What this chapter is about
This chapter helps you understand Earth not as a collection of separate parts, but as one connected system where energy flows and matter cycles continuously. You will learn how the Sun's energy drives processes in the atmosphere, hydrosphere, lithosphere and biosphere, and how living organisms interact with non-living components to keep our planet functioning.
At Class 9, you have already studied the structure of matter, basic life processes and forms of energy. Now you bring these ideas together to see how Earth maintains conditions suitable for life. Energy enters mainly from the Sun, passes through food chains, powers the water cycle, and eventually leaves as heat. Matter, on the other hand, is recycled — carbon, nitrogen, oxygen and water move in closed loops through air, water, soil and living things.
After studying this chapter you should be able to describe how energy flows through ecosystems, explain important biogeochemical cycles, give examples of how human activities disturb these cycles, and appreciate why protecting any one part of the Earth system helps protect the whole.
Key ideas
- Earth is a system of four interacting spheres: atmosphere (air), hydrosphere (water), lithosphere (land and rock) and biosphere (all living things).
- The Sun is Earth's main external energy source; geothermal heat from inside Earth is a secondary source.
- Energy flows in one direction: Sun → producers → consumers → decomposers → heat lost to space. It is not recycled.
- Matter is recycled through biogeochemical cycles: the carbon cycle, nitrogen cycle, oxygen cycle and water cycle.
- Producers (green plants, algae, some bacteria) capture solar energy by photosynthesis and store it in organic compounds.
- Decomposers (bacteria, fungi) break down dead matter, releasing nutrients back into soil, water and air.
- Human activities such as burning fossil fuels, deforestation and excessive fertiliser use can speed up or slow down parts of these cycles, causing problems like climate change and water pollution.
- Biodiversity — the variety of life — helps keep the Earth system stable because different organisms play different roles in cycling matter and energy.
Formulas and facts to remember
1. Photosynthesis equation (simplified): Carbon dioxide + Water + Light energy → Glucose + Oxygen. This captures solar energy into chemical energy.
2. Respiration equation (simplified): Glucose + Oxygen → Carbon dioxide + Water + Energy. This releases stored energy for life processes.
3. Energy transfer in food chains: Only about 10 % of energy at one trophic level passes to the next; the rest is lost as heat.
4. Carbon cycle steps: Carbon enters the biosphere through photosynthesis, moves through food chains, returns to the atmosphere via respiration, decomposition and combustion of fuels.
5. Nitrogen cycle steps: Atmospheric nitrogen (N₂) is fixed by bacteria into ammonia, converted to nitrates plants can absorb, passed to animals, then returned to the air by denitrifying bacteria.
6. Water cycle stages: Evaporation from water bodies → condensation forming clouds → precipitation (rain, snow) → collection in rivers, lakes and groundwater → evaporation again.
7. Greenhouse effect: Gases like carbon dioxide, methane and water vapour trap some of Earth's outgoing heat, keeping the planet warm enough for life.
8. Ozone layer: A region in the upper atmosphere containing O₃ molecules that absorbs harmful ultraviolet radiation from the Sun.
Worked examples
### Example 1: Energy loss along a food chain A field of grass stores 50 000 kJ of energy in a season. Grasshoppers eat the grass. How much energy is available to a frog that eats the grasshoppers?
Solution Energy passed to grasshoppers = 10 % of 50 000 kJ = 5 000 kJ. Energy passed to frogs = 10 % of 5 000 kJ = 500 kJ.
So the frog receives about 500 kJ — only 1 % of the original energy in the grass.
### Example 2: Carbon movement A farmer burns 5 kg of dry wood. Describe how carbon moves in this process.
Solution 1. The wood came from a tree that absorbed carbon dioxide from the air during photosynthesis and stored carbon in its tissues. 2. When the wood burns, the carbon combines with oxygen to form carbon dioxide (combustion). 3. This carbon dioxide escapes into the atmosphere. 4. Green plants may absorb it again, continuing the carbon cycle.
Thus, burning releases carbon that was locked in the tree back into the air.
### Example 3: Nitrogen fixation A farmer notices that growing pulses (legumes) improves soil fertility. Explain why.
Solution 1. Legume roots have nodules containing nitrogen-fixing bacteria (Rhizobium). 2. These bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which changes to nitrates in soil. 3. Nitrates are nutrients other plants can absorb. 4. After harvest, decaying roots leave nitrates in the soil, enriching it for the next crop.
This is why farmers rotate pulses with cereals — the legumes add nitrogen naturally.
Common mistakes
- Thinking energy is recycled like matter → Energy flows in one direction and is eventually lost as heat; only matter cycles.
- Believing all nitrogen in air can be used directly by plants → Plants absorb nitrogen only as nitrates or ammonium; bacteria must first fix atmospheric N₂.
- Confusing the greenhouse effect with the ozone hole → The greenhouse effect involves heat-trapping gases warming Earth; the ozone hole is a thinning of the UV-blocking ozone layer. They are different problems.
- Assuming decomposers are unimportant → Without decomposers, dead matter would pile up and nutrients would stay locked, stopping cycles.
- Forgetting that respiration releases carbon dioxide → Both plants and animals respire, returning carbon to the atmosphere day and night.
Quick revision
- Earth works as one system: energy flows through it, matter cycles within it.
- Sun → producers → consumers → decomposers → heat lost (energy path).
- Carbon, nitrogen, oxygen and water move in closed biogeochemical cycles.
- Only about 10 % of energy passes from one trophic level to the next.
- Nitrogen-fixing bacteria convert N₂ into forms plants can use.
- Human actions (burning fuels, deforestation, excess fertilisers) disturb natural cycles and harm the environment.