RowQ
The Vault
RowQ
The Vault
CBSE Class 9 Biology · 12 questions · 29 marks
Sunlight falling on a paddy field, rain draining into a river, nitrogen locked in the air and later found inside your own proteins — none of these belong to a single subject, and that is precisely the point of this chapter. The Earth behaves as one connected system in which the atmosphere, hydrosphere, lithosphere and biosphere constantly exchange matter and energy. Once you can trace a carbon atom or a unit of solar energy through that system, questions about pollution, soil loss and climate change stop being separate topics and become one story.
Which of the following best describes how energy and matter behave in the Earth system?
Answer
Energy flows in one direction, while matter is recycled is correct — solar energy enters the biosphere, moves through trophic levels and is finally lost as heat that cannot be reused, whereas elements such as carbon, nitrogen and oxygen are used repeatedly through biogeochemical cycles. The other options reverse or deny this fundamental distinction.
Rhizobium bacteria living in the root nodules of a pea plant are important because they:
Answer
Convert atmospheric nitrogen into compounds the plant can use is correct — these symbiotic bacteria fix inert nitrogen gas into ammonia and related compounds that the legume can build into proteins, which is why legumes enrich soil. Breaking nitrates down to nitrogen gas is the work of denitrifying bacteria, sunlight absorption depends on chlorophyll, and cellulose digestion is carried out by other decomposers.
In the food chain grass to grasshopper to frog to snake, if the grass captures 10000 units of energy, roughly how much reaches the snake?
Answer
10 units is correct — following the ten per cent law, the grasshopper receives about 1000 units, the frog about 100 units, and the snake about 10 units, since only about a tenth of the energy at each trophic level passes to the next. The remaining energy is lost as heat and used in the organisms' own life processes.
The thinning of the ozone layer over the polar regions is most directly caused by:
Answer
Chlorofluorocarbons released from refrigerants and aerosols is correct — these stable compounds drift into the upper atmosphere, where ultraviolet light frees chlorine atoms that break ozone molecules apart in a chain reaction. Carbon dioxide and methane contribute mainly to the greenhouse effect, and sulphur dioxide causes acid rain rather than ozone depletion.
Assertion (A): Cutting down forests on a hillside is usually followed within a few years by heavy loss of topsoil. Reason (R): Tree roots bind soil particles together and the leaf canopy slows the impact of falling rain.
Answer
Both A and R are true and R is the correct explanation of A — a forest canopy intercepts rainfall so that drops strike the ground gently instead of dislodging soil particles, and the dense root network physically holds the soil in place while allowing water to soak in. When the trees are removed, rain strikes bare ground directly and runs off the slope, carrying the fertile topsoil away. Since topsoil takes hundreds of years to form, this loss is effectively irreversible on a human timescale.
Differentiate between the flow of energy and the cycling of matter in an ecosystem.
Answer
Energy enters an ecosystem only from an external source, the Sun, and moves in a single direction from producers to herbivores to carnivores. At every transfer a large fraction, around ninety per cent, is lost as heat to the surroundings, and this heat cannot be recaptured by living organisms, so energy must be continually resupplied by sunlight. Matter, by contrast, is not created or lost. The same atoms of carbon, nitrogen, oxygen and hydrogen pass from the air and soil into plants, then into animals, and are returned to the environment by respiration, excretion and the action of decomposers, ready to be used again. Energy therefore flows through the ecosystem, while matter circulates within it.
Explain why the greenhouse effect is essential for life but its intensification is harmful.
Answer
Gases such as carbon dioxide, water vapour and methane allow incoming sunlight to reach the Earth's surface but absorb much of the heat radiated back, keeping the planet's average temperature at a level that supports liquid water and life; without this natural greenhouse effect the Earth would be far too cold for most organisms. The problem arises when human activities such as burning fossil fuels and clearing forests sharply raise the concentration of these gases. More heat is then trapped than before, raising global temperatures, melting glaciers and polar ice, causing sea levels to rise and disturbing rainfall patterns and crop seasons.
Describe the main steps of the water cycle and name the Earth spheres involved at each step.
Answer
Water in oceans, lakes and rivers is heated by the Sun and changes into vapour by evaporation, while plants add vapour through transpiration; this step links the hydrosphere and biosphere to the atmosphere. High in the atmosphere the vapour cools and condenses around dust particles to form clouds, and when the droplets grow heavy enough they fall as precipitation in the form of rain, snow or hail. The water that reaches the ground either runs off over the surface into streams and back to the sea, or infiltrates the soil and rock to become groundwater; this stage connects the atmosphere to the lithosphere and hydrosphere, and living organisms take up part of this water for their own use before returning it. The cycle then repeats, so the total quantity of water on Earth stays effectively constant.
Why are decomposers described as the recyclers of an ecosystem?
Answer
Decomposers such as bacteria and fungi feed saprophytically on dead plants, animal remains and excreta, breaking their complex organic compounds down into simple inorganic substances such as carbon dioxide, water, nitrates and mineral salts. These simple substances are released back into the soil and air, where producers can absorb and reuse them. Without decomposers, nutrients would stay locked in dead bodies, the soil would become infertile, and the biogeochemical cycles that keep matter in circulation would break down.
Explain the nitrogen cycle in detail, describing the role of each group of microorganisms involved, and state why nitrogen must be converted before plants can use it.
Answer
Nitrogen makes up about seventy-eight per cent of the atmosphere, yet plants and animals cannot use it in this gaseous form. The two nitrogen atoms in a nitrogen molecule are held together by an extremely strong triple bond, making the gas chemically inert, so it must first be converted, or fixed, into reactive compounds such as ammonia, nitrites and nitrates that plants can absorb through their roots. Nitrogen fixation is carried out chiefly by nitrogen-fixing bacteria such as Rhizobium, which live symbiotically in the root nodules of leguminous plants like peas, beans and gram, and by free-living soil bacteria. A smaller amount of fixation happens physically during lightning, when the intense energy makes atmospheric nitrogen combine with oxygen to form oxides that dissolve in rain and reach the soil. Ammonification follows the death of organisms. Decomposing bacteria and fungi break down proteins in dead bodies and in excreta, releasing nitrogen as ammonia into the soil. Nitrification is the work of nitrifying bacteria, which oxidise ammonia first into nitrites and then into nitrates. Nitrates are the form most readily taken up by plant roots and built into plant proteins, nucleic acids and other nitrogen-containing molecules. Assimilation then transfers this nitrogen along the food chain, as animals eat plants and convert plant protein into animal protein. Finally, denitrifying bacteria in poorly aerated, waterlogged soils convert nitrates back into free nitrogen gas, which returns to the atmosphere and completes the cycle. Because of this cycle the total quantity of nitrogen in the Earth system remains balanced, even though individual atoms move repeatedly between the atmosphere, soil, plants and animals.
The Earth is often described as a single interconnected system rather than four separate spheres. Justify this statement using examples that link the atmosphere, hydrosphere, lithosphere and biosphere.
Answer
The four spheres cannot be studied in isolation because a change in any one of them produces measurable changes in the others; the connections run through both matter and energy. The biosphere and atmosphere are linked by respiration and photosynthesis. Green plants remove carbon dioxide from the air and release oxygen, while all living things and every fire return carbon dioxide to it. The composition of the air we breathe is therefore itself a product of life. The atmosphere and hydrosphere are linked by the water cycle. Solar energy evaporates ocean water into the air, and the vapour returns as precipitation. This exchange also moves heat around the planet and drives winds, monsoons and ocean currents. The hydrosphere and lithosphere interact through weathering and erosion. Flowing water and freezing ice break rock down into fragments and dissolve minerals from it, gradually forming soil and carrying dissolved salts to the sea, which is one reason seawater is saline. The lithosphere and biosphere meet in the soil. Plant roots penetrate and further break up rock, decaying organisms add humus that holds water and nutrients, and in return the soil supplies plants with the mineral nitrogen, phosphorus and potassium they need. Human activity shows these links most sharply. Burning coal in a power station adds carbon dioxide to the atmosphere; this intensifies the greenhouse effect and warms the planet; warming melts glaciers in the lithosphere and raises sea level in the hydrosphere; and the resulting shifts in temperature and rainfall alter where crops can grow and which species survive in the biosphere. A single action therefore travels through all four spheres, which is exactly why the Earth is treated as one system.
The village of Tarapali sits below a hill that was densely wooded until timber cutting cleared most of it eight years ago. Since then villagers report three changes: the stream that once flowed all year now dries up by March, the fields at the foot of the hill are covered with coarse sand and give poor yields, and the well water level has dropped sharply. The panchayat is considering a plan to replant native trees along the slope and build low stone bunds across it. Answer the following: (a) Explain why the stream stopped flowing all year after the trees were cut. (b) Account for the coarse sand now covering the fields. (c) Why has the well water level fallen? (d) Explain how the panchayat's plan would help, naming the Earth spheres involved.
Answer
(a) A forested slope acts like a sponge. The canopy slows the rain, the litter layer holds it, and the roots keep the soil porous so that water soaks in rather than rushing away. This stored groundwater seeps out slowly and fed the stream through the dry months. With the trees gone, rainwater now runs straight off the bare slope during the monsoon and there is nothing left underground to release later, so the stream flows only while it is actually raining and dries up by March. (b) The fast-moving runoff carried away the fine, fertile topsoil from the hillside, a process called soil erosion. The heavier coarse particles and sand were dropped where the water slowed down at the foot of the hill, burying the fields. Coarse sand holds very little water and almost no humus or nutrients, which is why yields have fallen. (c) The well is fed by groundwater. Since rain now runs off the surface instead of infiltrating the soil, far less water percolates down to recharge the water table each year, while extraction has continued as before. The result is a steadily falling water level. (d) Replanting native trees will restore the canopy and root network, so rain will again strike the ground gently and soak in, erosion will slow, and leaf litter will begin rebuilding humus in the soil. The stone bunds act immediately: they break the speed of runoff across the slope, trap soil that would otherwise be washed away, and hold water long enough for it to infiltrate. Together these measures should return the stream to year-round flow and raise the well level over several seasons. The plan involves the biosphere (the replanted trees), the lithosphere (the soil being conserved and rebuilt), the hydrosphere (the stream, the groundwater and the well) and the atmosphere (the rainfall being intercepted and the water vapour returned by transpiration), showing how a single intervention works across all four spheres.
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