Question

Describe the light-dependent steps of photosynthesis. How are they linked to the dark reaction?

Answer 1

The light-dependent steps of photosynthesis are the formation of high-energy chemical intermediates, ATP and NADPH, and it includes light absorption, water splitting and release of oxygen. It takes place in the granal thylakoids of chloroplasts in the presence of sunlight.

Following are the light-dependent steps of photosynthesis:

• Production of assimilatory power in photosynthesis: It occurs through two processes:

1. Non-cyclic photophosphorylation: Non-cyclic photophosphorylation involves both Photosystem I and Photosystem II. These two photosystems work in series, first PS II and the PS I. The two photosystems are connected through an electron transport chain. Both ATP and $NADPH+H^{+}$ are synthesised by this kind of electron flow.
2. Cyclic photophosphorylation: Cyclic photophosphorylation is a process of photophosphorylation in which an electron released by the excited photocentre is returned to it after passing through a series of electron carriers. It takes place in the stroma lamellae membrane of chloroplasts. The process of cyclic photophosphorylation involves only photosystem I. The excited electron does not pass on to $NAD{P}^{+}$ and is cycled back to the PS I complex through the electron transport chain. It synthesises ATP only.

• Splitting of water and release of oxygen: The electrons that were removed from PS II must be replaced. This is achieved by electrons available due to the splitting of water. The water splitting complex is associated with the PS II, which itself is physically located on the inner side of the membrane. Water is split into $H^{+}$, [O] and electrons. The protons and oxygen formed by splitting of water are released into the lumen of the thylakoids. The oxygen produced is released as one of the net products of photosynthesis.

$2H_{2}O\to 4H^{+}+O_2+4e^{-}$

Dark reactions are the reactions which are not directly dependent on light. But they are linked to light-dependent steps of photosynthesis because they use assimilatory powers i.e., ATP and NADPH in fixation and reduction of carbon dioxide.