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.
Know your predicted grade
Take timed mock exams and get detailed feedback on every answer. See exactly where you're losing marks.
Try Mock Exams Free7-day free trial • No card required
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.