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 electrons one by one from chlorophyll and oxidizes water when four are collected.

Step-by-step solution

Understand the Problem

The oxidation of water requires four electrons, but chlorophyll molecules can transfer only one electron at a time. This means we need to find a mechanism that allows four electrons to be transferred while accommodating chlorophyll's one-electron capacity.

Concept of Electron Transfer

Chlorophyll molecules use light energy to excite electrons to a higher energy state. These excited electrons are then passed to an electron transport chain, one at a time.

Role of the Oxygen-Evolving Complex (OEC)

The Oxygen-Evolving Complex (OEC) in Photosystem II of chloroplasts plays a crucial role. It accumulates four electrons one at a time from four chlorophyll molecules.

The S-state Cycle

The OEC operates through a cycle of states called S-states (S0 to S4). Each transition from one state to the next involves the transfer of a single electron. When the OEC reaches S4, it releases O2 by oxidizing two water molecules and resetting back to S0.

Summarizing the Mechanism

By enabling a step-wise accumulation of four electrons and then releasing them in one batch to oxidize water, the OEC reconciles the single-electron capacity of chlorophyll with the four-electron requirement of water oxidation.

Key concepts

chlorophyll electron transfer

Chlorophyll is essential for photosynthesis. It absorbs light energy and uses it to drive the transfer of electrons. When light strikes chlorophyll, it excites electrons to a higher energy state. These excited electrons are then passed to an electron transport chain. This process transfers one electron at a time instead of multiple electrons at once.
The one-at-a-time electron transfer effectively harnesses the energy from light. By passing electrons through a series of molecules, chlorophyll ensures that the energy is gradually released.
This careful control prevents energy waste and allows the plants to maximize the energy extracted from light.

oxygen-evolving complex

The Oxygen-Evolving Complex (OEC) in Photosystem II is crucial for water oxidation. It helps resolve the issue of single-electron transfers by chlorophyll. The OEC collects four electrons one at a time from excited chlorophyll molecules.
Here's how it works:

• Located in Photosystem II within chloroplasts
• Receives electrons from chlorophyll
• Handles one electron at a time
• Stores up to four electrons

The OEC ensures that despite chlorophyll's single-electron transfer, the four-electron requirement for water oxidation can be met.
It acts like a storage unit, accumulating enough electrons to facilitate the release of oxygen by splitting water molecules.

S-state cycle

The S-state cycle in the OEC is integral to its function. It operates through a series of states (S0 to S4). Each transition involves transferring a single electron.
Here's a detailed look:

• S0: Initial state
• S1: After accepting the first electron
• S2: After accepting the second electron
• S3: After accepting the third electron
• S4: After accepting the fourth electron, splits water molecules and releases O2
• Resets to S0 after releasing O2

This cycle makes it possible to accumulate several electrons temporarily and then use them all at once.
By reaching S4, the OEC can release oxygen and then reset to repeat the cycle.
This ensures the ongoing process of photosynthesis and oxygen production, complementing chlorophyll's role.