IB Biology HLGas Exchange SystemsPaper 1 & 2~13 min read
Gas Exchange in Plants
A leaf has no lungs, no muscles and no blood, yet it solves exactly the same problem you do: get gases in and out fast, across a big moist surface. It also has a problem you do not have – every time it opens the door for carbon dioxide, water walks out.
📚 What you need to know
Gas exchange in plants happens in the leaf, through pores called stomata (singular: stoma).
A leaf has three tissue types: epidermal (outer boundary), mesophyll (the bulk of the inside) and vascular (transport).
Each stoma is controlled by two guard cells. Water in → turgid → open. Water out → flaccid → closed.
Palisade mesophyll is packed with chloroplasts for photosynthesis; spongy mesophyll has large air spaces for gas exchange.
The waxy cuticle is impermeable, so gases must use the stomata – which makes water loss controllable.
Xylem carries water and mineral ions to the leaf; phloem carries the products of photosynthesis away.
Transpiration is the unavoidable consequence of opening stomata; its rate is affected by air movement, temperature, light intensity and humidity, and is measured with a potometer.
Inside a leaf
Cut a leaf across and you find layers, each doing one job.
Layer
What it is
What it does
Waxy cuticle
A waterproof layer over the epidermis
Impermeable barrier; stops gases and water vapour leaving through the surface
Upper and lower epidermis
A single layer of tightly packed cells
Protects the inside of the leaf; the lower one contains the stomata
Palisade mesophyll
Column-shaped parenchyma cells just under the upper epidermis
Packed with chloroplasts, so most photosynthesis happens here
Spongy mesophyll
Loosely packed cells with big gaps between them
The air spaces give a huge moist internal surface for gas exchange
Guard cells
Two curved cells around each stoma
Open and close the pore, controlling gas exchange and water loss
Vascular bundle (vein)
Xylem and phloem together
Xylem brings water in, phloem takes sugars out
The air spaces in the spongy mesophyll are the plant’s real gas exchange surface – damp cell walls with air on the other side, exactly like an alveolus.
Look at where the stomata are: mostly on the underside. The lower surface is shaded and cooler, so less water evaporates from an open pore there. That single fact answers a lot of “suggest why” questions.
Guard cells: the door handle
Each stoma is a gap between two curved guard cells. They work by changing shape.
Water moves into the guard cells, they become turgid and bend apart – the pore opens.
Water is lost, they become flaccid and straighten against each other – the pore closes.
Open stomata let carbon dioxide diffuse in for photosynthesis and let oxygen diffuse out. Closed stomata stop that, but they also stop water escaping.
Blue arrows show water entering the turgid guard cells; amber arrows show water leaving as they go flaccid.
Transpiration: the price of the ticket
Here is the plant’s dilemma, and it is worth understanding rather than memorising.
Photosynthesis needs carbon dioxide, and the only way in is through open stomata.
Inside the leaf, the air spaces are saturated with water vapour from the damp cell walls.
Outside, the air is usually drier. So the moment a stoma opens, water vapour diffuses out down its own gradient.
The key sentence
Transpiration is the inevitable consequence of gas exchange in the leaf
The plant can shut its stomata to save water, but then carbon dioxide stops coming in and photosynthesis slows down. There is no setting where it wins both ways.
Transpiration is not all bad news, though:
Evaporation from the leaf cools the plant.
The transpiration stream carries mineral ions up from the roots.
Water keeps cells turgid, which supports leaves and the stems of non-woody plants. A turgid leaf stays flat and holds a bigger surface area to the light.
Diffusion, not osmosis. Water leaves the leaf as vapour, through a pore, not across a membrane. Osmosis needs a partially permeable membrane, so it is the wrong word here. Say water vapour diffuses out through the stomata.
What changes the rate of transpiration
Factor
Effect
Why
Air movement
More wind, faster transpiration
Still air lets water vapour build up outside the stomata, making a humid layer that flattens the gradient. Wind blows it away and the gradient gets steeper
Temperature
Faster, up to a point
More kinetic energy, so water evaporates from the cell walls faster. If it gets too hot the stomata close to save water and the rate drops sharply
Light intensity
Faster, then levels off
Stomata open in the light for photosynthesis. Once they are all open, more light makes no further difference
Humidity
Higher humidity, slower transpiration
Air outside is already close to saturated, so there is little concentration gradient. At high enough humidity the levels inside and outside are equal and there is no net loss
Measuring it: the potometer
A potometer measures how fast a cut shoot takes up water. Strictly speaking that is water uptake, not transpiration – a little of the water is used in photosynthesis. But that amount is tiny compared with what evaporates, so uptake is a fair stand-in for transpiration rate.
Two ways to take the reading: measure how far the bubble moves in a set time, or time how long it takes to move a set distance.
🧩 Using a potometer
Cut the shoot and set the apparatus up under water, so no air gets into the tube.
Seal the shoot into the tube with an airtight rubber bung and check every joint.
Let the shoot settle for a few minutes so the rate becomes steady.
Introduce a single air bubble into the capillary tube and note its starting position.
Time how far the bubble moves in a fixed time, then use the reservoir tap to reset it.
Repeat and take a mean, then change one factor and repeat the whole thing.
To change one factor at a time: a fan on different settings varies air movement, a plastic bag over the shoot raises humidity, a lamp at different distances varies light intensity, and a heater or air conditioner varies temperature. Whichever one you change, everything else has to be held constant – moving the plant into a different room to change temperature would also change light and humidity, and your results would mean nothing.
Worked examples
WE 1
Adaptations of the leaf
Explain two ways in which a leaf is adapted for efficient gas exchange. (4 marks)
Adaptation 1: air spaces
The spongy mesophyll contains large air spaces between the cells.
This gives a large moist internal surface area and lets gases move freely right up to every cell.
Adaptation 2: stomata
Pores in the lower epidermis, opened and closed by guard cells, let carbon dioxide diffuse in and oxygen diffuse out.
They are thin openings in a very thin epidermis, so the diffusion distance is short.
Big internal surface + short controlled route in and outtwo marks per adaptation: name it, then say what it does for diffusion
WE 2
Wind and transpiration
Explain why the rate of transpiration increases when a fan is switched on next to a plant. (3 marks)
Point 1: still air
In still air, water vapour builds up just outside the stomata, creating a humid layer.
Point 2: the gradient
This reduces the water vapour concentration gradient between the inside of the leaf and the air, so less water diffuses out.
Point 3: the fan
Moving air carries the vapour away, so the gradient becomes steeper again and more water vapour diffuses out of the leaf.
Wind removes the humid layer, so the gradient stays steepthe mark is for the gradient, not for saying “the wind dries the leaf”
WE 3
The plant’s dilemma
Suggest why a plant closes its stomata on a very hot, dry day, and what the cost of doing so is. (3 marks)
Point 1: the risk
On a hot dry day water evaporates from the leaf very quickly, so the plant risks losing more water than the roots can replace and wilting.
Point 2: the response
Guard cells lose water, become flaccid and close the stomata, greatly reducing transpiration.
Point 3: the cost
Carbon dioxide can no longer diffuse into the leaf, so the rate of photosynthesis falls.
Saving water and photosynthesising are a trade-off“suggest” questions want reasoning, so always name the cost as well as the benefit
💡 Exam tips
Say stoma for one and stomata for more than one. Getting this wrong looks careless.
Guard cells: turgid = open, flaccid = closed. Learn it as a pair.
Every transpiration explanation should end with the water vapour concentration gradient.
Water vapour diffuses out of a leaf. It does not leave by osmosis.
Know that a potometer measures water uptake, and be ready to say why that is close enough to transpiration.
For any practical question, name the independent, dependent and controlled variables.
⚠ Common mistakes
Saying plants only respire at night. Plants respire all the time; in the light photosynthesis simply masks it.
Writing that leaves take in oxygen only. In daylight the leaf takes in carbon dioxide and releases oxygen overall.
Calling transpiration a way of getting rid of waste water. It is an unavoidable side effect of opening stomata.
Saying humidity blocks the stomata. High humidity reduces the gradient; the pore is still open.
Putting most stomata on the upper surface. In a typical dicot leaf they are mainly on the lower surface.
Forgetting that a potometer needs to be set up under water. One air bubble in the wrong place ruins the readings.
Up next: Drawing Leaf Structure – the skill of turning a micrograph into a plan diagram that actually earns the marks.
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