Question

Which one is involved in Z-scheme of photosynthesis? (a) PSI (b) PS II (c) e-carriers (d) All of these

Short answer

(d) All of these

Step-by-step solution

Understand the Z-scheme in photosynthesis

The Z-scheme is a model that describes the oxidation/reduction changes during the light reactions of photosynthesis. It involves the flow of electrons through two types of photosystems (PS I and PS II) and a chain of electron carriers.

Identifying the components involved in the Z-scheme

Both photosystems (PS I and PS II) along with the electron carriers are part of the Z-scheme. PS II absorbs light and initiates the chain of electron transport, followed by the electron carriers that transfer the electrons to PS I, which further facilitates the electron transport towards NADP+ reduction.

Determine the correct answer

Given the roles of PSI, PS II, and the electron carriers (e-carriers) in the Z-scheme, it is clear that all these components are involved in the process.

Key concepts

Photosystems in photosynthesis

In the heart of the light-dependent reactions of photosynthesis are two molecular complexes known as photosystems—Photosystem I (PS I) and Photosystem II (PS II). These two complexes play a crucial role in capturing light energy and transforming it into chemical energy. Inside the photosystems, chlorophyll pigments absorb sunlight, with PS II acting first to absorb the light energy at a wavelength of 680 nm (P680). It in turn energizes electrons, which are then passed onto a series of proteins and molecules in the electron transport chain.

Photosystem I, absorbing light at a slightly different wavelength of 700 nm (P700), then re-energizes the electrons that have lost some energy while being transferred through the electron transport chain. This re-energization is necessary for the eventual production of ATP and NADPH, which are crucial molecules for fueling the Calvin cycle, where carbon fixation occurs.

Electron transport chain

The electron transport chain (ETC) is essentially an energy converter within the Z-scheme of photosynthesis. After the initial excitation by PS II, the high-energy electrons are transferred through a series of protein complexes and mobile electron carriers embedded in the thylakoid membrane of chloroplasts.

This ETC consists of a sequence of electron carriers, such as plastoquinone, cytochrome b6f complex, and plastocyanin, that shuttle electrons between PS II and PS I. During this process, protons are pumped across the thylakoid membrane, creating a proton gradient. The subsequent flow of protons back across the membrane through ATP synthase drives the synthesis of ATP. This intricate flow of electrons not only assists in ATP production but also sets the stage for the final steps in the light-dependent reactions—the reduction of NADP+ to NADPH.

Light reactions of photosynthesis

The light reactions, also known as the light-dependent reactions, represent the first stage of photosynthesis. Occurring in the thylakoid membranes of chloroplasts, these reactions harness solar energy to produce ATP and NADPH—molecules that store energy and reducing power needed later on for sugar synthesis during the Calvin cycle.

During the light reactions, water is split into oxygen, protons, and electrons in a process known as photolysis, carried out by PS II. The electrons released from this process are then passed through the electron transport chain, contributing to a proton gradient used for ATP synthesis, and finally re-energized by PS I for NADP+ reduction. The overall process converts the energy of photons into a stable chemical form, ready for the plant's photosynthetic needs.

NADP+ reduction

The reduction of NADP+ to NADPH is the final electron-stealing act in the light reactions of photosynthesis and an essential step in the Z-scheme. After traveling through the ETC, the twice-energized electrons from PS I are ultimately transferred to NADP+, a carrier molecule. The enzyme NADP+ reductase facilitates this transfer, combining the electrons with a proton to form NADPH. This molecule serves as a key player in the Calvin cycle, where it contributes its electrons for the reduction of CO₂, leading to the synthesis of glucose and other carbohydrates.

The production of NADPH, along with ATP generation through the light reactions is collectively referred to as the photosynthetic 'light reactions' output. This process ensures that the 'dark reactions', or the Calvin cycle, have the necessary ingredients to run smoothly and sustain plant growth by creating organic molecules from inorganic carbon dioxide.