CBSE Class X Science Chapter 13 Our Environment Questions and Answers

In-Text Questions & Answers

Section 13.1: Eco-system — What Are Its Components?

Q1 (Page 212): What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

Each step or level of a food chain forms a trophic level where transfer of food and energy takes place.

Example of a food chain (in a grassland):

Grass $\rightarrow$ Insects $\rightarrow$ Frog $\rightarrow$ Snake $\rightarrow$ Eagle

Trophic levels in this food chain:

• First trophic level (Producers): Grass
• Second trophic level (Primary Consumers / Herbivores): Insects
• Third trophic level (Secondary Consumers / Small Carnivores): Frog
• Fourth trophic level (Tertiary Consumers / Larger Carnivores): Snake
• Fifth trophic level (Top Carnivores): Eagle

Figure 13.1: Food chain in nature (a) in forest, (b) in grassland and (c) in a pond.

Figure 13.2: Trophic levels pyramid.

Q2 (Page 212): What is the role of decomposers in the ecosystem?

The decomposers, which comprise microorganisms like bacteria and fungi, play the following crucial roles in an ecosystem:

• They break down complex organic substances from dead remains and waste products of organisms into simple inorganic substances.
• These simple inorganic substances go back into the soil and are reused by plants, thereby facilitating the natural replenishment of soil nutrients.
• They act as natural cleansing agents by decomposing garbage and dead plants and animals.

Section 13.2.1: Ozone Layer and How It Is Getting Depleted

Q1 (Page 214): Why are some substances biodegradable and some non-biodegradable?

Substances are categorized as biodegradable or non-biodegradable based on the specificity of biological catalysts called enzymes:

• Biodegradable substances: These are substances that can be broken down into simpler substances by biological processes carried out by bacteria and other saprophytes (decomposers) using specific enzymes. Examples include plant waste, paper, and food leftovers.
• Non-biodegradable substances: These are substances that cannot be broken down by biological processes because decomposers do not possess specific enzymes to break them down. They persist in the environment for a long time and are only acted upon by physical processes like heat and pressure. Examples include plastics and synthetic chemicals.

Q2 (Page 214): Give any two ways in which biodegradable substances would affect the environment.

Two ways in which biodegradable substances can affect the environment are:

• Foul smell and health hazards: During decomposition, large amounts of biodegradable waste produce foul odors and serve as breeding grounds for flies and insects that spread diseases.
• Nutrient replenishment: Upon decomposition, they break down into simple inorganic nutrients that enrich the soil and maintain natural nutrient cycling in the ecosystem.

Q3 (Page 214): Give any two ways in which non-biodegradable substances would affect the environment.

Two ways in which non-biodegradable substances affect the environment are:

• Environmental pollution and persistence: Non-biodegradable waste (like plastics) persists in the environment for a long time, causing soil degradation and clogging drains and water bodies.
• Biological Magnification: Harmful non-biodegradable chemicals, such as pesticides, enter the food chain, accumulate progressively at each trophic level, and cause severe toxic effects in organisms, particularly top consumers like humans.

Section 13.2.2: Managing the Garbage We Produce

Q1 (Page 216): What is ozone and how does it affect any ecosystem?

What is ozone: Ozone ($\text{O}_3$) is a molecule formed by three atoms of oxygen. While oxygen gas ($\text{O}_2$) is essential for aerobic life, ozone is a deadly poison at ground level.

Effects on an ecosystem:

• Protective Shield (Stratosphere): At higher levels of the atmosphere, ozone performs an essential function by shielding the Earth's surface from harmful ultraviolet (UV) radiation from the Sun. UV radiation is damaging to organisms and causes skin cancer in humans.
• Harmful Effects (Ground level): At ground level, ozone acts as a toxic pollutant and is hazardous to living organisms.

Q2 (Page 216): How can you help in reducing the problem of waste disposal? Give any two methods.

Two methods to help reduce the problem of waste disposal are:

• Segregation of Waste: Separating waste at the source into biodegradable and non-biodegradable categories. Biodegradable waste can be converted into compost, while non-biodegradable materials can be sent for recycling.
• Using Eco-Friendly Materials: Replacing single-use non-biodegradable items with biodegradable alternatives, such as using cloth/paper bags instead of plastic bags and using paper cups instead of plastic cups.

End of Chapter Exercises

Q1: Which of the following groups contain only biodegradable items?
(a) Grass, flowers and leather
(b) Grass, wood and plastic
(c) Fruit-peels, cake and lime-juice
(d) Cake, wood and grass

Answer: Both (a) Grass, flowers and leather, (c) Fruit-peels, cake and lime-juice, and (d) Cake, wood and grass contain only biodegradable items.

Note: (b) is incorrect because plastic is non-biodegradable.

Q2: Which of the following constitute a food-chain?
(a) Grass, wheat and mango
(b) Grass, goat and human
(c) Goat, cow and elephant
(d) Grass, fish and goat

Answer: (b) Grass, goat and human

Explanation: Grass is the producer eaten by the primary consumer (goat), which in turn is eaten by the secondary consumer (human).

Q3: Which of the following are environment-friendly practices?
(a) Carrying cloth-bags to put purchases in while shopping
(b) Switching off unnecessary lights and fans
(c) Walking to school instead of getting your mother to drop you on her scooter
(d) All of the above

Answer: (d) All of the above

Explanation: All three practices help conserve resources, reduce non-biodegradable waste, and minimize pollution.

Q4: What will happen if we kill all the organisms in one trophic level?

If all organisms in one trophic level are killed, it will cause an ecological imbalance in the ecosystem:

• Organisms at higher trophic levels: They will suffer from starvation and die or migrate due to the unavailability of food.
• Organisms at lower trophic levels: Their population will increase abnormally because their natural predators have been removed.

This disruption breaks the food chain and disturbs the entire ecosystem balance.

Q5: Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem?

1. Will the impact be different for different trophic levels?
Yes, the impact will be different depending on which trophic level is removed:

• Removing producers collapses the entire ecosystem immediately, as no energy will be captured from sunlight for any subsequent level.
• Removing primary consumers causes producers to overgrow and higher carnivores to starve.
• Removing top carnivores leads to overpopulation of herbivores, causing overexploitation of plant producers.

2. Can any trophic level be removed without causing damage?
No, organisms of no trophic level can be removed without causing damage. Each trophic level plays an indispensable role in maintaining energy flow and ecosystem stability.

Q6: What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?

Biological Magnification: It is the phenomenon of progressive accumulation of non-biodegradable chemicals (such as pesticides and insecticides) at each higher trophic level in a food chain.

Levels at different ecosystem stages:
Yes, the concentration level of these harmful chemicals varies across trophic levels. Since these chemicals are non-degradable, their concentration increases progressively as we move up through the trophic levels. Consequently, the lowest concentration is at the producer level, and the maximum concentration is accumulated at the top trophic level (e.g., in human beings).

Figure 13.3: Food web consisting of many food chains.

Figure 13.4: Flow of energy in an ecosystem.

Q7: What are the problems caused by the non-biodegradable wastes that we generate?

The main problems caused by non-biodegradable waste include:

• Soil and Water Pollution: They persist in the environment for a very long time, polluting soil and aquatic bodies.
• Biological Magnification: Toxic chemicals like pesticides enter the food chain and harm top-level organisms including humans.
• Drainage Blockage: Plastic waste clogs drains, leading to waterlogging and breeding grounds for disease vectors.
• Harm to Wildlife: Animals like cows or aquatic organisms swallow plastic waste along with food, leading to severe health complications or death.

Q8: If all the waste we generate is biodegradable, will this have no impact on the environment?

No, it will still have an impact on the environment if generated in huge quantities:

• Overburdening Decomposers: Decomposers might not be able to break down enormous amounts of organic waste quickly enough.
• Pollution & Odour: Accumulation of undecomposed organic matter produces foul gases and creates unhygienic conditions.
• Disease Transmission: Stagnant decaying organic waste provides ideal breeding conditions for mosquitoes, flies, and harmful microbes.
• Oxygen Depletion in Water: Excessive organic waste reaching aquatic bodies leads to high biological oxygen demand during decomposition, causing depletion of oxygen and suffocation of aquatic life.

Q9: Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?

Why it is a cause for concern:
The ozone layer shields the Earth from harmful ultraviolet (UV) radiation coming from the Sun. Depletion of this layer allows UV radiation to reach the Earth's surface, which causes serious damage to organisms, such as skin cancer in human beings, cataracts, damage to immune systems, and harm to plants and aquatic organisms.

Steps taken to limit damage:

• In 1987, the United Nations Environment Programme (UNEP) forged an agreement among nations to freeze chlorofluorocarbon (CFC) production at 1986 levels.
• Manufacturing companies globally are now mandated to produce CFC-free refrigerators and fire extinguishers.