Breathing and gas exchange · Contrast
Stomata and gas exchange in plants
A plant respires every second of its life, exactly like you. In daylight it also does something else, and the two run at once.
Start here
Seal a healthy plant in a jar with a carbon dioxide sensor. At dawn, the reading stops falling.
Overnight the carbon dioxide in the jar rose steadily. As the light came up it began to fall. Then, at one particular light level in the early morning, it held perfectly steady for several minutes — neither rising nor falling — before starting to fall again.
What is the plant doing during those steady minutes?
Both processes at full tilt, cancelling out. The plant is respiring and producing carbon dioxide at exactly the rate photosynthesis is consuming it. A flat line does not mean nothing is happening — here it means two things are happening at identical rates, and it is the single best piece of evidence that plants never stop respiring.
At the bench · turn the light up
Two processes, one net figure
currently dark
Light level
darkness
Respiration — uses oxygen, makes carbon dioxide
2.0 units
Flat. Drag the light from one end to the other and this bar does not move — that is the whole point.
Photosynthesis — uses carbon dioxide, makes oxygen
0.0 units
Zero in darkness. No light, no photosynthesis — but the bar above is unaffected.
Rises with light, then levels off as other factors become limiting.
What a sensor outside the leaf measures
net CO₂ out 2.0 units
Only the difference is visible from outside. Both processes are always running underneath this figure.
The compensation point
Flat line — and both processes at full rate.
This is the dawn reading from the hook. Photosynthesis is consuming carbon dioxide at exactly the rate respiration produces it, so nothing appears to happen. A sensor cannot tell this apart from a dead plant, which is why the reasoning matters more than the reading.
Net uptake of carbon dioxide
Carbon dioxide moving in, oxygen moving out.
Photosynthesis is outrunning respiration, so the plant absorbs more carbon dioxide than it produces. Respiration has not stopped — look at the top bar. It is simply hidden underneath a larger opposite flow.
Net release of carbon dioxide
Carbon dioxide moving out, oxygen moving in — the same direction as you.
In darkness only respiration is running, so the plant takes in oxygen and gives out carbon dioxide, exactly as an animal does. This is the reading that shows plants and animals are not opposites.
How the gases get in
A hole you can shut, and the price of shutting it.
The pore · A stoma is a gap between two cells
Not a hole in a wall — a gap held open between a pair of curved guard cells. Most are on the underside of the leaf, where they are shaded and sheltered, and a square millimetre of leaf may carry several hundred.
The control · Guard cells open it by swelling
When water moves into the guard cells they become turgid and bow apart, opening the pore. When water leaves they go limp and the pore closes. The plant controls its gas exchange by moving water, not by moving muscle.
The trade-off · Every open stoma loses water
The inside of a leaf is wet, because diffusion needs a moist surface. So a pore open enough for carbon dioxide to diffuse in is open enough for water vapour to diffuse out — the two cannot be separated.
The consequence · Wilting is a decision, not a failure
A plant short of water closes its stomata, which stops water loss and simultaneously stops photosynthesis. It is choosing survival over growth, and it is why a hot dry afternoon slows a crop even in full sunshine.
Same four requirements, third organ A leaf meets the same four requirements as a villus and an alveolus: a large internal surface where the air spaces meet the cells, a very short diffusion distance from air space to cell, a maintained concentration difference — the cells consume whichever gas they need, keeping its level low inside — and a moist surface, which is exactly why water is lost through an open stoma. Three organs, three shapes, one set of rules.
Key fact
Gases diffuse in and out of a leaf through stomata, mostly on the underside, opened and closed by guard cells. Plants respire continuously and photosynthesise only in light; the net movement of gas is the difference between the two.
Think again
“Plants take in carbon dioxide and give out oxygen. Animals do the opposite.”
This describes photosynthesis and then calls it the whole of plant biology. Every living cell respires — glucose plus oxygen, releasing energy — and a plant cell is not an exception, because a plant needs energy to grow, to transport minerals and to make new tissue in exactly the way you do. What is true is that in good light photosynthesis runs several times faster than respiration, so the net flow reverses and a sensor outside the leaf records oxygen coming out. The word doing all the work in that sentence is net. Set the light to zero on the bench above and the plant releases carbon dioxide and absorbs oxygen — the same direction as you.
“Plants respire at night and photosynthesise in the day.”
Almost right, and wrong in the way that costs marks. Respiration does not switch on at dusk; it never switches off at all. In the middle of a bright afternoon a leaf is respiring at exactly the rate it was respiring at midnight — the top bar on the bench does not move when you drag the light. What changes is only the second bar. At night respiration is the only process running, which is why it becomes visible; during the day it is hidden underneath a larger opposite flow. A process you cannot detect is not a process that has stopped.
“Plants breathe through their stomata.”
Stomata are holes, and holes do not breathe. Nothing in a plant does the muscular work you met in How breathing works — there is no diaphragm, no rib cage, no pressure difference generated, and no ventilation. Gases arrive and leave entirely by diffusion, driven by nothing but the concentration difference the cells themselves create by consuming one gas or the other. A plant does not breathe, and this is not a small verbal distinction: it is why a plant can be metres tall with no circulation of air inside it at all, and why leaves have to be thin.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · When does a plant respire
When does a plant respire?
Rung 2 · The one that catches people
A sealed jar containing a plant shows no change in carbon dioxide for ten minutes in dim light. What is happening?
Rung 3 · Explain the net figure
Explain why a plant absorbs carbon dioxide in bright light but releases it in darkness, without saying that respiration stops or starts. Use the words net, rate and continuously.
Rung 4 · Take it somewhere new
A grower keeps greenhouse lights on for 24 hours to maximise growth. A colleague argues this wastes electricity because the plants also need a dark period. Using this lesson, say what continuous light does and does not achieve, and what evidence would settle the argument.
Key note
Stomata are pores in the leaf surface, mostly on the underside, controlled by a pair of guard cells. Gases move through them by diffusion only — plants do not breathe. Plants respire all the time; they photosynthesise only in light. The net gas movement is the difference between the two rates, and at one particular light level it is zero.
Going further
Cacti and pineapples solve the stomatal dilemma by moving it in time rather than solving it. Opening a stoma in desert daylight loses catastrophic amounts of water, so these plants keep them shut all day and open them at night, when the air is cool and damp. Carbon dioxide taken in at night is fixed into an acid and stored in the cells until morning, then released internally and fed into photosynthesis behind closed stomata. It is called CAM photosynthesis, and it costs energy and limits growth rate — which is why cacti grow so slowly, and why the strategy only makes sense where water, not time, is the thing in shortest supply. A pineapple leaf is at its most sour early in the morning and much less so by evening: that is the stored acid, used up over the course of the day.
Before this lesson
Connects to
At GCSE this becomes
- Leaf structure and adaptation, transpiration, limiting factors in photosynthesis, and the compensation point.
Where to next
Ask Mr Badmus AI
Want to check what the flat line at dawn actually means?
Rates on the bench are relative units chosen to make the two processes comparable, not measured values. The light level at which the two balance differs widely between species and conditions. B7 Photosynthesis owns the reaction itself; this lesson covers only the movement of gases.
Lesson content © MrBadmusAI.