Photosynthesis puts energy into storage. Respiration takes it back out. Every living thing does it, every hour of every day, and every time it happens a little heat escapes. That constant leak is the single biggest reason energy transfers in ecosystems are so inefficient.
📚 What you need to know
Respiration converts organic matter into carbon dioxide and water, releasing energy.
It happens in all living organisms, including plants, all of the time.
Aerobic respiration: glucose + oxygen → carbon dioxide + water (+ energy released).
Inputs: organic matter and oxygen. Outputs: energy for work, plus heat.
Transformation: stored chemical energy becomes kinetic energy and heat.
Respiration is not 100% efficient, so heat is always produced.
That heat cannot be converted back into chemical energy and is lost from the organism.
The equation, in words
Respiration is not the same thing as breathing. Breathing moves gases; respiration is the chemical reaction happening inside every cell.
Systems term
For aerobic respiration
Inputs
Organic matter (glucose) and oxygen
Processes
Oxidation reactions inside living cells
Outputs
Energy released for work such as movement, growth and repair, plus heat
Transformations
Stored chemical energy becomes kinetic energy and heat
The two processes are opposites
Put the two equations next to each other and the pattern is obvious. Photosynthesis stores energy and builds organic matter; respiration releases energy and breaks organic matter down.
The carbon atoms cycle round this loop endlessly. The energy makes one pass and exits as heat, which is why the loop cannot power itself.
Why respiration matters for the whole ecosystem
Cellular respiration is where the biggest energy losses in an ecosystem happen. An animal uses energy for moving, hunting, digesting, keeping warm and repairing tissue. All of that work generates heat, and heat generated inside a body cannot be turned back into chemical energy. It radiates away.
Only what is left after respiration can be stored as new tissue — and only stored tissue can be eaten by the next trophic level. That is the link between this page and everything that follows.
Entropy in one line: heat released by respiration increases the disorder (entropy) of the surroundings, and that is precisely what allows an organism to keep itself highly organised.
You will not be asked about ATP. What you do need is the idea that the energy released by respiration is used to carry out active processes inside living cells — growth, movement, repair.
Worked examples
WORKED EXAMPLE
Explain why respiration reduces the energy available to the next trophic level.
Step 1: what respiration does
Respiration releases energy from glucose so the organism can move, grow and repair itself.
Step 2: where the energy goes
Respiration is not fully efficient, so a large share of that energy becomes heat and is lost from the body.
Step 3: the consequence
Only the energy stored as new biomass can be eaten, so the next trophic level receives much less than the level below took in.
Energy lost as heat is unavailable to consumersSay “stored as biomass” — that phrase is what connects respiration to food chains.
WORKED EXAMPLE
A student writes: “Plants photosynthesise and animals respire.” Correct the statement.
What is wrong
Plants respire too, continuously, day and night. Respiration is how they release the energy in the glucose they make.
The correct version
Plants both photosynthesise and respire. Animals only respire.
The visible consequenceIn daylight a plant photosynthesises faster than it respires, so overall it releases oxygenAt night photosynthesis stops but respiration carries on, so a plant takes in oxygen then.
💡 Exam tip
Put energy released on the arrow of the respiration equation, not among the products.
When explaining energy loss, name respiration as the process and heat as the form. Both words are needed.
Remember respiration happens in producers as well as consumers. Many mark schemes test this.
Use “cannot be converted back into chemical energy” when explaining why heat is a permanent loss.
Keep breathing out of your answer entirely unless the question is about gas exchange.
⚠ Common mix-up
Respiration confused with breathing. Respiration is a chemical reaction in cells; breathing is a movement of air.
Thinking plants do not respire. They do, all the time.
Saying respiration produces energy. It releases energy that was already stored in glucose.
Writing the equation the wrong way round. Carbon dioxide and water on the left is photosynthesis.
Treating heat as useful output. For the ecosystem it is a loss, even though it keeps mammals warm.
Forgetting that respiration is the biggest loss in a food chain, larger than egestion or uneaten material.
Up next: Trophic Levels and Feeding Chains — naming the levels correctly, drawing arrows the right way, and where decomposers fit in.
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