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How Do Plants Use Water in Photosynthesis?

By Richard Hoyt ; Updated September 21, 2017


Plants use energy from sunlight to convert air and water into sugars that they need to grow. This chemical process is called photosynthesis. Plants don’t require soil to produce food. They can grow using only light and water, which is how they are grown hydroponically. Water is H-O-H (H2), a molecule that has two atoms of hydrogen and one atom of hydrogen. An electron binds these atoms together. Carbon dioxide is C-O-O (CO2), a molecule that contains two atoms of oxygen and one atom of carbon. The electron that binds the hydrogen and oxygen together in water is the key to photosynthesis. It binds the hydrogen from the water to the carbon atom of carbon dioxide to form sugars which the plant needs to grow. A biology textbook would write this as 6CO2 + 6H2O = C6H12O6 + 6O2. This formula means that six molecules of carbon dioxide, obtained from the air, plus six molecules of water equal sugar necessary for plant growth plus six molecules of oxygen that are released into the air as waste.


The conversion of water and carbon dioxide into sugar nutrients requires energy. The plant gets this energy from light. It can use sunlight or light produced artificially by grow lights. The chlorophyll in the leaves of plants absorbs photons from light and turns them into high-energy molecules needed to combine the hydrogen from water and the carbon from carbon dioxide. A photon is a measure of electromagnetic energy that has no electric charge, no mass and an indefinite lifetime. Synthesis means to “put something together.” The word "photosynthesis" literally means "putting together from light."

Getting the Water

A high concentration of minerals in the plant roots form a pressure that pulls water from the soil and pushes it up into the plant. The water travels through a series of vessels to the leaves, where it is used for photosynthesis. These vessels are called the xylum. The leaves also have veins that carry water. Dew on the grass in the morning is actually sap containing water that was pulled up during the night. This water is lost during the day during transpiration, a form of plant breathing. A plant only uses 1 percent of the water it takes in for photosynthesis. The rest is lost in transpiration in plant leaves and blades.