KS3 · Biology

Adaptations of leaves: stomata and gas exchange

Nothing in a leaf pumps gas in. So how does carbon dioxide get from the open air into the middle of a leaf? Step inside and find out.

Step inside a leaf

Follow the gases through a photosynthesising leaf

A thin leaf that is photosynthesising, with its stomata open. Start in the air outside and step down through the leaf, one zone at a time.

↑ Outside air

↓ Inside a leaf cell

Zone 1 of 4

Outside air

Here is where the journey starts. While the leaf is photosynthesising, the air outside has more carbon dioxide than the air inside the leaf, and less oxygen. Carbon dioxide has to get from here to the cells that use it.

More carbon dioxide than inside the leaf
Less oxygen than inside the leaf

Inside the leaf there is less carbon dioxide and more oxygen than outside, so carbon dioxide's net movement is in and oxygen's net movement is out.

Watch out: This is what happens while the leaf is photosynthesising. Plant cells also respire, so it is not the only exchange a plant makes.

Why the gases go the way they do

Reason it through

Why does carbon dioxide move into a photosynthesising leaf while oxygen moves out?

Link 1 of 4

First link · your turn

What are the leaf cells doing with carbon dioxide and water?

Locked — reveal the link above first

What do you think?

How do the gases get through?

A leaf is photosynthesising in the light with its stomata open. Carbon dioxide ends up inside the leaf and oxygen ends up outside it.

Which idea is closest to what you think is happening in the stomata?

The leaf sucks carbon dioxide in through the stomata and pushes oxygen out.Carbon dioxide particles only ever go into the leaf and oxygen particles only ever go out.The guard cells push the gases through the stoma when they open it.Particles move randomly in both directions through an open stoma, but more move from higher to lower concentration, so the net movement is in for carbon dioxide and out for oxygen.

How sure are you?

I'm sureI think soI'm guessing

The leaf sucks carbon dioxide in through the stomata and pushes oxygen out.

This is a common misconception

It's a tempting picture, because the gases do end up in the right places. But diffusion is caused by the continuous random movement of particles, and that is all the movement needs. Photosynthesising cells use up carbon dioxide, so there is less inside the leaf than outside, and more carbon dioxide particles move in than move out.

Carbon dioxide particles only ever go into the leaf and oxygen particles only ever go out.

This is a common misconception

Particles move continuously and randomly, so they head in every direction, including the 'wrong' way. What is true is that more particles diffuse from the higher concentration to the lower one than the other way. That difference is the net movement, so carbon dioxide still has some particles leaving even though its net movement is in.

The guard cells push the gases through the stoma when they open it.

This is a common misconception

Mixing up two jobs here. Guard cells change shape to open and close the stoma, so they decide whether the hole is open. The movement of the gases through an open stoma is caused by diffusion.

Particles move randomly in both directions through an open stoma, but more move from higher to lower concentration, so the net movement is in for carbon dioxide and out for oxygen.

This matches the evidence

That's the idea. Random movement both ways, plus a concentration gradient, gives a net movement from higher to lower concentration. Nothing else is needed.

Net movement is not one-way traffic. Particles go both ways, and more go from higher to lower concentration than back.

Open or closed?

Stoma openvsStoma closed

Each stoma is a trade-off: what the plant gains when it is open, and what it can lose.

Focus

What the two guard cells have done

Stoma open

Changed shape so the stoma is open

Stoma closed

Changed shape so the stoma is closed

The insight

Each stoma is surrounded by two guard cells. Their change of shape is what opens and closes it.

Carbon dioxide and oxygen

Stoma open

Carbon dioxide can move in and waste oxygen can move out

Stoma closed

Gases cannot move through a closed stoma

Water

Stoma open

Water in the gas state can move out of the leaf

Stoma closed

Closing can help prevent too much water being lost

When this is more likely

Stoma open

More likely in the daytime, while it is light and photosynthesis can take place

Stoma closed

Less likely in the daytime, when photosynthesis is taking place

Now you explain it

Write it in your own words

A leaf is photosynthesising with its stomata open. Explain how gas exchange happens through the stomata. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Stomata are small holes in the surface of the leaf that lead to air spaces inside the leaf. (1 mark)
The cells use up carbon dioxide in photosynthesis, so there is less carbon dioxide inside the leaf than outside. (1 mark)
So the net movement of carbon dioxide by diffusion is from outside to inside, through the open stomata. (1 mark)
The cells fill the air space with waste oxygen, so there is more oxygen inside than outside and its net movement is out through the stomata. (1 mark)

Model answer

The stomata are small holes in the leaf surface that lead to air spaces between the cells. Because the cells are photosynthesising, they use up carbon dioxide, so the air inside has less carbon dioxide than the air outside. Diffusion therefore gives a net movement of carbon dioxide from outside to inside through the open stomata. The cells also fill the air space with waste oxygen, so there is more oxygen inside than outside, and the net movement of oxygen is out through the stomata.

A good explanation links the hole, the reason (photosynthesis changes the concentrations) and the direction (net movement).

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

water + carbon dioxide → glucose + oxygen

A photosynthesising leaf takes in carbon dioxide and releases waste oxygen. Stomata are the way through.

What you need to know

Plants make their own food, glucose, by photosynthesis. Word summary: water + carbon dioxide → glucose + oxygen.
While a leaf photosynthesises, carbon dioxide moves into it and waste oxygen moves out. That is gas exchange in plants.
Stomata are small holes in the surfaces of a leaf. They lead to air spaces between the layers of cells inside.
Each stoma is surrounded by two guard cells. These can change shape to open and close it.
Diffusion is caused by the continuous random movement of particles of a gas or a liquid.
If particles are more concentrated in one area than another, that creates a concentration gradient.
More particles diffuse from higher to lower concentration than the other way. That difference is the net movement.
Have a goSay 10 carbon dioxide particles drift into a leaf through a stoma while 6 drift out. What is the net movement?
4 particles, into the leaf.
Particles go both ways, and net movement is only the difference between the two directions.
Photosynthesising cells use up carbon dioxide, so there is less inside the leaf than outside. Its net movement is inwards.
They also fill the air space with waste oxygen, so there is more inside than outside. Its net movement is outwards.
Have a goSpot the slip: 'There's more oxygen outside than inside, so oxygen diffuses out.' What's wrong?
It's backwards: the photosynthesising cells fill the air space, so there is more oxygen inside.
Net movement goes from higher to lower concentration, so the gradient has to be read the right way round first.
Water in the gas state can leave through open stomata. Closing them can help prevent too much water loss.
Have a goClosing stomata helps stop water loss. So why doesn't a plant simply keep them shut all the time?
Carbon dioxide could not move in through them for photosynthesis.
Stomata are the way carbon dioxide gets in, so the plant has to balance gas exchange against water loss.
Stomata are more likely to be open in the daytime, while it is light and photosynthesis can take place.
Leaves are thin, which keeps the diffusion distance short. Cells get carbon dioxide quickly and waste oxygen is removed quickly.

The big picture

While a leaf photosynthesises, its cells use up carbon dioxide and fill the air space inside with waste oxygen. That creates concentration gradients, so diffusion gives a net movement of carbon dioxide into the leaf and of oxygen out, through small holes called stomata. Guard cells open and close each stoma, and open stomata also let water in the gas state escape.

Key points

1While a leaf photosynthesises, carbon dioxide moves in and waste oxygen moves out through open stomata.

2The photosynthesising cells use up carbon dioxide and fill the air space with oxygen, which sets up the concentration gradients.

3Diffusion is random movement of particles both ways. The net movement is the extra that goes from higher to lower concentration.

4Guard cells open and close each stoma, and open stomata also let water in the gas state escape.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

A leaf is photosynthesising in the light with its stomata open. Explain why oxygen moves out of the leaf through the stomata.

⚠ Watch out

Saying the leaf pulls or sucks carbon dioxide in, or that carbon dioxide only ever goes one way. Particles move randomly both ways. Photosynthesising cells keep carbon dioxide lower inside, so the net movement is in.

🧠

Memory hook

Picture a doorway between a packed room and an empty one. People wander both ways, but more go from packed to empty. The stoma is the doorway, the guard cells decide if it is open, and photosynthesis keeps the carbon dioxide side inside the leaf emptier.

✓

Check yourself

Without looking back, trace carbon dioxide from the air outside to a leaf cell, then say what makes its net movement go that way.

Flashcards

(12)

What is the word summary of photosynthesis?

Water + carbon dioxide → glucose + oxygen.

In a photosynthesising leaf, which gas moves in and which moves out?

Carbon dioxide moves into the leaf. Waste oxygen moves out of the leaf.

What are stomata?

Small holes in the surfaces of a leaf that lead to air spaces between the layers of cells inside it.

What surrounds each stoma, and what can it do?

Two guard cells, which can change shape to open and close the stoma.

What causes diffusion?

The continuous random movement of particles of a gas or a liquid.

What is a concentration gradient?

A difference that exists when particles are more concentrated in one area than another.

What is the net movement?

More particles diffuse from higher to lower concentration than in the opposite direction. The difference is the net movement.

Why is there less carbon dioxide inside a photosynthesising leaf than outside?

The cells use it up in photosynthesis, so the net movement of carbon dioxide is from outside to inside.

Besides gases, what can leave a leaf through open stomata?

Water in the gas state.

How can closing the stomata help a plant?

It can help prevent the plant from losing too much water.

When are stomata more likely to be open?

In the daytime, while it is light and photosynthesis can take place.

Why does a thin leaf help gas exchange?

It keeps the diffusion distance short, so cells get carbon dioxide quickly and waste oxygen is removed quickly.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Follow the gases through a photosynthesising leaf

How do the gases get through?

Write it in your own words

More KS3 Biology topics