IB Biology SLGas Exchange SystemsPaper 1 & 2Core idea | Practical skill~12 min read
Gas Exchange in Plants
A plant faces a problem no animal has. To photosynthesise it must open pores and let carbon dioxide in — but the moment those pores open, water walks straight out. Everything about leaf structure is a compromise between those two facts.
📚 What you need to know
A leaf has three tissue types: epidermal (outer boundary), mesophyll (the bulk of the inside), and vascular (transport).
Stomata in the lower epidermis are the pores through which gas exchange happens. Each is controlled by two guard cells.
Guard cells become turgid when water enters and the stoma opens; they become flaccid when water is lost and it closes.
The waxy cuticle is impermeable, so gas exchange is forced to happen through the stomata where it can be controlled.
Palisade mesophyll is packed with chloroplasts for photosynthesis. Spongy mesophyll has large air spaces for gas exchange.
Transpiration is the loss of water vapour through the stomata — the inevitable consequence of gas exchange.
Rate of transpiration is affected by air movement, temperature, light intensity and humidity, and is measured with a potometer.
Inside a leaf
Notice where the chloroplasts are concentrated: in the palisade layer near the top, where the light is strongest. The spongy layer below is mostly gaps, because its job is moving gases, not catching light.
The three tissues
Epidermis. A single layer of tightly packed cells forming the upper and lower boundary, protecting the inside of the leaf. The lower epidermis contains the stomata. It is covered by a waxy cuticle, which is an impermeable barrier.
Mesophyll. Made of parenchyma cells containing chloroplasts, so this is where photosynthesis happens. Palisade mesophyll sits just under the upper epidermis and is packed with chloroplasts. Spongy mesophyll has large air spaces between the cells for gas exchange.
Vascular tissue. Arranged in vascular bundles, which form the veins. Xylem brings water and mineral ions up from the roots; phloem carries the products of photosynthesis away to the rest of the plant.
Adaptation
How it helps gas exchange
Waxy cuticle
Stops gases and water vapour leaving through the epidermis, so exchange is forced through the stomata where it can be controlled
Stomata in the lower epidermis
Provide the pores for diffusion. Being underneath keeps them cooler, which reduces water loss
Air spaces in spongy mesophyll
Allow gases to move freely inside the leaf, maintaining a concentration gradient right up to the cell surfaces
Spongy mesophyll cell shape
The irregular packing increases the total surface area available for gas exchange
Guard cells
Open and close the stomata, controlling gas exchange and water loss together
Veins
Xylem delivers the water needed for photosynthesis and lost in transpiration; phloem removes the sugars made
Transpiration: the price of gas exchange
Photosynthesis needs carbon dioxide, which can only get in when the stomata are open. But the air inside the leaf is much more humid than the air outside, so whenever the stomata open, water vapour diffuses out. That loss is transpiration.
The trade-off in one line
Stomata open → CO₂ in and water out | Stomata closed → water saved but photosynthesis stops
Guard cells let the plant choose which problem to have at any moment. Closing the stomata reduces water loss but also reduces gas exchange, and therefore the rate of photosynthesis. Transpiration is the inevitable consequence of gas exchange in the leaf.
It is not all cost, though. Transpiration brings some real advantages:
Cooling the plant, through evaporation.
The transpiration stream helps with the uptake of mineral ions.
Turgor pressure from the water moving up the plant supports the leaves and the stems of non-woody plants. Leaves with high turgor do not wilt, so they keep their full surface area for photosynthesis.
Get this wording right: water vapour diffuses out of the leaf during transpiration. It is not osmosis. Osmosis requires water to cross a partially permeable membrane, and that is not what is happening at the stoma.
What changes the rate of transpiration
Temperature is the odd one out: it turns downwards. That drop is not physics running out — it is the plant deliberately closing its stomata to save water.
Factor
Effect
Why
Air movement
More movement increases the rate, up to a plateau
In still air, water molecules accumulate just outside the stomata, creating a locally humid layer that reduces the concentration gradient. Wind carries them away, restoring a steep gradient
Temperature
Higher temperature increases the rate, then it falls
More kinetic energy means water evaporates out of the leaf faster. But if it gets too hot, the stomata close to prevent excessive water loss, which cuts the rate sharply
Light intensity
Brighter light increases the rate, then it levels off
Stomata close in the dark and open in the light to allow gas exchange for photosynthesis. Once every stoma is already open, more light makes no further difference
Humidity
Higher humidity decreases the rate
Humid air outside the leaf means a smaller difference in water vapour concentration between inside and outside, so the gradient is shallower. At high enough humidity an equilibrium is reached and there is no net loss
Measuring the rate: the potometer
🧩 How a bubble potometer works
A cut shoot is sealed into a water-filled tube with a rubber bung, so no air can leak in.
The tube runs into a capillary tube with a scale, whose far end sits in a beaker of water.
An air bubble is introduced into the capillary tube and its starting position recorded.
As the shoot transpires, water is drawn up and the bubble moves along the tube.
Either measure how far the bubble moves in a set time, or time how long it takes to move a set distance.
🤔 What a potometer actually measures
Strictly, a potometer measures the rate of water uptake, not the rate of transpiration — a small amount of the water taken up is used in photosynthesis rather than being lost as vapour. But that fraction is tiny compared with the total that passes through the plant, so water uptake is a reasonable stand-in for transpiration. Saying this in an evaluation question is an easy extra mark.
A mass potometer works differently: it measures the loss in mass of a water-filled tube attached to a shoot over a set time.
Investigating each factor
Air movement: a fan on different settings.
Humidity: enclose the shoot in a plastic bag to raise it, or use a humidifier or dehumidifier for measurable variation.
Light intensity: a lamp at different distances, or different bulbs.
Temperature: a heater or air conditioner, with a thermometer or temperature probe to record the actual value.
The control-variables trap: moving a shoot between rooms to change temperature also changes the light level and humidity. If more than one variable changes, you cannot say which caused the result, so the data is not valid.
Worked examples
WORKED EXAMPLE
Explain why the rate of transpiration decreases as humidity increases. [3]
Step 1: what humidity means hereHigh humidity means the air just outside the leaf is close to saturated with water vapour.Step 2: the gradientThe difference in water vapour concentration between the air spaces inside the leaf and the air outside becomes smaller.Step 3: the effectA shallower concentration gradient means slower diffusion of water vapour out of the stomataAt very high humidity the levels equalise and there is no net loss at all.
WORKED EXAMPLE
A student records a bubble moving 48 mm in 4 minutes in still air, and 90 mm in 5 minutes with a fan running. Calculate both rates and explain the difference. [4]
Step 1: still air48 ÷ 4 = 12 mm per minuteStep 2: with the fan90 ÷ 5 = 18 mm per minuteThe rate rises from 12 to 18 mm per minute, an increase of 50%Step 3: explain itIn still air, water molecules accumulate outside the stomata and create a locally humid layer, reducing the concentration gradient. The fan carries them away, so the gradient stays steep and more water vapour diffuses out.
WORKED EXAMPLE
Explain why transpiration is described as an inevitable consequence of gas exchange in a leaf. [3]
Point 1: why the stomata must openCarbon dioxide can only enter the leaf through the stomata, so they must be open for photosynthesis to continue.Point 2: what else happens when they are openThe air inside the leaf holds far more water vapour than the air outside, so water vapour diffuses out through the same open pores.Point 3: the trade-offClosing the stomata would stop the water loss but would also stop gas exchange and therefore photosynthesis
💡 Exam tip
Every transpiration explanation should end at the concentration gradient. That is the mechanism behind all four factors.
For temperature, give both parts: faster evaporation at first, then stomatal closure causing the fall.
Say water vapour diffuses out. Writing “osmosis” is a guaranteed lost mark.
Name the tissues precisely: palisade for photosynthesis, spongy for gas exchange.
In potometer questions, note that it measures water uptake and say why that is still a fair measure.
For any practical evaluation, name the variables that must be controlled, not just the one being changed.
⚠ Common mix-up
Calling transpiration osmosis. No membrane is crossed at the stoma, so it is diffusion.
Saying stomata are on the upper surface. Most are on the lower epidermis, where it is cooler.
Confusing palisade and spongy mesophyll. Palisade is tall, ordered and chloroplast-packed; spongy is loose with air spaces.
Thinking the cuticle helps gas exchange. It is impermeable; it forces exchange through the stomata.
Saying transpiration is purely harmful. It cools the plant, moves mineral ions and maintains turgor.
Swapping xylem and phloem. Xylem carries water up; phloem carries sugars away.
Reporting a potometer reading as a distance. A rate needs a time: mm per minute.
Up next: Drawing Leaf Structure — how to turn that cross-section into the plan diagram an examiner actually wants.
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