CBSE Class 10 Science Our Environment MCQs Set 13

Answer: (c) biotic and abiotic components. An ecosystem includes living (biotic) and non-living (abiotic) components that interact with each other.

Teacher's Note:
1. Examples of abiotic factors are temperature, rainfall, soil and light.
2. Name both groups in a descriptive answer.

Answer: (c) Aquarium. An aquarium or a crop field is made and maintained by humans, so it is artificial.

Teacher's Note:
1. Forests, ponds and lakes are natural ecosystems.
2. Check who maintains the system.

Answer: (c) green plants. Green plants make food by photosynthesis and form the first trophic level.

Teacher's Note:
1. They use sunlight as the energy source.
2. All other organisms depend on them directly or indirectly.

Answer: (a) Grass → Grasshopper → Frog → Snake. A chain starts with a producer and shows who is eaten by whom: grass is eaten by the grasshopper, then the frog, then the snake.

Teacher's Note:
1. Arrows point towards the eater.
2. A chain always begins with the producer.

Answer: (c) secondary consumer. Grass is the producer, the deer is the primary consumer and the tiger is the secondary consumer.

Teacher's Note:
1. The tiger occupies the third trophic level.
2. Count levels starting from the producer.

Answer: (a) primary consumers. Herbivores feed directly on producers, so they are primary consumers.

Teacher's Note:
1. Carnivores feeding on herbivores are secondary consumers.
2. Omnivores may occupy more than one level.

Answer: (c) unidirectional. Energy flows from the Sun to producers and then to consumers, and is not returned to the previous level.

Teacher's Note:
1. Materials like carbon and nitrogen are recycled, while energy is not.
2. State this difference in comparison questions.

Answer: (b) 10%. Only about 10 per cent of the energy at one level passes to the next level; the rest is lost as heat or used in life processes.

Teacher's Note:
1. The loss is mainly through respiration.
2. This explains why food chains are short.

Answer: (c) 10 J. Each step passes 10%: 10000 → 1000 (grasshopper) → 100 (frog) → 10 J (snake).

Teacher's Note:
1. Apply the 10% rule once for every arrow.
2. Count the arrows to get the number of steps.

Answer: (b) there is a great loss of energy at each level. With about 90 per cent energy lost at each step, very little remains to support more levels.

Teacher's Note:
1. Link the length of a chain to energy loss.
2. This is a standard reasoning point.

Answer: (a) food web. Many food chains linked together because organisms feed on more than one kind of food form a food web.

Teacher's Note:
1. A web is more stable than a single chain.
2. If one food source is lost, an alternative remains.

Answer: (c) decomposers. Bacteria and fungi decompose dead organisms and return nutrients to the soil, so they are natural cleaners of the environment.

Teacher's Note:
1. They are placed at the end of a food chain.
2. Without them dead matter would pile up and nutrients would not be recycled.

Answer: (c) Cotton cloth. Cotton is made from plant fibres and is broken down by microorganisms.

Teacher's Note:
1. Plastics, metals and glass are non-biodegradable.
2. Know three examples of each type.

Answer: (c) DDT. DDT is a persistent chemical that microbes cannot break down, so it remains in the environment.

Teacher's Note:
1. Pesticides and plastics are classic examples.
2. They harm the ecosystem by accumulating.

Answer: (a) biomagnification. Non-biodegradable chemicals like pesticides accumulate, and their concentration increases up the food chain.

Teacher's Note:
1. The topmost trophic level gets the maximum amount.
2. Humans, being at the top, have the highest concentration.

Answer: (c) top carnivores. Chemicals accumulate in the tissues and are passed on, so their level is highest in the top carnivores.

Teacher's Note:
1. Producers have the lowest amount.
2. Humans eat at several levels and thus take in a large amount.

Answer: (b) ultraviolet radiation. Ozone in the stratosphere absorbs the harmful ultraviolet rays of the Sun.

Teacher's Note:
1. UV radiation can cause skin cancer and cataract.
2. Mention the layer's location when asked.

Answer: (b) three atoms of oxygen. Ozone is O₃; it is formed when UV radiation splits O₂ into atoms which then combine with other O₂ molecules.

Teacher's Note:
1. Ordinary oxygen is O₂.
2. The formation and breakdown must be written correctly in an equation.

Answer: (b) chlorofluorocarbons (CFCs). CFCs used in refrigerators and aerosol sprays release chlorine in the stratosphere, which destroys ozone.

Teacher's Note:
1. The use of CFCs was limited by an international agreement in 1987.
2. UNEP played a key role in this.

Answer: (b) the Antarctic. A large drop in ozone levels was first noticed over the Antarctic in the 1980s.

Teacher's Note:
1. This discovery led to worldwide action.
2. Remember the continent, not just the pole.

Answer: (a) 1%. Green plants capture only about 1 per cent of the solar energy that falls on their leaves.

Teacher's Note:
1. This small amount supports the whole food chain.
2. Do not confuse it with the 10% transfer rule.

Answer: (b) Reduce, Reuse, Recycle. Reducing consumption, reusing items and recycling materials all cut down the amount of waste produced.

Teacher's Note:
1. Give an example of each when asked.
2. Reducing is the most effective of the three.

Answer: (b) are biodegradable. Paper breaks down naturally, while plastic remains in the environment for a very long time.

Teacher's Note:
1. Pay attention to the word 'biodegradable' in options.
2. Eco-friendliness is the reason for the change.

Answer: (b) top carnivore. The hawk feeds on other animals at the end of the chain, so it is a top carnivore.

Teacher's Note:
1. Top carnivores have no natural predator in the chain.
2. They hold the highest trophic level.

Answer: (c) more than one trophic level. An omnivore such as a human feeds as a primary consumer on plants and as a higher consumer on animals.

Teacher's Note:
1. The position of a human therefore varies.
2. Trophic levels are about feeding position.

Answer: (a) Both Assertion (A) and Reason (R) are true and R is the correct explanation of A. Both are true, and since the heat lost cannot be recaptured by plants, energy moves in one direction only.

Teacher's Note:
1. Producers can only use energy from sunlight.
2. This is why energy has to be supplied continuously.

Answer: (a) Both Assertion (A) and Reason (R) are true and R is the correct explanation of A. Both statements are true, and the persistence of plastics is the cause of the problem.

Teacher's Note:
1. Microbes cannot break plastic down, so it builds up in soil and water.
2. Think of ways to reduce its use.