The Leaf as the Main Organ for Gaseous Exchange
The leaf is the main site of gaseous exchange in a plant. Carbon dioxide and oxygen diffuse in and out through pores called stomata, moving through air spaces in the spongy mesophyll to reach the cells.
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Why the leaf is suited to gas exchange
- A thin, flat shape gives a short diffusion distance and a large surface area relative to volume.
- A high density of stomata, mainly on the lower epidermis, allow gases to pass in and out.
- Interconnected air spaces in the spongy mesophyll let gases circulate to reach every cell.
- A moist internal surface inside the leaf allows gases to dissolve before diffusing into cells.
- A waxy cuticle covering the leaf surface is impermeable to gases, so it forces almost all gas exchange to occur through the stomata rather than across the general leaf surface.
Gas exchange during the day and at night
During the day, when light is available, the rate of photosynthesis usually exceeds the rate of respiration. The leaf takes in carbon dioxide overall and releases oxygen, since photosynthesis uses more carbon dioxide than respiration produces.
At night, without light, only respiration occurs. The leaf takes in oxygen and releases carbon dioxide, the opposite of the daytime pattern.
At a particular light intensity, called the compensation point, the rate of photosynthesis exactly equals the rate of respiration, so there is no net exchange of carbon dioxide or oxygen between the leaf and the surrounding air.
The role of guard cells and stomata
Each stoma is bordered by two guard cells that change shape to open or close the pore, regulating how much gas (and water vapour) passes through. Stomata typically open in daylight to allow carbon dioxide in for photosynthesis and close in darkness or under water stress to limit water loss.
How it is examined
Questions often ask you to explain the net direction of gas exchange in light versus dark conditions, to state adaptations of the leaf for gas exchange, or to interpret an experiment using a suitable indicator, such as hydrogencarbonate indicator or a gas syringe, to measure gas exchange in a leaf.
Worked exam-style question
Question. The diagram for this question shows a transverse section through part of a leaf, with a labelled air space in the spongy mesophyll and a pore in the lower epidermis flanked by two curved cells. A student places two similar leaf discs into separate test tubes of hydrogencarbonate indicator solution: tube A is left in bright light, and tube B is wrapped in foil and kept in darkness, both for one hour.
(a) Name the pore and the two cells that surround it. (b) State and explain the colour change expected in tube A.
(c) State and explain the colour change expected in tube B. (d) Explain how the air spaces in the spongy mesophyll help gas exchange reach every cell in the leaf.
Model answer. (a) The pore is a stoma, bordered by two guard cells. (b) In tube A the indicator turns from red to purple, because the leaf disc's rate of photosynthesis exceeds its rate of respiration in bright light, so carbon dioxide is removed from the solution faster than respiration releases it, lowering the carbon dioxide concentration and raising the pH.
(c) In tube B the indicator turns from red to yellow, because in darkness only respiration occurs, releasing carbon dioxide into the solution and lowering the pH. (d) The air spaces form a continuous, interconnected pathway through the spongy mesophyll, allowing carbon dioxide and oxygen to diffuse freely between the stomata and the mesophyll cells, so every photosynthesising and respiring cell has access to the gases it needs.
Practice question
Try this. A plant is enclosed in a sealed, transparent plastic bag and left under bright light for six hours. State and explain what would happen to the carbon dioxide and oxygen concentrations inside the bag, and explain how the result would differ if the same set-up were left in complete darkness instead.
Exam tip
Key terms
Revise these linked terms to answer gaseous exchange questions precisely:
- Stoma, the pore through which gases enter and leave the leaf.
- Guard cell, one of the pair of cells that open and close each stoma.
- Gaseous exchange, the general movement of gases between an organism and its surroundings.
- Cuticle, the waxy layer that forces gas exchange through the stomata rather than across the leaf surface.
Source:SRC-DSKP-EN
Frequently asked questions
Why does a leaf release oxygen in the day but carbon dioxide at night?
How do stomata control gas exchange?
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