A green plant carries out two opposite gas-exchange processes at the same time. In the light it does photosynthesis (takes in carbon dioxide, gives out oxygen) and, like all living things, it always does respiration (takes in oxygen, gives out carbon dioxide). The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration.
Key idea
At the compensation point: rate of photosynthesis = rate of respiration. There is no net exchange of carbon dioxide or oxygen with the surroundings.
What happens at different light levels
- In darkness — only respiration occurs. The plant releases carbon dioxide and takes in oxygen.
- At the compensation point — the carbon dioxide made by respiration is exactly used up by photosynthesis, and the oxygen made by photosynthesis is exactly used up by respiration. No gases appear to enter or leave.
- Above the compensation point — photosynthesis is faster than respiration, so overall the plant takes in carbon dioxide and gives out oxygen, and makes surplus food.
Why it matters
Only above the compensation point does a plant build up a net gain of food (glucose) and grow. A plant kept below its compensation point for a long time uses up more food in respiration than it makes and will eventually die. Different plants have different compensation points: a shade plant reaches it at low light, while a sun plant needs brighter light. This helps explain why some plants thrive under a forest canopy while others survive only in open sunlight. Growers also use this idea when deciding how much light a crop or an indoor plant needs to keep a positive food balance.
Example
Around dawn and dusk, light intensity briefly passes through the compensation point. At those moments a leaf neither takes in nor gives out carbon dioxide overall, because photosynthesis and respiration are balanced.
Remember
Compensation point is about rates being equal, not about processes stopping. Respiration and photosynthesis both continue — they simply cancel out.