Photosynthesis: how energy enters ecosystems
Almost all energy in ecosystems originally comes from the Sun.
Photosynthesis allows producers to trap solar energy.
- Occurs in chloroplasts
- Uses carbon dioxide and water
- Produces glucose and oxygen
The chemical energy stored in glucose can later be passed along food chains.
Photosynthesis is the main entry point of energy into ecosystems.
Cellular respiration: releasing energy
Energy stored in food must be released before organisms can use it.
Cellular respiration happens in both plants and animals.
- Glucose is broken down using oxygen
- Energy is released for movement, growth, and repair
- Some energy is lost as heat
Heat energy cannot be reused by organisms — this is why energy transfers are inefficient.
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Energy loss in food chains
Key idea: As energy moves along a food chain, most of it is lost at each step.
Only a small amount of the energy eaten by an organism becomes new biomass and is passed on to the next trophic level.
- Incomplete consumption (e.g. bones, bark, shells)
- Inefficient digestion Energy leaves as faeces
- Respiration Energy is lost as heat
- Movement and activity require energy
Energy is not destroyed — it is mostly lost as heat.
Energy efficiency in food chains
Energy efficiency is low because energy is lost at every step.
Remember the 10% rule: only about 10% of energy is passed on.
This explains why food chains are short. There is not enough energy to support many trophic levels.