The Light Reactions of Photosynthesis

(Take place in the Grana)

As suggested by the name these reactions need the presence of light to be able to occur. Without light the plants would not be able to produce glucose. These are often referred to as Light Dependant Reactions (LDR)

The light energy is neede for two reasons:-

1. It splits the water molecules by the process of Photolysis into hydrogen and hydroxide ions [ H₂O ---> OH^- + H⁺ ]
2. It raises the energy levels of an electron in the chlorophyll molecule to the excited state. This electron then leaves the molecule. [ CHLOROPHYLL ----> e^- + CHLOROPHYLL⁺ ]

Once the four different ions are produced there are four possible combinations that can occur in the following stages:-

• OH^- + H⁺ → H₂O
• e^- + CHLOROPHYLL⁺ → CHLOROPHYLL
• CHLOROPHYLL⁺ + OH^- → CHLOROPHYLL + OH(x4) { 4OH ---> 2H₂O + O₂ }
• H⁺ + e^- → H₂

The hydrogen molecule formed in the last reaction, which is highlighted, will then go into the next stage of photosynthesis.

When the light hits the chlorophyll molecule the energy level of the electron is raised and when the electron returns to its normal energy level its has released enough energy to make one molecule of ATP.

As a result of the LDR:-

• 1 molecule of ATP is produced
• Oxygen gas is produced and given off
• 1 molecule of Hydrogen gas is made

The hydrogen molecule is immediately picked up by a chemical described as a hydrogen acceptor, which for photosynthesis is NADP. Once it picks up the hydrogen it is reduced and so can be described as reduced NADP, or NADPH₂.