Problem 162 - Objective Biology for NEET

Question

Assertion: Photolysis of water occur on inner side of the membrane of thylakoid. Reason: Water splitting complex is associated with PSII which is physically located on inner side of thylakoid Membrane.

Step-by-Step Solution

Answer

Both the assertion and reason are correct and the reason correctly explains the assertion.

Step 1: Understanding the terms

Understanding the meanings and relationships of 'Photolysis of water', 'Thylakoid membrane', 'Water splitting complex' and 'PSII'. Photolysis of water is the process where water molecules break down into hydrogen and oxygen in the presence of light. The Thylakoid membrane is a membrane within chloroplasts, the location where light-dependent reactions of photosynthesis occur. The water-splitting complex, also known as the oxygen-evolving complex (OEC), is a part of Photosystem II (PSII), a protein complex involved in the process of photosynthesis. Water-splitting complex is the site where photolysis happens.

Step 2: Verifying the assertion and reason

The assertion states 'Photolysis of water occur on inner side of the membrane of thylakoid'. This is correct as photolysis of water does indeed occur at the water-splitting complex which is located at the inner side of the thylakoid membrane, as is involved in the photosynthesis process, especially in the light-dependent reactions. The reason given is 'Water splitting complex is associated with PSII which is physically located on inner side of thylakoid Membrane'. This is also correct. The Water splitting complex, which is where photolysis happens, is part of PSII and is located on the inner side of the thylakoid membrane.

Step 3: Relating the assertion and reason

The reason 'Water splitting complex is associated with PSII which is physically located on inner side of thylakoid Membrane' correctly explains the assertion 'Photolysis of water occur on inner side of the membrane of thylakoid' as this association is why the photolysis happens at this location.

Key Concepts

Photolysis of Water

Photolysis of water is a vital reaction in the process of photosynthesis. It is the mechanism where water molecules (\[ H_2O \] break down into oxygen (\[ O_2 \] and hydrogen ions (\[ H^+ \] and electrons under the influence of light energy.
This process is essential because it generates the oxygen that plants release into the atmosphere, which is crucial for life on Earth.
Moreover, the electrons liberated during photolysis are used in the photosynthesis process to further produce energy-rich compounds.

• The splitting of water requires light energy, which is why it primarily occurs during the light-dependent reactions of photosynthesis.
• The reaction not only provides electrons but also contributes to the formation of a proton gradient required for ATP synthesis.

Understanding where and how photolysis occurs gives insight into its critical role in sustaining ecosystems.

Thylakoid Membrane

The thylakoid membrane is a fascinating and crucial part of a chloroplast, which are the organelles in plant cells responsible for photosynthesis. This membrane forms a series of stacked, disc-shaped structures known as thylakoids, which are rich in pigments, including chlorophyll.
The thylakoid membrane is the venue for the light-dependent reactions of photosynthesis.
It plays a dual role in capturing light energy and using that energy to convert water and carbon dioxide into glucose and oxygen.

• The interior of the thylakoid membrane, known as the thylakoid lumen, is where the proton gradient builds up, crucial for ATP synthesis.
• This membrane contains essential protein complexes like Photosystem II and Photosystem I, where vital processes like photolysis take place.

The structural setup of the thylakoid membrane is key to maximizing its efficiency in capturing light and facilitating photosynthesis events.

Photosystem II (PSII)

Photosystem II (PSII) plays a central role in the light-dependent reactions of photosynthesis. It is a large protein complex located in the thylakoid membrane of chloroplasts. PSII is remarkable for being the site where photolysis of water occurs. Its primary function is to capture light energy and use it to extract electrons from water molecules.
These electrons then pass through a series of proteins embedded in the thylakoid membrane, ultimately aiding in the production of ATP and NADPH.

Key Features of PSII

• Light Harvesting: PSII contains chlorophyll pigments that absorb light, initiating the processes of photosynthesis.
• Water Splitting: The water-splitting complex, also known as the oxygen-evolving complex, within PSII is responsible for splitting water molecules to release electrons, protons, and oxygen.

Understanding PSII not only explains the origins of molecular oxygen but also illustrates how plants convert light energy into chemical energy.

Light-dependent Reactions

Light-dependent reactions are the first phase of photosynthesis, taking place within the thylakoid membranes of chloroplasts. These reactions require sunlight to proceed, as they harness light energy to drive several critical processes. They predominantly culminate in the generation of ATP and NADPH, essential energy carriers used in the later stages of photosynthesis.

• Photophosphorylation: This process involves the conversion of ADP to ATP using the energy derived from light.
• Reduction of NADP⁺: These reactions also reduce NADP⁺ to NADPH, which is used in the Calvin cycle to synthesize glucose.
• Electron Transport Chain: The transfer of electrons through a series of proteins integrated into the thylakoid membrane leads to the production of ATP and NADPH.

These reactions underpin the energy supply in plants, setting the stage for the light-independent reactions where glucose is synthesized.