WAEC SSCE - Biology - 2009

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The statement "it occurs only in young cells" is not correct. Mitotic cell division can occur in cells at any stage of their life cycle, not just in young cells. Mitotic cell division is a type of cell division that occurs in somatic cells, which are all the cells in the body except the reproductive cells. During mitosis, the chromosomes in the parent cell replicate and then separate into two identical sets, each of which moves to opposite ends of the cell. The cell then divides into two daughter cells, each with the same chromosome number and genetic composition as the parent cell. Mitotic cell division plays an essential role in many processes, such as growth and repair of tissues in the body. It occurs throughout the lifespan of an organism, not just in young cells. Therefore, the statement that "it occurs only in young cells" is incorrect.

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The habitat that cannot be used in the study of succession is "a well-cultivated land". Succession is the process of ecological change over time in a particular habitat, and it occurs in stages, from the initial colonization of pioneer species to the establishment of a stable climax community. Ecologists study succession in a range of habitats to understand how ecosystems develop and change over time. Savanna grasslands and abandoned farmland are both examples of habitats that can be studied in the context of succession. In these habitats, ecologists can observe how plant and animal communities change over time in response to changing environmental conditions. A pond is also a useful habitat for studying succession, as it undergoes a process of natural succession from an open body of water to a wetland ecosystem. Ecologists can observe how different plant and animal species colonize the pond as it fills in and eventually becomes a marsh or swamp. However, a well-cultivated land is not a suitable habitat for studying succession because it has been heavily modified by human activity. The process of cultivation involves clearing the land, removing native vegetation, and introducing crops or other cultivated species. As a result, a cultivated land is not a natural habitat and is not representative of ecological succession in an undisturbed ecosystem.

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The physiological term that can be used to describe the regulation of the body temperature of a lizard is "poikilothermy". Poikilothermy refers to the characteristic of an organism whose body temperature varies with that of the external environment. Lizards, along with other cold-blooded animals such as snakes and turtles, are poikilothermic. Lizards are not able to regulate their body temperature internally like warm-blooded animals, such as mammals and birds. Instead, they rely on external sources of heat, such as basking in the sun or resting on warm surfaces, to raise their body temperature. Conversely, they may seek out shade or cooler areas to lower their body temperature. This reliance on external sources of heat means that the body temperature of a lizard can vary greatly depending on the temperature of its environment. However, lizards are able to maintain a degree of stability in their internal processes through other mechanisms such as behavioral adjustments and physiological changes. In summary, the regulation of body temperature in lizards is an example of poikilothermy, where the organism's body temperature varies with that of the external environment.

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Based on the given sequence, organism R is likely a producer. This is because the sequence P → Q → R → S → T represents a food chain, with each organism being consumed by the one that follows it. Organism P is usually a producer, which means it makes its own food through photosynthesis. Organism Q is likely a herbivore or primary consumer, feeding on the producer (P). Organism S is likely a carnivore or secondary consumer, feeding on organism Q. Finally, organism T is likely a tertiary consumer, feeding on organism S. Therefore, since organism R is located between the producer (P) and herbivore (Q), it is likely a producer as well. It is not a consumer, let alone a secondary consumer, since it is not located after the carnivore (S) in the food chain.

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Lichens are considered pioneer organisms in ecological succession. This is because they are among the first organisms to colonize an area that is devoid of life, such as bare rock. Lichens are unique in that they are composed of both a fungus and an alga, which allows them to grow in harsh environments with limited resources. Over time, lichens help to break down rock and create soil, which then allows other plants and organisms to colonize the area. As such, lichens play an important role in the early stages of ecological succession by paving the way for other species to establish themselves in a newly-formed ecosystem.

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The femur is the longest bone in the body. It is located in the thigh region and extends from the hip joint to the knee joint. It is the primary weight-bearing bone in the human body and helps to support the body's weight during activities like walking and running. The femur is also an important attachment site for many of the body's large muscle groups, including those of the thigh and hip.

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The reason why the flow of blood through the capillaries is very slow is to allow adequate time for exchange of materials. Capillaries are tiny, thin-walled blood vessels that are responsible for delivering oxygen and nutrients to the body's tissues and removing waste products. The slow flow of blood through the capillaries is important because it allows for sufficient time for diffusion and exchange of materials to occur between the blood and the surrounding tissue. This process is critical for maintaining proper cellular function and overall health. If the blood flowed too quickly through the capillaries, there would not be enough time for this exchange of materials to occur, which could lead to various health problems. Therefore, the slow flow of blood through the capillaries is essential for proper tissue function and overall health.

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The cytoplasm is a very important component of the cell because it suspends all the cell organelles, including the nucleus, which contains the genetic material of the cell. The cytoplasm is the gel-like substance that fills the cell and surrounds the organelles. It contains various enzymes that help to catalyze metabolic reactions and is also the site of many cellular processes such as protein synthesis, cell division, and transport of molecules within the cell. Furthermore, the cytoplasm contains various structures called cytoskeletons that help in maintaining the shape and structure of the cell, as well as aiding in cell division and movement. The cytoplasm also regulates the amount of energy in the cell, as it is the site of many metabolic reactions that produce ATP, which is the main source of energy for the cell. In summary, the cytoplasm of the cell is a very important component because it suspends all the cell organelles, regulates the amount of energy in the cell, and is the site of many cellular processes.

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The outward appearance of an organism is referred to as its phenotype. Phenotype refers to the observable traits of an organism, such as its physical appearance, behavior, and physiological characteristics, which are the result of the interaction between an organism's genetic makeup (genotype) and its environment. For example, a person's hair color, height, and eye color are all part of their phenotype. Phenotype is influenced by both genetic and environmental factors, and can vary within a population due to differences in genes and environmental conditions.

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DNA in eukaryotic cells is contained in the nucleus. The nucleus is a membrane-bound organelle that contains the cell's genetic material in the form of DNA. It is the control center of the cell, regulating gene expression and controlling the cell's activities. The DNA in the nucleus is organized into chromosomes, which are long strands of DNA wrapped around proteins called histones. Eukaryotic cells also have other organelles like lysosomes, Golgi bodies and central vacuoles, but none of them contain DNA.

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The part of the ear which contains nerve cells sensitive to sound vibrations is the cochlea. The cochlea is a spiral-shaped cavity located in the inner ear that is responsible for converting sound vibrations into electrical signals that can be interpreted by the brain. It contains thousands of tiny hair cells that are sensitive to different frequencies of sound. When sound waves enter the ear, they cause the hair cells in the cochlea to vibrate, which triggers the production of electrical signals that are transmitted to the brain via the auditory nerve.

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The group of organisms that can interbreed and produce offspring that can also reproduce is called a species. This means that members of the same species share similar genetic traits and can mate with each other to create fertile offspring. For example, dogs can interbreed with other dogs and produce puppies that can also reproduce, making them members of the same species. On the other hand, dogs cannot interbreed with cats, making them members of different species.

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Placing a potted plant under a fan in a room would increase the rate of transpiration. Transpiration is the process by which water is lost through the stomata of a plant. This process is dependent on factors such as temperature, humidity, light, and air movement. When a plant is placed under a fan, the air movement increases, which causes the water vapor around the leaves to move away from the plant quickly. As a result, the rate of transpiration increases, and the plant loses more water. This is why it's essential to keep plants well-watered when they are placed under fans, as the increased transpiration rate can cause them to dry out quickly. The other options - placing it in a cold room, putting it in a cupboard, or smearing grease on the lower surface of the leaves - would all have the opposite effect and decrease the rate of transpiration.

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An example of continuous variation is body size. Continuous variation refers to variation that can take on any value within a range, rather than being limited to certain discrete options. Body size is a good example of this because it can vary continuously from very small to very large, with many possible sizes in between. For instance, a person can be very short, moderately short, of average height, moderately tall, or very tall. This variation is not limited to a certain number of options, and each person's size can be measured and compared to others on a continuous scale.

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The process that removes carbon from the atmosphere is photosynthesis. During photosynthesis, plants take in carbon dioxide from the atmosphere and convert it into glucose and oxygen, with the carbon being stored in the plant's tissues. This process helps to regulate the amount of carbon dioxide in the atmosphere, which is important for maintaining a healthy balance in the Earth's atmosphere. The other options listed, putrefaction, volcanic eruption, and burning of fuels, all release carbon into the atmosphere, contributing to climate change and other environmental issues.

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An endangered species is a species whose populations have become so small that they may become extinct if not protected. This can be caused by a variety of factors, including habitat destruction, overhunting, and pollution. Endangered species are considered to be important to conserve because they contribute to biodiversity and the overall health of ecosystems. Therefore, the correct option is "endangered species".

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The phenomenon of red blood cells bursting open when placed in distilled water is called haemolysis. This is due to a process called osmosis, which is the movement of water molecules across a semi-permeable membrane from an area of high concentration to an area of low concentration. When red blood cells are placed in distilled water, the water concentration outside the cell is higher than inside the cell, causing water molecules to move into the cell by osmosis. This results in an increase in the volume of water inside the cell, which can lead to the cell membrane rupturing and the release of the cell's contents into the surrounding environment. The bursting of red blood cells is known as haemolysis, and this phenomenon is commonly used in laboratory experiments to study the properties of cell membranes and the effects of different solutions on cells. In summary, the bursting of red blood cells when placed in distilled water is known as haemolysis, which is caused by the process of osmosis where water molecules move into the cell, causing an increase in volume and ultimately leading to the rupture of the cell membrane.

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Scabies is not a hereditary disease. Scabies is a highly contagious skin infestation caused by a mite that burrows into the skin. It is spread through close contact with an infected person or by sharing personal items, such as clothing or bedding. Scabies is not passed down from parents to children through genetics, unlike the other options listed. Albinism is a genetic condition where a person has little or no pigment in their skin, hair, and eyes. Haemophilia is a genetic disorder that affects the blood's ability to clot, while color blindness is a genetic condition that affects a person's ability to perceive certain colors. In summary, while scabies is a serious skin condition, it is not hereditary and is caused by a parasite rather than a genetic mutation or disorder.

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An animal that is active during the day is known as a "diurnal animal." "Diurnal" comes from the Latin word "diurnus," which means "pertaining to a day." These animals have adapted to being active during the daytime, and often have specialized sensory organs, such as eyes and ears, that are optimized for daylight conditions. In contrast, "nocturnal animals" are active during the night, and often have adaptations to help them navigate and find food in low light conditions. "Terrestrial animals" are animals that live primarily on land, as opposed to aquatic or aerial animals. "Homoiothermic animals" are animals that can regulate their body temperature internally, maintaining a constant body temperature regardless of the temperature of the environment around them. In summary, a diurnal animal is an animal that is active during the day, while nocturnal animals are active at night.

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The statement that is valid is "The body temperature of the bird remained constant despite changes in environmental temperature." Birds are endothermic animals, which means that they can regulate their body temperature internally, independent of the environmental temperature. They maintain a constant body temperature through various physiological mechanisms, such as shivering, panting, and fluffing their feathers. In contrast, lizards are ectothermic animals, which means that they rely on the environmental temperature to regulate their body temperature. Their body temperature is usually close to that of the environmental temperature, and it can vary significantly depending on the ambient conditions. Therefore, the statement that "The body temperature of the bird remained constant despite changes in environmental temperature" is the only correct statement as it describes the ability of birds to maintain a constant body temperature, regardless of changes in the environmental temperature.

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Food chains are relatively short because of energy loss at each trophic level. As energy flows from one trophic level to another, it is lost as heat and through metabolic processes, reducing the amount of energy available to the next level. This energy loss means that there is not enough energy to support many trophic levels, leading to relatively short food chains. Additionally, as food chains become longer, the amount of energy available at the top of the chain becomes very limited, further limiting the number of trophic levels.

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The statement that is false is "All the energy entering the ecosystem is passed on to decomposers." Energy flows through an ecosystem in a one-way stream, from the sun or inorganic compounds to autotrophs, then to heterotrophs, and eventually to decomposers. However, not all of the energy that enters an ecosystem is passed on to the next trophic level. Some of the energy is lost as heat during metabolic processes, and some is used for growth and reproduction. As a result, energy transfer from one trophic level to the other is not 100% efficient. The primary producers, such as plants and algae, are the organisms that capture energy from the sun or inorganic compounds and convert it into organic compounds through photosynthesis. They provide the energy base for herbivores and carnivores, which are the primary and secondary consumers, respectively. Finally, decomposers break down dead organic matter and recycle nutrients back into the ecosystem. They play a critical role in nutrient cycling, but not all of the energy entering the ecosystem is passed on to them. Therefore, the statement that "All the energy entering the ecosystem is passed on to decomposers" is false.

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The structure that is not a skeletal material is muscle. Muscles are specialized tissues that are responsible for movement in animals. They are not considered skeletal materials because they do not provide structural support to the body. In contrast, chitin, cartilage, and bone are all skeletal materials that provide support and protection to different parts of the body. Chitin is a tough, semi-transparent polysaccharide that makes up the exoskeletons of insects, crustaceans, and other arthropods. It provides structural support and protection to these animals. Cartilage is a tough, flexible tissue that provides structural support and cushioning to joints and other parts of the body. It is found in the ears, nose, ribs, and joints of vertebrates. Bone is a hard, mineralized tissue that provides structural support, protection, and anchorage for muscles in vertebrates. It also serves as a mineral storage site and is involved in the production of blood cells. Therefore, the structure that is not a skeletal material is muscle, as it is not involved in providing structural support to the body.

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The instrument used to estimate the number of plants in a habitat is a quadrat. A quadrat is a square or rectangular frame made of metal or PVC, which is used to mark off a specific area in a habitat. This marked off area can be used to estimate the number of plants or other organisms within that habitat. To use a quadrat, the researcher places it randomly or systematically in the area of interest and then counts the number of plants or organisms within the quadrat. This count can then be extrapolated to estimate the total number of plants or organisms in the habitat. A pooter is used to collect and observe small insects, a pitfall trap is used to capture ground-dwelling insects, and a sweep net is used to capture insects flying close to the ground.

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The water cycle involves the process of precipitation and transpiration. Precipitation occurs when water vapor in the air condenses and falls to the ground as rain, snow, sleet, or hail. Transpiration is the process by which plants release water vapor into the air through small pores in their leaves. These two processes are part of the water cycle, which is the continuous movement of water on, above, and below the surface of the Earth. The water cycle is driven by the sun's energy and includes processes such as evaporation, condensation, precipitation, infiltration, and runoff. The other cycles listed (carbon, nitrogen, and oxygen) do not involve precipitation and transpiration as part of their processes.

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A primary consumer is an organism that feeds on producers (plants) as their source of food. Among the options provided, the organism that fits this description is the sheep. Sheep are herbivores that feed on plants as their primary source of food, making them a primary consumer in the food chain. The other options - dog and fungus - are not primary consumers as they do not feed primarily on plants.

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Population growth rate is defined as the net rate of natality (birth rate) and mortality (death rate) in a given habitat over a period of time. It is a measure of how fast a population is increasing or decreasing in size. The population growth rate takes into account both the number of individuals in the population and the amount of time it takes for the population to change in size. It is an important concept in ecology, as it allows us to better understand how populations of different species interact with their environment and with each other.

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The two organelles directly involved in mitotic cell division are the nucleus and the centriole. The nucleus is the organelle that contains the cell's genetic material in the form of chromosomes, and it plays a key role in regulating cell division. During mitosis, the nucleus divides into two identical copies, each containing a complete set of chromosomes. The centriole, on the other hand, is a small, cylindrical organelle that helps to organize the microtubules that pull the chromosomes apart during mitosis. The centrioles are located near the nucleus and are involved in the formation of the spindle fibers that attach to the chromosomes and help to pull them apart. Overall, the nucleus and the centriole are both essential for mitotic cell division and ensure that each new cell receives an identical copy of the genetic material.

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An autotrophic organism is likely to be an organism whose source of carbon is inorganic. Autotrophs are organisms that are capable of synthesizing their own organic molecules, including sugars and amino acids, from inorganic sources of carbon, such as carbon dioxide. This process is known as photosynthesis, which involves using energy from sunlight to convert carbon dioxide and water into organic molecules. On the other hand, heterotrophic organisms obtain their organic molecules from other organisms or from their organic waste products. Symbiotic organisms are those that live in a close relationship with another organism in which both benefit, while parasitic organisms obtain nutrients from another organism at the expense of that organism's health. Therefore, since autotrophs can produce their own organic molecules from inorganic sources, they are likely to be organisms whose source of carbon is inorganic.

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The immediate product of meiosis in flowering plants is the pollen grain. Meiosis is a type of cell division that results in the production of haploid cells. In flowering plants, meiosis occurs in the anthers of the flower, which are the male reproductive organs. The cells that undergo meiosis in the anthers are called microsporocytes, which produce microspores. Each microspore undergoes mitosis and develops into a pollen grain, which contains two haploid cells: the generative cell and the tube cell. The pollen grain is then transferred to the stigma of the flower, where it germinates and produces a pollen tube that grows down the style and into the ovary, allowing for fertilization to occur.

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Ascaris is an endoparasite. An endoparasite is a parasite that lives inside its host. Ascaris is a type of roundworm that can live in the intestines of humans and animals, where it feeds on the host's nutrients. It is transmitted through contaminated food or water and can cause a range of symptoms, including abdominal pain, diarrhea, and malnutrition. While mice and mosquitoes can be carriers of parasites, they are not endoparasites themselves. Dodder is a parasitic plant that feeds on the nutrients of other plants, but it is not an endoparasite because it does not live inside the host plant.

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Viruses are economically important because they can cause diseases in plants and animals, which can have significant economic consequences. When a virus infects a plant, it can damage crops and reduce yields, leading to lower profits for farmers. Similarly, when a virus infects an animal, it can cause illness or death, leading to losses for farmers and ranchers. Viruses can also be economically important in a positive way. For example, some viruses can be used to control pests that damage crops, reducing the need for pesticides and saving farmers money. Additionally, viruses can be used in biotechnology to produce vaccines and other medical treatments, which can have significant economic benefits. While viruses do not have cell structures and can only reproduce inside living cells, it is their ability to cause disease in plants and animals that makes them economically important.

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The group of rats placed in an air-tight box died because they used up the available oxygen in the air inside the box. Without fresh air entering the box, the rats were unable to obtain the oxygen they needed to sustain their bodily functions, such as respiration, and eventually died from lack of oxygen. This demonstrates the importance of air circulation and oxygen supply for living organisms.

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The earthworm is the organism that does not show color adaptation to its environment. Earthworms are typically brown or reddish-brown in color, and their color does not change to match their environment. This is because earthworms primarily live in soil or underground, where their color is not visible to predators or prey. Therefore, there is no evolutionary pressure for earthworms to adapt their coloration for camouflage or other purposes. In contrast, chameleons, fish, and toads are known for their ability to change color to blend in with their surroundings as a means of protection or hunting.

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The correct statement about sandy soil is that it is light and easy to dig. Sandy soil is characterized by its large particle size, which results in a soil structure with a lot of air space between the particles. This makes it easy to dig and work with. However, this also means that it has limited water-holding capacity and nutrients, and can drain quickly. Therefore, sandy soil is not ideal for certain types of plants that require more moisture or nutrients, but it can be suitable for others that prefer good drainage and aeration.

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The correct option is "possess valves at intervals throughout their length." This statement is not characteristic of arteries. Arteries are blood vessels that carry blood away from the heart to various parts of the body. They have thick, muscular and elastic walls to withstand the high pressure of blood pumped out of the heart. Arteries transport oxygenated blood to all parts of the body except the pulmonary artery which carries deoxygenated blood from the heart to the lungs. Unlike veins, arteries do not possess valves at intervals throughout their length. This is because their thick and muscular walls are capable of maintaining the pressure required to keep blood flowing in one direction.

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Most tropical plants shed their leaves during dry weather conditions in order to conserve water. When water is scarce, plants can lose water through transpiration, which is the process of water evaporating from the leaves. By shedding their leaves, plants can reduce the surface area exposed to the dry air and thereby minimize water loss. Leaves are responsible for a significant amount of water loss through transpiration, so shedding them during dry weather is an effective way for plants to conserve water. Additionally, shedding leaves can also reduce competition for limited resources, as other plants may struggle to survive during dry periods. While shedding leaves can reduce the rate of photosynthesis in plants, it is a necessary adaptation to survive in arid environments where water is scarce.

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The role of the membrane in plant cells is played by the cell wall. The cell wall is a tough and rigid layer that surrounds the cell membrane of plant cells. It provides structural support and protection to the cell, helps maintain the shape of the cell, and regulates the movement of materials into and out of the cell. Unlike the cell membrane, which is present in both plant and animal cells and is made up of lipids and proteins, the cell wall is unique to plant cells and is primarily made up of cellulose.

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The preservation and improvement of the environment is an important issue that involves practices that promote sustainability and protect the natural resources. Out of the options provided, the practice that does not contribute to the preservation and improvement of the environment is "Overgrazing." Overgrazing refers to the practice of allowing too many animals to graze on a particular area of land, which can lead to soil erosion, loss of vegetation, and reduced productivity of the land. Overgrazing can also cause negative impacts on the quality of the water and air, as well as on the health of the animals themselves. On the other hand, pollution reduction, material recycling, and reforestation are practices that contribute to the preservation and improvement of the environment. Pollution reduction aims to minimize the amount of pollutants that are released into the environment, which can have harmful effects on the ecosystem and human health. Material recycling helps to reduce waste and conserve natural resources by using recycled materials instead of new ones. Reforestation involves planting trees and restoring forests, which can help to reduce soil erosion, improve water quality, and provide habitat for wildlife. In summary, overgrazing does not contribute to the preservation and improvement of the environment, while pollution reduction, material recycling, and reforestation are practices that can help to protect and sustain the natural resources.

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The specimens of Euglena were still alive after being kept in a dark room for two weeks because they were able to carry out heterotrophic nutrition using holozoic nutrition. Euglena is a unicellular organism that is capable of both autotrophic and heterotrophic modes of nutrition. In the presence of light, it is able to carry out photosynthesis using chloroplasts, just like plants. However, when there is no light, Euglena is able to survive by carrying out holozoic nutrition, which involves engulfing and digesting other small organisms. In this case, since the Euglena specimens were kept in a dark room with no light available for photosynthesis, they must have switched to holozoic nutrition to survive. They were able to feed on other small organisms present in the pond water, which sustained them for the two-week period. Therefore, the correct answer is that the Euglena specimens were able to carry out holozoic nutrition in the absence of light, allowing them to survive for two weeks.

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The correct answer is "Fishes, amphibians, birds, man." These organisms all have kidneys as their excretory organ. Fish, amphibians, and birds have two kidneys, while humans have one on each side of the spine. The kidneys filter waste products from the blood and excrete them in the form of urine.

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The experiment described above is used to demonstrate the process of osmosis. Osmosis is the movement of water molecules across a selectively permeable membrane from an area of high water concentration to an area of low water concentration, in an attempt to balance the concentration of solutes on either side of the membrane. In the experiment, the potato acts as the selectively permeable membrane, and the water concentration is initially higher outside the potato. The potato cells contain solutes such as starch, which have a lower concentration outside the potato. As a result, water molecules move across the membrane from the area of high water concentration outside the potato to the area of low water concentration inside the potato, causing the potato to gain weight and become turgid. This experiment is often used in biology to illustrate the principles of osmosis, which is a fundamental process in the movement of water and nutrients in living organisms.

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Spectacles with convex lenses correct long-sightedness by converging the light rays before they enter the eye. Long-sightedness, or hyperopia, is a condition where the eye is too short or the cornea is too flat, causing light to focus behind the retina instead of on it. Convex lenses are thicker in the center than at the edges, causing them to converge incoming light rays towards a focal point. By placing a convex lens in front of a long-sighted eye, the lens will converge the incoming light rays and bring the focal point forward, allowing the image to be focused on the retina and improving vision clarity.

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The curvature movement of plants in response to the stimulus of water is called hydrotropism. This means that the plant is growing or moving in response to the presence of water in its environment. Hydrotropism is an important adaptation for plants, as it helps them to find sources of water and nutrients that are essential for their growth and survival. When a plant is exposed to water, the cells on one side of the root elongate more than the other side, causing the root to bend towards the water. This movement is controlled by plant hormones and is an example of how plants can sense and respond to changes in their environment.

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The photic zone is the upper layer of an aquatic ecosystem where sunlight can penetrate and support photosynthesis. Below the photic zone, light becomes increasingly scarce, making it the critical limiting factor for plant growth. Without adequate light, plants cannot photosynthesize and produce the energy they need to survive. Even if all other factors such as nutrients, water, and carbon dioxide are available in sufficient amounts, lack of light will prevent plants from growing and reproducing. Therefore, the correct answer is: the critical limiting factor for plants below the photic zone in an aquatic ecosystem is the intensity of light.

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The outcome of increased milk production in succeeding generations of cows is an example of artificial selection. Artificial selection is the process of selective breeding by humans in which desirable traits are chosen and bred in animals or plants. In this case, the dairy farmer allowed only the best milk producing cows to mate, which increased the likelihood that their offspring would inherit those desirable traits and produce more milk. Over time, by repeating this process of selective breeding, the farmer was able to produce a group of cows that were highly specialized for milk production. Natural selection, on the other hand, refers to the process by which organisms with advantageous traits are more likely to survive and pass on their traits to their offspring in a natural environment, without human intervention. Competition and cross fertilization are not directly related to the outcome of increased milk production in this scenario.

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The introduction of the sickle-cell allele into the United States due to the forced migration of individuals from Africa is an example of gene flow. Gene flow refers to the transfer of genetic material from one population to another through migration and interbreeding. In this case, the allele was transferred from African populations to the American population. This is an example of micro-evolution, which refers to small-scale evolutionary changes that occur within a population.

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The mesophyll layer of a leaf consists of photosynthetic parenchyma cells. These are specialized cells that contain chloroplasts, which are responsible for capturing light energy and converting it into chemical energy through the process of photosynthesis. The mesophyll layer is located between the upper and lower epidermis of the leaf and is where most of the photosynthesis in a leaf takes place. The photosynthetic parenchyma cells are typically thin and flat, with a large surface area that allows for maximum absorption of light. They are also closely packed together to facilitate the exchange of gases, such as carbon dioxide and oxygen, which are necessary for photosynthesis. Therefore, the mesophyll layer is a crucial part of a leaf's structure and plays a vital role in the plant's ability to produce energy through photosynthesis.

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