Gas Exchange in Plants
- Rachel Taylor
- Apr 11
- 4 min read
Updated: May 9
HSC Biology | Free Study Notes
In this lesson
what stomata are and what they do
how leaf structure supports gas exchange
how gases move by diffusion
how water loss happens through transpiration
how gas exchange and water balance are linked
Why plants need gas exchange
Plants need to exchange gases with their environment to carry out key life processes.
They need:
carbon dioxide for photosynthesis
oxygen for cellular respiration
They also produce gases that must move out of the plant:
oxygen from photosynthesis
carbon dioxide from respiration
Because of this, plants need structures that allow gases to move in and out efficiently.
Stomata
Stomata are tiny pores, usually found in the epidermis of leaves.
What stomata do
Stomata allow gases to move between the leaf and the air.
Through stomata:
carbon dioxide can enter the leaf
oxygen can leave the leaf
water vapour can leave the leaf
Guard cells
Each stoma is surrounded by guard cells.
Guard cells control whether the stoma is open or closed. This helps the plant balance:
gas exchange
water loss
Why stomata matter
Stomata are essential because plants need carbon dioxide for photosynthesis, but opening stomata also allows water to escape. That is why stomata are important in both gas exchange and water balance. Questions on HSC materials also link stomata directly to plant water balance.
Leaf structure
Leaves are adapted to allow efficient gas exchange.
Broad surface area
Leaves usually have a broad surface area.
This:
increases light absorption for photosynthesis
provides more area for gas exchange
Thin structure
Leaves are thin, which means:
gases do not have to travel very far
diffusion distances are short
This makes gas exchange faster and more efficient.
Internal air spaces
Leaves contain air spaces between some cells.
These spaces allow gases to move through the leaf more easily and reach photosynthesising cells.
Epidermis and cuticle
The leaf epidermis protects the leaf, and the cuticle helps reduce water loss. Most gas exchange therefore happens mainly through the stomata rather than directly across the whole outer surface.
Diffusion of gases
Gas exchange in plants happens mainly by diffusion.
What diffusion means here
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
Carbon dioxide
During photosynthesis:
carbon dioxide concentration is often lower inside photosynthesising cells because it is being used
carbon dioxide diffuses from the air into the leaf through open stomata
Oxygen
During photosynthesis:
oxygen is produced inside the leaf
oxygen diffuses out of the leaf through the stomata
Key point
Plants do not pump gases in and out in the same way animals ventilate lungs. In plants, gas exchange mostly depends on diffusion through stomata and internal air spaces.
Water loss and transpiration
Transpiration is the loss of water vapour from a plant, mainly through the stomata in the leaves.
How transpiration happens
Water evaporates from moist cell surfaces inside the leaf. The water vapour then diffuses out through the stomata.
Why transpiration matters
Transpiration:
helps move water through the plant
contributes to cooling
is linked to the transport of water and mineral ions
But too much transpiration can be harmful because the plant may lose more water than it can replace.
Stomata and water balance
Plants control water loss by regulating the opening and closing of stomata.
When water is limited, stomata may close to reduce transpiration. HSC materials also note adaptations such as sunken stomata and thick waxy cuticles, which reduce evaporation and help maintain water balance.
Gas exchange and transpiration are linked
This is one of the most important ideas in this topic.
The trade-off
When stomata open:
carbon dioxide can enter for photosynthesis
oxygen can leave
but water vapour also escapes
So plants must balance:
the need for gas exchange
the need to avoid excessive water loss
This is why stomatal control is so important.
Worked example
Exam-style question
Explain how stomata help plants carry out gas exchange, but also affect water loss.
Worked answer
Stomata are pores in the leaf that allow gases to move in and out by diffusion. When stomata are open, carbon dioxide can enter the leaf for photosynthesis and oxygen can leave. However, water vapour can also diffuse out through the stomata, causing transpiration. This means stomata help with gas exchange but also increase water loss.
Why this works
This answer:
explains the role of stomata clearly
includes diffusion
links stomata to transpiration
Common mistakes
Saying plants only exchange gases during photosynthesis. They also respire.
Forgetting that stomata are linked to water loss as well as gas exchange.
Saying gases are actively transported in and out of leaves.
Describing transpiration as liquid water leaving through stomata, rather than water vapour.
Ignoring the role of guard cells in controlling stomatal opening.
Quick quiz
What are stomata?
Which gas enters the leaf for photosynthesis?
How do gases move in and out of leaves?
What is transpiration?
Why does opening stomata create a problem for water balance?
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