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

What Are Photosystems?

Photosystems are protein and pigment complexes found 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 to absorb light most efficiently and transmit energy therein appropriately. These systems are involved in vital light-dependent reactions in photosynthesis. 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 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. The photosystems harvest the solar energy in a proper way.

Structure of Photosystems

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.

Photosystem I (PSI)

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

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+.

Key Details

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

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

Reaction: NADP⁺ + H⁺ + 2e⁻ → NADPH

Photosystem II (PSII)

Photosystem II is the first complex found in the light-dependent reactions.

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.

Key Details

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

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

Reaction: 2H₂O → 4H⁺ + 4e⁻ + O₂

Mechanism of Photosystems in Light-Dependent Reactions

The mechanism of photosystems in light dependent reactions is:

Step 1 – Photon Absorption and Excitation

• Photosystem II is the first complex and plays 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 is the complex that breaks down water molecules into electrons, protons, and oxygen.

Step 2 – Electron Transport Chain (ETC)

• The excited electrons from P680 are transferred to the primary electron acceptor.

• Then moved through an electron transport chain for the formation of a proton gradient to carry out ATP synthesis.

Step 3 – PSI Activation

• Photosystem I functions after PSII in the electron transport chain.

• Energy from light is absorbed by PSI to transfer the electrons of the P700 chlorophyll to a higher state.

Step 4 – ATP & NADPH Formation

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

• 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.

Cyclic vs Non-Cyclic Photophosphorylation

The difference between cyclic and non-cyclic photophosphorylation is given in the table below:

Role of Photosystems in Photosynthesis

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

• Energy Conversion: 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 helps in the synthesis of NADPH and ATP, which are the energy carriers involved in the Calvin cycle.

• Oxygen evolution: It is the source of atmospheric oxygen which is crucial for the sustenance of life in different ecosystems.

Photosystems NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Types of Photosystems (PSI & PSII)

• Mechanism of Photosystem

Practice Questions for NEET

Q1. In photosynthesis, oxygen is produced by

1. Photosystem I from carbon dioxide.

2. Photosystem II from carbon dioxide.

3. Photosystem I from water.

4. Photosystem II from water.

Correct answer: 4) Photosystem II from water.

Explanation:

The splitting of water is associated with PS II. The water splits into H+, [O], and electrons. This produces oxygen, one of the net products of photosynthesis. The water splitting complex is associated with PS II, which itself is physically located on the inner side of the membrane of the thylakoid. The electrons required to replace removed electrons from photosystem I are provided by photosystem II.

2H2O ------ 4H+ + O2 + 4e-

Hence, the correct answer is option 4) photosystem II from water.

Q2. Choose the incorrect statement regarding the photosystem II.

1. It is present in the non-appressed part of the thylakoid.

2. The reaction center of photosystem II is P700.

3. Photosystem II obtains electrons through the photolysis of water.

4. Photosystem II is involved in only non-cyclic photophosphorylation.

Correct answer: 2) The reaction center of photosystem II is P700.

Explanation:

Regarding Photosystem II, the phrase "Reaction center of Photosystem II is P700" is inaccurate. This is untrue since Photosystem II's response center is P680, not P700. Photosystem I's response center is P700. Whereas Photosystem I absorbs light at 700 nm, Photosystem II absorbs light best at 680 nm.

The other claims are true. In contrast to Photosystem I, which is found in the appressed sections of the thylakoid membrane, Photosystem II is found in the non-appressed part, or the area where the membranes are not firmly stacked. The process of photolysis, in which water molecules split to release electrons, protons, and oxygen, is another way that photosystem II gets electrons. Lastly, Photosystem II participates in non-cyclic photophosphorylation, a process that creates ATP and NADPH—two vital components for the plant's energy requirements—by allowing electrons to pass through both Photosystem II and Photosystem I.

Hence, the correct answer is option 2)The reaction center of photosystem II is P700.

Q3. Energy required for ATP synthesis in PS II comes from

1. Electron gradient

2. Reduction of glucose

3. Oxidation of glucose

4. Proton gradient

Correct answer: 4) Proton Gradient

Explanation:

On absorption of the photons, the antenna molecules get excited, and electrons are pushed into the outer orbitals. The excited antenna molecules hand over their energy to core molecules by resonance and come to the ground state. The energy picked up by the core molecules is supplied to the reaction center. On absorption of energy, the photo center gets excited and extrudes an electron after which it comes to the ground state to repeat the cycle. The frequency of excitation of the reaction center is very high. It cannot be met by its direct absorption and it cannot absorb a shorter wavelength. Therefore, the reaction center needs the help of harvesting molecules in the absorption of light energy. The energy required for ATP synthesis in PS II comes from Proton Gradient

Hence, the correct answer is option 4) Proton gradient.

Also Read:

• MCQs on Photosystems

• An Overview of Photosystem

• C4 and CAM Pathway