AQA GCSE Biology: Photosynthesis

Photosynthesis is how plants capture light energy and store it as chemical energy in glucose. This guide explains the reaction and its equations, why it counts as endothermic, and the many things a plant does with the glucose it makes. The key idea is that photosynthesis turns light energy into the chemical energy that feeds the whole plant.

The photosynthetic reaction

In photosynthesis, light energy is absorbed by chlorophyll, a green pigment in the chloroplasts of plant and algal cells. This energy is used to convert carbon dioxide and water into glucose (a sugar), with oxygen released as a by-product.

The reaction can be written as a word equation:

carbon dioxide + water → (light) → glucose + oxygen

and as a symbol equation:

6CO₂ + 6H₂O → (light) → C₆H₁₂O₆ + 6O₂

There are only two raw materials, carbon dioxide and water, and that is worth being strict about. Light and chlorophyll sit above the arrow rather than on the left of it, because neither is used up: the chlorophyll absorbs the light and is still chlorophyll afterwards. Writing either as a reactant is one of the easier marks to lose on this topic.

The symbol equation is better checked than memorised. Six carbons go in as carbon dioxide and six come out in a single glucose molecule, the twelve hydrogens in the water all end up in the glucose, and the oxygen left over leaves as six molecules of O₂.

Why photosynthesis is endothermic

An endothermic reaction takes in energy from its surroundings. Photosynthesis is endothermic because energy is transferred from the environment to the chloroplasts by light, and ends up stored in the chemical bonds of glucose.

That is the whole answer, and it is worth giving in those terms rather than saying the plant "makes" or "produces" energy. Nothing makes energy. It moves: light to chemical bonds here, and chemical bonds back out again in respiration.

How plants use the glucose

The glucose made in photosynthesis is used in several ways.

• Respiration: glucose is broken down to release energy for the cell's processes.
• Making starch: glucose is joined into starch, an insoluble storage carbohydrate. Because it is insoluble, starch does not affect the water potential of the cell, so it can be stored without drawing water in by osmosis.
• Making fats and oils: glucose is turned into lipids for long-term energy storage, which is especially important in seeds.
• Making cellulose: glucose is used to make cellulose, which strengthens the plant cell walls.
• Making amino acids and proteins: glucose is combined with nitrate ions (absorbed from the soil) to make amino acids, which are then built into proteins for growth and to make enzymes.

Two of those are about storing energy and two are about building the plant, which is a useful way to hold the list. If a question asks for two uses, give two different jobs rather than two versions of the same one.

The last of them is the only use that needs something from outside the reaction. Glucose contains carbon, hydrogen and oxygen but no nitrogen, so a plant cannot make an amino acid from glucose alone. That is why a plant short of nitrate ions shows stunted growth, which is covered in the plant disease and defences guide.

Photosynthesis and respiration together

Plants respire all the time, day and night, exactly as animals do. Photosynthesis happens only in the light. Keeping those two facts apart answers most questions about a plant in the dark: respiration carries on, photosynthesis stops.

In good light a plant photosynthesises faster than it respires, so overall it takes carbon dioxide in and gives oxygen out. At night only respiration is running, so the exchange reverses and the plant takes oxygen in and gives carbon dioxide out. A plant is not an oxygen source at three in the morning.

Set the two equations side by side and they are near enough reverses of each other, one storing energy in glucose and the other releasing it again. The respiration guide takes the second half.

How this fits together

Photosynthesis makes the glucose that a plant lives on, and its rate depends on the conditions. Those conditions, and the limiting factors that control the rate, are the subject of the rate of photosynthesis guide.