An Overview of Photosystem: Definition, Location, Function, Types, Steps, FAQs

What Are Photosystems?

Photosystems are protein and pigment complexes found in the thylakoid chloroplast membrane. These systems are involved in vital light-dependent reactions in photosynthesis. They capture light energy and convert it into chemical energy. There are mostly two major types of photosystems: Photosystem I (PSI) and Photosystem II (PSII).

The fact that photosystems are among the main parts of the process of photosynthesis is, on one hand, because they interact to trap light energy and later fuel the electron transfer chain. The latter leads to the generation of ATP and NADPH, both needed crucially for the light-independent reaction—that is, the Calvin cycle, which, in turn, is responsible for the synthesis of organic molecules. Had it not been for the photosystems, solar energy would not have found a proper way of getting harvested.

Photosystems are embedded in the thylakoid membrane in chloroplasts. The thylakoids are flattened sacs, which in turn, form stacks called grana where light-dependent reactions take place. This placement allows photosystems in the thylakoid membranes to absorb light most efficiently and transmit energy therein appropriately.

Photosystem Structure

Photosystems consist of a pigment and several components making up the complex, together specialising in capturing and converting light energy.

Light-Harvesting Complexes: These are simply arrays of pigments and protein molecules able to capture light energy, collecting it into the reaction centre.

Reaction Center: Consists of a cluster of chlorophylls and proteins where the principal photochemical reactions are initiated, such as electron transfer.

Accessory Pigments: Pigments which assist in the capturing of a wider wavelength range of light and protect the photosystem from damage caused by an over-excitation of light.

Photosystems

There exist two types of photosystems concerning their roles and features in the photosynthetic process.

Photosystem I (PSI)

Function and Significance: It is primarily meant for the capturing of light to oxidise plastocyanin and reduce NADP+ through electron transport from plastocyanin to ferredoxin and then to NADP+.

Absorption Spectrum: PSI peaks in absorption at 700 nm, known as P700.

Key Components of PSI: The reaction centre associated with P700 chlorophyll and the light-harvesting complex I plus related proteins.

Photosystem II (PSII)

Function and Significance: PSII is the photosystem that is responsible for initiating light-dependent reactions. It captures photons and utilises this energy to extract electrons from water molecules, producing oxygen as a by-product of the process.

Absorption Spectrum: PSII has a maximum absorption of 680 nanometers. This particular chlorophyll is called P680.

Notable Components: PSII comprises the reaction centre with P680 chlorophyll, light-harvesting complex II, as well as the oxygen-evolving complex.

Mechanism Of Function Of Photosystem II

Photosystem II represents the first complex that can be found in the whole line of light-dependent reactions and is intended to play a significant role in the initial stages of photosynthesis.

• PSII absorbs light energy that excites the electrons in P680 chlorophyll to an energy level higher than that of the ground state.

• Water-splitting complex: this is the complex that breaks down water molecules into electrons, protons, and oxygen.

• Initiation of Electron Transport Chain: The excited electrons from P680 are transferred to the primary electron acceptor and then moved through an electron transport chain for the formation of a proton gradient to carry out ATP synthesis.

Mechanism Of Photosystem I

Photosystem I functions after PSII in the electron transport chain of photosynthesis.

• Light Absorption and Energy Transfer: Energy from light is absorbed by PSI to transfer the electrons of the P700 chlorophyll to a higher state.

• Electron Transport and NADP+ Reduction: Excited electrons move to ferredoxin and then are used in the reduction of NADP+ to produce NADPH.

• Cyclic and Non-Cyclic Photophosphorylation: When the electron returns to PSI, it provides ATP. When the electron moves from PSII to PSI, it results in the production of both ATP and NADPH.

Role Of The Photosystems In Light-Dependent Reactions

These are roles in the light-dependent reaction that help in the process of changing light energy into chemical energy.

• Summary of Light Reactions: Energy derived from light absorbed by photosystems drives the generation of ATP and NADPH.

• Electron Transport in the Z-Scheme: This shows how electrons are carried away from H2O by the photosystems through PS II and PSI to NADP+, establishing the proton gradient that drives the synthesis of ATP.

• Production of NADPH and ATP: The energy that is absorbed by Photosystems I and II lies behind the synthesis of NADPH and ATP, which are the energy carriers involved in the light-independent reactions of the Calvin cycle.

Recommended Video On 'Photosystem Overview'