CBSE Class 10 Science Our Environment Assignment Set 08

This is a chapter-wise assignment for CBSE Class 10 Science Chapter 13, Our Environment, with the marks shown against each question group (1, 2, 3 and 5 marks) and step-by-step answers. It covers the components of an ecosystem, producers and decomposers, food chains and trophic levels, the 10% law of energy flow, the unidirectional flow of energy, biological magnification and the food web.

Very Short Answer Questions (1 Mark each)

Question 1. In the following food chain, 20,000 J of energy was available to the plants. How much energy will be available to man in this chain? Plants → Sheep → Man
Answer:
Sheep get 10% of 20,000 J = 2,000 J.
Man gets 10% of 2,000 J = 200 J.
In simple words: Each step passes on only one tenth of the energy.

Teacher's Note:
1. Apply the 10% law once for each step.
2. Two steps mean 20,000 → 2,000 → 200.

Question 2. Which will get more energy, secondary consumers or tertiary consumers?
Answer:
Secondary consumers get more energy, because energy decreases at each higher trophic level (tertiary consumers receive only 10% of what the secondary consumers have).
In simple words: The lower the trophic level, the more energy.

Teacher's Note:
1. Energy falls by about 90% at every step.
2. Producers always have the most energy.

Question 3. List two natural ecosystems.
Answer:
Forest and pond (also ocean, lake, river, grassland or desert).
In simple words: Natural ecosystems exist without human help.

Teacher's Note:
1. Name any two.
2. Gardens and crop fields are artificial ecosystems.

Question 4. List two biotic components of environment.
Answer:
Plants and animals (also micro-organisms such as bacteria and fungi).
In simple words: Biotic means living.

Teacher's Note:
1. Biotic = living; abiotic = non-living.
2. Producers, consumers and decomposers are the three biotic groups.

Question 5. Why are green plants called producers?
Answer:
Green plants prepare their own food from carbon dioxide and water by photosynthesis, using sunlight and chlorophyll. They produce food for all other organisms, so they are called producers.
In simple words: They make food for everyone else.

Teacher's Note:
1. Mention photosynthesis.
2. Producers form the first trophic level.

Question 6. Which of the following are always at second trophic level of food chains? Carnivores, Autotrophs, Herbivores.
Answer:
Herbivores are always at the second trophic level, as they feed directly on the producers (autotrophs) of the first level.
In simple words: Plant eaters are on the second step.

Teacher's Note:
1. Autotrophs are at the first level.
2. Carnivores are at the third or higher level.

Question 7. What will be the amount of energy available to the organisms of secondary consumer trophic level of food chain, if the energy available to producer level is 10000 Joules.
Answer:
Primary consumers get 10% of 10,000 J = 1,000 J.
Secondary consumers get 10% of 1,000 J = 100 J.
In simple words: 10,000 J → 1,000 J → 100 J.

Teacher's Note:
1. The producers are the first level, primary consumers the second, secondary consumers the third.
2. Two transfers are needed.

Question 8. What are various steps in the food chain called?
Answer:
The various steps in a food chain are called trophic levels.
In simple words: Each step of a food chain is a trophic level.

Teacher's Note:
1. Trophic levels carry the transfer of food energy.
2. Producers form the first trophic level.

Question 9. What is depicted in the below mentioned scheme? Sun → Phytoplankton (400 kJ) → Zooplankton (40 kJ) → Smaller fishes (4 kJ) → Big fish (0.4 kJ)
Answer:
The scheme shows the 10% law of energy transfer: only 10% of the energy of one trophic level passes to the next, and the energy decreases at every step of the aquatic food chain.
In simple words: Each level has one tenth of the energy of the level before it.

Teacher's Note:
1. Divide each value by the one before it; the ratio is 10.
2. This is an aquatic food chain.

Question 10. What is the functional unit of environment?
Answer:
The ecosystem is the functional unit of the environment.
In simple words: The ecosystem is the working unit of nature.

Teacher's Note:
1. It includes biotic and abiotic components.
2. The term ecosystem was coined by A. G. Tansley.

Short Answer Questions (2 Marks each)

Question 11. Define trophic level. Name the first trophic level.
Answer:
Each step or level of a food chain at which the transfer of food energy takes place is called a trophic level.
The first trophic level is occupied by the producers (green plants).
In simple words: A trophic level is one feeding step of a food chain.

Teacher's Note:
1. Name the producers as the first level.
2. Herbivores form the second level.

Question 12. Give an example of food chain and state the different trophic levels in it.
Answer:
Example: Grass → Grasshopper → Frog → Snake.
Grass is the producer (first trophic level), grasshopper is the primary consumer (second), frog is the secondary consumer (third) and snake is the tertiary consumer (fourth).
In simple words: A food chain lists who eats whom, and each organism is on a different trophic level.

Teacher's Note:
1. Give a chain of at least three organisms.
2. Label the producer and each consumer.

Question 13. Why do most food chains have 3-5 steps only?
Answer:
Only about 10% of the energy passes from one trophic level to the next; the rest is lost as heat or used in life processes.
After 3 to 4 steps, so little energy is left that it cannot support another level, so food chains are short.
In simple words: The energy runs out after a few steps.

Teacher's Note:
1. Quote the 10% law.
2. A chain with 1000 J at the producer gives 100, 10, 1 J at the next levels.

Question 14. Energy flow is unidirectional in food chain? Explain.
Answer:
The energy of the Sun is trapped by plants and cannot be given back to the Sun. Herbivores eat plants and cannot give energy back to plants, and carnivores cannot give it back to herbivores.
So energy flows only from the lower trophic level to the higher one; it is unidirectional.
In simple words: Energy moves forward only and never returns.

Teacher's Note:
1. Give the order: Sun, producer, herbivore, carnivore.
2. Matter, unlike energy, is recycled.

Question 15. Name the organisms which belong to first and third trophic level in food chain comprising of the following: Insects, birds, frog and grass.
Answer:
Food chain: Grass → Insects → Frog → Birds.
First trophic level: grass. Third trophic level: frog.
In simple words: Arrange the organisms in order, then count the levels.

Teacher's Note:
1. Arrange the chain first.
2. The producer is always at the start.

Short Answer Questions (3 Marks each)

Question 16. What will happen if we kill all the organisms in one trophic level?
Answer:
(i) The transfer of energy to the next level stops, so the population of the next trophic level decreases.
(ii) The organisms of the previous trophic level increase greatly because nothing eats them.
(iii) The food chain is broken and the ecosystem becomes unbalanced.
In simple words: Removing one level damages the whole chain.

Teacher's Note:
1. Mention the effect on the levels above and below.
2. This is an NCERT question.

Question 17. Differentiate between autotrophs, heterotrophs and decomposers and give one example of each.
Answer:
Autotrophs prepare their own food from CO₂ and H₂O with sunlight and chlorophyll, e.g. green plants.
Heterotrophs cannot make their own food and depend on other organisms, e.g. animals and human beings.
Decomposers break down the complex organic matter of dead plants and animals into simple substances, e.g. bacteria and fungi.
In simple words: Autotrophs make food, heterotrophs eat food, decomposers break down dead matter.

Teacher's Note:
1. Give one example for each.
2. Decomposers return nutrients to the soil.

Question 18. List three characteristics of energy flow in an ecosystem.
Answer:
(i) The Sun is the main source of energy; plants use about 1% of the sunlight that falls on them for photosynthesis.
(ii) The flow is unidirectional, from producers to herbivores to carnivores; it cannot be reversed.
(iii) The energy available decreases at each trophic level, as only about 10% is passed to the next one.
In simple words: Energy comes from the Sun, moves one way and decreases at each step.

Teacher's Note:
1. Any three are enough.
2. The 10% law is the key point.

Question 19. (i) Create a terrestrial food chain depicting four trophic levels. (ii) Why do we not find food chains of more than four trophic levels in nature?
Answer:
(i) Grass → Grasshopper → Frog → Snake (producer, primary, secondary and tertiary consumer).
(ii) According to the 10% law, the energy available becomes too small after four trophic levels to sustain organisms of a fifth level.
In simple words: There is not enough energy left for a fifth level.

Teacher's Note:
1. Label the four trophic levels.
2. Link the second part to the 10% law.

Long Answer Questions (5 Marks each)

Question 20. (i) What is food web? Illustrate with the help of example.
Answer:
A food web is a network of food chains which are interconnected at various trophic levels, so that an organism may be eaten by several others and may eat several kinds of organisms.
Example: Grass → Grasshopper → Frog → Snake → Hawk; Grass → Mouse → Snake → Hawk; Grass → Rabbit → Hawk. Here the snake and the hawk belong to more than one food chain.
Features: (1) it provides alternative pathways of food; (2) it makes the ecosystem more stable; (3) it keeps a check on the over-population of species.
In simple words: A food web is many food chains joined together.

Teacher's Note:
1. Give an example with at least three chains.
2. Mention stability as the advantage.

Question 21. What is an ecosystem? What are the components of an ecosystem? Also discuss the types of ecosystem?
Answer:
An ecosystem is the structural and functional unit of the biosphere in which living and non-living components interact with each other.
Biotic components: (a) producers, the green plants that make food by photosynthesis; (b) consumers (herbivores, carnivores and omnivores), which depend on producers for food; (c) decomposers, which break down dead organisms into simple substances.
Abiotic components: the physical factors such as temperature, light, wind, water and humidity, and soil and minerals.
Types: (a) natural ecosystems made by nature, e.g. forest, grassland, pond, lake, ocean; (b) artificial ecosystems made and maintained by man, e.g. garden, park, crop field and aquarium.
In simple words: An ecosystem is living things plus non-living things interacting; it may be natural or artificial.

Teacher's Note:
1. Name the three biotic groups.
2. Give two examples of each ecosystem type.