Photosynthesis

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.18 — Photosynthesis and its importance in converting light energy to chemical energy
• 2.19 — The word equation and balanced symbol equation for photosynthesis
• 2.22 — Mineral ions for plant growth: magnesium for chlorophyll, nitrate for amino acids

Specification section 2(d) Movement of substances into and out of cells — also taught in Enzymes and pH, Diffusion, Osmosis and Active Transport.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A mature oak weighs many tonnes. It grew from an acorn you could hold in one hand, standing in the same patch of ground for two hundred years — and the ground around it did not sink. So where did all that material come from? The answer is one of the strangest facts in biology, and it is worth being properly surprised by it before you learn the equation: almost all of a tree is air that has been assembled into wood.

The experiment that settled it

1. In the 1640s Jan van Helmont planted a willow shoot weighing 2.3 kg in a pot of dried soil weighing 90.7 kg, and for five years gave it nothing but water.
2. He then weighed both again. The tree had gained about 74 kg. The soil had lost about 57 g — roughly the mass of a small egg.
3. So the tree had not eaten the soil. Something else had supplied 74 kg of new plant.
4. Van Helmont concluded it must be the water, and he was partly right: water supplies the hydrogen. But the element a plant is mostly built from is carbon, and carbon arrives as carbon dioxide from the air.
5. The 57 g the soil did lose turns out to matter enormously — but not as food. That comes at the end of this lesson.

Now the harder half. Carbon dioxide and water do not join themselves together into sugar; left alone in a jar they sit there indefinitely. Building a large molecule out of small stable ones costs energy, and that energy comes from sunlight. It is captured by chlorophyll, the green pigment packed inside chloroplasts. Chlorophyll absorbs light, and the absorbed energy is what drives the reaction. Notice what that means about the colour: green is the light chlorophyll does NOT absorb, reflected back at you. You are looking at the part of the sunlight the plant had no use for.

What the equation is actually saying

1. Raw materials in: carbon dioxide from the air, and water taken up from the soil by the roots. Between them these contain only carbon, hydrogen and oxygen.
2. Energy in: light, absorbed by chlorophyll. Light is not a raw material — no part of it ends up in the sugar, only its energy does. That is why it is written above the arrow and never on the left.
3. Product out: glucose. The energy that arrived as light is now held in the bonds of that molecule, as chemical energy the plant can release later by respiration.
4. Left over: oxygen, which the plant produces far more of than it needs, so most of it diffuses out. Every breath you have ever taken was a waste product of this reaction.

One thing is still missing. Glucose is made of carbon, hydrogen and oxygen — that is all air and water can supply. But a plant is not built only from sugar. It needs proteins, and every protein contains nitrogen. It needs chlorophyll itself, and every chlorophyll molecule is built around a single magnesium atom. Neither element is in carbon dioxide or in water, and plants cannot use the nitrogen gas in the air directly, so both must come from somewhere else. This is where the soil finally earns its place: not as food, but as the only available source of the mineral ions a plant cannot make or breathe in. Van Helmont's 57 g was not the tree — it was the part of the tree that could not have come from anywhere else.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Photosynthesis

The process by which plants use light energy, absorbed by chlorophyll, to convert carbon dioxide and water into glucose. Light energy is transferred to chemical energy stored in the glucose, and oxygen is released as a by-product.

carbon dioxide + water → glucose + oxygen

Conditions: light energy absorbed by chlorophyll — written above the arrow, never as a reactant

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Conditions: light, chlorophyll. Check the balance: 6 C, 12 H and 18 O on each side.

Where each part of the equation comes from, and where it goes

Carbon dioxide

diffuses into the leaf from the air through the stomata, then through the air spaces to the mesophyll cells

Water

absorbed from the soil by root hair cells and carried up to the leaf in the xylem

Light

absorbed by chlorophyll in the chloroplasts; an energy source, not a raw material, so none of it appears in the glucose

Glucose

the product; the light energy is now chemical energy stored in its bonds

Oxygen

the by-product; most diffuses out through the stomata

What the plant does with the glucose — the five fates worth knowing

• Respired, to release energy for the cell
• Converted to starch for storage — insoluble, so it does not affect osmosis or the water balance of the cell
• Converted to cellulose, to build cell walls
• Converted to lipids, stored in seeds
• Combined with nitrate ions to make amino acids, which are built into proteins

The two mineral ions Edexcel names — learn the ion, the use and the deficiency

Magnesium ions

needed to make chlorophyll. Without them the plant cannot make chlorophyll, so the leaves turn yellow and the rate of photosynthesis falls.

Nitrate ions

combined with glucose to make amino acids, which are built into proteins. Without them growth is stunted and the older leaves may yellow.

Why a plant needs soil at all — the chain that earns the marks

1. Air supplies carbon dioxide and the soil water supplies water; between them they contain only carbon, hydrogen and oxygen.
2. Glucose is made of carbon, hydrogen and oxygen, so glucose can be made from air and water alone.
3. Proteins contain nitrogen and chlorophyll contains magnesium, and neither element is present in carbon dioxide or water.
4. Nitrogen and magnesium must therefore be absorbed from the soil, as nitrate ions and magnesium ions, by active transport into the root hair cells.

Model answer [4 marks]

Explain why a plant grown in distilled water, with plenty of light and carbon dioxide, eventually stops growing. [4]

Distilled water contains no mineral ions. The plant can still photosynthesise, because carbon dioxide and water supply everything glucose is made from. But it cannot make proteins, because these require nitrogen supplied as nitrate ions, so growth stops. It also cannot make new chlorophyll, because that requires magnesium ions, so the leaves yellow and the rate of photosynthesis falls as well.

Model answer [3 marks]

State the word equation for photosynthesis and explain what happens to the light energy. [3]

Carbon dioxide + water → glucose + oxygen, with light energy absorbed by chlorophyll. The light energy is not a raw material: it is transferred into the chemical bonds of the glucose. It is therefore stored as chemical energy, which the plant can release later by respiration.

Mark-losing trap. Light is not a reactant. It supplies the energy, so it goes above the arrow — never on the left-hand side of the equation.

Mark-losing trap. Magnesium makes chlorophyll; nitrate makes amino acids. Swapping those two is the most common mineral-ion mistake there is.

Mark-losing trap. A plant respires as well as photosynthesising. Photosynthesis does not replace respiration — it supplies the glucose respiration uses.

Mark-losing trap. Glucose is the product. Starch, cellulose, lipids and proteins are all made from it afterwards, and none of them appears in the equation.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — A young tree gains 60 kg of dry mass over ten years while growing in the same pot of soil. Where has most of that mass come from?
2. Grade 7 · Name [1 mark] — A plant's leaves are turning yellow because it cannot make enough chlorophyll. Name the mineral ion it is short of.
3. Grade 8 · Explain [4 marks] — A gardener grows two identical seedlings in the same light and the same air. One is watered with a solution containing all the mineral ions it needs; the other is watered with distilled water only. After six weeks the second seedling is much smaller and its leaves are pale. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — In van Helmont's experiment the willow gained 74.4 kg over five years, while the dried soil in the pot lost 57 g. Calculate the mass the soil could have contributed, as a percentage of the tree's total gain. Give your answer to two decimal places.
5. 9+ · Analyse [5 marks] — A plant is sealed in a chamber with bright light, plenty of carbon dioxide and pure distilled water. For the first two weeks it grows normally. Over the following weeks its growth slows, its leaves become pale yellow, and the volume of oxygen it releases each day falls. Select every statement that belongs in a full-mark explanation.

The people behind this science

Three ways into the same idea — the one whose answer held for two thousand years and was wrong, and the one who found the first evidence that this happens at all, and the one who worked out what light actually delivers. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Aristotle — the one whose answer held for two thousand years and was wrong

Aristotle taught that a plant draws ready-made food out of the earth through its roots, and because that idea is so intuitive it went essentially unchallenged until van Helmont weighed his willow. Understanding why such a reasonable answer survived for two millennia tells you exactly what the experiment in this lesson had to overturn — and why the real answer sounds so implausible at first hearing.

• “Why did it seem so obvious that a plant takes its food from the soil?”
• “What would you have needed to see to start doubting that idea?”
• “If a plant is not eating the soil, what did you think the roots were for?”
• “How would you answer someone who told you a tree is mostly built out of air?”
• “Why can a wrong explanation survive for two thousand years?”

Joseph Priestley — the one who found the first evidence that this happens at all

Every claim on this page rests on plants doing something to the air, and Priestley supplied the first evidence that they do. In 1771 he burned a candle in a sealed jar until it went out, put a sprig of mint into the spoiled air, and found days later that a candle would burn in it again. He had no idea what had happened — he thought in terms of phlogiston to the end of his life and rejected Lavoisier's explanation of his own discovery. What he had was the observation that a plant restores something a flame destroys, which is where this entire subject begins.

• “What did you do to the air in the jar before you put the mint in?”
• “How could you tell the air had been restored?”
• “Why would the experiment work on some occasions and fail on others?”
• “What did you think the plant had actually done?”
• “Why did you never accept Lavoisier's account of the air you discovered?”

Albert Einstein — the one who worked out what light actually delivers

This lesson asks you to accept that light carries energy and that a green pigment can capture it and put it into a sugar molecule. In 1905 Einstein showed that light arrives in discrete packets and that whether a substance absorbs one depends on the energy of the packet rather than the brightness of the beam — which is precisely why chlorophyll takes some colours and hands back the green you can see.

• “What is actually arriving at a leaf when sunlight lands on it?”
• “Why does a pigment absorb some colours of light and not others?”
• “How can energy from the Sun end up locked inside a sugar molecule?”
• “Does making a lamp brighter change the energy each packet of light carries?”
• “Why is green the one colour a leaf gives back to us?”

Carry on through the course

• Previous lesson: Diffusion, Osmosis and Active Transport
• Next lesson: What Limits Photosynthesis
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification