Earth as a System: Energy, Matter and Life — CBSE Class 9 Science Important Questions

Answer: (b) producers.

Green plants make their own food by photosynthesis and form the first trophic level, so they are called producers.

Answer: (c) oxygen.

During photosynthesis plants use carbon dioxide and water and release oxygen, replenishing atmospheric oxygen and driving the oxygen cycle.

Answer: (b) nitrogen fixation.

Rhizobium bacteria in the root nodules of legumes convert atmospheric nitrogen ($\text{N}_2$N_2) into nitrogen compounds (nitrates) that plants can absorb — this is biological nitrogen fixation.

Answer: (b) $100\text{ J}$100 J.

Only 10% passes to each next level. Producers $=10000\text{ J}$=10000 J; primary consumer $=10\%\text{ of }10000=1000\text{ J}$=10\% of 10000=1000 J; secondary consumer $=10\%\text{ of }1000=100\text{ J}$=10\% of 1000=100 J.

Answer: (a) Both A and R are true and R is the correct explanation of A.

Since only about 10% of energy passes to the next level, so little usable energy remains after four or five links that it cannot support a further level. Hence food chains are short.

An ecosystem is a self-contained unit in which living organisms interact with one another and with their non-living surroundings, exchanging matter and energy.

Its two main components are: (i) biotic components (all living things — producers, consumers, decomposers) and (ii) abiotic components (non-living factors like air, water, soil, light, temperature).

Energy flow is unidirectional because energy enters from the Sun, passes from producers to consumers, and at each step much of it is lost as heat that cannot be reused or returned to the Sun; it never flows back.

Matter is cyclic because elements like carbon, nitrogen and oxygen are used by organisms and then returned to the environment (through respiration, decay and excretion), so the same atoms are used again and again in biogeochemical cycles.

The water cycle is the continuous movement of water between the oceans, atmosphere, land and living things.

Heat from the Sun causes evaporation of water from oceans, rivers and lakes, and transpiration from plants, forming water vapour. The vapour rises, cools and condenses into clouds, then falls back as precipitation (rain, snow). Some water flows over the land as run-off into rivers and seas, and some infiltrates the soil to become ground water, which plants absorb and later release by transpiration. Thus water is endlessly recycled.

In the carbon cycle, carbon moves between the atmosphere (as $\text{CO}_2$CO_2), living organisms and fossil fuels. Plants take in $\text{CO}_2$CO_2 for photosynthesis and fix it into carbohydrates; this carbon passes along food chains. Respiration, decay and burning return $\text{CO}_2$CO_2 to the air, keeping the cycle balanced.

Processes that add $\text{CO}_2$CO_2 to the atmosphere:

1. Respiration by plants and animals.
2. Combustion (burning) of fuels, wood and fossil fuels; also decomposition by decomposers.

Processes that remove $\text{CO}_2$CO_2 from the atmosphere:

1. Photosynthesis by green plants.
2. Dissolving of $\text{CO}_2$CO_2 in oceans and its use in forming shells and, over long times, fossil fuels.

Leguminous plants such as gram have root nodules containing nitrogen-fixing bacteria (Rhizobium). These bacteria convert inert atmospheric nitrogen ($\text{N}_2$N_2) into nitrogen compounds (nitrates) in the soil.

When the legume crop is harvested and its roots and residues decay, this fixed nitrogen enriches the soil. The next crop, wheat, absorbs these nitrates to build proteins, so it grows well even without much added nitrogen fertiliser. This is why farmers practise crop rotation with legumes to restore soil fertility naturally.

Nitrogen makes up about 78% of the air as inert $\text{N}_2$N_2, which most organisms cannot use directly. The nitrogen cycle converts it into usable forms and back.

Nitrogen fixation: atmospheric $\text{N}_2$N_2 is converted into nitrates by lightning and by nitrogen-fixing bacteria (Rhizobium in legume nodules, and free-living soil bacteria).

Assimilation: plants absorb nitrates from the soil and use them to make proteins; animals get this nitrogen by eating plants.

Ammonification / Nitrification: when organisms die or excrete waste, decomposers convert the nitrogen to ammonia, and nitrifying bacteria change ammonia to nitrites and then nitrates, returning them to the soil.

Denitrification: denitrifying bacteria convert nitrates back into free $\text{N}_2$N_2 gas, which returns to the atmosphere, completing the cycle.

(a) A food chain is a sequence of organisms through which energy and nutrients pass, each feeding on the one before it. A grassland food chain:
$\text{Grass}\rightarrow\text{Grasshopper}\rightarrow\text{Frog}\rightarrow\text{Snake}$Grass→Grasshopper→Frog→Snake
Trophic levels: Grass = producer (T1); Grasshopper = primary consumer (T2); Frog = secondary consumer (T3); Snake = tertiary consumer (T4).

(b) By the 10% law, only one-tenth of the energy passes to the next level:

• Producers (grass): $20000\text{ J}$20000 J
• Primary consumer (grasshopper): $2000\text{ J}$2000 J
• Secondary consumer (frog): $200\text{ J}$200 J
• Tertiary consumer (snake): $20\text{ J}$20 J

Since energy decreases sharply at every level, the amount available (and hence the number of organisms it can support) is largest at the base and smallest at the top. The pyramid of energy is therefore always upright.

(i) Plants release oxygen during photosynthesis.

(ii) (a) Burning of fossil fuels (coal, petrol, diesel) and (b) large-scale cutting/burning of forests; increased industrial and vehicular activity is also acceptable.

(iii) Deforestation removes trees that take in $\text{CO}_2$CO_2 and give out $\text{O}_2$O_2. So less carbon dioxide is absorbed and less oxygen is released, causing $\text{CO}_2$CO_2 to build up and disturbing both the carbon and oxygen cycles, which enhances global warming.

(iv) Planting more trees (afforestation) and reducing the burning of fossil fuels by using cleaner or renewable energy sources helps restore the balance.