The Photosynthesis Equation and Chlorophyll

What photosynthesis actually is

Photosynthesis is the process by which plants synthesise carbohydrate from carbon dioxide and water, using energy transferred from light. The two raw materials are carbon dioxide and water — nothing else counts as a raw material here. Light is the energy source, not a substance the plant consumes, and that distinction is worth holding onto carefully: a raw material is something used up and incorporated into the product; light is a condition the reaction needs in order to run at all. Oxygen is produced alongside the carbohydrate.

The word equation

Learn this exactly, including which two things sit above the arrow rather than in it:

carbon dioxide + water → glucose + oxygen
conditions: light, chlorophyll

Everything to the left of the arrow is used; everything to the right is produced. Light and chlorophyll go above the arrow because they are conditions the reaction needs, not chemical reactants — writing them into the equation as if they were consumed alongside carbon dioxide and water is one of the most common ways this equation loses a mark. The carbohydrate product the syllabus wants named is glucose, not a vaguer word like “food” or “sugar.”

It is worth noticing how closely this mirrors — and inverts — aerobic respiration, because the two equations use exactly the same four substances and examiners enjoy testing whether you know which direction each one runs:

A plant cell does not switch respiration off while it photosynthesises — it does both at once whenever there is light. “Plants respire at night and photosynthesise in the day” is a genuinely common answer, and it is wrong on both halves: respiration never stops, and photosynthesis runs throughout daylight, not only at some specific time of day. You can read the full respiration equation and process on the respiration pages.

Supplement: the balanced chemical equation

Extended candidates also need the same reaction written in chemical formulae, with coefficients that balance the atoms on each side:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
conditions: light, chlorophyll

Those coefficients are not a free choice — they are the simplest whole-number ratio that makes the atom count match on both sides: 6 carbons, 12 hydrogens and 18 oxygens, before and after. Glucose itself is C₆H₁₂O₆, not CH₂O and not C₆H₁₂O₂ — a formula examiners specifically check candidates can reproduce correctly. Core candidates are not assessed on this equation at all; the word equation above is the required version at Core level.

Chlorophyll and chloroplasts

Chlorophyll is a green pigment found inside chloroplasts, and its required role goes well beyond “it makes plants green.” Chlorophyll is what transfers energy from light into chemical energy, so that carbohydrate synthesis can actually happen: light energy → chlorophyll → energy stored in chemicals. Saying “chlorophyll makes glucose” skips the actual job it does — it does not build the carbohydrate itself, it makes the light energy usable for the reactions that do. Cells that carry out large amounts of photosynthesis pack in many chloroplasts as a result, which is exactly why the palisade mesophyll — the layer nearest the upper surface of a leaf — is the tissue built to hold the most of them.

What happens to the carbohydrate a plant makes

Glucose does not simply accumulate once it is made. Cambridge limits the required list of what happens to it to five specific fates, and an answer that blurs two of them together — storage and transport are the pair most commonly confused — loses marks even when the biology is roughly right.

• Starch, for energy storage — carbohydrate the plant does not need immediately is converted to starch and kept in reserve.
• Cellulose, for cell walls — carbohydrate is built into cellulose, a structural component of plant cell walls.
• Glucose, used directly in respiration — releasing usable energy for the cell’s own activities.
• Sucrose, for transport — carbohydrate is converted to sucrose, the specific sugar carried in the phloem to other parts of the plant.
• Nectar, to attract pollinators — carbohydrate is used to make nectar, which can attract insects that transfer pollen between flowers.

Keep starch and sucrose apart in particular: starch is the storage form, sucrose is the transport form, and writing “glucose is transported in the phloem” where the question wants the named transport sugar will not get the mark. You can follow sucrose’s journey through the phloem on the transport in plants pages, and see how nectar fits into the bigger picture of pollination on the reproduction pages.

Mineral ions plants need

Photosynthesis supplies carbohydrate, but carbohydrate is mostly carbon, hydrogen and oxygen — it contains no nitrogen. A plant cannot build every molecule it needs out of photosynthetic product alone, which is why two mineral ions absorbed from the soil matter so much here.

Keep the pairing exact in both directions: nitrate is not the ion required to make chlorophyll, and magnesium is not the ion required to make amino acids. Magnesium is incorporated into the chlorophyll molecule itself — it is a building component, not an energy supply, and it cannot substitute for light.

The causal chain is what actually earns marks if a question pushes further than the bare fact: short of nitrate, a plant makes fewer amino acids, builds less protein, and grows poorly or looks stunted; short of magnesium, it makes less chlorophyll, its leaves look pale or yellow, and — because there is now less chlorophyll to transfer light energy — its rate of photosynthesis falls too. Notice the shape of both explanations: ion → molecule it builds → job that molecule does → what fails without it. Jumping straight from “no magnesium” to “yellow leaves” without the chlorophyll step in the middle is the version that loses marks.

Once you have the requirements pinned down this precisely, the natural next question is how you would actually prove them in a lab — which is exactly what investigating what photosynthesis needs covers.