Biology 20 · The biosphere
Energy passes through; matter goes round
That one sentence is the whole unit. Energy arrives from the Sun, gets used, and leaves as heat — it never comes back. Atoms do come back, endlessly, which is why cycles exist and food chains do not.
- 1. The words, first
- 2. Energy flow and the ten per cent rule
- 3. The carbon and nitrogen cycles
- 4. Climate, biomes and the greenhouse effect
- 5. What costs marks
The words, first
The idea: Trophic vocabulary is precise, and a question will punish using consumer where it wanted heterotroph.
| Word | What it means |
|---|---|
| Biosphere | The zone of Earth where life is found — where atmosphere, hydrosphere and lithosphere overlap enough to support it. |
| Autotroph / producer | An organism that builds organic molecules from inorganic sources, usually by photosynthesis. |
| Heterotroph / consumer | An organism that must take in organic matter made by something else. |
| Decomposer | A heterotroph that breaks down dead organic matter, returning nutrients to the abiotic pool. It is a kind of consumer, not a separate category. |
| Trophic level | A feeding position: producers, primary consumers, secondary consumers, and so on. |
| Food chain | A single line of who eats whom. A food web is the realistic version, where most organisms eat several things. |
| Biomass | The total dry mass of living material at a trophic level. |
| Biogeochemical cycle | The path an element takes between living things and the non-living environment — carbon, nitrogen, water, phosphorus. |
| Nitrogen fixation | Converting atmospheric N₂ into ammonia or ammonium, mostly by bacteria. |
| Denitrification | Converting nitrate back into N₂ gas, removing nitrogen from the ecosystem. |
| Greenhouse effect | Atmospheric gases absorbing the infrared Earth re-radiates, warming the surface. |
| Biome | A large region defined by its climate and the community adapted to it. |
Energy flow and the ten per cent rule
The idea: About a tenth of the energy at one level reaches the next. Everything else is spent on respiration, lost as heat, or never eaten.
Where the 90% goes. Most of it is spent by the organism on staying alive and is released as heat during respiration. Some is in parts that are never eaten — bones, bark, roots. Some passes through undigested.
Why food chains are short. Not because big predators run out of space, but because there is not enough energy left to support another level. Four or five links is close to the practical limit.
Pyramids. A pyramid of energy is always widest at the bottom. A pyramid of numbers can be inverted — one tree supports thousands of insects — which is a favourite exam trap.
The carbon and nitrogen cycles
The idea: Matter is conserved, so every cycle is a closed loop of the same atoms. The interesting questions are always about the rate of one step relative to another.
Carbon in. Photosynthesis takes inorganic CO₂ and fixes it into organic molecules. That is essentially the only doorway into the living world.
Carbon out. Respiration, decomposition and combustion all return it. In a steady ecosystem, in and out balance.
Why fossil fuels matter. They are carbon that left circulation over millions of years. Burning them returns it over a couple of centuries — far faster than photosynthesis and ocean uptake can absorb it, so atmospheric CO₂ rises. Nothing creates carbon; a slow store is simply being emptied quickly.
The nitrogen cycle, in order.
- Fixation: N₂ → NH₃/NH₄⁺, by bacteria, including those in legume root nodules. Atmospheric nitrogen is nearly 80% of the air and useless to most organisms until this step happens.
- Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻, also by bacteria. Nitrate is what plants take up most readily.
- Assimilation: plants build nitrate into protein and nucleic acids; animals get theirs by eating.
- Ammonification: decomposers return nitrogen from dead tissue as ammonium.
- Denitrification: NO₃⁻ → N₂, in oxygen-poor soils. This is the only step that removes nitrogen from the ecosystem.
Climate, biomes and the greenhouse effect
The idea: Plot average temperature against annual precipitation and you can predict the biome. Everything else about a region follows from those two numbers.
The greenhouse effect is normal. Short-wavelength sunlight passes through the atmosphere and warms the surface; the surface re-radiates longer-wavelength infrared; greenhouse gases absorb that infrared and re-emit it in all directions, including back down. Without it Earth would average about −18 °C.
The enhanced greenhouse effect is the problem: more greenhouse gas means more of that infrared is intercepted. CO₂, methane, nitrous oxide and water vapour all do it.
Biomes. Tundra is cold and dry; boreal forest cold and moderately wet; grassland warm and dry; desert hot and very dry; tropical rainforest hot and very wet. Latitude and altitude matter because of what they do to temperature and rainfall, not in themselves.
Limiting factors. In any biome, one factor is usually in shortest supply and sets the ceiling — water in a grassland, temperature and the growing season in the tundra, light on a forest floor.
What costs marks
The idea: Most of them come from blurring energy and matter.
- Saying energy is recycled. It is not. It passes through once and leaves as heat.
- Saying matter is used up. It is not. Atoms cycle.
- Confusing a pyramid of numbers with one of energy. Only the energy pyramid can never be inverted.
- Getting nitrification and fixation the wrong way round. Fixation starts from N₂ gas; nitrification starts from ammonium.