Why Do Plants Need Water in Photosynthesis?

Photosynthesis is a wondrous and yet simple chemical reaction that occurs when plants use sunlight, water and carbon dioxide to make energy-packed food molecules. Plants pull water from their roots and absorb molecules of atmospheric carbon dioxide to gather the necessary ingredients for synthesizing glucose (sugar).

Water (H2O) molecules split and donate electrons to carbon dioxide molecules as light energy from the sun is converted into the chemical bonds of glucose (sugar) during photosynthesis.

Photosynthesis Equation

The recipe for glucose is six molecules of water (H2O) plus six molecules of carbon dioxide (CO2) plus exposure to sunlight. Photons in light waves initiate a chemical reaction in the cell that breaks the bonds of water and carbon dioxide molecules and reorganizes these reactants into glucose and oxygen – a by-product.

The formula for photosynthesis is commonly expressed as an equation:

6H2O + 6CO2 + sunlight → C6H12O6 + 6O2

Early Origins of Photosynthesis

Nearly 3.5 billion years ago, cyanobacteria changed the course of the world with their photosynthetic power to convert light energy and inorganic substances into chemical energy for food. According to Quanta Magazine, archaic micro-organisms created the planetary conditions that gave rise to a cascade of diverse plants with a shared ability to photosynthesize and release oxygen.

Although the details are still being studied and debated, adaptation of photosynthetic centers in early life forms such as unicellular plants and algae appears to have jump-started evolution.

Why Is Photosynthesis Important?

Photosynthesis is essential for life and sustainability in a balanced ecosystem. Photosynthetic organisms are at the bottom of the food web, meaning they directly or indirectly produce food energy for herbivores, omnivores, secondary and tertiary consumers, and apex predators. When water molecules split during the photosynthetic reaction, oxygen molecules are formed and released into the water and air.

Without oxygen, life would not exist as it does today.

Further, photosynthesis plays a vital role in sinking carbon dioxide. The process of converting carbon dioxide to carbohydrates is called carbon fixation. When carbon-based living organisms die, their buried remains can become compressed, and over time, turn to fossil fuel.

Water Requirements of Plants

Water helps transport food and nutrients within cells and between tissues to provide nourishment to all parts of a living plant. Large vacuoles within cells contain water that strengthens the stem, fortifies the cell wall and facilitates osmosis in leaves.

Undifferentiated cells in the meristem could not properly specialize into leaves, blooms or stems if cells in the tissue were badly dehydrated. Stems and leaves droop when water needs are unmet, and photosynthesis slows.

Plants and Water: Related Science Projects

Students interested in learning more about plants and water requirements may enjoy experimenting with sprouted bean seeds. Lima beans and pole beans grow quickly, which makes them well-suited for a feeding plants science project or classroom demonstration. Teachers can plant the seeds about a week before students start experimenting to determine which environmental factors, such as adequate water, influence plant growth.

For instance, a science class could continue growing, watering and measuring five or more bean sprouts next to a window for two weeks or longer. For purposes of comparison, they could introduce variables in experimental groups of sprouts and develop a hypothesis. Experimental groups of five plants or more are recommended for a bigger sample size.

For example:

  • Experimental group 1: Withhold water to see how soon bean sprout growth is impacted by dehydration.
  • Experimental group 2: Place a paper bag over the bean sprouts to observe how low light can affect photosynthesis and chlorophyll production. 
  • Experiment group 3: Wrap plastic sandwich bags around bean sprouts to study effects of disrupted exchange of gases.
  • Experimental group 4: Place bean sprouts in a refrigerator each night to see how colder temperatures may affect growth.


About the Author

Dr. Mary Dowd studied biology in college where she worked as a lab assistant and tutored grateful students who didn't share her love of science. Her work history includes working as a naturalist in Minnesota and Wisconsin and presenting interactive science programs to groups of all ages. She enjoys writing online articles sharing information about science and education. Currently, Dr. Dowd is a dean of students at a mid-sized university.