Washington State University

Ask Dr. Universe

Not (quite) by air alone

January 6th, 2012

Dear Dr. Universe,
How do plants get their food?
Catherine
San Diego, California

Sorghum, a C4 plant. Robert Hubner

Sorghum, a C4 plant that's very good at photosynthesis under adverse conditions. Robert Hubner (Read more about C4 plants in in Washington State Magazine)

How do plants get their food? Out of thin air, says Ernest Uribe, a plant physiologist here at Washington State University. A plant physiologist studies how plants work.

All living things, you realize, need energy. Animals get their energy and the materials they need to grow their bodies through the food they eat, which generally includes plants. Since plants have no mouths or digestive systems, how do they get their energy and nutrition?

Plants make themselves out of carbon dioxide from the air and water and a few minerals from the soil. They do this with the aid of sunlight, in a process called PHOTOSYNTHESIS, which means “putting together with light.”

This process was not always obvious to people. In fact, up until about 350 years ago, everybody basically agreed with Aristotle’s idea that plants just sucked their food up out of the soil as a pure “nutrient fluid.”

That’s really not a bad guess. After all, most plants do grow out of the ground. But think about it. Think about the trees in your yard. Every year the leaves fall, and you and your parents rake them up and haul them away. If that tree got its food from the soil, eventually it would suck itself a big hole, right?

This was what Jan Baptista van Helmont finally realized in the 17th century. In one of the first recorded experiments, he weighed a young willow tree, then grew it in a pot for five years. When he weighed it again, he found that it had gained 165 pounds. However, the soil in the pot had lost only a few ounces.

Van Helmont decided that it must be water that led to the tree’s weight gain and growth. Although he was wrong about the water, he was right about the tree’s food not coming from the soil—and he managed to completely change the way we think about plant nutrition.

What other scientists after van Helmont discovered is that plants use the energy of the sun to capture carbon dioxide from the air. Plants use carbon dioxide as a building block to make sugars and other carbohydrates.

How do they do this? Well, here’s where things start getting complicated, says Professor Uribe. And pretty neat.

Pigments in the leaves of plants absorb sunlight. The most important pigment is chlorophyll. Sunlight, as you might know, is actually different “wavelengths” of different colors of light. Chlorophyll absorbs blue and red light. The wavelengths of light that not absorbed are reflected—as green.

The energy absorbed from the sunlight dislodges electrons from the pigment molecules. The electrons then organize within the leaf cells into tiny electric currents. THIS is the energy that powers a series of very complicated chemical reactions. The first thing that happens is the plant splits water molecules into hydrogen and oxygen. Then it transfers hydrogen and electrons to carbon dioxide molecules.

This results in two things that are very important to us. First is the release of oxygen into the atmosphere. Second, sugar (glucose) forms from the combination of carbon dioxide, hydrogen and electrons. Here’s how chemists describe this sugar: C6H12O6. In other words, the glucose molecule is made up of 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen.

The glucose that the plant does not use immediately for food is used to make other kinds of storage carbohydrates and CELLULOSE fibers for plant structure.

However, even plants can’t live on air alone. Even though they do not suck nutrient fluid out of the soil, they do need some nutrients contained in soil. The main one is nitrogen. Nitrogen is necessary for making protein and nucleic acids. Nucleic acids are the main ingredient of DNA, the material that holds genetic information in every cell.

Plants also need phosphorus, potassium, sulfur, calcium, iron and magnesium, and a list of “micronutrients”: molybdenum, copper, zinc, manganese, boron, chlorine and nickel. And probably others in amounts too small for use to detect.

But mainly, plants get their food from the air, which is a lot more than the nothing it seems!

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