Washington State University

Ask Dr. Universe

The Whole World Is Breathing!

January 6th, 2012

Dear Dr. Universe,
Plants give off oxygen and take in carbon dioxide. Do they do the reverse at night?

Alexis Bellido/Flickr

Alexis Bellido/Flickr

That’s right. At night, they breathe in oxygen and breathe out carbon dioxide, a process called respiration. Basically, at night they act like animals, says Gerry Edwards, a botanist here at Washington State University.

But wait a minute! Don’t plants provide us with oxygen and take in carbon dioxide?

Yes, of course. That’s photosynthesis.

So what’s going on here? Well, let’s think this through. Photosynthesis, as you probably know, is the process by which plants absorb light energy from the Sun through their leaves to make food from carbon dioxide that they absorb from the atmosphere.

Act like animals at night? Well, not quite, says Professor Edwards. Of course, plants don’t have the lungs that enable animals to breathe in oxygen and then transport it throughout their bodies via hemoglobin in their blood. Plants just suck it in as best they can.

Actually, oxygen diffuses into the plant tissue through membranes and air spaces around their cells. Plants also exchange, or “inhale” and “exhale,” carbon dioxide, oxygen and water through STOMATA, openings on the “skin” of their leaves that they can close to prevent water loss.

Lungs or not, the process of respiration is basically the same in plants and animals. Sugars break down through a process called GLYCOLYSIS and carbon enters the very tiny mitochondria within the cells, which help convert food to energy.

But photosynthesis MAKES the food in the first place. And how do plants get food to parts other than their leaves? They transport sugar through their veins to parts of the plant that do not photosynthesize, such as roots and seeds, which need energy to grow. Also, if you think about it long enough, you realize that this transfer of energy itself requires energy.

And back to your question, the whole plant also needs energy at night so it can keep on growing. Without sunlight, it has no energy source—except what it has stored during the day in the form of carbohydrates, which can be converted to sugar.

Respiration also does a lot more than just provide energy. Respiration involves at least 50 different steps. Each step in the break-up of sugars results in different compounds. Some of these compounds lead to other compounds used by the plant. Fats, oils and hormones are produced indirectly by respiration. So are compounds such as caffeine and nicotine. And rubber. And amino acids, which are needed for proteins, and nucleotides, which are building blocks for making DNA and RNA.

In spite of the surprising fact that plants produce carbon dioxide by respiration—and in spite of us animals pumping it out constantly—the proportion of carbon dioxide in the atmosphere is actually very small. Seventy-eight percent of the air is nitrogen. Twenty-one percent is oxygen. Carbon dioxide makes up only a fraction of the atmosphere, about .035 percent!

However, even a slight change in that proportion not only can lead to global climate change, but it can affect photosynthesis. More carbon dioxide doesn’t necessarily mean more photosynthesis. Some plants will sense the increase in carbon dioxide and figure they’ve had enough and, through a process called “negative feedback,” slow down their photosynthesis.

Professor Edwards is very interested in how the increase of carbon dioxide in the atmosphere will affect plant growth. Many scientists are worried that some plants may not be able to adjust on their own to the different makeup of the warmer atmosphere caused by increased carbon dioxide and other “greenhouse gases,” at least quickly enough to keep us in food.

So Professor Edwards and others are particularly interested in helping plants, through genetic modifications, adjust. The breakfast cereal you eat 50 years from now could very well be the result of work they are doing right now. But that’s another story. So stay tuned.

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