Question

During linear electron flow, the high-energy electron from \(\mathrm{P} 680^{*}\) a. eventually moves to NADP'. b. becomes incorporated in water molecules. c. is pumped into the thylakoid space to drive ATP production. d. provides the energy necessary to split water molecules. e. falls back to the low-energy state in photosystem II.

Short answer

The high-energy electron from \(P680^{*}\) eventually moves to NADP+ (option a).

Step-by-step solution

Understanding the Photosynthesis System

The \(P680^{*}\) is an excited state of chlorophyll, which plays a key role in photosynthesis within Photosystem II by absorbing light energy. It's important to know that the excited electron needs to return to the normal state, and the path it follows is pivotal to the light-dependent reactions of photosynthesis.

Recalling the Linear Electron Flow Path (Z scheme)

In the Z scheme, the high-energy electron from \(P680^{*}\) is transferred through the electron transport chain. It passes through various molecular intermediates like plastoquinone (PQ), cytochrome complex (cyt) and plastocyanin (PC) before reaching Photosystem I (P700). This transition leads to the generation of a proton gradient which drives ATP synthesis (photophosphorylation). The electron then gets re-excited in Photosystem I and eventually goes to NADP+ to reduce it to NADPH.

Matching the Processes with Options

Given our knowledge of Linear Electron Flow, we can now match this understanding with the given options. The electron neither gets incorporated into water molecules (option b), nor is directly pumped into the thylakoid space (option c). It doesn't provide the energy necessary to split water molecules (option d), nor does it fall back to the low-energy state in photosystem II (option e). Thus the conclusion drawn, aligns with the option a; which states that the high-energy electron from \(P680^{*}\) eventually moves to NADP+.

Key concepts

Photosystem II

Photosystem II is where the magic of photosynthesis begins. It's a protein complex found in the thylakoid membranes of plants, algae, and cyanobacteria. This is the first step in the light-dependent reactions where light energy is captured by chlorophyll, creating an excited state known as \(P680^*\). This high-energy state is crucial because it allows the system to split water molecules, releasing oxygen and providing electrons for further processes.

• Located in thylakoid membranes
• First step in capturing light energy
• Splits water molecules, releasing oxygen

The significance of Photosystem II lies in its ability to harness light energy and start the electron flow, setting the stage for the entire photosynthetic process.

Electron Transport Chain

The electron transport chain is a series of protein complexes and molecules that transfer electrons from one to another via redox reactions. After being excited in Photosystem II, electrons are passed along this chain. Key players include plastoquinone (PQ), cytochrome complexes, and plastocyanin (PC).

• Transfers electrons through redox reactions
• Creates a proton gradient essential for ATP synthesis

As electrons move down this chain, they lose energy, which is used to pump protons across the thylakoid membrane. This process creates a proton gradient that will drive ATP production in a later stage, making this chain an essential component in energy conversion.

NADPH Production

NADPH is a crucial molecule in photosynthesis, acting as an electron carrier. During linear electron flow, after electrons leave the electron transport chain, they enter Photosystem I, where they are re-excited. Here, they are ultimately transferred to NADP\(^+\), reducing it to NADPH.

• Produced at the end of linear electron flow
• Serves as a reducing agent in the Calvin Cycle

NADPH is vital for the light-independent Calvin Cycle, where it helps convert carbon dioxide into glucose. This shows how interconnected the light-dependent reactions are with the plant's overall energy needs.

Photophosphorylation

Photophosphorylation is the process of converting light energy into chemical energy in the form of ATP. This occurs in the thylakoid membrane, driven by the proton gradient established by the electron transport chain. As protons flow back into the stroma via ATP synthase, ATP is generated.

• Uses energy from proton gradient
• Occurs in the thylakoid membrane

This process ensures that the energy captured from sunlight is stored as ATP, which can be used in the Calvin Cycle to synthesize glucose. Photophosphorylation ties together the light-dependent reactions and energy production in plants.