Photosynthesis puts energy into store. Respiration takes it back out again — in every living cell, day and night, in plants as much as in animals. It is also the main reason energy drains out of food chains, so this page quietly explains a lot of what comes later.
📘 What you need to know
Respiration converts organic matter into carbon dioxide and water, releasing energy.
It happens in all living organisms, including producers.
Inputs: glucose and oxygen. Outputs: carbon dioxide, water, usable energy and heat.
Transformation: stored chemical energy becomes kinetic energy and heat.
It is not 100% efficient, so heat is always produced as a by-product.
That heat cannot be turned back into chemical energy — it leaves the organism for good.
The reaction, in plain words
Glucose and oxygen go in. The cell breaks the glucose apart in a controlled way, releasing the energy that photosynthesis put in. Carbon dioxide and water are left over.
Aerobic respiration word equation
glucose + oxygen → carbon dioxide + water (+ energy released)
Compare this with the photosynthesis equation on the previous page. Same four substances, opposite direction, opposite effect on energy.
Where the released energy actually goes
Here is the part that matters for ecology. The energy released does not all go into useful work. Respiration is not 100% efficient at moving energy from glucose into the chemical form cells can use, so some of it is degraded into heat straight away.
That heat warms the organism for a moment and then leaves. It cannot be converted back into chemical energy, and no organism further up the food chain can eat it. It is gone from the ecosystem’s energy budget.
This split is why a herbivore never passes on everything it eats. A large slice was already spent just keeping the animal alive.
When an exam asks why energy transfer between trophic levels is inefficient, the first thing to write is heat lost during respiration. It is the biggest single loss, and it is the one students most often leave out.
Respiration, heat and entropy
There is a neat link back to the second law here. A living body is highly ordered — low entropy. Keeping it that way is not free. Organisms pay for their internal order by releasing heat into the surroundings, which raises the entropy of the wider ecosystem.
One-line version: organisms stay organised on the inside by making a mess on the outside. The heat they dump into the environment is that mess.
WORKED EXAMPLE
A crop stores 9 500 kJ m−2 yr−1 of chemical energy through photosynthesis. It uses 3 800 kJ m−2 yr−1 in respiration. How much energy is left in the plants’ biomass?
Step 1: Identify what each number is9 500 is the energy captured; 3 800 is the respiratory lossStep 2: Subtract9 500 − 3 800 = 5 7005 700 kJ m−² yr−¹This is the amount actually available to herbivores
WORKED EXAMPLE
A mouse takes in 1 400 kJ of chemical energy in its food. It loses 1 040 kJ as heat from respiration and 260 kJ in faeces. How much is stored as new body tissue?
Step 1: Add up the losses1 040 + 260 = 1 300 kJStep 2: Subtract from the energy taken in1 400 − 1 300 = 100 kJ100 kJ stored as biomassThat is about 7% – and only this part can be passed on
💡 Exam tip
Always add the words “as heat”. “Energy is lost in respiration” is vague; “lost as heat during respiration” is complete.
Remember producers respire too. This is the whole reason NPP is smaller than GPP later in the topic.
You do not need to mention ATP by name, though it is fine if you do.
If a question gives energy in and several losses, add the losses first, then subtract once. Fewer slips that way.
Link respiration to the second law if the question asks about efficiency or entropy.
⚠ Common mix-up
“Only animals respire.” Every living cell respires, plants included, all day and all night.
Confusing respiration with breathing. Breathing moves air; respiration is the chemical release of energy inside cells.
Thinking the heat is recycled. It raises entropy in the surroundings and is unavailable to any organism.
Writing that respiration produces energy. It releases energy that was already stored — nothing is created.
Forgetting faeces. Respiration is the biggest loss, but undigested material leaves too, and questions often give you both.
Up next: Trophic Levels and Feeding Chains — how we map who eats whom, and why the arrows point the way they do.
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