Edexcel IGCSE Biology · Spec 2.18-2.19
Photosynthesis
The word and balanced equations for photosynthesis, its raw materials and products, and the role of chlorophyll.
Biology revision video
Photosynthesis
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Photosynthesis, and what the equation is really saying
Photosynthesis is the process by which plants make their own food. Carbon dioxide and water are combined, using energy from light, to produce glucose and oxygen.
The word equation is carbon dioxide plus water goes to glucose plus oxygen, with light and chlorophyll written above and below the arrow. They belong there rather than on the left, because neither is used up. Light supplies the energy and chlorophyll absorbs it, and both are still present at the end.
The balanced symbol equation is 6CO2 plus 6H2O goes to C6H12O6 plus 6O2. Six of each on the left, one glucose and six oxygen on the right. Count the carbons if you are unsure: six on the left, and six in C6H12O6, which is what fixes the number.
Where it happens
In chloroplasts, which are found mainly in the palisade mesophyll cells near the top of the leaf, and in the guard cells around each stoma.
Chloroplasts contain chlorophyll, a green pigment. It is green because it absorbs red and blue light well and reflects green, which is why leaves look the colour they do and why plants grow poorly under green light.
Root cells have no chloroplasts, because there is no light underground. This is a favourite question, and the answer is about light rather than about roots.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks for the energy conversion that takes place during photosynthesis, for two marks. The credited answers are light, sunlight or solar for the first mark, and chemical for the second.
The examiner report on that paper shows what happened. Responses giving sunlight and chemical scored both. Responses giving light and chemical scored both. Then it records one that gained one mark for light but no credit for water.
Water is a reactant, not a form of energy. The question asked what the energy became, and half the answer named a substance instead.
That distinction runs through the whole topic. Carbon dioxide and water are the raw materials. Light is the energy. Chlorophyll is what captures it. Glucose is where the energy ends up, stored as chemical energy in its bonds. Four different roles, and a question will usually be asking about exactly one of them.
What happens to the glucose
Very little of it stays as glucose. Knowing the five destinations turns a recall question into an explanation.
Some is used straight away in respiration, releasing energy for the plant's own processes.
Some is converted to starch for storage. Starch is insoluble, which is why it is used rather than glucose: it will not dissolve and draw water into the cell by osmosis, and it will not affect the concentration of the cell contents.
Some is converted to cellulose to build cell walls.
Some is converted to sucrose for transport in the phloem, sucrose being the form plants move around rather than glucose.
And some is combined with nitrate from the soil to make amino acids, and then proteins. That step is the one that needs a mineral, because glucose contains no nitrogen.
Why any of this matters beyond plants
Photosynthesis is the entry point for almost all the energy in almost every food chain. Producers capture light energy and store it as chemical energy, and everything above them in the chain is living on energy that a plant captured first.
It is also the source of the oxygen in the atmosphere, and the process that removes carbon dioxide from it, which is why deforestation appears in questions about greenhouse gases as well as in questions about habitats.
Spec 2.18-2.19
What you need to know
- Describe what photosynthesis does
- Recall the word equation
- Recall the balanced symbol equation
Active recall
Quick check
Answer each question before opening the answer.
Give the word equation for photosynthesis.
Carbon dioxide + water → glucose + oxygen (requires light and chlorophyll).
Where does photosynthesis happen and which pigment absorbs the light?
In chloroplasts; the pigment chlorophyll absorbs light energy.
Biology revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.