Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy. This process uses sunlight, carbon dioxide, and water to create the sugars that fuel life. While sunlight provides the energy and carbon dioxide supplies the carbon, water’s role is both chemical and structural. It is an active participant in the energy-converting reactions that sustain most life on Earth.
Water as the Primary Electron Donor
During the light-dependent reactions of photosynthesis, energy from sunlight is captured by chlorophyll within the chloroplasts. This energy powers a process known as photolysis, or “splitting with light.” In a protein complex called Photosystem II, a water molecule (H₂O) is oxidized, meaning it loses electrons.
The splitting of a water molecule releases two electrons, two protons (hydrogen ions), and an oxygen atom. These electrons are important because they replace electrons lost from the chlorophyll in Photosystem II. The newly supplied electrons are then excited by light and passed along an electron transport chain in the thylakoid membrane.
As these high-energy electrons move from one protein to the next, they release energy. This energy is used to create two energy-carrying molecules: ATP and NADPH. These molecules are then used in the Calvin cycle to convert carbon dioxide into sugars. Without water to donate the initial electrons, this energy-capturing sequence would be impossible.
The Release of Oxygen and Protons
When water molecules are split during photolysis, electrons are not the only products. The remaining components, oxygen atoms and protons (H⁺), also have important roles. The oxygen is a byproduct of photosynthesis, and for every two water molecules split, one molecule of diatomic oxygen (O₂) is released into the atmosphere.
Simultaneously, the protons (H⁺) from water are released into the thylakoid lumen, the space inside the thylakoid discs. This action, combined with protons pumped in by the electron transport chain, creates a high concentration of protons. This buildup establishes an electrochemical gradient, similar to water behind a dam, which represents stored energy.
The protons flow back into the stroma through a protein channel called ATP synthase. As protons rush through this channel, they cause it to spin, driving a reaction that generates ATP. This process, known as chemiosmosis, produces the cell’s main energy currency.
How Water Scarcity Halts Photosynthesis
Water’s importance extends beyond its chemical role. Plants absorb water from the soil through their roots and transport it to their leaves. Much of this water is lost to the atmosphere as vapor through pores on the leaf surface called stomata. These same pores are the entry point for the carbon dioxide (CO₂) required for photosynthesis.
During water stress, such as a drought, a plant must conserve water. To do this, it closes its stomata, reducing water loss through transpiration. While this is an effective defense against dehydration, it hinders the plant’s ability to photosynthesize.
With the stomata closed, the plant’s access to atmospheric CO₂ is restricted. Without a supply of CO₂, the Calvin cycle cannot produce sugars. This shutdown creates a feedback loop where unused ATP and NADPH can cause damage, stopping photosynthesis and inhibiting plant growth.