RowQ
The Vault
RowQ
The Vault
CBSE Class 10 Biology · 11 questions · 26 marks
Nothing in nature exists in isolation — a single leaf, a deer that eats it, and the bacteria that eventually decompose them are all threads in the same web of an ecosystem. This chapter examines how energy and matter flow through food chains, why nature never wastes anything, and how human activities like using non-biodegradable plastics or ozone-depleting chemicals can disturb this balance.
Green plants, which manufacture their own food, occupy which position in a food chain?
Answer
Producers is correct — green plants trap solar energy through photosynthesis and form the first trophic level, on which all other organisms in the food chain ultimately depend.
Which group of chemicals, once widely used in refrigerators and aerosol sprays, is chiefly responsible for depleting the ozone layer?
Answer
Chlorofluorocarbons (CFCs) is correct — these stable but harmful gases rise to the upper atmosphere and break down ozone molecules, thinning the protective ozone layer.
According to the 10 percent law of energy flow, if a producer level has 10,000 kJ of energy, how much energy is approximately available to primary consumers?
Answer
1,000 kJ is correct — only about 10% of the energy at one trophic level is transferred to the next, so 10% of 10,000 kJ is 1,000 kJ.
Which of the following substances is non-biodegradable?
Answer
Plastic bag is correct — plastics are not broken down by microorganisms and persist in the environment for a very long time, unlike banana peel, cotton, or paper.
Assertion (A): Top carnivores in a food chain, such as large fish-eating birds, often accumulate the highest concentration of certain harmful pesticides in their bodies. Reason (R): Non-biodegradable chemicals undergo biological magnification, becoming more concentrated at each successive trophic level as they pass up the food chain.
Answer
Both A and R are true and R is the correct explanation of A — since these chemicals are not broken down or excreted efficiently, they accumulate progressively as one organism eats many of the level below, reaching the highest concentration in top consumers.
Define food chain and food web, and state one key difference between them.
Answer
A food chain is a single, straight-line sequence showing the transfer of food energy from producers through a series of consumers, such as grass to deer to lion. A food web is a more realistic network formed by many interconnected and overlapping food chains within an ecosystem, since most organisms eat, and are eaten by, more than one type of organism. The key difference is that a food chain shows only one linear feeding pathway, while a food web shows multiple interconnected pathways occurring simultaneously in an ecosystem.
Why are decomposers considered essential components of an ecosystem?
Answer
Decomposers, mainly bacteria and fungi, break down the complex organic compounds present in dead plants, dead animals, and waste products into simpler inorganic substances. This decomposition releases nutrients like nitrogen, phosphorus, and minerals back into the soil, air, and water, making them available again for uptake by producers. Without decomposers, nutrients would remain locked in dead matter, and the natural recycling of materials in the ecosystem would stop.
Explain the 10 percent law of energy transfer in an ecosystem with a suitable example.
Answer
The 10 percent law states that at each trophic level in a food chain, only about 10% of the energy received from the trophic level below is available for transfer to the next level; the remaining 90% is lost mainly as heat during respiration, used up in the organism's own life processes, or remains in body parts that are not consumed. For example, if producers (grass) fix 10,000 kJ of solar energy, only about 1,000 kJ becomes available to the herbivores (deer) that eat the grass, and only about 100 kJ is available to the carnivores (lion) that eat the deer. This large loss of usable energy at each step is why food chains rarely extend beyond four or five trophic levels.
Discuss the causes and harmful effects of ozone layer depletion, and suggest two measures to help control it.
Answer
The ozone layer, present in the stratosphere, absorbs most of the sun's harmful ultraviolet (UV) radiation before it reaches the Earth's surface. Its depletion is caused mainly by chlorofluorocarbons (CFCs), which were widely used in refrigerants, air conditioners, aerosol sprays, and foam packaging; when released into the air, these stable compounds slowly rise to the stratosphere, where UV radiation breaks them down and releases chlorine atoms that destroy ozone molecules. As the ozone layer thins, particularly noticeable as the ozone hole over Antarctica, more UV radiation reaches the Earth's surface, increasing the risk of skin cancer, cataracts (eye damage), weakened immunity in humans and animals, and harm to crop yields and plankton, which in turn affects entire food chains. Measures to control this include phasing out and replacing CFCs with less harmful alternatives in refrigerators, air conditioners, and aerosol products, and enforcing international agreements, such as the Montreal Protocol, that limit and phase out the production and use of ozone-depleting substances worldwide.
Explain the flow of energy through a four-step terrestrial food chain, and state why such chains rarely have more than four or five trophic levels.
Answer
In a typical terrestrial food chain — for example, grass (producer) to grasshopper (primary consumer) to frog (secondary consumer) to snake (tertiary consumer) — energy enters the ecosystem when producers like grass trap a small fraction of the sun's energy through photosynthesis and convert it into chemical energy stored in their tissues. When the grasshopper eats the grass, only about 10% of the energy stored in the grass is transferred into the grasshopper's body, since most of the rest is lost as heat during the grass's own respiration or remains in parts not eaten. Similarly, when the frog eats the grasshopper, only about 10% of the grasshopper's stored energy passes on to the frog, and when the snake eats the frog, again only about 10% is transferred to the snake. Because roughly 90% of the available energy is lost at every single step, the amount of usable energy remaining becomes extremely small after just a few transfers, which is why most food chains are limited to four or five trophic levels — beyond that point, too little energy remains to support a viable population at the next level.
A residential society decides to start segregating its household waste into two separate bins: one for leftover food, vegetable peels, and garden waste, and another for plastic wrappers, bottles, and broken glass. Explain the scientific basis for this segregation, and describe the concept of the 3 Rs that could further help this society manage its waste responsibly.
Answer
This segregation is based on the difference between biodegradable waste (leftover food, vegetable peels, garden waste), which can be broken down naturally by decomposer microorganisms and even converted into compost, and non-biodegradable waste (plastic wrappers, bottles, glass), which persists in the environment for a very long time and cannot be broken down by microbes, so it needs separate collection for proper disposal or recycling. To manage waste more responsibly, the society could follow the 3 Rs: Reduce, by using fewer disposable or single-use items, such as avoiding excess plastic packaging; Reuse, by using items like glass jars, cloth bags, and containers multiple times before discarding them; and Recycle, by sending recyclable materials such as plastic, paper, glass, and metal for processing into new products instead of sending them to a landfill.
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