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What is photosynthesis, where does it happen, and what happens to the glucose that plants make?

Photosynthesis as an endothermic reaction, the word and symbol equations, the site of photosynthesis, and the uses of the glucose produced by the plant.

A focused answer to AQA GCSE Biology 4.4.1.1, covering photosynthesis as an endothermic reaction, the word and symbol equations, where it happens, and how plants use the glucose they make.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The photosynthesis reaction
  3. Uses of glucose
  4. Try this

What this dot point is asking

AQA wants you to describe photosynthesis as an endothermic reaction, recall the word and (higher tier) symbol equation, state where photosynthesis happens, and list the uses of the glucose the plant makes.

The photosynthesis reaction

carbon dioxide+water→lightglucose+oxygen\text{carbon dioxide} + \text{water} \xrightarrow{\text{light}} \text{glucose} + \text{oxygen}

The symbol equation (higher tier):

6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2

Light energy is absorbed by the green pigment chlorophyll found in the chloroplasts, which are most concentrated in the palisade mesophyll cells near the upper surface of the leaf. The leaf is adapted for this: it is broad and flat to capture light, thin so gases diffuse quickly, and its lower surface has stomata (pores controlled by guard cells) that let carbon dioxide diffuse in. Water arrives through the xylem from the roots.

Why light energy matters

Light does not become part of the glucose. Instead, the energy of light is captured by chlorophyll and used to drive the chemical reactions that join small molecules (carbon dioxide and water) into a larger, energy-rich molecule (glucose). Because the products store more energy than the reactants, energy must be supplied, which is what makes the reaction endothermic. A useful check in the exam: if a reaction needs an energy input to keep going, it is endothermic.

Uses of glucose

The glucose made in photosynthesis is used in several ways, and AQA expects you to be able to list and explain them:

  • For respiration, releasing energy for the plant's life processes such as active transport of mineral ions into root cells.
  • Converted into insoluble starch for storage. Starch is ideal for storage because it is insoluble, so it does not affect the water balance of the cell by osmosis and does not diffuse out.
  • Used to make cellulose, which strengthens plant cell walls. Plants that grow quickly use a lot of glucose this way.
  • Used to make amino acids (glucose combined with nitrate ions taken up from the soil), which are then joined into proteins for enzymes and growth.
  • Used to make lipids (fats and oils) for storage, for example in seeds such as sunflower or rapeseed.

Try this

Q1. Write the word equation for photosynthesis. [2 marks]

  • Cue. Carbon dioxide + water, with light, gives glucose + oxygen.

Q2. Give two uses of the glucose made by a plant. [2 marks]

  • Cue. Any two of: respiration, stored as starch, making cellulose, proteins or fats and oils.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksDescribe what happens during photosynthesis. In your answer refer to the raw materials, the products, where the reaction takes place and the energy change involved.
Show worked answer →

A 4-mark describe question on Paper 1 rewards four distinct, accurate points.

  1. Carbon dioxide (from the air through the stomata) and water (from the soil through the roots) are the raw materials.
  2. The reaction takes place in the chloroplasts, where the green pigment chlorophyll absorbs light.
  3. The products are glucose and oxygen.
  4. Photosynthesis is endothermic, so energy is transferred from the surroundings (light) to the chloroplast.

Markers reward correct raw materials and products, the site (chloroplast or chlorophyll), and the word endothermic. A common loss is writing energy is released, which describes respiration, not photosynthesis.

AQA 20212 marksGlucose made in photosynthesis is converted into other substances in a plant. Explain why a plant needs nitrate ions from the soil to make proteins.
Show worked answer →

A 2-mark explain question wants a cause and a consequence.

Glucose contains only carbon, hydrogen and oxygen, but proteins also contain nitrogen. The plant absorbs nitrate ions from the soil to supply the nitrogen, and combines the nitrogen with glucose to make amino acids, which are then joined to build proteins.

Markers reward the idea that glucose lacks nitrogen and that nitrate supplies the nitrogen for amino acids. Saying nitrate provides energy or minerals in general would not score.

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