Photosynthesis is the fundamental biological process by which plants, algae, and certain bacteria convert light energy into chemical energy, primarily in the form of sugars. This conversion uses water and carbon dioxide as raw materials for the organism’s growth and metabolism. A byproduct of this energy transformation is the release of molecular oxygen (\(\text{O}_2\)), which sustains aerobic life across the planet.
The Two Stages of Photosynthesis
The entire process of photosynthesis is organized into two interconnected main stages, each occurring in a different region of the cell’s chloroplast organelle. The initial stage is known as the light-dependent reactions, because it directly requires the energy input from sunlight. These reactions take place within the stacked membrane structures inside the chloroplast called thylakoids.
The light-dependent reactions capture photons and use that energy to create two temporary energy-carrying molecules: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). During this phase, water is consumed and oxygen is released as a gaseous byproduct. The second stage, known as the light-independent reactions or the Calvin Cycle, does not require light directly. It occurs in the fluid-filled space surrounding the thylakoids, called the stroma.
The Calvin Cycle utilizes the chemical energy stored in the ATP and NADPH molecules. This energy powers carbon fixation, where carbon dioxide (\(\text{CO}_2\)) is converted into a three-carbon sugar molecule. The light-dependent reactions are responsible for oxygen production, while the light-independent reactions are responsible for sugar synthesis.
Identifying the Oxygen Source
A common misconception regarding photosynthesis is the belief that the released oxygen originates from the carbon dioxide molecule (\(\text{CO}_2\)). However, scientific evidence confirms that the source of the molecular oxygen is exclusively the water molecule (\(\text{H}_2\text{O}\)) absorbed by the plant. This was proven through experiments utilizing isotopic labeling.
Researchers supplied photosynthetic organisms with water tagged with the heavy isotope \(\text{Oxygen-18}\) (\(\text{^{18}O}\)), rather than the common \(\text{Oxygen-16}\) isotope. When this \(\text{^{18}O}\)-labeled water was used, the molecular oxygen (\(\text{O}_2\)) released by the plant also contained the \(\text{^{18}O}\) isotope. Conversely, when the label was placed on the carbon dioxide molecule, the released oxygen did not contain the \(\text{^{18}O}\) tag.
This established that the oxygen atoms in the carbon dioxide molecule are incorporated into the final sugar product. The oxygen atoms in the water molecule are freed and released into the atmosphere. The splitting of water is the specific reaction that contributes oxygen to the air.
The Mechanism of Water Splitting (Photolysis)
The molecular process for generating oxygen is called photolysis, or water oxidation, and it occurs within a protein complex embedded in the thylakoid membrane known as Photosystem II (PSII). PSII acts as the initial entry point for light energy, containing chlorophyll and other pigments that capture photons. The absorbed light energy is funneled to a specific reaction center, exciting an electron to a higher energy level, which then leaves the complex to travel down the electron transport chain.
To replace the lost, high-energy electron, PSII must find a new source of electrons, which is provided by the water molecule. This electron replacement is handled by a specialized structure within PSII called the Oxygen-Evolving Complex (OEC). The OEC is a cluster of inorganic ions, specifically four manganese (\(\text{Mn}\)) atoms and one calcium (\(\text{Ca}\)) atom, which together form the catalyst for the water-splitting reaction.
The OEC operates through four single-electron oxidation steps, often described as the Kok cycle, where the cluster cycles through five different states labeled \(\text{S}_0\) through \(\text{S}_4\). Each state represents the accumulation of a single positive charge after the absorption of a photon excites an electron away from the reaction center. The manganese ions are suited for this role because they can exist in multiple oxidation states, allowing them to sequentially strip electrons from water.
The complex accumulates four positive charges by absorbing four separate photons, which provides the immense oxidizing power required to break the strong bonds in two water molecules. In the final step, the \(\text{S}_4\) state reacts with the two water molecules, causing the splitting reaction: \(2\text{H}_2\text{O} \to 4\text{H}^+ + 4\text{e}^- + \text{O}_2\).
The four electrons (\(\text{4e}^-\)) are immediately supplied to the reaction center of PSII to replace the lost electrons and continue the electron transport chain. The four protons (\(\text{4H}^+\)) are released into the thylakoid lumen, the space inside the membrane, contributing to a high concentration gradient that is then used to synthesize ATP. The molecular oxygen (\(\text{O}_2\)) is released into the internal air spaces of the plant and diffuses out into the atmosphere.