Question

During non-cyclic photophosphorylation, electrons are continuously lost from the reaction centre of PS II. Which source is used to replace these electrons? (a) Sunlight (b) \(\mathrm{O}_{2}\) (c) \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CO}_{2}\)

Short answer

The source used to replace the lost electrons from the reaction center of PS II during non-cyclic photophosphorylation is (c) \(\mathrm{H}_{2}\mathrm{O}\).

Step-by-step solution

Understanding non-cyclic photophosphorylation

Begin by understanding that non-cyclic photophosphorylation is a process where light energy is used to produce ATP and NADPH. During this process, when the reaction center of Photosystem II (PS II) loses electrons, they must be replaced to continue the photosynthetic process.

Identifying the source of replacement electrons

Identify the correct source of replacement electrons. The electrons lost from PS II are replaced by electrons obtained from the splitting of water molecules, a process known as photolysis. Oxygen is released as a byproduct.

Choosing the correct answer

Knowing that water is the molecule that gets split in photolysis to replace the lost electrons, we can ascertain that the correct answer must be the component that is involved in this process. The molecular formula for water is \(\mathrm{H}_{2}\mathrm{O}\), which corresponds to option (c).

Key concepts

Photosystem II

Photosystem II (PS II) is a vital component in the light-dependent reactions of photosynthesis. Located within the thylakoid membranes of chloroplasts, PS II is a complex of proteins and pigments capable of absorbing sunlight. The absorbed light energy excites electrons within PS II, which are then transferred through a series of proteins known as the electron transport chain.

These excited electrons enable the production of ATP and NADPH, which are essential for the light-independent reactions. photolysis is essential for sustaining the electron flow. When electrons leave PS II, they must be replenished to maintain the continuity of the photosynthesis process. Thus, understanding the role of PS II is crucial for grasping the intricate symphony of events that take place during photosynthesis.

Photolysis

Photolysis is the process whereby water molecules (H_2O) are split into their constituent components—hydrogen ions, electrons, and oxygen—using the energy from light. This fascinating phenomenon takes place in the thylakoid membrane, where water molecules are oxidized to replace the electrons lost by PS II.

During photolysis, sunlight strikes PS II and catalyzes the splitting of water molecules, which releases vital replacement electrons, protons, and free oxygen. The oxygen is then expelled as a waste product, which, fortunately for aerobic life on Earth, provides the atmosphere with oxygen. Hydrogen ions, or protons, contribute to creating a proton gradient that is later used to generate ATP.

Photosynthesis

Photosynthesis is the remarkable process through which green plants, algae, and certain bacteria convert light energy into chemical energy, stored in glucose and other organic compounds.

This complex process occurs in two primary stages: the light-dependent reactions and the light-independent reactions, also known as the Calvin Cycle. The light-dependent reactions, comprising of events like the activation of PS II and photolysis, occur within the thylakoid membranes and result in ATP and NADPH, which are then used in the Calvin Cycle to synthesize glucose. By using carbon dioxide (CO_2), ATP, and NADPH, plants are able to produce glucose and other carbohydrates, fostering the rich web of life on our planet.

ATP and NADPH production

The production of ATP and NADPH is a key outcome of the light-dependent reactions of photosynthesis. ATP (adenosine triphosphate) serves as a universal energy currency within cells, while NADPH (nicotinamide adenine dinucleotide phosphate) is a crucial cofactor used in anabolic reactions, such as those that build carbohydrates in the Calvin Cycle.

The generation of these molecules is driven by the electron transport chain and the accompanying proton gradient established across the thylakoid membrane. Electrons from photolysis move through the chain, and as they do, their energy is used to pump protons into the thylakoid lumen, building up a concentration gradient. ATP synthase, an enzyme embedded in the membrane, then utilizes the energy released as protons flow back across the membrane to synthesize ATP from ADP and inorganic phosphate (Pi). Meanwhile, NADP+ is reduced to NADPH by accepting the excited electrons at the end of the transport chain, thus completing this phase of photosynthesis.