What Are the Light-Dependent and Light-Independent Reactions?

The light-dependent reactions and light-independent reactions are two linked stages of photosynthesis. The light-dependent reactions use light energy to produce ATP and reduced NADP. The light-independent reactions use ATP and reduced NADP to make organic substances from carbon dioxide.

Light-dependent reactions

These reactions take place on the thylakoid membranes inside chloroplasts.

1. Chlorophyll absorbs light energy, causing photoionisation and releasing electrons.
2. The electrons pass along an electron transport chain. Their energy is used to move protons across the thylakoid membrane.
3. The proton gradient drives protons through ATP synthase, producing ATP by chemiosmosis.
4. Water undergoes photolysis, producing protons, electrons and oxygen:

$${\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}+\frac{1}{2}{\mathrm{O}}_2$$

5. The electrons and protons reduce NADP to reduced NADP.

The products are ATP, reduced NADP and oxygen. Oxygen is released as a by-product; it comes from the photolysis of water, not from carbon dioxide.

Light-independent reactions

These reactions take place in the chloroplast stroma and are also called the Calvin cycle. They do not directly require light, but they depend on ATP and reduced NADP from the light-dependent reactions.

1. Carbon dioxide combines with ribulose bisphosphate, or RuBP, in a reaction catalysed by rubisco.
2. The unstable six-carbon compound immediately splits into two molecules of glycerate 3-phosphate, or GP.
3. ATP and reduced NADP are used to reduce GP to triose phosphate, or TP.
4. Some TP is used to make useful organic substances, such as carbohydrates. The remaining TP uses ATP to regenerate RuBP, allowing the cycle to continue.

Common exam mistake

Do not write that the light-independent reactions happen only in darkness. They can occur in light or darkness, provided ATP and reduced NADP are available. In an intact chloroplast, they usually depend on products made by the light-dependent reactions.