My teacher marked me down for saying plants respire at night. What is wrong with that?

The statement implies plants respire only at night, and that is the error. Plants respire continuously, day and night, exactly as animals do. Every living cell needs ATP every moment it is alive.

The confusion comes from mixing up two different processes that both involve the same gases.

Respiration is not breathing

These are separate things and the words are not interchangeable, which is the underlying problem in most answers on this topic.

Respiration is a chemical reaction inside every cell, releasing energy from glucose. It happens in mitochondria. Plants do it, animals do it, bacteria do it, and it never stops while the organism is alive.

Breathing — properly called ventilation — is the physical movement of air into and out of lungs. It is a mechanism for supplying the oxygen that respiration needs. Plants have no lungs and do not breathe at all.

If you write "plants breathe through their stomata", that is wrong twice over. Gas exchange through stomata is diffusion, not ventilation.

What actually changes at night

Not whether the plant respires, but whether photosynthesis is also happening.

In daylight, both processes run. Photosynthesis is usually faster, so the plant produces more oxygen than it consumes and takes in more carbon dioxide than it releases. The net movement of gases looks like photosynthesis alone.

At night, photosynthesis stops because there is no light. Respiration continues unchanged. Now the only gas exchange is respiratory, so the plant takes in oxygen and releases carbon dioxide.

The plant did not start respiring. The other process stopped masking it.

The compensation point

This is the idea the better questions are really after. At dawn and dusk there is a light intensity at which photosynthesis and respiration proceed at exactly the same rate, so there is no net gas exchange at all.

Below it, the plant is a net consumer of oxygen. Above it, a net producer. A question giving you a graph of gas exchange against light intensity is asking you to find that point and say what it means.

Writing it correctly

Compare these two sentences:

• "Plants respire at night." — implies they do not during the day. Marked wrong.
• "Plants respire constantly, but at night photosynthesis stops, so the net gas exchange reverses." — complete, and worth the full marks.

The second one takes six more words and says something true. Precision about net versus total exchange is most of what this topic tests.