Photosynthesis · Grade 7
What is the equation of photosynthesis and where does the oxygen come from?
Six molecules of carbon dioxide and six of water, driven by light energy, become one molecule of glucose and six of oxygen. The oxygen the plant releases comes from splitting the water, not from the carbon dioxide — and that is the point most people remember the wrong way round.
Learning objectives
- State the raw materials and the products of photosynthesis, each in its right place
- Explain where the oxygen a plant releases actually comes from
- Follow the carbon atoms from the air into a molecule of glucose
- Describe what the plant does with the glucose it makes, including storing it as starch
The equation in words
On one side: carbon dioxide and water, with light energy driving it. On the other: glucose and oxygen. Chlorophyll does not appear in the equation because it is not consumed — it is the tool, not a raw material.
The numbers matter less than the order. Someone who knows what goes in and what comes out, and why, can reconstruct the numbers; someone who only memorised them will swap the sides on the first question phrased differently.
Where the oxygen comes from
The natural assumption is that the oxygen comes from the carbon dioxide, since oxygen is in its name. It is wrong. The plant splits the water molecules, releasing the oxygen that was in them.
This was settled experimentally, using water whose oxygen was labelled as a heavy isotope. The oxygen released carried the label — so it came from the water.
The carbon, by contrast, does come from the gas. The carbon atoms in carbon dioxide are the ones that bond together to build the skeleton of the glucose molecule.
What the sugar is for
Some of the glucose is burned immediately in respiration and supplies the plant with energy. Some is built into cellulose, the material of the cell wall — and that is where most of the solid mass goes.
The surplus is stored as starch, in the leaf itself and also in roots and tubers. A potato is exactly that: a starch store built from sugar made in the leaves.
Worked examples
How many oxygen molecules are made per molecule of glucose?
- Six water molecules go into the equation
- Each water molecule releases its oxygen
- So six oxygen molecules come out
Answer: Six
A plant is watered with water whose oxygen is labelled. Where does the label show up?
- The plant splits the water molecules
- The oxygen released is the oxygen that was in the water
- So the label appears in the gas the plant gives off
Answer: In the oxygen released — and that is the proof the source is water
Where does the carbon in a potato come from?
- A potato is mostly starch, built from glucose
- The glucose was made in the leaves from carbon dioxide
- From there it travelled to the tuber and was stored
Answer: From the air, through the leaves — not from the soil around it
Common mistakes
- Saying the oxygen comes from the carbon dioxide
- It comes from splitting water, and that was shown by isotope labelling. The name carbon dioxide misleads, because its oxygen stays bonded to the carbon.
- Writing chlorophyll into the equation
- Chlorophyll is neither consumed nor produced; it captures the light and enables the process. It goes above the arrow, if anywhere.
- Thinking the plant makes oxygen for us
- The oxygen is a by-product of splitting water. The plant itself consumes oxygen in respiration and releases the surplus.
What to remember
- Carbon dioxide and water in; glucose and oxygen out.
- The oxygen comes from the water.
- The carbon comes from the air.
- The sugar is burned, built into cellulose, or stored as starch.