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Breathing and gas exchange · Contrast

Stomata and gas exchange in plants

Start here

A plant sealed in a jar shows a steady carbon dioxide reading at dawn.

If you had to guess, during those steady minutes, is the plant making or using any carbon dioxide at all?

Key words

  • stomata
  • guard cells

  • The spreading of a substance from where it is more concentrated to where it is less concentrated, with nothing pushing it.

Key knowledge

Stomata are tiny pores in a leaf. Most are on the underside.

In the light, carbon dioxide diffuses in through the stomata.

Oxygen and water vapour diffuse out.

A pair of guard cells, open and closed, and where the pores are in the leafTwo panels side by side, each showing the same pair of guard cells seen from the surface of the leaf. In the left panel, water has moved in: each cell is bowed away from its partner and the gap between them, the pore, is wide open. In the right panel water has moved out: the cells are straight and limp, their inner edges meet along a line, and there is no gap at all. In both panels the inner edge of each cell is drawn as a much thicker wall than the outer edge, which is why a filling cell bends outwards instead of swelling evenly. Arrows show water moving into the cells on the left and out of them on the right. Underneath both panels, a wide shallow slice through a leaf shows three pores, every one of them in the lower surface, while the upper surface runs unbroken from one side to the other. The slice is drawn in layers: a thin skin along the top, a row of block-shaped cells beneath it, a band of rounded cells through the middle, and along the bottom another row of block cells and a second thin skin. Only those two bottom rows are interrupted, and they are interrupted three times. At each interruption a pair of rounded guard cells sits one either side of the gap, the same pair the two panels above show from the surface. A line of text beneath the slice states the point in words: three pores, every one of them on the underside, and the top surface unbroken.THE SAME PAIR OF GUARD CELLS, SEEN FROM THE SURFACEOpenwater in · firmpore openwater inwater inthickerinner wallClosedwater out · limpwater outwater outthe two innerwalls meet —no gap leftTHE SAME LEAF, SLICED — TOP OF THE LEAF AT THE TOPThree pores, every one of them on the underside — and the top surface unbroken.
Water moves in, the guard cells curve and the pore opens. Water moves out and it closes.

A pair of guard cells sits around each stoma.

Water moves into the guard cells and they swell. They bend apart and the stoma opens.

Water moves out and they go limp. The stoma closes.

A leaf through the day

RowIn bright lightIn the dark
Stomata
Photosynthesis
Respiration
Overall, the leaf takes in
Overall, the leaf gives out

In bright light, the rate of photosynthesis is higher than the rate of respiration.

So overall the leaf takes in carbon dioxide.

In the dark, only respiration happens. So overall the leaf gives out carbon dioxide.

Respiration happens continuously. That means all the time.

Net means overall.

  • Air spaces inside the leaf give a large surface for diffusion.
  • The leaf is thin, so gases only travel a short distance.
  • Stomata let gases in and out.

Open stomata let water vapour escape.

A plant short of water closes its stomata.

This saves water, but it also stops carbon dioxide getting in.

  • Wrong: Plants breathe.

    Right: Plants have no lungs or breathing muscles. Gases move by diffusion.

  • Wrong: Plants only respire at night.

    Right: Plants respire all the time, day and night.

  • Wrong: Plants only ever take in carbon dioxide.

    Right: In the dark, a plant gives out carbon dioxide overall.

Key fact

Gases diffuse through stomata, which guard cells open and close. Plants respire all the time and photosynthesise only in light.

Practice

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1

When does a plant respire?

Rung 2

A sealed jar containing a plant shows no change in carbon dioxide for ten minutes in dim light. What is happening?

Rung 3

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

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.

In bright light, photosynthesis is faster than respiration.

In dim light, the two rates can be equal.

Then the gas in the air around the leaf does not change overall.

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

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