11.6.2. Cyclic and Non-cyclic Photo-phosphorylation

Living organisms have the capability of extracting energy from oxidisable substances and store this in the form of bond energy. Special substances like ATP, carry this energy in their chemical bonds. The process through which ATP is synthesised by cells (in mitochondria and chloroplasts) is named phosphorylation. Photo-phosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

Photo-phosphorylation is an essential process wherein ATP (adenosine triphosphate), the energy currency of the cell, is produced. This occurs in two main types: cyclic and non-cyclic photo-phosphorylation. ATP is synthesized through a process that either relies on the flow of electrons in PS I and PS II or only uses PS I in a cyclic manner. This process is crucial because cells utilize ATP for various energy-consuming activities, functioning almost like a battery that stores energy.

Imagine charging a battery using sunlight. When sunlight hits the solar panel (analogous to chlorophyll in plants), it generates electricity (ATP), which can then be stored for later use in different devices, similar to how ATP provides energy for cellular processes.

When the two photosystems work in a series, first PS II and then the PS I, a process called non-cyclic photo-phosphorylation occurs. The two photosystems are connected through an electron transport chain. Both ATP and NADPH + H+ are synthesised by this kind of electron flow.

Non-cyclic photo-phosphorylation involves both Photosystem II (PS II) and Photosystem I (PS I). In this process, light energy excites electrons in PS II, which then travel through an electron transport chain, ultimately reaching PS I. This movement generates ATP and NADPH, essential for the next steps of photosynthesis where carbon fixation occurs. This process enhances the efficiency of photosynthesis as both energy carriers (ATP and NADPH) are generated simultaneously.

Think of non-cyclic photo-phosphorylation as a relay race. The first runner (PS II) starts the race by receiving light energy and passing the baton (electron) down the line until the last runner (PS I) completes the race, resulting in the production of energy drinks (ATP and NADPH) that will energize the rest of the team (the plant) for future activities.

When only PS I is functional, the electron is circulated within the photosystem and phosphorylation occurs due to cyclic flow of electrons. A possible location where this could be happening is in the stroma lamellae.

Cyclic photo-phosphorylation occurs when only Photosystem I (PS I) is active. Here, the excited electron from PS I is cycled back into the system instead of being used to reduce NADP+ to NADPH. This results in the production of only ATP. This process ensures that when energy demands are high, but there is not enough light to support both ATP and NADPH production, the plant can still generate ATP to maintain vital functions.

Imagine a treadmill in a gym. When you walk on it, you generate energy and work out without actually moving forward in space (cyclic phosphorylation), as you are constantly returning to the same spot on the treadmill. However, in a running race where you are moving towards a goal (non-cyclic phosphorylation), every step forward takes you further until you reach a finish line.

Chemiosmotic hypothesis has been put forward to explain the mechanism. ATP synthesis is linked to development of a proton gradient across a membrane. The ATP synthase enzyme consists of two parts: one called the CF0 embedded in the thylakoid membrane and the other portion, CF1, protrudes on the outer surface.

The chemiosmotic hypothesis explains how ATP is synthesized in chloroplasts during photosynthesis. This occurs through the establishment of a proton gradient across the thylakoid membrane. Specifically, as electrons move through the electron transport chain, protons are pumped into the thylakoid lumen, creating a high concentration of protons. ATP synthase utilizes this proton flow back into the stroma to drive the conversion of ADP and inorganic phosphate into ATP.

Imagine a waterwheel that generates power as water flows from a higher reservoir to a lower one. The flow of water represents the movement of protons through ATP synthase, providing the energy necessary to turn the wheel and produce electricity (ATP), storing energy that can be used later.

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