Question

Describe various steps of light reaction.

Answer 1

The photochemical phase of photosynthesis is also known as the light reaction. It is responsible for 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.

Following are the main steps of photochemical phase:

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. First, in PS II, the $P_{680}$ molecule absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit which is farther from the atomic nucleus. These electrons are picked up by an electron acceptor which passes them to an electron transport system of cytochromes. This movement of electrons is downhill on redox potential scale. The electrons are then passed onto the pigments of PS I, without being used as they pass through the electron transport chain. Simultaneously, electrons in the reaction center of PS I ($P_{700}$) are excited when they receive light of wavelength 700 nm and these electrons are transferred to another acceptor molecule that has a greater redox potential. These electrons are then moved downhill again to a molecule of $NAD{P}^{+}$. The addition of these electrons reduces the $NAD{P}^{+}$ to $NADPH+H^{+}$.
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 membrane or lamella of the grana have both PS I and PS II, the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. 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.

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