SciencePhotosynthesis and respiration30 min★★★ difficulty

Photosynthesis: making food from light

Almost every food chain on Earth starts here. Plants build glucose out of carbon dioxide and water using light energy - and the oxygen you just breathed in is their waste product.

Part of your national curriculum

Biology: material cycles and energy: Understand the reactants in, and products of, photosynthesis, and a word summary for photosynthesis

Lesson overview

What you'll learn in this lesson

Understand the reactants in, and products of, photosynthesis, and a word summary for photosynthesis

Key learning points

• The word summary
• Where it happens
• Limiting factors
• Testing a leaf

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

reactantsproductsphotosynthesissummaryhappens

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%

Watch & discover

Part 1 of 19

Watch first

Photosynthesis: making food from light

Watch a leaf turn light, water and carbon dioxide into sugar.

Do it for real

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

What happens to a leaf kept in the dark?Warming up anaerobic respiration in yeast

In a nutshell

Understand the reactants in, and products of, photosynthesis, and a word summary for photosynthesis

What you already know

Part 2 of 19

Before we start

What you already know

You should know plants need light and water to grow.

Recap: Digestion, enzymes and gut bacteria

Key words

Part 3 of 19

Key words

Words you'll need today

Photosynthesis: The reaction in which plants make glucose from carbon dioxide and water using light.
Chlorophyll: The green pigment in chloroplasts that absorbs light.
Reactant: A substance used up in a reaction.
Product: A substance made in a reaction.
Limiting factor: The factor currently holding back the rate of a reaction.

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

The reaction itself

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

Explore the idea

Part 5 of 19

Learn

The word summary

Carbon dioxide + water, with light energy absorbed by chlorophyll, produces glucose + oxygen. Learn which side each substance is on: carbon dioxide and water are reactants, glucose and oxygen are products. Light is not a reactant - it is the energy source that drives the reaction.

Explore the idea

Part 6 of 19

Learn

Where it happens

Photosynthesis takes place in chloroplasts, most densely in the palisade layer near the top of the leaf where light arrives first. Carbon dioxide enters through stomata on the underside; water arrives from the roots through the xylem. Glucose is used for respiration, converted to starch for storage, or built into cellulose for cell walls.

Quick check

Part 7 of 19

Quick check

The reactants of photosynthesis are...

Quick check

Part 8 of 19

Quick check

Chlorophyll's job is to...

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

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

What limits the rate

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

Explore the idea

Part 11 of 19

Learn

Limiting factors

Increase light intensity and the rate rises, then levels off - something else is now limiting, usually carbon dioxide concentration or temperature. Commercial greenhouses raise all three deliberately. Temperature has a ceiling because the enzymes involved denature, so the rate drops sharply rather than levelling off.

Explore the idea

Part 12 of 19

Learn

Testing a leaf

A destarched plant kept in the dark makes no starch. Iodine solution turns blue-black where starch is present, so covering part of a leaf with foil and testing afterwards shows light was needed. That is a fair test only if the plant, temperature and water are the same in each part - the same logic you will use in every investigation.

Quick check

Part 13 of 19

Quick check

The rate stops rising with more light because...

Quick check

Part 14 of 19

Quick check

Iodine solution turning blue-black shows the presence of...

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

Light is the energy source, not a reactant in the equation.
Plants respire all the time as well as photosynthesising.
Oxygen is a waste product for the plant, not its food.
Rate levelling off means a different factor has become limiting, not that light stopped working.

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

Challenge round

Part 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

Learn which side each substance is on: carbon dioxide and water are ____, glucose and oxygen are products.

____ takes place in chloroplasts, most densely in the palisade layer near the top of the leaf where light arrives first.

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Photosynthesis: making food from light

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

1. Photosynthesis happens in...
2. Oxygen in photosynthesis is a...
3. Carbon dioxide enters a leaf through...
4. Glucose can be stored as...
5. Too high a temperature lowers the rate because...
6. Almost all life depends on photosynthesis because it...

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

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

Name the variable the club changed the second time, and one variable they kept the same.
Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
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

Write your answer, notes or working here

Saved as you type.

Photos of paper work, diagrams or working outHow many minutes did the written work take?

Part 1 of 19 · Watch & discover

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Understand the reactants in, and products of, photosynthesis, and a word summary for photosynthesis