SciencePlants: the quiet engines30 min★★ difficulty

How a leaf eats sunlight

A leaf is a factory that runs on light. This lesson opens the factory doors: what goes in, what comes out, and why the whole animal world is a customer.

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 equation
• Where each part comes from
• Testing a leaf for starch
• Everything eats plants

This lesson at a glance

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

Words to know

reactantsproductsphotosynthesissummaryequation

Scroll down — the lesson carries on below

Part 1 of 20 · Discover5%

Watch & discover

Part 1 of 20

Watch first

How a leaf eats sunlight

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

In a nutshell

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

What you already know

Part 2 of 20

Before we start

What you already know

You know plants need light and water to grow, and that animals eat plants or other animals.

Key words

Part 3 of 20

Key words

Words you'll need today

photosynthesis: The process plants use to make glucose from carbon dioxide and water, using light energy.
chlorophyll: The green pigment in leaves that traps light energy.
glucose: The sugar plants make — their food and building material.

Learning cycle

Part 4 of 20

Learning cycle 1 of 2

Part 1 — The recipe

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

Explore the idea

Part 5 of 20

Learn

The word equation

Carbon dioxide + water, with light energy trapped by chlorophyll, becomes glucose + oxygen. Learn it as a recipe: two ingredients in through the air and the roots, two products out — food for the plant and the oxygen we breathe. The light is not an ingredient; it is the energy that makes the reaction go.

Explore the idea

Part 6 of 20

Learn

Where each part comes from

Carbon dioxide enters through tiny pores called stomata, mostly on the underside of the leaf. Water arrives from the roots through tubes called xylem. Oxygen leaves through the same stomata, and glucose is used straight away for energy or stored as starch — which is why a potato is basically a buried energy store.

Quick check

Part 7 of 20

Quick check

The two raw materials of photosynthesis are...

Quick check

Part 8 of 20

Quick check

Which pigment traps the light?

Reset break

Part 9 of 20

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 20

Learning cycle 2 of 2

Part 2 — Why it matters

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

Explore the idea

Part 11 of 20

Learn

Testing a leaf for starch

The classic experiment: boil a leaf to kill it, soften it in hot ethanol to remove the green, then add iodine. Blue-black means starch is present — proof the leaf has been photosynthesising. A leaf kept in the dark stays brown: no light, no glucose, no starch.

Explore the idea

Part 12 of 20

Learn

Everything eats plants

Trace any meal back far enough and you reach a photosynthesiser. The beef in a burger ate grass; the grass caught sunlight. Even the oxygen in this sentence came from a leaf or ocean algae. That is why scientists call plants producers — everything else is a consumer.

Quick check

Part 13 of 20

Quick check

A leaf kept in the dark for two days tests negative for starch because...

Quick check

Part 14 of 20

Quick check

Plants are called producers because...

Investigate evidence

Part 15 of 20

Evidence 1 of 1

primary

Van Helmont's willow tree experiment

Jan Baptist van Helmont, Flemish physician, 1648

I took an earthen pot and placed in it 200 pounds of soil dried in an oven, and planted a willow shoot weighing 5 pounds. After five years of watering with rainwater only, the tree weighed 169 pounds. The soil, dried again, had lost barely 2 ounces.

Van Helmont concluded the tree's mass came from water alone. He was half right — he could not know about carbon dioxide in the air, which is where most of a plant's mass actually comes from.

Think like a historian

Van Helmont thought the willow's new mass came only from water. Using what you know about photosynthesis, explain what he missed — and why his soil result is still a clever clue.

Source: Published in van Helmont's collected works, Ortus Medicinae, after his death.

View the archive record ↗

Common mix-ups

Part 16 of 20

Common mix-ups

Things people often get wrong

Thinking plants get food from the soil — soil supplies water and minerals, not food.
Writing light as an ingredient in the equation — it is energy, not a substance.
Believing plants only make oxygen — they also use oxygen in respiration, day and night.

Challenge round

Part 17 of 20

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 18 of 20

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

Learn it as a recipe: two ingredients in through the air and the roots, two ____ out — food for the plant and the oxygen we breathe.

Mastery quiz

Part 19 of 20

Marked quiz

End of lesson quiz: How a leaf eats sunlight

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

1. Write the word equation for photosynthesis.
2. Where does the water in a leaf come from?
3. Iodine turns blue-black when it touches...

Lesson round-up

Part 20 of 20

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.

Source for this lesson

Use this for the source work task below.

Van Helmont's willow tree experiment

primary

Jan Baptist van Helmont, Flemish physician, 1648

I took an earthen pot and placed in it 200 pounds of soil dried in an oven, and planted a willow shoot weighing 5 pounds. After five years of watering with rainwater only, the tree weighed 169 pounds. The soil, dried again, had lost barely 2 ounces.

Van Helmont concluded the tree's mass came from water alone. He was half right — he could not know about carbon dioxide in the air, which is where most of a plant's mass actually comes from.

Source: Published in van Helmont's collected works, Ortus Medicinae, after his death.

View the original record ↗

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 20 · Watch & discover

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