Photosynthesis overview and ATP

The overall reaction

6CO2 + 6H2O → C6H12O6 + 6O2 (light energy absorbed by chlorophyll)

Light energy is used to split water (photolysis) into hydrogen ions (protons), electrons and oxygen. The hydrogen is used to reduce carbon dioxide, so the energy ends up stored in the chemical bonds of glucose. The oxygen is released as a waste product.

Experiments with water labelled with the isotope oxygen-18 show that the oxygen gas given off comes from water, not from carbon dioxide.

ATP: the energy currency

ATP (adenosine triphosphate) is made of adenine, ribose and three phosphate groups.

• Hydrolysis: ATP + H2O → ADP + Pi, catalysed by ATPase. This releases energy that the cell can use straight away, for example for active transport, muscle contraction or making proteins.
• Phosphorylation: ADP + Pi → ATP, a condensation reaction catalysed by ATP synthase. This needs an energy input.

ATP is an immediate energy source: it releases energy in small, manageable amounts in a single reaction, and it is quickly remade, so cells hold only a small amount at any time.

Photophosphorylation

In plants, the energy to make ATP during photosynthesis comes from light, so it is called photophosphorylation. In respiration the energy comes from the oxidation of food molecules (oxidative phosphorylation).

The ATP and reduced NADP made using light are then used to turn carbon dioxide into sugars, which you meet in the next two lessons.

Worked example

How many molecules of oxygen are released when a plant makes 3 molecules of glucose?

1. The equation releases 6 O2 for every 1 glucose.
2. 3 x 6 = 18

Answer: 18 molecules of oxygen.

Check yourself

1. How many molecules of carbon dioxide are fixed to make one molecule of glucose?
   Show answer

   6. The equation has 6CO2 for each C6H12O6: one carbon atom from each CO2.

2. How many molecules of oxygen are released when 5 glucose molecules are made?
   Show answer

   30. 6 O2 per glucose, so 5 x 6 = 30.

3. Where does the oxygen released in photosynthesis come from?
   • Carbon dioxide
   • Glucose
   • Water, split by light energy
   • The air spaces in the leaf
   Show answer

   Water, split by light energy. Photolysis of water releases O2; isotope labelling confirms this.

4. What are the products of ATP hydrolysis?
   • AMP and water
   • ADP and inorganic phosphate
   • Adenine and ribose
   • Glucose and oxygen
   Show answer

   ADP and inorganic phosphate. ATP + H2O → ADP + Pi, releasing energy.

5. Why is ATP described as an immediate source of energy?
   • It stores huge amounts of energy for long periods
   • It is the only molecule that contains energy
   • It cannot be remade once used
   • Its hydrolysis releases energy in small amounts in a single step
   Show answer

   Its hydrolysis releases energy in small amounts in a single step. One reaction releases a usable packet of energy; glucose needs many steps.

6. What is photophosphorylation?
   • Breaking down ATP using light
   • Making ATP from ADP and Pi using energy from light
   • Splitting water using ATP
   • Using light to fix carbon dioxide
   Show answer

   Making ATP from ADP and Pi using energy from light. Photo- means light; phosphorylation means adding phosphate to ADP.

7. Which enzyme catalyses the formation of ATP from ADP and Pi?
   • ATPase
   • RuBisCO
   • Amylase
   • ATP synthase
   Show answer

   ATP synthase. ATP synthase makes ATP; ATPase hydrolyses it.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.