International A Level Biology / Photosynthesis

Photosynthesis: the overall reaction

Edexcel International A Level Biology (YBI11), chapter 14: Photosynthesis, specification point 5.1. Free lesson.

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Photosynthesis absorbs light energy and stores it as chemical energy in glucose. Understanding the balanced equation, where its atoms come from, and how the two stages work helps you explain the whole process.

How light energy becomes stored in glucose

Light energy is absorbed by chlorophyll, the green pigment in chloroplasts. It is used to split water. The hydrogen from water joins carbon dioxide, which is reduced to make glucose. The energy is now stored as chemical energy in this fuel and can later be released by respiration. Splitting water using light is called photolysis: photo means light and lysis means splitting. Oxygen is left over when water splits, so it is released into the air. The sequence is light absorbed, water split, oxygen released, hydrogen joined to carbon dioxide, then glucose made.

How to write and balance the overall equation

The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy over the arrow. Carbon dioxide enters from the air through the stomata. Water travels from the soil up the xylem to the leaves. Glucose is a hexose, a sugar with six carbon atoms, and oxygen is a by-product that can diffuse out through the stomata. Check the atoms on both sides: there are 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side. The glucose stores the energy that began as light energy.

Where the atoms in the products come from

Track atoms separately, rather than assuming that a whole molecule stays together. All the carbon in glucose comes from carbon dioxide, and the hydrogen in glucose comes from water. The oxygen gas released comes from water, not carbon dioxide. The oxygen in glucose comes from carbon dioxide. A fuller equation shows the oxygen atoms more clearly: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O. Six oxygen atoms from carbon dioxide enter glucose. The other six end up in new water. Cancel 6H₂O from both sides to get the usual overall equation.

What happens in the two stages

Photosynthesis has light-dependent and light-independent reactions. The first stage needs light directly. The second does not use light directly, but depends on products made by the first stage. The light-dependent stage needs light, water, ADP, inorganic phosphate (Pᵢ) and NADP. It makes ATP and reduced NADP, and releases oxygen from water. The light-independent stage needs carbon dioxide, ATP and reduced NADP. It uses them to make sugars such as glucose. The light-independent stage stops soon after darkness begins, when its supplies of ATP and reduced NADP run out.

Where the stages happen and what water does

Both stages take place inside a chloroplast, which is wrapped in an envelope, a double membrane. The light-dependent reactions happen on the thylakoid membranes. These flattened sacs form stacks called a granum. The light-independent reactions happen in the surrounding fluid, the stroma. ATP and reduced NADP pass from the thylakoid membranes into the stroma. Do not just write that water is needed. Say what it does: light energy splits water in photolysis, providing hydrogen for glucose and oxygen that is released. If water contains oxygen-18, the label is found in the oxygen gas…

More lessons in Photosynthesis

• ATP: the immediate energy supply
• Chloroplast structure and function
• Absorption and action spectra
• Separating leaf pigments by chromatography
• The light-dependent reactions
• Cyclic and non-cyclic photophosphorylation
• The Calvin cycle
• What the plant makes from GALP
• Core practical 10: measuring the rate of photosynthesis
• Limiting factors: light, carbon dioxide, temperature and wavelength