Early experiments and the overall reaction

Where does a plant get its food from?

A seed in wet soil grows into a tall plant. Its mass goes up by many kilograms, yet nobody feeds it. The soil hardly loses any mass. So where does the new body come from?

The answer is photosynthesis. It is the process by which green plants use light energy to make food from carbon dioxide and water, and release oxygen. It is the ultimate source of nearly all the food on Earth. It is also the source of the oxygen in the air.

It took about two hundred years of experiments to understand it. Let us follow them in order.

The early experiments

• Joseph Priestley (1770). He kept a burning candle in a closed jar, and it soon went out. A mouse in a closed jar soon became restless and died. But when a mint plant was kept in the jar with the candle, the candle kept burning. A mouse with the plant stayed alive. Priestley concluded that plants restore to the air whatever a burning candle or a breathing animal takes out of it.
• Jan Ingenhousz (1779). He repeated the work with an aquatic plant in a glass trough. In bright light, bubbles of gas rose from the green parts. In the dark, there were none. So sunlight is needed, and only the green parts release the gas. The gas was later shown to be oxygen.
• Julius von Sachs (1854). He showed that glucose is made in the green parts of a plant, and that it is usually stored as starch. A starch-filled leaf turns blue-black with iodine.
• T. W. Engelmann (1843 to 1909). He split light into its colours with a prism and let it fall on a green filament of the alga Cladophora. Around it he placed bacteria that need oxygen. The bacteria gathered in the blue and red regions, where the alga made most oxygen. This was the first action spectrum. It matched the light that chlorophyll absorbs.
• Cornelius van Niel (1897 to 1985). He studied purple and green bacteria. They make food from carbon dioxide, but they use hydrogen sulphide ($\text{H}_2\text{S}$) in place of water, and they give out sulphur, not oxygen.

What van Niel's result means

Van Niel wrote the reaction in a general form. Here $\text{H}_2\text{A}$ is the hydrogen donor.

In sulphur bacteria, A is sulphur. In green plants, the donor is water, so A is oxygen. The oxygen of photosynthesis must therefore come from water, not from carbon dioxide. Later experiments with a heavy isotope of oxygen proved it.

The overall reaction

Why $12$ water molecules on the left and $6$ on the right? All $12$ water molecules are split to give $6$ oxygen molecules. The $6$ water molecules on the right are new. Their oxygen atoms come from carbon dioxide. Writing it this way keeps track of every oxygen atom.

Photosynthesis is also an oxidation and reduction. Water is oxidised to oxygen. Carbon dioxide is reduced to carbohydrate. The energy for this uphill change comes from light.

You should find that $36$ millimoles of carbon dioxide make only $6$ millimoles of glucose, which is $1080$ mg. The same $36$ millimoles of oxygen weigh $1152$ mg. The oxygen outweighs the glucose, because six oxygen molecules of $32$ mg each are released for every glucose of $180$ mg.

Try it in Python

The program counts the atoms on each side of the equation. It also checks where the oxygen gas comes from.

You should see the same count of carbon, hydrogen and oxygen on both sides, so the equation is balanced. The $12$ oxygen atoms of the oxygen gas match the $12$ oxygen atoms of the $12$ water molecules. With only $6$ water molecules, the two sides no longer match.

Practise with new numbers

Common mistakes

• Saying the oxygen comes from carbon dioxide. It comes from water. This is the main point of van Niel's work.
• Mixing up the two early experiments. Priestley showed that plants restore the air. Ingenhousz showed that light and green parts are needed.
• Forgetting the six new water molecules. The equation with $12$ water on the left and $6$ on the right is the full record.
• Thinking one carbon dioxide makes one glucose. Six are needed for one glucose.
• Mixing up Sachs and Engelmann. Sachs found glucose stored as starch. Engelmann found the colours of light that work best.

Remember these

• Priestley: plants restore the air. Ingenhousz: light and green parts. Sachs: glucose, stored as starch. Engelmann: blue and red light work best. Van Niel: oxygen comes from water.
• Overall: $6\,\text{CO}_2 + 12\,\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{H}_2\text{O} + 6\,\text{O}_2$.
• Water is oxidised. Carbon dioxide is reduced.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In Priestley's experiment, what did the mint plant do for the candle?

2. What two things did Ingenhousz find out?

3. What did the bacteria in Engelmann's experiment show?

4. In van Niel's equation, what is A in a green plant, and what does that tell you?

5. In the overall reaction, which substance is oxidised and which is reduced?