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 one electron at a time from chlorophyll until it has four electrons to oxidize water.

Step-by-step solution

Understand the Problem

The problem states that the oxidation of water requires four electrons, while chlorophyll molecules can transfer only one electron at a time. The task is to explain how both can occur simultaneously in the process of photosynthesis.

Recognize the Photosystem II Process

Photosystem II (PSII) in the chloroplasts is where water oxidation takes place. It involves a complex of proteins and pigments, including chlorophyll, which absorbs light energy to drive the reaction.

Identify the Role of Chlorophyll

Chlorophyll molecules in PSII absorb photons of light, which excites electrons to a higher energy state. These excited electrons are then transferred one at a time.

Understand the Oxygen-Evolving Complex (OEC)

The Oxygen-Evolving Complex (OEC) is a part of PSII which accumulates the necessary electrons. The OEC collects four electrons (one at a time from chlorophyll) before it can oxidize two water molecules to produce one mole of oxygen (O₂), four protons (H⁺), and four electrons.

Electron Donation Sequence

The process happens in a sequence where each chlorophyll molecule, upon absorbing light, donates a single electron to the OEC. This continues until four electrons have been transferred.

Final Reconciliation

Although a single chlorophyll molecule can only transfer one electron at a time, multiple transfers take place sequentially. The OEC in PSII is able to hold the intermediate states until it has received all four electrons needed to oxidize water.

Key concepts

Photosystem II

Photosystem II (PSII) is one of the essential components in the photosynthesis process. It is located in the thylakoid membranes of the chloroplasts in plant cells. PSII is the site where the oxidation of water occurs. This system consists of a complex of proteins and pigments, one of which is chlorophyll, that work together to absorb light energy and initiate the reaction.

When light photons hit the PSII, they excite electrons in the chlorophyll molecules. This energy is harnessed to split water molecules (H₂O) into oxygen (O₂), protons (H⁺), and electrons. The light-driven oxidation of water is crucial because it replenishes the supply of electrons for the photosynthesis process and produces oxygen as a byproduct, which is essential for life on Earth.

Chlorophyll

Chlorophyll is the green pigment found in chloroplasts that plays a key role in photosynthesis. It is primarily responsible for absorbing light energy from the sun and converting it into chemical energy. There are different types of chlorophyll, but chlorophyll a and b are the most common.

When chlorophyll absorbs light, its electrons become excited and reach a higher energy state. These high-energy electrons are then transferred through a series of proteins and molecules in the thylakoid membrane. Each chlorophyll molecule in PSII can transfer only one electron at a time, which starts the chain of events leading to the oxidation of water.

Oxygen-Evolving Complex

The Oxygen-Evolving Complex (OEC) is a crucial part of Photosystem II in the photosynthetic process. The OEC is responsible for gathering the four electrons required to oxidize two water molecules into one mole of oxygen (O₂), four protons (H⁺), and four electrons.

This complex works by collecting one electron at a time from the excited chlorophyll molecules. While a single chlorophyll molecule can only transfer one electron at a time, the OEC holds these electrons through intermediate states until all four required electrons are accumulated. Once it has gathered all four electrons, the OEC can proceed with the oxidation of water. This careful management of electrons ensures a continuous flow of the photosynthetic process and the consistent production of oxygen.