Photosynthesis · Process
The photosynthesis reaction
A tree is mostly made of air. That sentence sounds like a trick, and it is the literal truth: the wood, the bark and the leaves were assembled from a gas that makes up four hundredths of one per cent of the atmosphere.
Start here
The tree gained 74 kilograms. The soil lost 57 grams.
In the 1640s Jan van Helmont weighed a willow shoot — 2.3 kg — and planted it in 90 kg of dried soil in a pot he covered to keep dust out. He gave it nothing but water for five years, then weighed both again. The willow was 77 kg. The soil was 90 kg, short by about 57 grams.
Where did the other 74 kilograms come from?
Mostly from the air. Van Helmont proved it was not the soil, which was the point of weighing it, and then concluded it must all have been the water — half right, and the best available answer for another century. Nearly all the dry mass of that willow was carbon dioxide taken from the air through the leaves. The hydrogen came from the water; the 57 grams of soil were minerals, needed in tiny amounts and not the plant's food at all.
At the bench · take one thing away
Four things it needs. Remove any one.
not tested yet
A pot plant under a bell jar, kept for a day, then one leaf tested for starch.
Light
Carbon dioxide
Water
The leaf tested
Everything present
Rate 100% of maximum
Glucose being made
100 units an hour
Oxygen released into the jar
100 units an hour
Carbon dioxide taken from the jar
100 units an hour
Iodine test on a leaf
Orange-brown — no starch
The soda lime absorbed the carbon dioxide, so the carbon the glucose is built from never arrived. This is the one that convinces people the mass really does come from the air.
Iodine test on a leaf
Orange-brown — no starch
No chlorophyll in the white tissue, so no light energy was absorbed there and no glucose was built. Every other condition was identical to the green part a centimetre away, which is what makes this the cleanest test of the four.
Iodine test on a leaf
Orange-brown — no starch
Chlorophyll, carbon dioxide and water all present, and nothing made. Light supplies the energy the reaction needs, and no amount of raw material substitutes for it.
Iodine test on a leaf
Faint blue-black — a little starch
Some starch, faintly. Everything the reaction needs is present and the light is dim, so the rate is low rather than zero — the plant is limited, not stopped.
Iodine test on a leaf
Orange-brown — no starch
More than one thing is missing: {missing}. The reaction needs all four, so removing any of them is enough on its own — for a fair test, take away one at a time.
Iodine test on a leaf
Blue-black — starch present
Starch is present, so glucose was made and stored. Starch is the storage form, which is why a leaf is tested for starch rather than for glucose: glucose is used or converted within hours, and starch stays put.
Iodine test on a leaf
Orange-brown — no starch
No water means no hydrogen for the glucose — and in a real plant the stomata also close, which shuts the carbon dioxide out as well. Two reasons, one result.
The word summary
Two reactants in, two products out.
carbon dioxide + water
glucose + oxygen
Reactant · from the air
Carbon dioxide
Diffuses in through the stomata on the underside of the leaf. This is where nearly all of a plant’s mass comes from.
Reactant · from the soil
Water
Taken up by root hair cells and carried to every leaf. It supplies the hydrogen in glucose.
Product · kept
Glucose
A sugar, and an energy store. Used for respiration, converted to starch for storage, or built into cellulose for new cell walls.
Product · released
Oxygen
A waste product of the reaction, diffusing out through the stomata. Almost all the oxygen in the atmosphere was made this way.
Key fact
Photosynthesis builds glucose from carbon dioxide and water, using light energy absorbed by chlorophyll, and releases oxygen. Light and chlorophyll are needed for the reaction but are not raw materials — they appear above and below the arrow, not in it.
Think again
“Plants get their food from the soil.”
A plant takes water and dissolved minerals from the soil, and neither of those is food. Food means something that can be broken down to release energy, and minerals cannot — they are needed in milligram quantities to build particular molecules, in the same way that the iron in your diet is essential and is not a meal. Van Helmont's pot settled this in the 1640s: 74 kilograms of new willow appeared and the soil weighed almost exactly what it had at the start. The word plant food on a bottle in a garden centre is a marketing name for a mineral mix, and it is the reason this misconception survives. A plant is the one kind of organism on Earth that does not need to find food, because it makes its own; that is what producer means, and it is the whole reason the rest of us can exist.
“Photosynthesis makes energy.”
Nothing makes energy. You met the rule in Conservation of energy: energy is transferred between stores and never created, and a plant is not an exception to the laws of physics. What photosynthesis does is take energy that is already arriving as light and store it chemically, by building glucose molecules that hold more energy than the carbon dioxide and water they were made from. The sunlight is not a reactant — no atom of it ends up in the glucose — which is exactly why it sits above the arrow in the summary rather than on the left of it. Getting this straight now matters, because respiration in the next unit is the reaction that takes that energy back out of the store, and neither reaction can be understood if energy is being invented in one of them.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · The summary
Which line is the word summary for photosynthesis?
Rung 2 · The one that catches people
A sunflower seed weighing 0.1 g grows into a plant weighing 3 kg. Where has almost all that mass come from?
Rung 3 · Explain the variegated leaf
A variegated leaf, green in the middle and white at the edges, is destarched in the dark for two days, left in bright light for a day, then tested with iodine. The middle goes blue-black; the edges stay orange-brown. Explain the result, and say why this plant is a better test than using two separate plants.
Rung 4 · Take it somewhere new
A grower has a greenhouse of tomatoes in good soil, watered automatically, in a bright and warm greenhouse in June. Yields have stopped rising. She is offered four upgrades: more mineral feed, extra lighting, a carbon dioxide supply, or a bigger pot for each plant. Say which you would advise and why, and how you would test your advice before buying for the whole greenhouse.
Key note
Carbon dioxide + water gives glucose + oxygen, using light energy absorbed by chlorophyll. The carbon dioxide comes from the air through the leaves, the water from the soil through the roots, and almost all of a plant's mass comes from the carbon dioxide rather than from the soil. Remove light, carbon dioxide, water or chlorophyll and the reaction stops.
Going further
Carbon dioxide is about 0.04% of the air — four molecules in every ten thousand. Every oak beam in a cathedral roof was assembled out of that trace, one molecule at a time, by leaves working at a rate slow enough that the tree took two centuries to do it. Two consequences follow. The first is agricultural: because the supply is so thin, carbon dioxide is often the factor holding a greenhouse crop back, and growers pipe it in deliberately to raise the concentration. The second is planetary: a growing forest is taking carbon out of the atmosphere and locking it into wood, which is why what happens to the world's forests appears in every calculation about the climate. Burning that wood, or letting it rot, returns exactly the carbon the tree removed.
Before this lesson
Connects to
At GCSE this becomes
- The balanced symbol equation, limiting factors and rate graphs, and the light-dependent and light-independent stages.
Where to next
- Next: Leaves built for the job
- Previous: Substance misuse and decisions
Health and drugs
Ask Mr Badmus AI
Want to work through why light is not a reactant?
The bench is a simplified model. Rate is shown as a percentage of this plant's maximum with one condition changed at a time; real rates depend on temperature and on several conditions at once, and a plant deprived of water wilts and closes its stomata rather than switching off cleanly. Van Helmont's figures are as he reported them, converted from pounds and ounces.
Lesson content © MrBadmusAI.