Energy input and release at HL: The energy ideas here are the same for SL and HL, and they set up the more detailed nutrient and carbon cycle work that's unique to HL in this unit.
No HL-only material here — just learn how energy enters and leaves a system.
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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.
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Photosynthesis is the main entry point of energy into ecosystems.
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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.
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.
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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.
IB-style question — Calculate NPP from GPP and respiration
An Amazonian rainforest plot has a gross primary productivity (GPP) of 9 400 g dry mass m⁻² yr⁻¹. The same plot loses 6 100 g dry mass m⁻² yr⁻¹ through plant respiration (R). Calculate the net primary productivity (NPP) and state what NPP represents. [2]
How to answer it, step by step
- Formula → substitution → answer
• NPP = GPP − R = 9 400 − 6 100 = 3 300 g dry mass m⁻² yr⁻¹ - State what NPP means
• NPP is the biomass available to consumers (herbivores and decomposers) after the plant has used some for its own respiration.
Final answer
Always write NPP = GPP − R before plugging in numbers. Include units. Common error: writing NPP = GPP + R — respiration is a LOSS.