway in some organisms, such as yeasts and bacteria. During anaerobic respiration, glucose is broken down into pyruvate, which is then converted into ethanol and carbon dioxide in the absence of oxygen. The process of anaerobic respiration produces energy in the form of ATP, which the organism can use for various cellular activities. The production of ethanol and carbon dioxide during anaerobic respiration is also used in various industrial processes, such as brewing and baking. Overall, anaerobic respiration is a crucial process that enables some organisms to survive and thrive in environments where oxygen is limited.

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Maelezo ya Majibu

Tall trees with unbranched trunks which grow above the upper storey of the forest are commonly referred to as emergents. Emergents are a type of tree that grow above the canopy layer of a forest. They typically have unbranched trunks with a small crown of leaves at the top, which allows them to access more sunlight and better compete for resources. Emergents are found in tropical rainforests, where they can grow up to 70 meters or more in height. Emergent trees play an important role in the ecosystem by providing habitat and food for a variety of animals, as well as helping to regulate the climate by absorbing carbon dioxide from the atmosphere. Because they grow above the canopy layer, they also have a unique view of the forest and provide a different perspective on the ecosystem as a whole. Therefore, tall trees with unbranched trunks which grow above the upper storey of the forest are commonly referred to as emergents.

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The specific role of a species within its environment is known as its niche. A niche refers to the way an organism interacts with its environment, including the biotic (living) and abiotic (non-living) factors. It describes the specific set of resources that a species needs to survive, grow, and reproduce, as well as its interactions with other species, such as predation, competition, and mutualism. The niche of a species is determined by a combination of its physical and biological requirements, such as food, water, shelter, temperature, and reproduction. For example, the niche of a hummingbird includes feeding on nectar from flowers and hovering in mid-air to collect it, which requires specialized beaks and wings. The concept of niche is important in ecology because it helps to explain how different species coexist in the same ecosystem without competing for the same resources. Each species occupies a unique niche that allows it to avoid competition with other species and to use the resources available in a specific way. If two species have the same niche, they will compete for the same resources, and one may be driven to extinction or forced to evolve to occupy a different niche.

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Cholera is mostly spread by contaminated water. Cholera is a bacterial infection that affects the small intestine, and it is caused by the bacterium Vibrio cholerae. When a person consumes contaminated water or food, they can become infected with the bacterium and develop cholera. Cholera can spread quickly and easily in areas with poor sanitation and hygiene practices. When people use contaminated water sources for drinking, cooking, or washing, they can become infected with cholera. Additionally, cholera can be spread through the fecal-oral route, which means that if an infected person does not practice good hygiene, they can spread the bacterium to others through their feces. In summary, cholera is mostly spread by contaminated water, and it can also be spread through poor hygiene practices and the fecal-oral route. It is important to practice good sanitation and hygiene practices to prevent the spread of cholera.

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Maelezo ya Majibu

The structure that will NOT be found in the nucleus of a cell is lysosome. The nucleus is a membrane-bound organelle found in eukaryotic cells that contains genetic material in the form of DNA. It is typically surrounded by a double membrane called the nuclear envelope, which has pores that allow for the exchange of materials between the nucleus and the cytoplasm. The nucleolus is a substructure within the nucleus that is involved in the production of ribosomes. It is made up of proteins and RNA, and is not surrounded by a membrane. Centrioles are cylindrical structures made up of microtubules that play a role in cell division. They are found in pairs, and are typically located near the nucleus of the cell. Lysosomes, on the other hand, are membrane-bound organelles that contain digestive enzymes. They are involved in breaking down and recycling cellular waste and foreign materials that have been taken up by the cell. Therefore, of the structures listed, lysosome is the one that will NOT be found in the nucleus of a cell. The other structures, including DNA, nucleolus, and centrioles, are all either found within the nucleus or in close proximity to it.

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Maelezo ya Majibu

Maelezo ya Majibu

Maelezo ya Majibu

The part labelled I in the diagram above is the cerebrum. The cerebrum is the largest part of the brain and is responsible for many important functions, such as consciousness, sensation, movement, perception, and learning. It is divided into two hemispheres, the left and right hemispheres, which are connected by a bundle of nerve fibers called the corpus callosum. The cerebrum is also divided into four lobes: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. Each lobe is responsible for different functions, such as motor control, sensory processing, language, memory, and visual perception. The cerebrum is one of the most complex and important organs in the human body, and its functions are essential for our daily lives and interactions with the world around us.

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The salinity of a brackish habitat can vary depending on the amount of freshwater and saltwater that mix together in that environment. Brackish water is a mixture of freshwater and saltwater, which can occur in estuaries, mangrove swamps, and other coastal areas. The salinity of the habitat may increase during the dry season when the water evaporates and the salt concentration becomes more concentrated. Conversely, it may decrease when there is a lot of rainfall and freshwater flows into the habitat, diluting the saltwater. Therefore, the correct answer is that the salinity of a brackish habitat increases during the dry season.

Maelezo ya Majibu

Maelezo ya Majibu

Organisms in an ecosystem are usually grouped according to their trophic level into producers and consumers. Producers, also known as autotrophs, are organisms that are capable of producing their food using energy from the sun (photosynthesis) or chemical energy (chemosynthesis). Examples of producers include plants, algae, and some bacteria. They form the base of the food chain in an ecosystem. Consumers, on the other hand, are organisms that cannot produce their food and rely on other organisms for their energy. They are classified into different levels, also known as trophic levels, based on the source of their food. The primary consumers, also known as herbivores, eat plants and other producers. The secondary consumers, or carnivores, eat herbivores, while the tertiary consumers, or top carnivores, eat other carnivores. By grouping organisms into producers and consumers based on their trophic level, ecologists can study the flow of energy and nutrients through an ecosystem. This information can be used to understand how changes in one part of the ecosystem can affect other parts and to make predictions about the impact of human activities on the environment.

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The correct answer is "homeostasis." Homeostasis refers to the process of maintaining a stable and consistent internal environment within an organism. This is important because cells and organs require a specific environment to function properly, and changes to that environment can disrupt the organism's ability to survive. Homeostasis is achieved through various mechanisms that monitor and regulate important physiological parameters such as temperature, pH, and nutrient levels. For example, the body may sweat to cool down when it gets too hot, or shiver to generate heat when it gets too cold. Similarly, the body may release hormones to regulate blood sugar levels, or increase breathing rate to remove excess carbon dioxide. Overall, homeostasis is a complex and dynamic process that allows organisms to adapt to changing environmental conditions and maintain a stable internal environment that supports optimal physiological function.

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The form of energy utilized during photosynthesis is "solar" energy. Photosynthesis is the process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy stored in glucose and other organic compounds. This process takes place in specialized organelles called chloroplasts, which contain chlorophyll, a pigment that absorbs light energy. The light energy is used to power a series of chemical reactions that convert carbon dioxide and water into glucose and oxygen. Therefore, the energy used during photosynthesis is solar energy, which is captured by chlorophyll and converted into chemical energy.

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To focus on a distant object, the ciliary muscle of the eye relaxes, and the eye lens gets thinner. The ciliary muscle is a ring of muscle fibers that surrounds the lens of the eye, and it helps to change the shape of the lens to adjust the focus. When we look at a distant object, the ciliary muscle relaxes, causing the suspensory ligaments to pull on the lens and make it thinner. This allows the lens to refract the light less and focus the light on the retina at the back of the eye. This process is called accommodation, and it enables us to see objects clearly at different distances. So, the correct answer is that the ciliary muscle of the eye relaxes, and the eye lens gets thinner to focus on a distant object.

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Maelezo ya Majibu

The digestion of proteins starts in the stomach. When we eat protein-rich foods such as meat, eggs, or dairy products, the proteins are broken down into smaller units called amino acids. This process of breaking down proteins into smaller units is called digestion. In the stomach, proteins are mixed with gastric juices that contain hydrochloric acid and a digestive enzyme called pepsin. The hydrochloric acid helps to break down the proteins into smaller units, while pepsin cuts the proteins into even smaller pieces. Once the proteins have been broken down into smaller units, they move into the small intestine, where they are further broken down into individual amino acids by digestive enzymes produced by the pancreas and the small intestine. In summary, the digestion of proteins starts in the stomach, where they are broken down into smaller units by the action of gastric juices containing hydrochloric acid and pepsin. The smaller units are then further broken down into individual amino acids in the small intestine.

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Maelezo ya Majibu

The stage of cell division illustrated in the diagram is Metaphase. Metaphase is the second stage of mitosis, which is the process of cell division that results in the formation of two daughter cells from a single parent cell. In metaphase, the chromosomes, which were duplicated in the previous stage (prophase), line up along the equator of the cell, called the metaphase plate. In the diagram, we can see that the chromosomes are aligned in the middle of the cell, which is characteristic of metaphase. The spindle fibers, which attach to the centromeres of the chromosomes and pull them apart in the next stage (anaphase), are also visible in the diagram. During metaphase, the chromosomes are at their most condensed and visible under a microscope. This allows for the accurate separation of genetic material between the two daughter cells during the subsequent stages of cell division. After metaphase, the cell progresses to anaphase, where the spindle fibers pull the sister chromatids of each chromosome apart to opposite poles of the cell. Finally, in telophase, the cell divides into two daughter cells, each with its own set of chromosomes.

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The underground storage stems that grow horizontally in the soil are called "rhizomes." Rhizomes are a type of stem that grow underground and horizontally, sending out roots and shoots from nodes along their length. They store nutrients and energy for the plant, which can help it survive adverse conditions such as drought or cold temperatures. Examples of plants that have rhizomes include ginger, bamboo, and some types of grasses. Because they grow horizontally, rhizomes can also help plants spread and colonize new areas.

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The complex relationship between the members of a community and their physical environment is known as Ecology. Ecology is the study of the interactions between living organisms and their environment. It looks at how plants, animals, and other organisms depend on each other and their surroundings for survival. The physical environment includes things like air, water, soil, and climate. Ecologists study the flow of energy and matter through these relationships, as well as how different species adapt to changes in their environment. In simple terms, ecology is about understanding how all the parts of an ecosystem (a community of living and nonliving things) fit together and depend on each other.

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In a complete dominance monohybrid cross between a pure breeding yellow-flowered plant Y and a pure breeding white-flowered plant y, the result of the first filial generation is 100% yellow flowers. This is because yellow flower color is dominant over white flower color, and the Y allele is dominant over the y allele. As a result, all the offspring in the first generation, also known as the F1 generation, will have one Y allele from the yellow-flowered parent and one y allele from the white-flowered parent, making them all yellow-flowered heterozygotes. However, if we cross the F1 generation plants with each other, we will get a 3:1 ratio of yellow to white flowers in the second filial generation (F2), because the yellow-flowered heterozygotes can produce both yellow and white-flowered offspring.

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The resource that is renewable is trees. Renewable resources are resources that can be replenished naturally over time, either through natural processes or human intervention. Trees are a renewable resource because they can be replanted and grown again after they are harvested for wood or other products. When forests are managed sustainably, meaning they are harvested in a way that does not exceed the forest's ability to regenerate, trees can provide an ongoing supply of wood products without depleting the resource. On the other hand, petroleum, coal, and iron are non-renewable resources. These resources are finite and can only be extracted from the earth's crust once. Once they are extracted and used, they cannot be replenished. As a result, they are considered non-renewable resources. Therefore, the resource that is renewable is trees.

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Tooth decay is usually caused by the bacteria in our mouth that metabolize sugar and other carbohydrates from our food and produce acids that damage the enamel of our teeth. These bacteria live in our mouth and form a sticky film on our teeth called plaque. When we eat sugary or starchy foods, the bacteria in the plaque use them as fuel to produce acids. Over time, these acids can dissolve the minerals in our tooth enamel, leading to cavities and decay. So, it's important to maintain good oral hygiene, limit sugary foods, and visit a dentist regularly to prevent tooth decay. Eating proteinous foods does not directly cause tooth decay, but neglecting oral hygiene after eating such foods can lead to plaque formation and eventually tooth decay.

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The yellow and black stripes on the body of wasps is an example of "warning coloration". Warning coloration, also known as aposematism, is a type of protective mechanism used by animals to warn potential predators that they are harmful, poisonous, or dangerous to consume. Animals that display warning coloration often have bright, vibrant, and easily identifiable markings or patterns, such as stripes, spots, or bold coloration. In the case of wasps, the bright yellow and black stripes on their bodies serve as a warning to predators that they are equipped with a powerful stinger and can defend themselves if threatened. These markings help to deter predators from attacking, which reduces the risk of injury or death to the wasp. Other examples of animals that use warning coloration include poison dart frogs, monarch butterflies, and skunks. These animals have evolved to use bright colors or strong odors as a signal to potential predators that they are not to be messed with. By doing so, they increase their chances of survival in the wild.

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The heart can beat continuously without being fatigued because it is composed of cardiac muscle. The heart is a special type of muscle called cardiac muscle. Cardiac muscle is different from skeletal muscle and smooth muscle in that it has unique properties that allow it to contract and relax repeatedly without becoming fatigued. One of the key differences between cardiac muscle and other types of muscle is that cardiac muscle has its own intrinsic electrical conduction system, which generates electrical signals that coordinate the contraction and relaxation of the heart. This allows the heart to beat continuously without relying on signals from the nervous system, which can become fatigued. In addition, cardiac muscle also has a high number of mitochondria, which are the energy-producing structures in cells. This allows cardiac muscle to produce energy continuously, enabling it to maintain its rhythmic contractions. In summary, the heart can beat continuously without being fatigued because it is composed of a unique type of muscle called cardiac muscle, which has its own intrinsic electrical conduction system and a high number of mitochondria.

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Muscle fatigue in the body of an athlete is due to the "accumulation of lactic acid". When an athlete engages in intense physical activity, their muscles require a large amount of energy to contract and relax rapidly. This energy is supplied by a process called cellular respiration, which involves the breakdown of glucose to produce ATP (adenosine triphosphate), the main source of energy for muscle contraction. However, during intense exercise, the body may not be able to supply enough oxygen to the muscles to sustain cellular respiration. In the absence of oxygen, the body switches to a different metabolic pathway called anaerobic respiration, which produces energy more quickly but also generates lactic acid as a byproduct. As lactic acid accumulates in the muscles, it can lead to a drop in pH, which can interfere with the chemical reactions required for muscle contraction. This can cause the muscles to become fatigued, making it more difficult for the athlete to continue exercising at a high level of intensity. In summary, muscle fatigue in athletes is caused by the accumulation of lactic acid in the muscles, which can interfere with the chemical reactions required for muscle contraction. This buildup of lactic acid is a result of the switch to anaerobic respiration during intense exercise, which occurs when there is not enough oxygen available to sustain cellular respiration.

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The nucleotide that is not found in DNA molecules is uracil. DNA (deoxyribonucleic acid) is a type of nucleic acid that contains the genetic instructions used in the development and function of all living organisms. DNA is made up of repeating units called nucleotides, which consist of a sugar molecule (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), or thymine (T). Uracil (U) is a nitrogenous base that is similar in structure to thymine, but it does not occur in DNA. Instead, uracil is found in RNA (ribonucleic acid), which is another type of nucleic acid that is involved in the synthesis of proteins. RNA is similar to DNA in many ways, but it contains a different sugar molecule (ribose) and can have uracil in place of thymine. This difference in nucleotide composition allows RNA to perform a range of functions that are distinct from those of DNA. Therefore, the nucleotide that is not found in DNA molecules is uracil. The other three nucleotides - adenine, cytosine, and guanine - are all found in DNA.

Maelezo ya Majibu

Maelezo ya Majibu

(a) Completed table of organisms, mode of feeding and the adapting mouthpart feature:

(Butterfly, with a long coiled proboscis for sucking nectar, and honeybee, with mandibles plus a long tongue for chewing and lapping, are also acceptable entries.)

(b) Five parts of the alimentary canal of an earthworm: pharynx, oesophagus, crop, gizzard, intestine (the full canal runs mouth, buccal cavity, pharynx, oesophagus, crop, gizzard, intestine, anus).

(c) Six ways water can be polluted by agricultural practices:

• Chemical fertilizers washed (leached) into rivers and lakes, causing eutrophication.
• Insecticides and pesticides running off farmland into water bodies.
• Herbicides washed from sprayed crops into streams.
• Animal droppings and livestock waste washed into water.
• Soil erosion from cultivated land silting up water bodies.
• Improper disposal of decaying farm and organic waste into water.

(d) Life cycle of a housefly (complete metamorphosis): The adult female lays batches of eggs on decaying organic matter such as refuse or faeces. Each egg hatches into a larva (maggot) that feeds actively, grows and moults. The larva then changes into an inactive pupa enclosed in a puparium, within which the body is reorganised. Finally an adult fly (imago) emerges, matures and mates, and the cycle repeats: egg to larva to pupa to adult.

(e) Hypogeal germination in maize: the diagram should show the grain remaining below the soil (the cotyledon and food-storing endosperm stay underground), the coleoptile pushing vertically up through the soil, the first foliage leaf emerging from the tip of the coleoptile above ground, the primary (radicle) root growing downward, and the fibrous seminal/adventitious roots. The soil line should be marked, with the grain and roots below it and the shoot above it.

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(a) Three features of the small intestine that increase the rate of absorption

1. It is very long and coiled, providing a large surface area for absorption.
2. Its inner wall is folded and carries numerous finger-like villi (and microvilli) which greatly increase the absorbing surface.
3. The villi have thin (one-cell-thick) walls and a rich supply of blood capillaries and lacteals, so absorbed food passes quickly into the blood and lymph.

(b) Fate of the glucose formed during photosynthesis

Some of the glucose is used at once in respiration to release energy. The rest is converted to starch for temporary storage in the leaf, or changed to sucrose and translocated through the phloem to other parts of the plant, where it is used for growth or stored (as starch, oils or proteins) in fruits, seeds, roots and stems. Glucose is also used to build cellulose for cell walls and, with nitrogen, to make proteins.

(c) Mode of feeding

(i) Euglena: Euglena is mainly holophytic (autotrophic). It contains chloroplasts and manufactures its own food by photosynthesis in the presence of sunlight. In darkness it can feed saprophytically (saprozoically) by absorbing dissolved organic matter through its body surface. This double mode of nutrition makes it mixotrophic.

(ii) Spirogyra: Spirogyra is holophytic (autotrophic). It has a spiral chloroplast containing chlorophyll and manufactures its own food by photosynthesis, using water, carbon dioxide and sunlight.

(d) Three characteristics of an estuarine habitat

1. The water is brackish (a mixture of fresh and salt water), so salinity varies.
2. Salinity, water level and temperature change with the tides.
3. The water is often muddy/turbid and rich in nutrients and silt.

(e) Three differences between an aquatic habitat and a terrestrial habitat

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The data to reason from is the composition table. Fat yields about \(37\ \text{kJ g}^{-1}\) of energy while protein yields about \(17\ \text{kJ g}^{-1}\), so fat is the richer energy source and protein is the tissue builder. Approximate energy per 100 g is \(E = (\text{fat}\times 9) + (\text{protein}\times 4)\) kcal.

(a)(i) Meat types D and A.

(a)(ii) They have the lowest fat content (3.1% and 7.2%), so they add the least fat and energy to the body and will not worsen the obesity.

(b)(i) Meat types B and C.

(b)(ii) (1) They carry the highest fat contents (25.3% and 20.0%), and fat provides more than twice the energy per gram of protein or carbohydrate. (2) Their calculated energy values (about 270 kcal/100 g) are the highest in the table.

(c)(i) Meat types D, C and A.

(c)(ii) Kwashiorkor is a protein-deficiency disease, and these three have the highest protein contents (28.2%, 22.5%, 21.3%), so they best replace the missing protein.

(d)(i) An active teenager: C (it combines high energy with high protein, 22.5%, needed for activity and growth).

(d)(ii) A 70-year-old person: D (very low fat, 3.1%, to avoid cardiovascular strain, yet high protein for tissue repair).

(e) Carbohydrates also provide energy.

(f) Three uses of fat in the human body:

• It is a concentrated reserve store of energy.
• It insulates the body against heat loss (fat layer beneath the skin).
• It cushions and protects delicate internal organs such as the kidneys.

(g) Testing for fat with white paper: rub a small piece of the meat firmly on the white paper so that any oil is transferred. Allow it to stand (or gently warm it) so that water and moisture evaporate. Hold the paper up to the light. A permanent translucent (greasy) spot that does not dry off shows that fat is present.

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(a)(i) Respiratory surfaces of the organisms

Organism	Respiratory surface
Tadpole (2 days old)	External gills
Cockroach	Tracheae and tracheoles
Domestic fowl	Lungs
Talinum	Stomata
Earthworm	Moist skin or body surface
Amoeba	Cell membrane or body surface

(a)(ii) Characteristics of respiratory surfaces

1. They have a large surface area for rapid gaseous exchange.
2. They are thin, usually one cell thick, giving a short diffusion distance.
3. They are moist so that oxygen and carbon dioxide can dissolve before diffusion.
4. They are permeable to respiratory gases.
5. They are well ventilated to maintain a steep concentration gradient.
6. In organisms with circulatory systems, they are richly supplied with blood capillaries.

(b) Respiratory organ of Tilapia: gill

The labelled drawing below should be made about 4 cm to 6 cm long.

The numerous gill filaments provide a large, thin, moist and richly supplied surface for gaseous exchange.

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(a) Five vertebrae in mammals with their locations

(b) One function each in a dicotyledonous leaf

• Palisade: contains many chloroplasts and is the main site of photosynthesis.
• Vascular bundle (vein): conducts water and mineral salts to the leaf (xylem) and carries manufactured food away (phloem); it also supports the leaf.
• Epidermis: protects the inner tissues and, being covered by a cuticle, reduces water loss.

(c) One function each in the stem of a flowering plant

• Sieve tube: conducts (translocates) manufactured food from the leaves to other parts of the plant.
• Cortex: stores food and water and gives support to the stem.
• Pith: stores food and water in the centre of the stem.

(d)(i) Two external structural differences between the stem and root of a maize seedling

Also: stem grows upward, root grows downward; root has root hairs and a root cap, the stem does not.

(d)(ii) Two internal structural differences between the stem and root of a maize seedling

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(a) Classification of the biological associations:

In parasitism one partner (the parasite) benefits while the host is harmed; in mutualism both partners benefit; in commensalism one partner benefits while the other is neither helped nor harmed.

(b)(i) Three adaptive features of parasites:

• Presence of hooks and suckers for firm attachment to the host so they are not dislodged.
• A thick, resistant cuticle (and anti-enzymes) that protects them from the host's digestive juices.
• High reproductive capacity (production of very many eggs) to ensure that some offspring reach new hosts.

Other acceptable features: reduced or absent locomotory organs, reduced sense organs, and a reduced digestive system since food is absorbed already digested.

(b)(ii) Two effects of parasites on their hosts:

• They deprive the host of nutrients, causing malnutrition and weakness.
• They cause diseases and damage host tissues or organs (for example anaemia from blood loss).

(c)(i) Saprophytes are organisms that feed on dead and decaying organic matter, secreting digestive enzymes onto the material externally and absorbing the soluble digested products.

(c)(ii) Four examples of saprophytes: Mushroom, Rhizopus (bread mould), Mucor, Yeast (also Penicillium and many bacteria of decay).

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b. The five parts of the alimentary canal of an earthworm are the mouth, pharynx, esophagus, crop, and gizzard.

c. Six ways by which water could be polluted by agricultural activities are:

1. Runoff from fields carrying pesticides and fertilizers into nearby water bodies.
2. Livestock waste and manure runoff into rivers and streams.
3. Leaching of nutrients and chemicals from fields into groundwater.
4. Erosion from tilled fields, which can carry sediment and nutrients into waterways.
5. Irrigation runoff carrying chemicals and salts.
6. Discharge of untreated or poorly treated agricultural wastewater into surface waters.

d. The life cycle of a housefly involves four stages: egg, larva, pupa, and adult. Adult female houseflies lay eggs on organic matter such as garbage, animal waste, or decaying food. The eggs hatch into larvae, commonly known as maggots, which feed on the organic matter for several days. The larvae then move to a dry location to pupate, forming a hard outer casing known as a puparium. Within the puparium, the larvae transform into an adult fly. The adult emerges from the puparium, mate, and begin the cycle anew.

e. Hypogeal germination is a type of seed germination where the cotyledons (seed leaves) remain below the soil surface. In a mature maize seedling, the embryo is located at the base of the kernel, and the radicle (embryonic root) emerges first during germination. The radicle grows downward, elongates, and forms lateral roots. The shoot also emerges from the embryo and grows upward, carrying the plumule (embryonic shoot) and the first true leaves above the soil surface. The diagram of hypogeal germination in a mature maize seedling should show the radicle, the embryonic shoot, the first true leaves, the kernel, and the soil.

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(a)(i) Lightning: The high electrical energy of lightning supplies enough energy to make atmospheric nitrogen combine directly with oxygen to form oxides of nitrogen (nitric oxide and nitrogen dioxide). These oxides dissolve in rain water to form dilute nitric acid, which reaches the soil and forms nitrates that plants absorb. Lightning therefore fixes free nitrogen into a usable form.

(a)(ii) Nitrosomonas: It is a nitrifying bacterium that oxidises (converts) ammonia or ammonium compounds in the soil to nitrites.

(a)(iii) Azotobacter: It is a free-living, nitrogen-fixing soil bacterium that fixes atmospheric nitrogen into nitrogenous compounds, enriching the soil with usable nitrogen.

(b) Excretory organs:

• (i) Insects: Malpighian tubules.
• (ii) Earthworm: Nephridia.

(c) Definitions:

• (i) Hepatitis is the inflammation of the liver, usually caused by a viral infection, which damages liver cells and may lead to jaundice.
• (ii) Kidney stones are hard, solid crystalline deposits of mineral salts (such as calcium oxalate and urates) that form inside the kidney from substances concentrated in the urine.

(d)(i) Three effects of lack of sense receptors in the skin:

• Inability to feel pain, so injuries occur without the person being aware.
• Inability to detect temperature changes, leading to burns or excessive cold exposure.
• Inability to detect touch and pressure, so harmful contact is not sensed.

(d)(ii) Three layers of the epidermis of human skin: the cornified (horny) layer, the granular layer, and the Malpighian (germinative) layer.

(e)(i) Metamorphosis is the series of distinct changes in body form and structure that an insect undergoes during its development from the egg to the adult stage.

(e)(ii) Type of metamorphosis in each insect:

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(a)(i) Genetic cross

Let Hb^A represent the allele for normal haemoglobin and Hb^S represent the allele for sickle-cell haemoglobin.

Female heterozygote: Hb^AHb^S
Male sickler: Hb^SHb^S

The female produces gametes Hb^A and Hb^S, while the male produces only Hb^S gametes.

Therefore, the possible genotypes of the offspring are:

• Hb^AHb^S, heterozygous normal carrier or sickle-cell trait, 2 out of 4;
• Hb^SHb^S, sickler, 2 out of 4.

Genotypic ratio: Hb^AHb^S : Hb^SHb^S = 2:2 = 1:1.

(a)(ii) Phenotypic ratio

Normal carriers (non-sicklers) : sicklers = 1:1.

(b) Rhesus incompatibility and loss of the second pregnancy

A Rhesus-negative woman has no Rh antigen on her red blood cells. If her first foetus is Rhesus-positive, some foetal red blood cells may enter her circulation, especially at delivery when the placenta separates. The Rh antigen stimulates the mother to form anti-Rhesus antibodies. In a later pregnancy with a Rhesus-positive foetus, these antibodies cross the placenta and destroy the foetal red blood cells. The resulting severe haemolysis, called erythroblastosis foetalis, may cause death of the foetus and miscarriage.

(c) Features explained by Lamarck's use and disuse theory

1. The long neck of the giraffe, attributed to continual stretching to reach high leaves.
2. The webbed feet of ducks or geese, attributed to constant use in swimming.

(d) Vestigial structures in mammals

1. Vermiform appendix in humans.
2. Coccyx, or reduced tail vertebrae, in humans.
3. Pelvic and hind-limb bones in whales.

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(a)(i) The cross is a dihybrid cross.

(a)(ii) It involves two pairs of contrasting characters, namely plant height (tall/dwarf) and flower colour (red/white).

(b)(i) Evolution is the gradual change in the inherited characteristics of a population over many generations, which may result in new forms or species.

(b)(ii) Genetic diagram

Let T represent tallness and t dwarfness; R represent red flowers and r white flowers. Tallness and red flower colour are dominant.

P generation: Tall, red plant (TTRR) × Dwarf, white plant (ttrr)

Gametes: TR × tr

F₁ generation: all TtRr, tall and red.

Selfing of F₁: TtRr × TtRr

Each F₁ plant produces the four types of gametes: TR, Tr, tR and tr.

From the genetic diagram, the F₂ phenotypic ratio is:

9 tall red : 3 tall white : 3 dwarf red : 1 dwarf white.

(b)(iii) Tall, white plants are represented by 3 out of 16 plants.

\[\text{Number of tall, white plants}=\frac{3}{16}\times160=30\]

Therefore, 30 tall, white-flowered plants would be obtained in the F₂ generation.

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(a)(i) Five challenges that make conservation of resources difficult

1. Rapid population growth that increases demand for land, food and fuel.
2. Poverty, which forces people to over-exploit resources for immediate survival.
3. Ignorance and lack of awareness about the value of conservation.
4. Weak enforcement of conservation laws and corruption.
5. Inadequate funds and trained personnel to manage protected areas.

Others: industrialisation and urbanisation, and high demand for timber and bushmeat.

(a)(ii) Five ways a government ensures conservation of natural resources

1. Enacting and enforcing laws that protect wildlife and forests.
2. Establishing national parks, game reserves and forest reserves.
3. Educating the public on the need for conservation.
4. Encouraging afforestation and reforestation programmes.
5. Controlling hunting, logging and fishing through licences and closed seasons.

(b) What is an endangered species?

An endangered species is a species whose population has fallen so low that it is in danger of becoming extinct (dying out completely) if the conditions threatening it are not controlled. Examples include the gorilla and certain species of elephant.

(c) Blood group genetics

Blood group O has the genotype OO. A child of blood group O must receive one O allele from each parent.

(c)(i) Genotype of the child: OO.

(c)(ii) Possible genotypes of the father: Since the child is OO, the father must supply an O allele. His possible genotypes are therefore any group carrying an O allele:

• IᴬO (blood group A carrying O)
• IᴮO (blood group B carrying O)
• OO (blood group O)

(d) Three reasons why genetic variation is important to plants

1. It enables some plants to survive changes in the environment (better adaptation).
2. It provides the raw material for natural selection and evolution.
3. It allows breeders to select and produce improved varieties (higher yield, disease resistance).