CBSE Class 10 Science HOTs Our Environment Set 04

Question. We often use the word environment. What does it mean?
Answer: Environment refers to the sum total of all external factors (both living and non-living) and conditions that surround and affect an organism.

Question. Which of the following are always at the second trophic level of food chains?
Carnivores, Autotrophs, Herbivores
Answer: Herbivores.

Question. The following organisms form a food chain. Which of these will have the highest concentration of non-biodegradable chemicals? Name the phenomenon associated with it.
Insects, Hawk, Grass, Snake, Frog
Answer: The Hawk will have the highest concentration. The phenomenon is called biological magnification (or biomagnification).

Question. List two examples of natural ecosystem.
Answer: Forest and Desert (or Ponds/Lakes).

Question. Use of several pesticides which results in excessive accumulation of pesticides in rivers or ponds, is a matter of deep concern. Justify this statement.
Answer: Pesticides are non-biodegradable chemicals. When they enter water bodies, they are taken up by aquatic plants and animals. Through food chains, they undergo biological magnification, reaching very high concentrations in top consumers (like humans), causing serious health issues including nervous system damage and reproductive problems.

Question. (i) Why are crop fields considered as artificial ecosystems? (ii) Write a common food chain of four steps operating in a terrestrial ecosystem.
Answer: (i) Crop fields are considered artificial because they are created and maintained by humans; they are not self-sustaining like natural ecosystems.
(ii) \( \text{Grass} \rightarrow \text{Grasshopper} \rightarrow \text{Frog} \rightarrow \text{Snake} \).

Question. (i) List two human-made ecosystems. (ii) “We do not clean a pond in the same manner as we do in an aquarium.” Give reason to justify this statement.
Answer: (i) Gardens and Aquariums.
(ii) A pond is a natural, self-sustaining ecosystem containing decomposers (bacteria and fungi) that naturally break down waste and dead matter. An aquarium is an artificial ecosystem that lacks a complete set of natural cleaners and decomposers, so it requires regular manual cleaning to remove waste.

Question. In the following food chain, only \( 2 \text{ J} \) of energy was available to the peacocks. How much energy would have been present in Grass? Justify your answer.
\( \text{Grass} \rightarrow \text{Grasshopper} \rightarrow \text{Frog} \rightarrow \text{Snake} \rightarrow \text{Peacock} \)
Answer: Applying the \( 10\% \) law in reverse:
Energy at Peacock \( = 2 \text{ J} \)
Energy at Snake \( = 20 \text{ J} \)
Energy at Frog \( = 200 \text{ J} \)
Energy at Grasshopper \( = 2,000 \text{ J} \)
Energy at Grass \( = 20,000 \text{ J} \).

\( \implies 20,000 \text{ J} \)

Question. What are decomposers? List two important roles they play in the environment.
Answer: Decomposers (saprotrophs) are microorganisms like bacteria and fungi that break down the dead remains and waste products of organisms. Roles: (i) They clean the environment by disposing of dead organic matter. (ii) They replenish the soil with nutrients (nutrient recycling), maintaining soil fertility.

Question. List two reasons to show that the existence of decomposers is essential in an ecosystem.
Answer: (i) Without decomposers, dead bodies of plants and animals would pile up, polluting the environment. (ii) They return essential minerals to the soil, without which producers could not continue to grow, leading to the collapse of the food chain.

Question. State with reason any two possible consequences of elimination of decomposers from the earth.
Answer: (i) Accumulation of waste: The Earth would be covered with dead remains and organic waste. (ii) Stoppage of nutrient cycling: The soil would lose its fertility as nutrients would not be recycled, eventually causing all life to perish due to lack of food.

Question. What are human-made ecosystems? Give an example. Can a human-made ecosystem become a self-sustaining ecosystem? Give reason to justify your answer.
Answer: Human-made ecosystems are artificial environments created and maintained by human beings, such as a crop field or an aquarium. Generally, they cannot become fully self-sustaining because they require constant human input (like watering, weeding, or providing food) and lack the complex, diverse interactions and complete nutrient cycles found in natural ecosystems.

Question. (a) Name the group of organisms which form in the first trophic level of all food chains. Why are they called so? (b) Why are the human beings most adversely affected by biomagnification? (c) State one ill-effect of the absence of decomposers from a natural ecosystem.
Answer: (a) Producers (or autotrophs). They are called producers because they produce organic food from inorganic substances using sunlight.
(b) Humans are top consumers. Since harmful chemicals increase in concentration at each trophic level (biomagnification), humans consume the most concentrated toxins.
(c) The natural recycling of materials would stop, leading to soil exhaustion.

Question. What are consumers? Name the four categories under which the consumers are further classified.
Answer: Consumers (heterotrophs) are organisms that depend on other organisms for food. Categories: (i) Herbivores, (ii) Carnivores, (iii) Omnivores, (iv) Parasites.

Question. From the following group of organisms create a food chain which is most advantageous for human beings in terms of energy.
Hawk, Rat, Cereal plant, Goat, Snake, Human being
Answer: The shortest food chain is most efficient: \( \text{Cereal plant} \rightarrow \text{Human being} \). According to the \( 10\% \) law, energy loss is minimized when there are fewer steps in the chain.

Question. (a) Create a food chain of the following organisms.
Insect, Hawk, Grass, Snake, Frog
(b) Name the organism at the third trophic level of the created food chain.
(c) Which organism of this food chain will have the highest concentration of non-biodegradable chemicals?
(d) Name the phenomenon associated with it.
(e) If 10,000 Joules of energy is available to frogs, how much energy will be available to snakes in this food chain?
Answer: (a) Grass \( \rightarrow \) Insect \( \rightarrow \) Frog \( \rightarrow \) Snake \( \rightarrow \) Hawk
(b) Frog is at the third trophic level.
(c) Hawk will have the highest concentration of chemicals.
(d) Biological magnification.
(e) According to the 10% law, if 10,000 J is available to frogs, then 10% of that will be available to snakes.
\( \implies \) Energy for snakes = \( 10\% \text{ of } 10,000 \text{ J} = 1,000 \text{ J} \).

Question. (a) What is an ecosystem?
(b) List any two natural ecosystems.
(c) We do not clean ponds or lakes but an aquarium needs to be cleaned regularly. Why?
Answer: (a) An ecosystem is a functional unit of nature where living organisms (biotic factors) interact among themselves and with the surrounding physical environment (abiotic factors).
(b) Forests and Ponds (or Oceans).
(c) Ponds and lakes are natural, complete ecosystems with a balanced population of producers, consumers, and decomposers that recycle waste. An aquarium is an artificial, incomplete ecosystem that lacks a balanced natural recycling system, so waste must be removed manually to keep the fish healthy.

Question. What is meant by trophic level in a food chain? Construct a terrestrial food chain with trophic levels. The energy flow in a food chain is always unidirectional. Why?
Answer: Each step or level of a food chain where transfer of food and energy takes place is called a trophic level.
Terrestrial food chain: Grass (T1) \( \rightarrow \) Grasshopper (T2) \( \rightarrow \) Frog (T3) \( \rightarrow \) Snake (T4).
Energy flow is unidirectional because energy captured by autotrophs from sunlight does not revert back to the sun, and energy passed to herbivores does not go back to autotrophs. As it moves up the trophic levels, it is lost as heat and used for metabolic activities, making it unavailable to the previous level.

Question. (a) Construct a terrestrial food chain comprising four trophic levels.
(b) What will happen if we kill all organisms in one trophic level?
(c) Calculate the amount of energy available to the organisms at the fourth trophic level. If the energy available to the organisms at the second trophic level is 2000 J.
Answer: (a) Plants \( \rightarrow \) Grasshopper \( \rightarrow \) Frog \( \rightarrow \) Snake.
(b) If all organisms in one trophic level are killed, the food chain will be disrupted. The population of organisms in the previous level will increase uncontrollably (due to lack of predators), and the organisms in the next level will die of starvation.
(c) Energy at 2nd level = 2000 J
Energy at 3rd level = \( 10\% \text{ of } 2000 = 200 \) J
Energy at 4th level = \( 10\% \text{ of } 200 = 20 \) J.

Question. (a) A food chain generally has three or four trophic levels. Explain.
(b) What is biological magnification ? Explain.
Answer: (a) A food chain usually has only 3-4 levels because, according to the 10% law, very little usable energy remains after 4 transfers. By the time energy reaches the 5th level, it is insufficient to support a population of organisms.
(b) Biological magnification is the process by which the concentration of harmful, non-biodegradable chemicals increases at each successive trophic level of a food chain. As these chemicals cannot be broken down, they accumulate in higher amounts in top-level predators.

How do Our Activities Affect the Environment?

Question. What is a food chain? Why is the flow of energy in an ecosystem unidirectional? Explain briefly.
Answer: A food chain is a linear sequence of organisms where each is dependent on the next as a source of food. Energy flow is unidirectional because energy flows from the sun to producers and then to consumers. It cannot move in the reverse direction (e.g., from herbivores back to plants) because once energy is consumed and transformed into heat or used for metabolism, it is lost to the environment and cannot be recaptured by the previous level.

Question. “Energy flow in food chains is always unidirectional.” Justify this statement. Explain how the pesticides enter a food chain and subsequently get into our body.
Answer: Energy flow is unidirectional because energy captured from sunlight by producers is lost as heat as it moves up trophic levels and cannot be reused by the sun or plants. Pesticides enter the food chain when they are sprayed on crops or soil; they are absorbed by plants along with water and minerals. When primary consumers eat these plants, the pesticides enter their bodies, and the concentration increases at each higher level, eventually reaching humans who eat those plants or animals.

Question. “Our food grains such as wheat and rice, the vegetables and fruits and even meat are found to contain varying amounts of pesticide residues.” State the reason to explain how and why it happens.
Answer: This happens due to the widespread use of pesticides and insecticides in agriculture to protect crops from pests. These chemicals are non-biodegradable and are absorbed by plants from the soil or water. They accumulate in the plant parts (grains, fruits) and in the bodies of animals that consume them, a process called biological magnification.

Question. What is meant by food chain? “The number of trophic levels in a food chain is limited.” Give reason to justify this statement.
Answer: A food chain is a sequence of organisms in an ecological community that represents who eats whom. The number of levels is limited because only 10% of energy is transferred to the next level. After 4 or 5 levels, the amount of available energy becomes too small to sustain the life of an organism at a higher level.

Ozone Layer and How it is Getting Depleted

Question. Choose the incorrect statement from the following:
(a) Ozone is a molecule formed by three atoms of oxygen.
(b) Ozone shields the surface of the earth from ultraviolet radiations.
(c) Ozone is deadly poisonous.
(d) Ozone gets decomposed by UV radiations.
Answer: (d) Ozone gets decomposed by UV radiations.

Question. The depletion of ozone layer is a cause of concern. Why?
Answer: Ozone layer depletion is a concern because it allows harmful ultraviolet (UV) radiation from the sun to reach the Earth's surface. This can cause skin cancer, cataracts, and damage the immune system in humans, as well as harm crops and aquatic life.

Question. Why is excessive use of CFCs a cause of concern?
Answer: CFCs (Chlorofluorocarbons) are stable compounds that reach the upper atmosphere (stratosphere) where they are broken down by UV radiation, releasing chlorine atoms. These chlorine atoms react with and destroy ozone molecules, leading to the thinning of the ozone layer.

Question. What is the function of ozone in the upper atmosphere?
Answer: The primary function of ozone in the upper atmosphere is to absorb the majority of the sun's harmful ultraviolet (UV) radiation, protecting living organisms on Earth from its damaging effects.

Question. Write the full name of the group of compounds mainly responsible for the depletion of ozone layer.
Answer: Chlorofluorocarbons (CFCs).

Question. What is ozone? How is it formed in the upper layers of the earth’s atmosphere? How does ozone affect our ecosystem?
Answer: Ozone (\( O_3 \)) is a gas composed of three oxygen atoms. It is formed in the upper atmosphere when high-energy UV radiation splits oxygen molecules (\( O_2 \)) into free oxygen atoms (\( O \)). These atoms then combine with molecular oxygen to form ozone (\( O + O_2 \rightarrow O_3 \)). Ozone protects the ecosystem by filtering out UV rays; its depletion leads to harmful effects on plants, animals, and human health.

Question. How is ozone formed in the higher levels of the atmosphere? “Damage to the ozone layer is a cause of concern.” Justify this statement.
Answer: Ozone is formed by the action of UV radiation on \( O_2 \) molecules. High energy UV splits \( O_2 \) into atomic oxygen, which then reacts with \( O_2 \) to form \( O_3 \). Damage is a concern because the ozone layer protects us from UV radiation which can cause skin cancer, mutations, and disrupt ecological balances.

Question. (a) We do not clean ponds or lakes, but an aquarium needs to be cleaned regularly. Why?
(b) Why is ozone layer getting depleted at the higher levels of the atmosphere? Mention one harmful effect caused by its depletion.
Answer: (a) Ponds are natural ecosystems with decomposers that manage waste, whereas aquariums are artificial and lack such self-cleaning mechanisms.
(b) The ozone layer is depleting due to the release of synthetic chemicals like CFCs. One harmful effect is an increased incidence of skin cancer in humans.

Question. How is ozone layer formed? State its importance to all life forms on earth. Why the amount of ozone in the atmosphere dropped sharply in the 1980s?
Answer: Ozone is formed in the stratosphere by the photochemical reaction of UV rays on oxygen. It is important because it shields the Earth from harmful UV-B radiation. The sharp drop in the 1980s was due to the increased use of CFCs in refrigeration, fire extinguishers, and aerosol sprays, which released ozone-destroying chlorine.

Question. You have been selected to talk on “Ozone layer and its protection” in the school assembly on ‘Environment Day’.
(a) Why should ozone layer be protected to save the environment?
(b) List any two ways that you would stress in your talk to bring in awareness amongst your fellow friends that would also help in protection of ozone layer as well as the environment.
Answer: (a) The ozone layer should be protected because it prevents harmful UV rays from reaching Earth, protecting all life from DNA damage, cancer, and ecosystem disruption.
(b) 1. Avoid using products (like old refrigerators or certain sprays) that contain CFCs.
2. Promoting the use of eco-friendly and ozone-safe appliances.

Question. What is ozone? How and where is it formed in the atmosphere? Explain how does it affect an ecosystem.
Answer: Ozone is a triatomic molecule of oxygen (\( O_3 \)). It is formed in the stratosphere (upper atmosphere) by the action of UV light splitting \( O_2 \) into free oxygen atoms that join other \( O_2 \) molecules. It affects the ecosystem by acting as a protective shield; without it, UV radiation would damage crops, phytoplankton (affecting marine food chains), and cause health issues in animals.

Managing the Garbage we Produce

Question. Disposable plastic plates should not be used because
(a) they are made of non-biodegradable materials
(b) they are made of biodegradable materials
(c) they are made of toxic materials
(d) they are made of materials with light weight.
Answer: (a) they are made of non-biodegradable materials

India today is facing the problem of overuse of resources, contamination of water and soil and lack of methods of processing the waste. The time has come for the world to say goodbye to “single use plastics”. Steps must be undertaken to develop environment-friendly substitutes, effective plastics waste collection and methods of its disposal. Indore treated 15 lakhs metric tonnes of waste in just 3 years, through biomining and bioremediation techniques. Bioremediation involves introducing microbes into a landfill to naturally ‘break’ it down and biomining involves using trommel machines to sift through the waste to separate to ‘soil’ and the waste component. The city managed to chip away 15 lakh metric tonnes of waste at a cost of around Rs. 10 crore. A similar experiment was successfully carried out in Ahmedabad also.

Question. State two methods of effective plastic waste collection in your school.
Answer: 1. Placing dedicated bins specifically for plastic waste in every classroom and corridor.
2. Organizing regular "plastic-free" drives where students collect and segregate plastic for recycling.

Question. Name any two uses of ‘single use plastic’ in daily life.
Answer: Plastic straws and disposable water bottles.

Question. If we discontinue the use of plastic, how can an environment-friendly substitute be provided?
Answer: We can use substitutes like cloth or jute bags for shopping, paper or bamboo straws, and metal or glass containers for storage.

Question. Do you think microbes will work similarly in landfill sites as they work in the laboratory? Justify your answer.
Answer: Not necessarily. In the laboratory, conditions like temperature, moisture, and pH are controlled to be optimal for microbes. In a landfill, environmental conditions fluctuate, and the presence of toxic non-biodegradable materials may inhibit microbial activity, making the process slower or less efficient than in a lab.