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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
The key difference
Photosynthesis converts light energy into chemical energy by building glucose from carbon dioxide and water. Respiration releases that chemical energy from glucose for use by the cell. They are essentially reverse reactions.
Photosynthesis and respiration are often described as "opposite" processes, but this is a simplification. They use different pathways, occur in different organelles, and serve fundamentally different purposes. Understanding how they relate: and how they differ: is essential for IGCSE Biology.
Feature
Photosynthesis
Respiration
Word equation
carbon dioxide + water → glucose + oxygen
glucose + oxygen → carbon dioxide + water
Energy change
Endothermic: absorbs light energy and stores it as chemical energy in glucose
Exothermic: releases chemical energy from glucose
Where in the cell
Chloroplasts (in cells containing chlorophyll)
Cytoplasm and mitochondria
Which organisms
Plants, algae, and some bacteria only
All living organisms
When it occurs
Only in the light (daytime)
Continuously: day and night
Gas exchange
Takes in CO₂, releases O₂
Takes in O₂, releases CO₂
Reactants
Carbon dioxide and water
Glucose and oxygen
Products
Glucose and oxygen
Carbon dioxide and water
Do Plants Respire?
Yes: this is one of the most common misconceptions. All living cells respire, including plant cells. Plants carry out respiration continuously (24 hours a day), just like animals. During the day, plants also photosynthesise, and the rate of photosynthesis usually exceeds the rate of respiration. This means that during the day, a plant is a net absorber of CO₂ and a net producer of O₂. At night, only respiration occurs, so the plant takes in O₂ and releases CO₂.
The Compensation Point
At a particular light intensity, the rate of photosynthesis exactly equals the rate of respiration. This is the compensation point. At this point:
CO₂ absorbed by photosynthesis = CO₂ released by respiration
O₂ released by photosynthesis = O₂ used in respiration
There is no net gas exchange with the environment
Above the compensation point, photosynthesis exceeds respiration and the plant has a net gain of glucose. Below it (dim light or darkness), respiration exceeds photosynthesis and the plant uses its stored glucose.
Try a comparison question (4 marks)
A student places a water plant in a sealed container. In bright light, bubbles of gas are produced. In the dark, no bubbles are seen. Explain these observations. [4]
Show the answer and marking guidance
In bright light, the plant photosynthesises, producing oxygen as a by-product: the bubbles are oxygen
The rate of photosynthesis exceeds the rate of respiration, so there is net oxygen production
In the dark, photosynthesis cannot occur (no light energy available)
Only respiration occurs in the dark, which uses oxygen rather than producing it, so no bubbles are seen
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Saying plants do not respire: all living organisms respire continuously
Claiming photosynthesis is the "opposite" of respiration without qualification: they are reverse in equation but different in mechanism, location, and function
Forgetting that photosynthesis only occurs in the light but respiration occurs all the time
Saying photosynthesis produces energy: it stores light energy as chemical energy in glucose