Question

The oxidation of water requires four electrons, but chlorophyll molecules can transfer only one electron at a time. Describe how these two statements can be reconciled.

Short answer

The oxygen-evolving complex accumulates four electrons from chlorophyll one at a time, then oxidizes water in one step.

Step-by-step solution

Understanding the problem

The oxidation of water involves the transfer of four electrons, while chlorophyll can only transfer one electron at a time. We need to explain how these disparate electron transfer numbers can be reconciled.

Chlorophyll's Electron Transfer

Chlorophyll molecules are capable of transferring one electron at a time. This process occurs during the light reactions of photosynthesis.

Water Oxidation Reaction

The oxidation of water produces oxygen gas, protons, and electrons. The overall reaction can be written as: and results in the transfer of four electrons.

Role of the Water-Splitting Complex

The water-splitting complex, also known as the oxygen-evolving complex (OEC), plays a critical role in this process. OEC accumulates the electrons one at a time from chlorophyll until four electrons are collected.

Combining the Process

Once the OEC has collected four electrons from four consecutive electron transfers by chlorophyll molecules, it can oxidize one molecule of water. This mechanism ensures that even though chlorophyll transfers one electron at a time, the necessary four electrons are eventually gathered to facilitate water oxidation.

Key concepts

chlorophyll electron transfer

Chlorophyll is a crucial molecule in the process of photosynthesis. It captures light energy and uses this energy to transfer electrons. This electron transfer happens one electron at a time. Inside the chlorophyll molecule, when light hits, it excites the electrons to a higher energy level. These high-energy electrons are then transferred to various carriers within the photosynthetic electron transport chain.
Even though chlorophyll can only transfer one electron at a time, this single electron transfer is part of a series of reactions that eventually lead to the production of energy necessary for the synthesis of organic molecules in plants.
The key point is that chlorophyll doesn't work alone; it is part of a larger system that ensures the continual flow of electrons.

water oxidation

Water oxidation is an integral part of photosynthesis and occurs in the thylakoid membrane of chloroplasts. The chemical reaction for water oxidation can be represented as:
2 H₂O → 4 H⁺ + 4 e⁻ + O₂
This reaction shows how water molecules are split to release oxygen, protons, and electrons. This process is essential because it replenishes the electrons that are lost by chlorophyll molecules during the light reactions.
Since chlorophyll can only transfer one electron at a time, but water oxidation requires the transfer of four electrons, there must be a mechanism to collect these electrons efficiently. This is where the oxygen-evolving complex plays a pivotal role.

oxygen-evolving complex

The oxygen-evolving complex (OEC), also known as the water-splitting complex, is a remarkable cluster of proteins and metal ions within photosystem II. It serves as a crucial component that bridges the gap between single electron transfers by chlorophyll molecules and the need for four electrons for water oxidation.
The OEC collects one electron at a time from successive transfers by chlorophyll. Once the OEC has accumulated four electrons, it triggers the oxidation of one water molecule. This leads to the production of oxygen gas, protons, and electrons.
Here is a simplified breakdown of the role of the OEC:

• Step 1: Chlorophyll absorbs light energy.
• Step 2: Excited chlorophyll transfers one high-energy electron to the electron transport chain.
• Step 3: The OEC collects this electron while chlorophyll transfers three more electrons in subsequent light reactions.
• Step 4: With four collected electrons, the OEC oxidizes one water molecule, releasing oxygen gas.

This finely tuned system ensures that photosynthesis can operate efficiently, even with chlorophyll's one electron at a time transfer.