Photosynthetic reaction — AQA GCSE Combined Science
Test yourself on Photosynthetic reaction with AQA GCSE practice questions.
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Photosynthetic reaction explained
Photosynthesis is the process by which plants, algae and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen.
Read the full explanation
The overall change is summarised by the word equation carbon dioxide + water → glucose + oxygen, and by the balanced symbol equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The reaction is endothermic because it transfers energy from the environment as light is absorbed. It occurs mainly in the chloroplasts of leaf cells, where chlorophyll absorbs light. The glucose produced is used for respiration, converted to starch for storage, or used to make cellulose, amino acids and lipids. Oxygen is released as a by-product. Students should be able to state the reactants and products, identify the energy source, and explain that the reaction stores energy in glucose.
light glucose + oxygen
This statement is part of the word equation for photosynthesis, showing that light energy is needed and that glucose and oxygen are produced. The full word equation is carbon dioxide + water → glucose + oxygen, with light energy shown above the arrow. Light is absorbed by chlorophyll in chloroplasts and provides the energy to convert carbon dioxide and water into glucose. Oxygen is released as a by-product. The glucose formed is used in respiration, stored as starch, or used to build cellulose, amino acids and lipids. Students should be able to place light correctly above the arrow, name glucose and oxygen as products, and explain that the reaction stores energy in glucose.
carbon dioxide + water
This word equation gives the reactants of photosynthesis: carbon dioxide and water. In the presence of light, absorbed by chlorophyll in chloroplasts, these two reactants are converted into glucose and oxygen. Carbon dioxide enters the leaf by diffusion through stomata; water is drawn up the xylem from the roots. The balanced symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, so six molecules of each reactant yield one glucose molecule and six oxygen molecules. Energy is transferred from light to chemical stores in glucose, so photosynthesis is endothermic overall. A useful check is that the equation is not respiration reversed in wording only: respiration uses glucose and oxygen and releases carbon dioxide and water.
Students should recognise the chemical symbols: CO₂, H₂O, O₂ and C₆H₁₂O₆.
Recognising these symbols means reading each formula as a substance and using it correctly in the photosynthesis equation. CO₂ is carbon dioxide, H₂O is water, O₂ is oxygen and C₆H₁₂O₆ is glucose. The subscript numbers show atoms in one molecule: CO₂ has one carbon and two oxygen atoms; H₂O has two hydrogen and one oxygen; O₂ has two oxygen; C₆H₁₂O₆ has six carbon, twelve hydrogen and six oxygen. In the balanced equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, the large numbers count molecules, so atoms balance on both sides. A quick check is to count carbon, hydrogen and oxygen atoms on each side.
Students should be able to describe photosynthesis as an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light.
Photosynthesis is the process by which plants, algae and some bacteria make glucose and oxygen from carbon dioxide and water. It is endothermic: overall, energy is transferred from the environment to the organism, not released to the surroundings. Light is absorbed by chlorophyll inside chloroplasts, so the energy transfer is from the environment to the chloroplasts by light. The balanced equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The glucose stores chemical energy in its bonds, and oxygen is released as a by-product. A useful check is that energy must be supplied continuously for the reaction to continue; in darkness it stops. Compare this with exothermic respiration, which transfers energy to the surroundings.
Your focus
- Write the word equation for photosynthesis.
- Write and balance the symbol equation for photosynthesis.
- Describe the role of light energy and chlorophyll in the reaction.
Show all 15 objectives
- Identify light, glucose and oxygen in the photosynthesis equation.
- Explain why light is shown above the arrow rather than as a reactant.
- Describe the fate of glucose and oxygen produced in photosynthesis.
- State the word equation for photosynthesis, naming carbon dioxide and water as reactants.
- Explain how light energy absorbed by chlorophyll is transferred to chemical stores in glucose.
- Link the reactants and products to the balanced symbol equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Name the substances represented by CO₂, H₂O, O₂ and C₆H₁₂O₆.
- Use these formulas correctly in the balanced photosynthesis equation.
- Count atoms from subscripts and coefficients to confirm the equation balances.
- Describe photosynthesis as an endothermic reaction using the correct word and balanced symbol equation.
- Explain that light energy is transferred from the environment to the chloroplasts by light absorbed by chlorophyll.
- Compare photosynthesis with respiration in terms of the direction of energy transfer.
Photosynthetic reaction exam tips
Marking Points
- States the word equation: carbon dioxide + water → glucose + oxygen.
- Writes the balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Identifies light as the energy source absorbed by chlorophyll in chloroplasts.
- Explains that photosynthesis is endothermic and stores energy in glucose.
- Names glucose and oxygen as the products and carbon dioxide and water as the reactants.
- Links the process to its role in supplying glucose for respiration and growth and releasing oxygen.
- Places light above the arrow in the word equation, showing it is required but not a reactant.
- Names glucose and oxygen as the products of photosynthesis.
- States that light energy is absorbed by chlorophyll in chloroplasts.
- Explains that the reaction is endothermic and stores energy in glucose.
- Links glucose to uses such as respiration, starch storage and growth.
- Identifies oxygen as a by-product released into the environment.
- Identifies carbon dioxide and water as the reactants of photosynthesis.
- States that light energy is absorbed by chlorophyll and transferred to chemical stores in glucose.
- Names glucose and oxygen as the products, linking them to the full equation.
- Describes carbon dioxide entering through stomata and water arriving via the xylem.
- Recognises that photosynthesis is endothermic overall because energy is taken in from light.
- Names each formula correctly: CO₂ carbon dioxide, H₂O water, O₂ oxygen, C₆H₁₂O₆ glucose.
- Uses the formulas in the correct positions in the photosynthesis equation.
- Counts atoms from subscripts, for example six carbon atoms in one C₆H₁₂O₆ molecule.
- Distinguishes large numbers in front of formulas, which count molecules, from subscripts, which count atoms within a molecule.
- Checks that the balanced equation has equal numbers of each type of atom on both sides.
- States that photosynthesis is endothermic, meaning energy is transferred from the environment to the organism overall.
- Identifies the reactants carbon dioxide and water and the products glucose and oxygen, with the balanced equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Explains that light energy is absorbed by chlorophyll in the chloroplasts and transferred to the chloroplasts.
- Describes the energy transfer pathway: environment → light → chlorophyll in chloroplasts → chemical energy in glucose.
- Recognises that the energy is stored in the chemical bonds of glucose and that oxygen is released as a by-product.
- Contrasts endothermic photosynthesis with exothermic respiration to justify the direction of energy transfer.
Examiner Tips
- 💡Learn both the word equation and the balanced symbol equation, and practise writing them from memory.
- 💡When asked to explain the equation, name the energy source and the location of the process, not just the reactants and products.
- 💡Check that every chemical formula in the symbol equation is balanced before moving on to the next part of the question.
- 💡Draw the word equation with light clearly above the arrow to show it is a requirement, not a reactant.
- 💡When explaining the equation, state where light is absorbed and what happens to the energy.
- 💡Use the products to explain why photosynthesis is important for life, for example glucose for respiration and oxygen for aerobic respiration.
- 💡Write the word equation on one line and the balanced symbol equation directly beneath it to show the link.
- 💡When asked to compare, set out photosynthesis and respiration side by side rather than describing them separately.
- 💡Use the phrase 'energy transferred from light to chemical stores' rather than saying energy is 'made'.
- 💡Underline each formula in the equation and write its name above it before answering.
- 💡Count atoms on each side of the arrow to confirm the equation balances.
- 💡Check subscripts carefully; H₂O and H₂O₂ are different substances.
- 💡Use the word endothermic explicitly and link it to the direction of energy transfer from the environment.
- 💡When asked to describe, give the equation and name the energy transfer pathway rather than only naming the reactants.
- 💡If asked to compare with respiration, state that respiration is exothermic and transfers energy to the surroundings.
Common Mistakes
- Writing the word equation with oxygen and glucose as reactants; correction: carbon dioxide and water are the reactants, glucose and oxygen are the products.
- Omitting the 6 in front of CO₂, H₂O or O₂ in the symbol equation; correction: balance each element so that 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Stating that photosynthesis is exothermic; correction: it is endothermic because it takes in light energy and stores it in glucose.
- Writing light as a reactant on the left of the arrow; correction: light is a condition shown above the arrow, not a chemical reactant.
- Confusing the products with the reactants; correction: glucose and oxygen are produced, while carbon dioxide and water are used up.
- Stating that oxygen is used by the plant in photosynthesis; correction: oxygen is released, and the plant uses oxygen in respiration.
- Writing oxygen as a reactant: correct this by stating oxygen is a product of photosynthesis and a reactant of aerobic respiration.
- Omitting light or chlorophyll: correct this by noting both are needed even though neither appears in the word equation.
- Confusing the direction of gas movement: correct this by stating carbon dioxide diffuses in through stomata while oxygen diffuses out.
- Reading C₆H₁₂O₆ as six separate elements: correct this by treating it as one glucose molecule containing 6 C, 12 H and 6 O atoms.
- Treating the 6 in 6CO₂ as a subscript: correct this by noting it multiplies the whole molecule, giving 6 C and 12 O atoms.
- Writing O instead of O₂ for oxygen gas: correct this by using O₂ because oxygen gas is diatomic.
- Saying photosynthesis is exothermic because it makes glucose: correct this by stressing that energy is taken in from the environment, so the overall transfer is endothermic.
- Writing the equation with the wrong products or unbalanced formulae, such as CO₂ + H₂O → C₆H₁₂O₆ + O₂: correct this by balancing to 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Claiming energy is transferred to the chloroplasts by carbon dioxide or by heat: correct this by stating that light is absorbed by chlorophyll in the chloroplasts.