C1.3.11 Generation of oxygen by the photolysis of water in photosystem II (HL) Notes

Photolysis in Photosystem II Splits Water to Produce Oxygen, Electrons, and Protons

1. Photolysis occurs in photosystem II (PSII), specifically in a protein complex called the oxygen-evolving complex (OEC).
2. This process is part of the light-dependent reactions of photosynthesis and takes place in the thylakoid membranes of chloroplasts.

The reaction can be summarized as:

$$
2H_2O \rightarrow O_2 + 4H^+ + 4e^-
$$

How Photolysis Works

Step 1: Light Absorption

1. Chlorophyll molecules in photosystem II absorb photons of light.
2. This energy excites electrons in a special chlorophyll molecule called P680 (named for the wavelength of light it absorbs—680 nm).
3. The excited electrons are boosted to a higher energy level and leave P680.

Step 2: Electron Replacement

1. When P680 loses electrons, it becomes positively charged and highly reactive.
2. P680 must be "recharged" to continue functioning.
3. The oxygen-evolving complex splits water molecules to provide replacement electrons.
4. These electrons fill the "electron holes" left in P680, restoring it so it can be excited again by light.

Step 3: Release of Products

1. As water is split, three products are generated:
   1. Electrons (e⁻): Replace electrons lost by P680, allowing the light-dependent reactions to continue.
   2. Protons (H⁺): Released into the thylakoid space, contributing to the proton gradient used for ATP synthesis via chemiosmosis (covered in the previous article).
   3. Oxygen (O₂): Oxygen atoms from split water molecules combine to form O₂ gas, which diffuses out of the chloroplast.

The Three Products Are Used Differently

1. Electrons: Used in Photosynthesis
   1. Replace electrons lost by chlorophyll in photosystem II when light excites them.
   2. Flow through the electron transport chain.
   3. Eventually used to produce NADPH (covered in C1.3.13).
2. Protons: Used in ATP Synthesis
   1. Protons are released into the thylakoid space, increasing the H⁺ concentration.
   2. This contributes to the proton gradient across the thylakoid membrane that drives ATP synthesis via chemiosmosis (covered next in C1.3.12).
3. Oxygen: A Waste Product
   1. Oxygen is not used in photosynthesis, it's simply a byproduct.
   2. It diffuses out of the chloroplast and into the atmosphere.
   3. Although oxygen is waste for the plant, it has had profound consequences for life on Earth.

Consequences of Oxygen Generation for Earth

The release of oxygen through photolysis transformed Earth's environment and enabled complex life.

The Great Oxidation Event (~2.4 Billion Years Ago)

1. Cyanobacteria began producing oxygen through photolysis.
2. Oxygen accumulated in the atmosphere, the Great Oxidation Event.
3. Consequences for organisms:
   1. Many anaerobic organisms (for which O₂ is toxic) went extinct.
   2. Aerobic respiration evolved, producing far more ATP than anaerobic processes.
   3. Complex life with high energy demands became possible.

Geological Consequences: Banded Iron Formations

1. Before oxygen accumulation, Earth's oceans contained dissolved iron (Fe²⁺).
2. Oxygen reacted with iron: Fe²⁺ + O₂ → Fe₂O₃ (iron oxide).
3. Iron oxides precipitated and settled on ocean floors, creating banded iron formations, visible in geological records today.
4. The bands reflect cycles of oxygen production over millions of years.

Ozone Layer Formation

1. Oxygen in the upper atmosphere converted to ozone (O₃).
2. The ozone layer absorbed harmful UV radiation.
3. This allowed life to colonize land, where UV exposure had previously been lethal.
4. Enabled diversification of terrestrial ecosystems.