Plant Excretion Through Stomata

Explained

What counts as waste in a plant

Excretion is the removal of the waste products of metabolism, meaning substances the organism has made itself. Plants excrete, and what they excrete is unusual: their two waste products are gases, and each is the raw material of the other process.

The two waste gases

Respiration produces carbon dioxide. When photosynthesis is not using all of it, the surplus is waste and leaves the leaf.

Photosynthesis produces oxygen. When respiration is not using all of it, the surplus is waste and leaves the leaf.

So which gas is the waste product depends entirely on the light. In darkness only respiration runs, so carbon dioxide is the waste. In bright light photosynthesis is faster, so oxygen is the waste and carbon dioxide is being taken in rather than removed.

That is why a question about plant excretion usually specifies the conditions, and why an answer that names only one gas is answering only one half of the day.

How the gases leave

Through the stomata, which are pores mostly on the lower surface of the leaf, each controlled by a pair of guard cells.

The movement is by diffusion, down a concentration gradient, and it needs no energy. Inside the leaf are air spaces between the spongy mesophyll cells, connected to the stomata, so a gas produced by any cell can reach the outside air.

In bright light, oxygen builds up inside the leaf to a higher concentration than in the air outside, so it diffuses out. At the same time carbon dioxide inside is being used up, falling below the outside concentration, so it diffuses in. Both gradients exist at once, in opposite directions, through the same pores.

Why it is not called egestion

Excretion is the removal of something the organism made. Egestion is the removal of material that passed through without being absorbed.

A plant has no gut and takes in nothing it does not absorb, so it does not egest at all. Both of its waste gases were made by its own reactions, which makes them excretion by definition.

Other waste

Plants also produce waste that is not a gas, and they deal with it differently from animals. Rather than removing it continuously, they store it in cells that are going to be lost anyway, particularly in leaves before those leaves fall.

Losing a leaf therefore removes stored waste along with it. That is a slower system than an animal's, and it works because a plant produces far less metabolic waste in the first place, having no need to break down excess protein into urea.

Answering the standard question

Name the gas, name the process that made it, and say that it leaves by diffusion through the stomata.

In the dark, carbon dioxide from respiration diffuses out through the stomata. In bright light, oxygen from photosynthesis diffuses out through the stomata. Three clauses, and each one is usually a mark.