Good Learning Co.
SavannahYear 8
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ScienceReproduction, plant gas exchange and particles on the move30 min★★ difficulty

How a leaf breathes, and why everything depends on it

A leaf has to let carbon dioxide in without drying out. The solution is thousands of adjustable pores on the underside. Once you understand them, you can explain almost every food chain on Earth.

Part of your national curriculum
  • Biology: material cycles and energy: Understand the role of leaf stomata in gas exchange in plants and the dependence of almost all life on Earth on the ability of photosynthetic organisms to use sunlight

Lesson overview

What you'll learn in this lesson

Understand the role of leaf stomata in gas exchange in plants and the dependence of almost all life on Earth on the ability of photosynthetic organisms to use sunlight

Key learning points

  • • The pores and their guards
  • • Which gas goes which way
  • • Producers at the base of everything
  • • Break the base and the rest follows

This lesson at a glance

  • 30 minutes
  • 19 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

stomataexchangeplantsdependencealmost

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%
1

Watch & discover

Part 1 of 19

Watch first

How a leaf breathes, and why everything depends on it

An English teaching video on this topic from Archie Science Show. Watch it, then scroll down for the lesson.

Do it for real

The practicals below use what the film shows, with a full method you can follow at home.

The hot-and-cold diffusion race

In a nutshell

Understand the role of leaf stomata in gas exchange in plants and the dependence of almost all life on Earth on the ability of photosynthetic organisms to use sunlight

2

What you already know

Part 2 of 19

Before we start

What you already know

You should know that photosynthesis uses carbon dioxide and water to make glucose and oxygen.

Recap: Photosynthesis: making food from light
3

Key words

Part 3 of 19

Key words

Words you'll need today

Stoma
A tiny pore in a leaf surface that lets gases in and out (plural: stomata).
Guard cell
One of a pair of cells that opens and closes a stoma.
Transpiration
The loss of water vapour from a leaf.
Producer
An organism that makes its own food using light.
Biomass
The mass of living material an organism builds up.
4

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

Gas exchange in a leaf

A short piece of teaching, then a check to make sure it has landed.

5

Explore the idea

Part 5 of 19

Learn

The pores and their guards

Most stomata sit on the shaded underside of a leaf, where less water is lost. Each one is opened and closed by a pair of guard cells: when they take in water they curve apart and the pore opens; when they lose water they straighten and it closes. Open in daylight, mostly closed at night and in drought - a permanent trade-off between taking in carbon dioxide and keeping water.

6

Explore the idea

Part 6 of 19

Learn

Which gas goes which way

Gases move by diffusion, always from higher to lower concentration. In bright light photosynthesis uses carbon dioxide up fast, so carbon dioxide diffuses in and surplus oxygen diffuses out. In darkness only respiration is happening, so the direction reverses: oxygen in, carbon dioxide out. Plants respire all the time - they do not stop at night and switch to photosynthesis by day.

7

Quick check

Part 7 of 19

Quick check

A stoma opens when its guard cells...

8

Quick check

Part 8 of 19

Quick check

In bright light, carbon dioxide diffuses into a leaf because...

9

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

Stop here for now
10

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

Why all life leans on producers

A short piece of teaching, then a check to make sure it has landed.

11

Explore the idea

Part 11 of 19

Learn

Producers at the base of everything

Photosynthetic organisms - plants, algae and some bacteria - are the only large-scale route by which sunlight becomes chemical energy in food. Every consumer, including us, is eating that trapped energy at second hand. Ocean algae do a huge share of it, which is why the phrase 'the Amazon is the lungs of the Earth' is only part of the story.

12

Explore the idea

Part 12 of 19

Learn

Break the base and the rest follows

If light, water or carbon dioxide is limited, producers build less biomass, and every organism above them has less to eat. That is why deforestation, drought and ocean warming show up as food shortages far from where they start, and why the oxygen in the air we breathe was itself produced by photosynthesis over billions of years.

13

Quick check

Part 13 of 19

Quick check

Most stomata are on the underside of a leaf because...

14

Quick check

Part 14 of 19

Quick check

Why does almost all life depend on photosynthetic organisms?

15

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

  • Plants respire all the time, including in daylight.
  • Stomata are pores, not cells; guard cells are the cells that work them.
  • Gases move by diffusion; the leaf does not pump them.
  • Most photosynthesis on Earth happens in the oceans, not only in forests.
16

Challenge round

Part 16 of 19

Game · Sort it

Which of these are true?

Drag each card into the right column. Tap a card first if dragging is fiddly.

True

Not true

17

Challenge round

Part 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

Most ____ sit on the shaded underside of a leaf, where less water is lost.

____ respire all the time - they do not stop at night and switch to photosynthesis by day.

18

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: How a leaf breathes, and why everything depends on it

6 questions, marked with the reasoning shown. No timer.

  1. 1. Transpiration is the loss of...

  2. 2. At night, which gas mainly diffuses into a leaf?

  3. 3. Stomata close in drought in order to...

  4. 4. Which group carries out a very large share of global photosynthesis?

  5. 5. Less biomass built by producers means...

  6. 6. Which statement is correct?

19

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

0

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

  1. Name the variable the club changed the second time, and one variable they kept the same.
  2. Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
  3. Write a conclusion for the club's report, saying how far the evidence supports it.

  • Method and results

    Writing

    Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.

    1 page + results table

  • Conclusion

    Writing

    Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.

    6–8 sentences

  • Labelled diagram

    Diagram

    Draw the apparatus or the process in pencil, label it fully, and photograph it.

    1 labelled diagram

  • Reading and notes

    Reading

    Read the lesson text again and write the key ideas in your own words.

    Half a page of notes

Saved as you type.

Part 1 of 19 · Watch & discover

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    Understand the role of leaf stomata in gas exchange in plants and the dependence of almost all life on Earth on the ability of photosynthetic organisms to use sunlight

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