Why Do Plants Need Water in Photosynthesis?

By David Chandler
Oxygen is produced during the light reactions of photosynthesis.
plants image by Dave from Fotolia.com

Photosynthesis is the process by which plants, algae, and some bacteria and protists manufacture food from sunlight and carbon dioxide. A third key ingredient in this process is water. Electrons are stripped from the water molecule breaking the water molecule apart. The protons (hydrogen atoms without the electrons) are added to a proton gradient which is then used to generate the energy molecule ATP. The oxygen atom from a water molecule is combined with another oxygen atom from another water molecule and released as diatomic oxygen (O2).


The process of photosynthesis may be divided into two stages. The first stage of photosynthesis, often called the light reactions since they are dependent on light, generates energy for the later stage of photosynthesis. The second stage of photosynthesis is frequently referred to as the dark reactions and uses the energy harvested in the light reactions to fix carbon dioxide into sugar molecules.


Photosynthesis in plants occurs in specialized structures called chloroplasts. Within the chloroplasts are stacked membranes called thylakoid membranes. The thylakoid membranes enclose small spaces that allow protons to be concentrated on one side of the membrane. These protons are packed into the space through the decomposition of water molecules, and by enzymes that use the electrons released from the photosystem to add to the proton gradient. This proton gradient can then be used to generate chemical energy.


Photosystems embedded in the membranes house pigments that capture energy and energize electrons. As the electrons are energized, they are passed to membrane molecules that create the proton gradient. The protons then flow back across the membrane through an enzyme called ATP synthase which generates ATP, an energy molecule.


The heart of the photosystem is the pigment chlorophyll. While other pigments such as carotenoids and xanthophylls are present in the photosystem, it is chlorophyll that provides the electron that is energized with the light energy. As chlorophyll releases the electron to produce the proton gradient, the electron is replaced by the ion center of the chlorophyll. The ion center of the chlorophyll binds to water and removes the electrons from the water molecule.

Chlorophyll-Light Process

As the light reaches chlorophyll, an electron is released and replaced. Four electrons are energized and released as to form one oxygen molecule from two water molecules. The remaining hydrogen ions (protons) are released into the building proton gradient while the oxygen atoms from the two molecules combine to form diatomic oxygen and released from the photosystem. It is important to note that it is this initial step in photosynthesis where water is consumed and oxygen is released and this process requires the presence of light.

About the Author

David Chandler has been a freelance writer since 2006 whose work has appeared in various print and online publications. A former reconnaissance Marine, he is an active hiker, diver, kayaker, sailor and angler. He has traveled extensively and holds a bachelor's degree from the University of South Florida where he was educated in international studies and microbiology.