What are the stages of the light-dependent reactions in photosynthesis?

The stages of the light-dependent reactions in photosynthesis are photoactivation, electron transport, and chemiosmosis.

In the first stage, photoactivation, light energy is absorbed by chlorophyll, the green pigment in plants. This energy is used to excite electrons in the chlorophyll molecule, causing them to gain energy and move to a higher energy level. This process also leads to the splitting of water molecules, a process known as photolysis, which releases oxygen gas and hydrogen ions.

The second stage, electron transport, involves the movement of these high-energy electrons along a series of proteins embedded in the thylakoid membrane, known as the electron transport chain. As the electrons move along the chain, they gradually lose energy. This energy is used to pump hydrogen ions across the thylakoid membrane, from the stroma into the thylakoid space. This creates a concentration gradient of hydrogen ions across the membrane.

The final stage, chemiosmosis, involves the movement of hydrogen ions down their concentration gradient, back across the thylakoid membrane and into the stroma. This movement is facilitated by a protein complex called ATP synthase. As the hydrogen ions pass through ATP synthase, their energy is used to combine ADP and inorganic phosphate to form ATP, the energy currency of the cell.

Simultaneously, the high-energy electrons that have passed along the electron transport chain are transferred to a molecule called NADP+ to form NADPH, another energy-carrying molecule. Both ATP and NADPH are then used in the light-independent reactions of photosynthesis to synthesise glucose from carbon dioxide.

In summary, the light-dependent reactions of photosynthesis involve the absorption of light energy by chlorophyll, the use of this energy to transport electrons and pump hydrogen ions across a membrane, and the generation of ATP and NADPH. These reactions are crucial for the energy needs of the plant and also contribute to the production of oxygen in our atmosphere.