Gas Exchange in Plants

Explained

Two processes at once, and which one wins

A plant respires all the time, day and night, like every living thing. It photosynthesises only in the light.

That single sentence is the whole topic. What you observe going in and out of a leaf is the net result of the two happening together, and which way it goes depends on how bright the light is.

The two processes are opposites

Respiration uses oxygen and glucose, and produces carbon dioxide and water.

Photosynthesis uses carbon dioxide and water, and produces oxygen and glucose.

So each one produces exactly what the other consumes, which is why they cancel each other out to some degree whenever both are running.

In darkness

Only respiration is happening. The plant takes in oxygen and gives out carbon dioxide, just like an animal.

This surprises people who have learned that plants give out oxygen. They do, but only in the light, and never exclusively.

In bright light

Both are happening, but photosynthesis is much faster.

The plant uses all the carbon dioxide its own respiration produces and still needs more from the air, and it produces more oxygen than its respiration uses, so oxygen diffuses out.

Net gas exchange is therefore carbon dioxide in and oxygen out, even though respiration has not stopped for a moment.

The compensation point

Between those two there is a light intensity at which the rate of photosynthesis exactly equals the rate of respiration.

At that point every molecule of carbon dioxide produced by respiration is used by photosynthesis, and every molecule of oxygen produced by photosynthesis is used by respiration. There is no net exchange with the air at all, and a measuring instrument would record nothing entering or leaving.

That is the compensation point, and it typically occurs around dawn and again around dusk. It does not mean nothing is happening: it means two busy processes are cancelling.

How the gases move

Gases enter and leave through the stomata, which are pores mostly on the underside of the leaf, each controlled by a pair of guard cells.

They move by diffusion, down their concentration gradients, and no energy is required. In bright light the carbon dioxide inside the leaf is being used up quickly, so its concentration falls below that of the air outside, and more diffuses in. The gradient is created by the plant using the gas.

Answering the standard question

If asked what a plant is doing at a given time, say both, then say which is faster, then give the net result.

At midnight the plant is respiring only, so it takes in oxygen and releases carbon dioxide. At midday it is respiring and photosynthesising, but photosynthesis is faster, so the net movement is carbon dioxide in and oxygen out.

Answers that say a plant photosynthesises during the day and respires at night are the common error. It respires at both.