Photochemical and biosynthetic phase

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Photosynthesis comprises of two phases; first phase is the photochemical phase or light dependent process (light reaction) and second phase is the biosynthetic phase or dark reaction of photosynthesis. The whole process of photosynthesis takes place within the chloroplast.

Light reaction is a light dependent process which includes a series of events such as light absorption, hydrolysis, the release of oxygen, formation of ATP and NADPH. The light reaction of photosynthesis initiates only when it is supplied with light energy. There are two photosystems in plants, Photosystem I (PS I) and Photosystem II (PS II). Photosystem I absorbs light at a wavelength of 700 nm whereas Photosystem II absorbs light at a wavelength of 680 nm. On receiving a photon of light energy the photocentre expels an electron with a gain of energy. It is the primary reaction of photosynthesis which involves the conversion of light energy into chemical form. This phenomenon is known as quantum conversion. When the light hits, chlorophyll a get excited to higher energy state followed by a series of reactions, this energy is converted into energy molecules ATP and NADPH by using PS I and PS II.

Dark reaction occurs in the stroma of the chloroplast where they utilize the products of the light reaction and reduction of CO2 takes place result in the formation glucose. Assimilatory powers (ATP and NADPH) produced during photochemical phase is used in the fixation of CO2.

Electron Transport Chain in Photosynthesis

The electron transport chain of photosynthesis is initiated by the absorbance of light by the photosystem II (P680). The P680 becomes exited by absorbing light and its electrons are transferred to an electron acceptor molecule. As a result, P680 becomes a strong oxidizing agent and splits a molecule of water to release oxygen. This light dependent splitting of water molecule is called photolysis. It require magnesium, calcium and chlorine ions. The electrons generated by the breakdown of water, are then passed to the oxidized P680. Thus, the electron deficient P680 is able to restore its electrons from the water molecule. After accepting electrons from the excited P680, the primary electron acceptor is reduced. The reduced acceptor now becomes a strong reducing agent, and hence donates the electrons to the downstream components of the electron transport chain. Like the photosystem II (P680), photosystem I (P700) is excited on absorbing light and get oxidized. It transfers its electron to the primary electron acceptor, which in turn gets reduced while the oxidized P700 draws electron from photosystem II, the reduced electron acceptor of photosystem I, transfers electrons to ferredoxin and ferredoxin-NADP reductase to reduce NADP. The latter is reduced to NADPH2 using protons (H+) released during photolysis of water. The NADPH2 is a powerful reducing agent is then utilized in the reduction of carbon dioxide to carbohydrates in the carbon reaction of photosynthesis. This process requires energy, which is provided by ATP produced via electron transport chain.

The process of ATP formation from ADP and Pi (inorganic phosphate) in the presence of sunlight in chloroplasts is called photophosphorylation. Its occurs in two ways; Non-cyclic and cyclic.

Non-cyclic is the normal process of photophosphorylation in which the electron expelled by the excited photocentre does not return to it. It is carried out by the interaction of photosystem I and photosystem II and ultimately reduces NADP to NADPH2. ATP is formed from ADP and Pi. Since the flow of electrons from water to NADP is unidirectional, the process of ATP formation is called non-cyclic photophosphorylation. The electrons pass through the primary acceptor, plastoquinone (PQ), cytochrome complex, plastocyanin (PC) and finally to P700.

Image showing Non-cyclic photophosphorylation.

Image Showing Non-Cyclic Photophosphorylation.

Image showing Non-cyclic photophosphorylation.

Cyclic phosphorylation occurs when the carbon fixation is stopped due to limited/no supply of carbon dioxide and NADPH2 accumulates. This reaction starts when PS I absorbs sufficient amount of light to pass on to p700. When P700 receives enough quanta of light, it gets excited and emits electrons, which are accepted by an unknown primary electron acceptor designated as A. Ferredoxin has to pass on the electrons to plastoquinone, as there is no NADP+ to reduce. From plastoquinone, electrons pass through cyt b – cyt f complex and plastocyanin and as de-energised electrons come back to PS I; the exited PS I brought back to the ground state. Since the electrons lost by PS I come back to it, the process us termed cyclic.

Image showing Cyclic photophosphorylation.

Image Showing Cyclic Photophosphorylation.

Image showing Cyclic photophosphorylation.

Differences between Cyclic and Non-Cyclic Photophosphorylation

Table Showing Cyclic and Non-Cyclic Photophosphorylation.
Table showing cyclic and non-cyclic photophosphorylation.

Cyclic photophosphorylation

Non-cyclic photophosphorylation

It involves photosystem I only

It involves both photosystem I and II

Electron comes from the chlorophyll molecule and returns to the chlorophyll

Water is the source of the electrons and NADP is the final acceptor of the electron. The electron goes out the system

Reduced NADP (NADPH2) is not formed

Reduced NADP i.e. NADPH2 is formed which is used in the reduction of carbon dioxide

It is connected with photolysis of water. Therefore, oxygen is not evolved

It is connected with photolysis of water and oxygen is evolved as a by product

This process is formed mainly in photosynthetic bacteria

This mainly takes place in green plants

In higher photosynthetic plants, extra ATP can be made via cyclic photophosphorylation if cyclic and non-cyclic photophosphorylation occurs side by side. The efficiency of energy conversion in the light reactions of photosynthesis is high and estimated at about

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