WAEC SSCE - Biology - 1989

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The statement that is not true of the nucleus of a living cell is that it contains ribosomes. The nucleus is a membrane-bound organelle that is found in eukaryotic cells. It contains the genetic material of the cell in the form of chromosomes, which are made up of DNA. The nucleolus is a small, dense region within the nucleus that is responsible for producing ribosomes. The nucleoplasm is the fluid-like substance that fills the nucleus and contains the chromatin, which is the uncondensed form of DNA. However, ribosomes are not found within the nucleus. Ribosomes are small, non-membrane-bound organelles that are responsible for protein synthesis, and they are found either floating freely in the cytoplasm or attached to the endoplasmic reticulum. Therefore, the statement that is not true of the nucleus of a living cell is that it contains ribosomes.

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The statement that is not true of osmotic process is "The two solutions are of equal concentration at the beginning of experiment." Osmosis is the movement of water molecules across a selectively permeable membrane from an area of high concentration to an area of low concentration. For osmosis to occur, there must be a concentration gradient between the two solutions, which means that they must be of different concentrations initially. The process involves only the movement of water molecules and equilibrium is reached when there is equal distribution of water molecules. Therefore, is not true of osmotic process.

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Structure II is responsible for osmoregulation. Osmoregulation is the process by which organisms regulate the concentration of water and solutes in their cells to maintain internal balance or homeostasis. Structure II is most likely a cell or an organ that is involved in this process. The other structures may have different functions such as support, protection, or movement, but they are not directly involved in osmoregulation.

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Pests can be controlled without the subsequent danger of polluting the ecosystem by increasing the population of the natural enemies of the pest. Natural enemies of pests can be other organisms such as predators, parasites, or pathogens, that can help to control the pest population naturally. By increasing the population of these natural enemies, pests can be controlled without using harmful chemicals that can pollute the environment. This method of pest control is known as biological control or biocontrol, and it involves the release of natural enemies or the creation of conditions that encourage their natural presence in the ecosystem. Biocontrol is an effective and sustainable method of pest control that can help to reduce the use of pesticides and promote a healthy ecosystem.

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The process that will not introduce carbon dioxide into the atmosphere is photosynthesis. Photosynthesis is the process by which plants, algae, and some bacteria use sunlight to produce glucose from carbon dioxide and water. In this process, carbon dioxide is actually removed from the atmosphere and converted into organic matter. On the other hand, the other options listed involve the release of carbon dioxide into the atmosphere. Breathing, respiration, and putrefaction are all processes that involve the breakdown of organic matter, which releases carbon dioxide as a byproduct. Burning, whether it be of fossil fuels or biomass, also releases carbon dioxide into the atmosphere, contributing to the increase in atmospheric CO2 levels and climate change.

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The inability of some individuals to perceive the taste of some substances is an example of physiological variation. Physiological variation refers to differences in physiological processes, such as metabolism or hormone production, between individuals. In the case of taste perception, physiological variation can affect an individual's ability to taste certain substances, such as bitterness or sweetness. For example, some individuals may have a genetic variation that affects the structure or function of taste receptors on the tongue, making them less sensitive to certain tastes. This can lead to differences in food preferences and dietary habits among individuals. Physiological variation is a natural occurrence in populations and is a result of genetic and environmental factors. It allows individuals to adapt to their environments and survive in different conditions.

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The autonomic nervous system comprises the sympathetic and parasympathetic systems. The autonomic nervous system is responsible for controlling the body's internal functions, such as heart rate, blood pressure, digestion, and respiratory rate, which are all necessary for maintaining homeostasis in the body. The sympathetic nervous system is responsible for the "fight or flight" response, which prepares the body for physical activity and stress by increasing heart rate, blood pressure, and respiratory rate while decreasing digestion. The parasympathetic nervous system, on the other hand, is responsible for the "rest and digest" response, which slows down heart rate, blood pressure, and respiratory rate while increasing digestion and promoting relaxation. Both the sympathetic and parasympathetic systems work together to maintain balance and stability in the body.

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The statement that is not correct about food chains is: "There is no energy loss in a food chain." In reality, energy is lost as it moves through a food chain, and this is due to the second law of thermodynamics, which states that energy cannot be created or destroyed, but it can be converted from one form to another. As organisms consume other organisms in a food chain, they convert some of the energy they receive into other forms of energy, such as heat or movement, which are not available to be passed on to the next organism in the chain. This means that energy is lost at each stage of the food chain, and this loss of energy is one reason why food chains tend to be relatively short. The longer a food chain is, the more energy is lost, and the less efficient it becomes.

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The cell membrane is a thin, flexible layer that surrounds the cells of all living organisms. It separates the inside of the cell from the outside environment and regulates the movement of substances in and out of the cell. The cell membrane is primarily composed of two types of molecules: lipids and proteins. The most common type of lipid in the cell membrane is phospholipids. They have a hydrophilic (water-loving) head and a hydrophobic (water-hating) tail. The phospholipids are arranged in a bilayer with the hydrophilic heads facing outward and the hydrophobic tails facing inward. The proteins in the cell membrane have a variety of functions, such as transporting molecules across the membrane, serving as receptors for chemical signals, and anchoring the membrane to the cell's cytoskeleton. Carbohydrates are also present in the cell membrane, but they are attached to the surface of the lipids and proteins, forming glycolipids and glycoproteins. They play a role in cell-to-cell recognition and communication. Therefore, the correct option is (C) lipids and proteins, as they are the main components of the cell membrane, while carbohydrates are also present but in a minor proportion.

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The bone that articulates with the part labelled III is the femur. The part labelled III is the acetabulum, which is a cup-shaped socket located on the lateral side of the pelvis. The femur is the thigh bone and is the longest and strongest bone in the human body. The head of the femur is shaped like a ball and articulates with the acetabulum to form the hip joint, which allows for a wide range of motion in the hip. Therefore, the femur is the bone that articulates with the part labelled III (acetabulum).

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The cross between a plant with genotype Rr (where R is the dominant allele for round seed and r is the recessive allele for wrinkled seed) and another Rr plant can be represented as follows: | | R | r | | - | - | - | | R | RR | Rr | | r | Rr | rr | The Punnett square above shows that the offspring can have the genotypes RR, Rr, or rr in a ratio of 1:2:1 respectively. Since the round seed trait is dominant, the plants with genotypes RR and Rr will have a round seed phenotype. Only the plants with genotype rr will have the wrinkled seed phenotype. Therefore, the phenotypic ratio of the offspring is 3:1 (round seed : wrinkled seed) as only one out of four possible outcomes has the recessive trait (wrinkled seed). Thus, option (D) represents the correct phenotypic ratio for this cross.

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The term used in describing the effective management of national resources by man is conservation. Conservation refers to the sustainable use and management of natural resources such as water, forests, wildlife, and minerals. It involves balancing the needs of humans with the needs of the environment, to ensure that natural resources are not depleted or destroyed over time. Conservation aims to protect the environment and the natural resources it provides, while also ensuring that these resources are used in a responsible and sustainable way. This can involve a range of strategies, such as setting aside protected areas for wildlife, implementing sustainable forestry practices, and developing renewable sources of energy. Conservation is an important concept in environmental science, as it recognizes the importance of preserving natural resources for future generations. By conserving natural resources, we can ensure that they will continue to be available for the benefit of all, while also protecting the environment and the many species that depend on it.

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The loss of water vapor through the aerial parts of the plant to the atmosphere is called transpiration. Transpiration is a natural process by which plants release water vapor from their leaves, stems, and flowers into the atmosphere. It occurs due to the movement of water through the plant from the roots to the leaves, where it is eventually lost to the air through small openings called stomata. Transpiration serves several essential functions in plants, including regulating temperature, transporting nutrients and minerals, and maintaining turgor pressure in cells. However, excessive transpiration can lead to water stress and dehydration, especially in drought conditions.

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Group instinct or social behavior is not displayed in chickens. While chickens do have some social tendencies, such as flocking together for safety and warmth, they do not exhibit the highly organized and cooperative behaviors seen in other social animals like soldier ants, bees, weaver birds, and termites. For example, soldier ants work together to protect the colony and forage for food, while bees have a complex system of communication and division of labor within the hive. Weaver birds build intricate nests together and termites cooperate in the construction of their elaborate underground colonies. Chickens, on the other hand, do not have such complex social structures or behaviors.

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The theory that new organs or characteristics develop in organisms when there is a need for them was postulated by Jean Lamarck. Lamarck believed that an organism's characteristics can be changed during its lifetime in response to its environment and these changes could be passed on to its offspring. For example, he proposed that a giraffe's long neck evolved because its ancestors stretched their necks to reach high leaves and this characteristic was passed on to their offspring. This idea is now known as the inheritance of acquired characteristics and has been largely discredited, but it was an important early attempt to explain the process of evolution.

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The pigment that transports oxygen in the blood of mammals is called "haemoglobin". Haemoglobin is a protein molecule found in red blood cells that has a high affinity for oxygen. When oxygen enters the lungs, it binds to the haemoglobin molecules in the red blood cells and is transported throughout the body. As the oxygen-depleted blood returns to the lungs, the haemoglobin releases the oxygen, which is then exhaled. This cycle repeats with each breath, allowing oxygen to be transported to all the cells in the body that require it. The other options given, chlorocruorin, melanin, haemocyanin, and haemoerythrin, are also pigments found in various animals, but they are not involved in transporting oxygen in mammalian blood. Chlorocruorin is found in some marine worms and transports oxygen in their blood, melanin is a pigment responsible for skin coloration, and haemocyanin and haemoerythrin are pigments found in the blood of some invertebrates that transport oxygen.

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Maize is best stored against insect attack in fumigated silos. Insects can easily infest stored maize, leading to losses and reduced quality of the maize. Fumigation involves using chemicals to kill insects that may be present in the maize. Fumigated silos are designed to be airtight and prevent re-infestation of the maize by insects after fumigation. Raised platforms, a sunny area, and humid metal tanks are not ideal storage options for maize. Maize stored on raised platforms is still susceptible to insect infestation, and exposure to moisture and humidity can cause the maize to spoil. Storing maize in a sunny area can also lead to moisture build-up, and this can cause the maize to mold. Humid metal tanks can also cause the maize to spoil due to moisture build-up, and they may not be airtight enough to prevent insect infestation. Leaving the maize still on the cob is not a suitable storage option either. The cob can provide a good breeding ground for insects, which can then infest the maize kernels.

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The most effective way to prevent measles is by vaccinating young children. Measles is a highly contagious viral disease that spreads through the air when an infected person coughs or sneezes. Vaccination creates immunity in the body, which helps to protect against the virus. This means that if a person comes into contact with someone who has measles, they are less likely to become infected. Vaccination not only protects the individual who receives the vaccine, but also helps to prevent the spread of the disease within the community. Therefore, vaccinating young children is an important public health measure to prevent measles outbreaks.

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The process through which hereditary characters are passed on from parent-cell to daughter-cell is called mitosis. Mitosis is a type of cell division that occurs in somatic (non-reproductive) cells and is responsible for the growth, repair, and maintenance of tissues in the body. During mitosis, the DNA in the parent-cell duplicates itself, and the two copies are separated into two new daughter-cells. Each daughter-cell receives an exact copy of the genetic material (chromosomes) from the parent-cell, ensuring that hereditary characters are passed down from one generation of cells to the next. This process occurs in both plants and animals and is essential for the survival of organisms.

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The correct option is "formation of glucose using energy from NADPH." In photosynthesis, the light reaction occurs in the chloroplasts of plant cells and involves the absorption of light by chlorophyll, which excites electrons. These electrons are passed through a series of carriers, releasing energy that is used to produce ATP and NADPH. The splitting of water molecules provides electrons to replace those lost from chlorophyll, and this also produces oxygen gas. Glucose is formed during the dark reaction (also known as the Calvin cycle) which uses energy from ATP and NADPH produced in the light reaction.

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The activities of an organism that affect the survival of another organism in a habitat can be described as biotic factors. Biotic factors are living things such as animals, plants, fungi, and bacteria that interact with each other in an ecosystem. These interactions can be either positive (mutualism or commensalism) or negative (predation, parasitism, or competition) and can have a significant impact on the survival and well-being of the organisms involved. For example, a predator (biotic factor) may hunt and kill prey (another organism) to survive, while a plant (biotic factor) may provide food and shelter for insects that pollinate it.

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The answer to the question is (c) Commensalism. Commensalism is a type of symbiotic relationship between two organisms where one organism benefits without affecting the other. Overcrowding can lead to several negative effects on a population such as increased competition for resources, aggression among individuals, and even cannibalism in some cases. Death can also occur due to the spread of diseases in overcrowded populations. However, commensalism is not a negative effect of overcrowding. It is a relationship that occurs between two organisms in a neutral or positive way, without any harm or benefit to the other.

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The stage of cell division where the cell can be said to be resting is interphase. Interphase is the stage of the cell cycle where the cell grows and replicates its DNA in preparation for cell division. During this stage, the cell is not actively dividing, but rather carrying out its normal functions. This is the stage where the cell spends most of its time before proceeding to the next stage of cell division. The other four stages of cell division (prophase, metaphase, anaphase, and telophase) are collectively known as mitosis. These stages involve the actual division of the cell, and therefore the cell is not considered to be resting during any of these stages.

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The correct sequence of levels of organization in organisms in an increasing order of complexity is: III, I, IV, II. Firstly, cells are the basic unit of life and are the simplest level of organization. Tissues are groups of cells that work together to perform a specific function, such as muscle tissue or nervous tissue. Organs are made up of multiple tissues that work together to perform a specific function, such as the heart or lungs. Finally, systems are groups of organs that work together to perform a specific function for the whole organism, such as the respiratory system or the circulatory system. Therefore, the correct sequence of levels of organization in organisms in an increasing order of complexity is: cells (III) -> tissues (I) -> organs (IV) -> systems (II).

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Commensalism is an association between two organisms of different species where one organism benefits while the other is neither benefited nor harmed. Therefore, the characteristic associated with commensalism is that the association is beneficial to one of the organisms while the other is unaffected. Option C describes this characteristic of commensalism.

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The scientist who discovered the honeycomb structure of the cell was Robert Hooke. In 1665, Robert Hooke observed thin slices of cork under a microscope and noticed small compartments that reminded him of the cells in a monastery. He called these compartments "cells", and his book "Micrographia" is considered a landmark in the history of microscopy. Hooke also examined other materials such as feathers, hair, and insects, and he discovered the honeycomb structure of the cell, which refers to the empty spaces that exist between the cells in a tissue. Therefore, the scientist who discovered the honeycomb structure of the cell was Robert Hooke.

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Salting and drying the food is a method used in food preservation for a long period. Salting and drying is an ancient method of food preservation that involves removing the moisture from food, which inhibits the growth of bacteria and other microorganisms that cause food to spoil. Salt acts as a preservative by drawing moisture out of the food and creating an environment that is inhospitable to bacteria. Drying the food complements the preserving effects of salt by reducing the water content to a level where microorganisms cannot survive. This method has been used for centuries to preserve a wide variety of foods, including fish, meat, and fruits. Once the food is salted and dried, it can be stored for months or even years without spoiling. Therefore, of the options given, salting and drying the food is the method used in food preservation for a long period.

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The statement that is not correct about the ecological effects of agricultural activities is that monocropping is one of the cultural methods of controlling pests. Monocropping, which is the practice of growing only one crop on a large area of land, can actually increase pest problems because it creates a favorable environment for pests to multiply and spread. This can lead to increased use of pesticides, which can have harmful effects on the environment and human health. The other statements are correct about the ecological effects of agricultural activities. The use of herbicides and pesticides can cause pollution, bush-burning can encourage desertification, excessive use of fertilizers can have harmful effects, and tillage of tropical soils with ploughs and harrows can encourage erosion.

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An association between living organisms in which one lives on and feeds at the expense of the other organism is known as parasitism. In parasitism, the parasite benefits from the relationship by deriving nutrients from the host, while the host is harmed by the relationship. Parasites may live on the surface of the host's body or inside its tissues, and can cause a range of negative effects, from mild irritation to severe disease or even death. Parasitism is a common type of interaction in nature, with many species of animals and plants acting as both parasites and hosts. For example, ticks and fleas are parasites that feed on the blood of mammals, while mistletoe is a parasitic plant that feeds on the sap of trees. Parasitism is different from predation, where one organism kills and eats another for food. In parasitism, the parasite feeds on the host's tissues, but typically does not kill the host outright.

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Breathing roots are characteristic structures of plants growing in mangrove swamps. Mangrove plants grow in an area where the soil is waterlogged, which leads to a lack of oxygen in the soil. Breathing roots, also called pneumatophores, are vertical structures that emerge from the soil and act as a way for the plant to take in oxygen from the air. These roots are typically seen in mangrove forests, which are characterized by shallow, brackish water and soft, muddy soil. The breathing roots of mangrove plants are essential for their survival in this unique and challenging environment.

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The translocation of food materials in plants takes place in the phloem. The phloem is a type of plant tissue that transports sugars and other nutrients produced in the leaves to other parts of the plant, such as the roots and fruits. This movement of nutrients is called translocation, and it occurs through a process known as pressure flow. During pressure flow, sugars are loaded into the phloem at the source, which is usually the leaves, creating a high concentration of sugars. This high concentration of sugars creates a pressure gradient that drives the flow of sugars through the phloem to the sink, which is usually a growing part of the plant or a storage organ. Along the way, the sugars are unloaded from the phloem and used by the plant for growth and energy.

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The evidence for evolution cannot be obtained from history because history refers to events that have occurred in the past, but it does not necessarily provide evidence for the process of evolution. The other options listed are all examples of evidence for evolution. Fossils provide evidence for the existence of organisms that lived in the past and show how they have changed over time. Anatomy shows similarities and differences between different organisms and how they are related. Embryology shows how different organisms develop and provides evidence for common ancestry. Taxonomy shows how different organisms are related and how they have evolved over time.

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The term afforestation refers to the process of planting trees in areas where there were previously no trees. This is usually done to increase forest cover, combat deforestation, and create new forests. It involves planting trees of value to man in areas that were previously devoid of forests. Therefore, the answer is option B, "planting trees of value to man."

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The spaces between neurons through which impulses cross by means of chemical reaction are called "synapses". Neurons are nerve cells that transmit information to other nerve cells or muscles in the body. Synapses are the connection points between two neurons or between a neuron and a muscle cell. When a neuron fires an electrical impulse, it releases chemical messengers called neurotransmitters into the synapse. These neurotransmitters then bind to receptors on the receiving neuron or muscle cell, causing it to either fire its own electrical impulse or trigger a muscle contraction. Synapses play a crucial role in the communication between neurons and are essential for normal brain function and behavior.

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Overcrowding in plants can lead to competition for resources such as water, nutrients, and sunlight. When seedlings of different species are overcrowded, there is less competition for resources because the different species may have different nutritional needs or be active at different times. This means that each species can obtain the necessary resources to grow and develop without being hindered by the other species. On the other hand, when overcrowding involves one species, they will be in direct competition with each other since they have similar nutritional needs, and this can hinder their growth and development. Therefore, overcrowding in seedlings of different species is less serious than overcrowding involving one species.

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The similarity between the bone structure of the arm of a human and the fore-limb of a rabbit indicates a common origin. This means that both the human and the rabbit have evolved from a common ancestor that had a similar bone structure. This is supported by the fact that the bone structure of the arm of a human and the fore-limb of a rabbit have the same basic arrangement of bones, such as the humerus, radius, and ulna. This similarity in bone structure is a result of the two species sharing a common evolutionary history.

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The main internal tissue of a leaf is the mesophyll. The mesophyll is the primary site of photosynthesis in the leaf, where light energy is used to convert carbon dioxide and water into glucose and oxygen. The mesophyll tissue is located between the upper and lower epidermis of the leaf and consists of two distinct layers: the palisade mesophyll and the spongy mesophyll. The palisade mesophyll is made up of long, narrow cells that are densely packed with chloroplasts, which are the organelles responsible for photosynthesis. The spongy mesophyll, on the other hand, is made up of loosely packed cells that contain air spaces, which allow for the exchange of gases during photosynthesis. Vascular tissue, which consists of the xylem and phloem, is responsible for transporting water, minerals, and nutrients throughout the plant and is also present in the leaf. The upper and lower epidermis are the outermost layers of the leaf, which serve to protect the mesophyll tissue and regulate gas exchange through specialized pores called stomata. The cuticle is a waxy layer that covers the outer surface of the leaf and helps to prevent water loss.

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The adaptation of the small intestine that is not related to the absorption of digested food is that the small intestine leads into the large intestine. The small intestine is a highly specialized organ of the digestive system that is responsible for the final stages of digestion and the absorption of nutrients into the bloodstream. The small intestine is well adapted for its function as it has a large surface area due to the presence of numerous and thin-walled finger-like projections called villi. These villi are well supplied with blood and lymph vessels, which allow for efficient absorption of nutrients. Additionally, the small intestine is long and coiled, which provides a longer path for the food to travel and increases the time available for absorption. However, the fact that the small intestine leads into the large intestine is not directly related to the absorption of digested food. Instead, the large intestine is responsible for the absorption of water and electrolytes from the remaining food material, and the formation and elimination of feces.

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A dehiscent fruit formed from several fused carpels with many seeds is classified as a capsule. A capsule is a type of fruit that is formed from multiple carpels that are fused together. When the fruit is mature, the carpels split open, usually along a seam, to release the seeds. Capsules can contain many seeds and are often found in plants such as poppies and lilies. Capsules can also be dry or fleshy and can have different shapes, sizes, and textures depending on the plant species. Overall, capsules are a common type of fruit in the plant kingdom and are an important structure for dispersing seeds.

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Cells that are actively respiring and photosynthesizing require a significant amount of energy to carry out these processes. Chloroplasts and mitochondria are the two organelles that are primarily responsible for energy production in eukaryotic cells. Chloroplasts are the site of photosynthesis, where light energy is used to convert carbon dioxide and water into glucose and oxygen. The glucose produced in the chloroplasts is then used as an energy source for the cell. Mitochondria are the site of cellular respiration, where glucose is broken down to produce ATP (adenosine triphosphate), the main energy currency of the cell. This process requires oxygen and generates carbon dioxide as a waste product. Therefore, cells that are actively respiring and photosynthesizing would require both chloroplasts and mitochondria to produce energy and carry out these processes efficiently. Thus, option (D) represents the organelles that are likely to be present in cells that are actively respiring and photosynthesizing.

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Autotrophs are described as producers. This is because they are able to produce their own food through the process of photosynthesis or chemosynthesis. They are the base of the food chain and provide food for all other organisms in the ecosystem. Unlike consumers that consume other organisms for their food, and decomposers that break down dead organisms, autotrophs are able to produce their own organic molecules from inorganic substances in the environment, making them an essential component of the ecosystem.

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Rhizobium is the bacterium that fixes atmospheric nitrogen. Nitrogen is a very important element for living organisms, and it is a major component of DNA, RNA, and proteins. However, most living organisms cannot use atmospheric nitrogen directly, as it is in a stable form. Nitrogen must first be "fixed" or converted into a usable form, such as ammonia or nitrate. Rhizobium is a type of bacteria that has the ability to convert atmospheric nitrogen gas into a form that plants can use. It does this by forming a symbiotic relationship with certain types of plants, such as legumes. The Rhizobium bacteria infect the roots of these plants and form structures called nodules. Within the nodules, the Rhizobium bacteria convert atmospheric nitrogen into a usable form of nitrogen that the plant can use to grow and develop. In summary, Rhizobium is the bacterium that fixes atmospheric nitrogen by forming a symbiotic relationship with certain types of plants and converting nitrogen gas into a form that the plant can use.

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The group of organisms that feeds directly on green plants are primary consumers. Primary consumers are herbivores, which means they eat plants as their primary source of food. Green plants are producers, which means they produce their own food through photosynthesis. Primary consumers, such as grasshoppers or caterpillars, feed directly on these plants to obtain the energy and nutrients they need. Secondary consumers are animals that feed on primary consumers, while tertiary consumers are animals that feed on secondary consumers. Decomposers, such as bacteria and fungi, feed on dead and decaying matter and play an important role in breaking down organic material and returning nutrients to the soil.

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The large intercellular air spaces in most hydrophytes (water plants) provide a pathway for gases to move through the plant. This allows for oxygen produced in the leaves to reach the submerged parts of the plant, and for carbon dioxide absorbed by the leaves to diffuse to the roots. However, these air spaces do not play a role in the transport of salts, plant hormones, or manufactured food.

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Chlorophyll is not likely to be found in the cell of a ripe tomato fruit. Chlorophyll is a green pigment found in chloroplasts that is essential for photosynthesis, the process by which plants make their own food. However, ripe tomato fruits do not photosynthesize, as they are no longer attached to the plant and cannot produce their own energy. Therefore, chlorophyll is not needed in the cells of a ripe tomato fruit. The other options, such as plastids, cellulose cell wall, mitochondrion, and mineral salts, are all commonly found in plant cells, including those of ripe tomato fruits.

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The name given to the structure labelled A at this stage of cell division is "chromosome". Chromosomes are structures made up of DNA and proteins that carry an organism's genetic information. They are visible during cell division when the DNA condenses and becomes visible under a microscope. At this stage, each chromosome consists of two identical copies, called chromatids, that are held together at a region called the centromere. Chromosomes play a crucial role in cell division, as they ensure that each daughter-cell receives a complete set of genetic information. After cell division is complete, the chromosomes unravel into their chromatin form, which allows for normal cellular activities, such as gene expression, to take place.

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