Question

680 Refer the given reaction. $$ 2 \mathrm{H}_{2} \mathrm{O} \rightarrow 4 \mathrm{H}^{+}+\mathrm{O}_{2}+4 \mathrm{e}^{-} $$ Where does this reaction take place in the chloroplasts of plants? (a) Outer surface of thylakoid membrane (b) Inner surface of thylakoid membrane (c) In the matrix (stroma) (d) Intermembrane space

Short answer

(b) Inner surface of thylakoid membrane

Step-by-step solution

Identify the Process and Location

First, identify that the given reaction is the process of photolysis of water, which is a part of the light-dependent reactions of photosynthesis. During these reactions, water is split into oxygen, protons, and electrons. This photolysis occurs in the chloroplasts of plants.

Determine the Specific Location of Photolysis

Knowing that photolysis is part of the light-dependent reactions, we locate these reactions within the thylakoid membranes of the chloroplasts. Water is specifically split at the inner surface of thylakoid membranes, where the photosystem II complex is situated, to facilitate the light-dependent reactions.

Select the Correct Answer

Based on the understanding of where photolysis occurs within the chloroplasts, we can conclude that the correct answer is: (b) Inner surface of thylakoid membrane.

Key concepts

Light-Dependent Reactions

In the grand scheme of photosynthesis, light-dependent reactions serve as the initial stage. These reactions are dependent on sunlight and take place within the specialized membranes found inside chloroplasts, known as thylakoid membranes. During these reactions, light energy is captured by pigments like chlorophyll and converted into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

The process of photolysis, which is the splitting of water molecules into hydrogen ions, oxygen, and electrons, is a crucial part of this stage. These products play essential roles in photosynthesis, with electrons being used to generate ATP and NADPH, and oxygen being released as a byproduct into the atmosphere. The hydrogen ions contribute to the creation of a proton gradient that powers ATP synthase to synthesize ATP.

Chloroplast Structure

The chloroplasts are cellular powerhouses found in the cells of green plants and algae, where photosynthesis predominantly occurs. They have a unique structure composed of an external envelope and a lumen filled with stroma, a semi-fluid substance. Within the stroma are disc-shaped structures called thylakoids, stacked into columns known as grana.

Key Components of Chloroplasts:

• Outer membrane: A semi-permeable membrane that interfaces with the cell's cytosol.
• Inner membrane: Regulates the entry and exit of substances.
• Stroma: Contains enzymes essential for carbon fixation during the Calvin cycle.
• Thylakoids: Site of light-dependent reactions; contain pigments like chlorophyll.
• Grana: Stacks of thylakoids, enhancing the chloroplast's ability to capture light.

The compartmentalization within the chloroplast ensures maximum efficiency of photosynthesis, with light-dependent reactions and light-independent reactions taking place in distinct areas.

Photosystem II

Photosystem II (PSII) is a vital protein complex within the light-dependent reactions of photosynthesis. Located in the thylakoid membrane, it plays a key role in absorbing light and initiating the process of converting light energy into chemical energy.

PSII is specifically responsible for the photolysis of water molecules. It uses the energy from absorbed photons to extract electrons from water, releasing oxygen as a byproduct and contributing to the electron transport chain that generates ATP and NADPH. PSII contains a special pair of chlorophyll molecules, known as P680, due to their peak absorption at 680 nm. It is this pigment that primarily traps the solar energy and drives the electron transport. The efficient operation of PSII is critical for the sustenance of life on Earth, as it not only fuels the production of organic molecules but also maintains the planet's oxygen levels.