Photosynthesis equation: carbon dioxide + water โ glucose + oxygen
The word equation for photosynthesis is carbon dioxide + water โ glucose + oxygen, with light written above the arrow. Carbon dioxide comes from the air and water comes up from the soil. Light gives the energy. The plant makes glucose, a sugar it uses for food, and gives off oxygen. The balanced symbol equation is 6COโ + 6HโO โ CโHโโOโ + 6Oโ.
What happens in a leaf
Photosynthesis is how green plants and algae make their own food. It happens inside chloroplasts, small parts of a plant cell that contain chlorophyll. The reaction needs both light and chlorophyll to start. You can see where they sit on our plant cell diagram.
The reaction takes in energy from its surroundings, so scientists call it endothermic. AQA puts it this way: light carries energy from the environment into the chloroplasts. Take the light away and the reaction stops.
Glucose is the useful product. AQA lists what a plant does with it:
- uses it in respiration to release energy
- turns it into starch, which does not dissolve, to store it
- makes fat or oil to store
- makes cellulose to strengthen its cell walls
- makes amino acids, and then proteins, using nitrate ions from the soil
Oxygen is a waste product for the plant. It leaves the leaf and goes into the air, where living things use it for aerobic respiration.
The balanced symbol equation, atom by atom
Each substance has a chemical formula. Carbon dioxide is COโ, water is HโO, glucose is CโHโโOโ and oxygen is Oโ. The big number in front of a formula says how many molecules take part.
In 6COโ + 6HโO โ CโHโโOโ + 6Oโ, count each kind of atom on both sides:
- carbon: 6 on the left, 6 in the glucose
- hydrogen: 12 in six waters, 12 in the glucose
- oxygen: 12 + 6 = 18 on the left, 6 + 12 = 18 on the right
The numbers match, so the equation balances. No atoms are made or lost in a reaction, as the chemistry part of AQA 8464 puts it. They only swap partners. The periodic table gives the symbol for each element if you want to check C, H and O.
What each board prints
AQA (8461 and 8464) prints the word equation, with light over the arrow. It then lists the formulae COโ, HโO, Oโ and CโHโโOโ for you to recognise. It does not print the balanced version with the 6s.
Edexcel (1BI0 and 1SC0) gives no equation at all. It describes the reaction in a sentence: light energy makes carbon dioxide and water react to form glucose and oxygen.
OCR Gateway (J247) asks you to recall the word equation but does not write it out. OCR Twenty First Century (J257) splits photosynthesis into two stages. In the first stage, light and chlorophyll split water into hydrogen and oxygen. In the second, the hydrogen joins carbon dioxide to make glucose.
At KS3 the National Curriculum asks for a word summary of the reactants and products. None of the six GCSE specifications prints the balanced equation above. Your teacher or textbook will show you the form your board expects.
How it links to respiration
Put the aerobic respiration equation next to this one and you will see that it runs the other way. Respiration uses glucose and oxygen and gives out carbon dioxide and water. Plants do both. OCR Gateway notes that many learners think plants do not respire. They do, all the time, in every living cell.
The rate of photosynthesis depends on how much light, carbon dioxide and warmth the plant gets. Our page on limiting factors shows the graphs.
Common mistakes
- Writing oxygen as a reactant. Oxygen is made by photosynthesis, so it goes on the right of the arrow.
- Putting chlorophyll in the equation. The reaction needs chlorophyll, but the reactants are only carbon dioxide and water.
- Writing CO for carbon dioxide or O for oxygen. The gas formulae are COโ and Oโ.
- Calling photosynthesis exothermic. It takes in energy from light, so it is endothermic.
Try Photosynthesis Lab to find the limiting factor on rate graphs and build the word equation. The biology hub lists every page in this set.
Questions people ask
Why is light not written as a reactant?
Light is energy, not a substance. It has no formula and no atoms, so AQA writes it above the arrow to show the reaction needs it.
Where does the oxygen come from?
OCR J257 answers this. In the first stage, light and chlorophyll split water molecules. The oxygen from the water leaves the plant as waste.
Do algae photosynthesise too?
Yes. Edexcel, OCR and the KS3 curriculum all name plants and algae as organisms that carry out photosynthesis.