Photophosphorylation

Photophosphorylation

Phosphorylation taking place during respiration is called as oxidative phosphorylation and ATP produced by the breakdown of substrate is known as substrate level phosphorylation. In this topic, we are going to learn about phosphorylation taking place in chloroplast with the help of light. During the movement of electrons through carrier molecules ATP and NADPH + H⁺ are produced. Phosphorylation is the process of synthesis of ATP by the addition of inorganic phosphate to ADP. The addition of phosphate here takes place with the help of light generated electron and so it is called as photophosphorylation. It takes place in both cyclic and non-cyclic electron transport.

1. Cyclic Photophosphorylation

Cyclic photophosphorylation refers to the electrons ejected from the pigment system I (Photosystem I) and again cycled back to the PS I. When the photons activate P700 reaction centre photosystem II is activated. Electrons are raised to the high energy level. The primary electron acceptor is Ferredoxin Reducing Substance (FRS) which transfers electrons to Ferredoxin (Fd), Plastoquinone (PQ), cytochrome b6-f complex, Plastocyanin (PC) and finally back to chlorophyll P700 (PS I). During this movement of electrons Adenosine Di Phosphate (ADP) is phosphorylated, by the addition of inorganic phosphate and generates Adenosine Tri Phosphate (ATP). Cyclic electron transport produces only ATP and there is no NADPH + H⁺ formation. At each step of electron transport, electron loses potential energy and is used by the transport chain to pump H⁺ ions across the thylakoid membrane. The proton gradient triggers ATP formation in ATP synthase enzyme situated on the thylakoid membrane. Photosystem I need light of longer wave length (> P700 nm). It operates under low light intensity, less CO₂ and under anaerobic conditions which makes it considered as earlier in evolution (Figure 13.15).

2. Non-Cyclic Photophosphorylation

When photons are activated reaction centre of pigment system II(P680), electrons are moved to the high energy level. Electrons from high energy state passes through series of electron carriers like pheophytin, plastoquinone, cytochrome complex, plastocyanin and finally accepted by PS I (P700). During this movement of electrons from PS II to PS I ATP is generated (Figure 13. 16). PS I (P700) is activated by light, electrons are moved to high energy state and accepted by electron acceptor molecule ferredoxin reducing Substance (FRS). During the downhill movement through ferredoxin, electrons are transferred to NADP1 and reduced into NADPH + H⁺ (H⁺ formed from splitting of water by light).

Electrons released from the photosystem II are not cycled back. It is used for the reduction of NADP1 in to NADPH + H+. During the electron transport it generates ATP and hence this type of photophosphorylation is called non-cyclic photophosphorylation. The electron flow looks like the appearance of letter ‘Z’ and so known as Z scheme. When there is availability of NADP1 for reduction and when there is splitting of water molecules both PS I and PS II are activated (Table 13.3). 

Non-cyclic electron transport PS I and PS II both are involved co-operatively to transport electrons from water to NADP1 (Figure 13.6). In oxygenic species non-cyclic electron transport takes place in three stages.

i. Electron transport from water to P680:

Splitting of water molecule produce electrons, protons and oxygen. Electrons lost by the PS II (P680) are replaced by electrons from splitting of water molecule.

ii. Electron transport from P680 to P700:

Electron flow starts from P680 through a series of electron carrier molecules like pheophytin, plastoquinone (PQ), cytochrome b₆- f complex, plastocyanin (PC) and finally reaches P700 (PS I).

iii. Electron transport from P700 to NADP1

PS I(P700) is excited now and the electrons pass to high energy level. When electron travels downhill through ferredoxin, NADP1 is reduced to NADPH + H+.

3. Bio energetics of light reaction

•             To release one electron from pigment system it requires two quanta of light.

•             One quantum is used for transport of electron from water to PS I.

•             Second quantum is used for transport of electron from PS I to NADP⁺

•             Two electrons are required to generate one NADPH + H⁺.

•             During Non-Cyclic electron transport two NADPH + H⁺ are produced and it requires 4 electrons.

•             Transportation of 4 electrons requires 8 quanta of light.

Check your grasp!

Name the products produced from Non-Cyclic photophosphorylation?

Why does PS II require electrons from water?

Can you find the difference in the Pathway of electrons during PS I and PS II?