What is the biosphere?
Earth can be seen as four connected spheres:
- Geosphere (lithosphere): rocks, soil and the solid Earth.
- Hydrosphere: oceans, rivers, lakes, groundwater and ice.
- Atmosphere: the layer of air around Earth.
- Biosphere: all living things and the places where they live.
The biosphere is not a separate shell. It is the zone where the other three overlap. Plants need soil (geosphere), water (hydrosphere) and air (atmosphere) at the same time. The word comes from Greek bios = life.
All the ecosystems on Earth (a pond, a forest, a coral reef) joined together make the biosphere. It is the largest level of ecological organisation: organism → population → community → ecosystem → biome → biosphere.
Characteristics and limits of the biosphere
- Thin: life is found from about 10 km up in the air (spores, bacteria, high-flying birds) to about 11 km deep in ocean trenches. That is roughly 20 km, compared with Earth's radius of 6371 km.
- Most life is near the surface: in the top few metres of soil and the sunlit top 200 m of the sea.
- Limits are set by: low temperature, lack of liquid water, lack of light, low oxygen and air pressure (high up), very high pressure (deep down), and harmful ultraviolet rays (the ozone layer protects us).
- Open to energy, closed to matter: sunlight comes in and heat goes out, but almost no matter enters or leaves Earth.
- Self-regulating: living things keep conditions fairly stable, for example plants take in CO₂.
- Diverse: millions of species, unevenly spread; most in warm, wet places.
Living matter shapes the planet
Scientist Vladimir Vernadsky (1926) said that life is a powerful force that changes Earth's surface. All living organisms together are called living matter. Its main jobs:
- Energy: plants capture sunlight in photosynthesis and store it in food.
- Gas: living things make oxygen and use CO₂; they made today's oxygen-rich air.
- Concentration: they collect elements, for example shells collect calcium, which later forms chalk and limestone.
- Redox (oxidation–reduction): bacteria change iron, sulfur and nitrogen compounds.
- Destruction: decomposers break dead matter down and return nutrients; lichens slowly break rock into soil.
Coal, oil, soil and limestone all exist because of past life.
Biomes, energy flow and matter cycles
Biomes
A biome is a large area with a similar climate, plants and animals. Temperature and rainfall decide the biome. From equator to poles: tropical rainforest, savanna and desert, temperate forest and grassland, taiga (conifer forest), tundra, polar ice. Height works the same way: climbing a mountain is like walking towards a pole.
Energy flow
Energy enters as sunlight, is fixed by producers, passes to consumers and decomposers, and at each step most of it is lost as heat. Only about 10% passes to the next level. Energy does not cycle.
Matter cycles
Carbon, water, nitrogen and oxygen are used again and again: the carbon cycle (photosynthesis, respiration, burning, decay), the water cycle (evaporation, condensation, rain) and the nitrogen cycle (bacteria in soil).
Humans and the biosphere
People change the biosphere fast: cutting forests, farming, building cities, polluting air and water, burning fossil fuels (climate change) and overfishing. This reduces biodiversity. Ways to protect it: protected areas and biosphere reserves (core zone, buffer zone, transition zone), saving energy, less waste, sustainable farming and fishing.
Try it yourself: a jar biosphere
You need: a clear jar with a lid, some soil, small stones, a small plant or moss, and a little water.
- Put stones, then soil, then the plant. Add a few spoons of water. Close the lid.
- Predict: will the plant live for weeks without fresh air or water?
- Keep it in bright light, not direct sun. Check every few days for water drops on the glass.
- The water cycles (drops form and fall), the plant makes oxygen and uses CO₂, and only light enters. That is the biosphere in a jar: open to energy, closed to matter.
Key formulas and definitions
- Biosphere = zone where geosphere + hydrosphere + atmosphere meet and life exists
- Limits: about +10 km (air) to about −11 km (deepest ocean); most life in top soil and top 200 m of sea
- Levels: organism → population → community → ecosystem → biome → biosphere
- 10% rule: about 10% of energy passes to the next feeding level
- Energy flows one way (Sun → producers → consumers → heat); matter cycles (C, H₂O, N, O)
- Photosynthesis: carbon dioxide + water + light → glucose + oxygen
Worked examples
1. Name the spheres that a fish in a river uses.
Hydrosphere: the river water it lives in. Atmosphere: oxygen from air dissolves in the water and the fish breathes it. Geosphere: the river bed gives minerals and shelter. The fish itself is part of the biosphere.
2. Plants in a field fix 20,000 kJ of energy. Using the 10% rule, how much reaches the goats that eat them, and the lion that eats the goats?
Goats (primary consumers) get about 10% of 20,000 = 2,000 kJ. Lions (secondary consumers) get about 10% of 2,000 = 200 kJ. The rest is lost mostly as heat at each step.
3. Why is there almost no life at the top of Mount Everest (8.8 km)?
It is very cold (often below −30 °C), there is no liquid water (only ice), the air is thin with little oxygen and strong ultraviolet light. These limiting factors stop most life, so it is near the upper edge of the biosphere.
4. Explain why the biosphere is called 'open to energy but closed to matter'.
Sunlight enters every day and heat leaves to space, so energy passes through. But almost no atoms arrive or leave (only a few meteorites and gas). So carbon, water and nitrogen must be reused in cycles.
Common mistakes
- Thinking the biosphere is a separate layer above the atmosphere. It is the overlap of the other spheres where life exists.
- Saying energy cycles like matter. Energy flows one way and leaves as heat; only matter cycles.
- Confusing ecosystem and biosphere. An ecosystem is one area such as a pond; the biosphere is all ecosystems together.
- Believing life is spread evenly. Most life is in a thin band near the surface, mainly in warm and wet places.