MSU Extension

Most people would agree that photosynthesis is a great thing. I’ve never heard anyone argue against it. However, some folks have missed the purpose of photosynthesis. It’s not oxygen production.

The primary function of photosynthesis is to convert solar energy into chemical energy and then store that chemical energy for future use. For the most part, the planet’s living systems are powered by this process. It’s not particularly efficient by human engineering standards, but it does the job. Photosynthesis happens in regions of a cell called chloroplasts. The chemistry and physics are complex.

It’s a bit humbling to consider that the energy in our bodies travels 93 million miles in a little more than eight minutes, and that life has tapped into that energy stream. For a short time that energy is tied up in biological systems before it continues on its merry way into the dark of space.

In essence, green plants take carbon, hydrogen and oxygen from the molecules of carbon dioxide and water, and then recombine them into a new molecule called glucose. This happens in the presence of sunlight, of course. Energy is stored in the bonds of the glucose molecule. Glucose is a fairly simple sugar, easy to break down. Ever wonder why kids bounce off the walls and ceilings soon after a good dose of sugar?

Chemically speaking, the inputs to photosynthesis are six carbon atoms, 12 hydrogen atoms and 18 oxygen atoms. Glucose uses six carbon, 12 hydrogen, and six oxygen molecules. Simple math shows 12 leftover oxygen atoms, or six oxygen molecules. Oxygen atoms prefer mates.

Interestingly, and not coincidentally, the process of respiration breaks apart the glucose molecule. Respiration occurs in the cells of nearly all living things. The released energy is then used for all sorts of metabolic activity, including the energy that you are using to read this article. Respiration happens in regions of a cell called mitochondria. The chemical reactions are the reverse of photosynthesis, using a glucose molecule and six oxygen molecules (12 atoms) as inputs. Energy is released along with some carbon dioxide and water.

But this is enough chemistry.

Trees and other green plants practice respiration, too, just like animals, but they also practice photosynthesis. This is why ecologists categorize green plants as “producers” and most every other life form as a “consumer.” It’s about the energy. OK, there are decomposers, too, but that’s another story and they’re still dependent upon the energy captured by the producers.

Oxygen is a byproduct of photosynthesis and, correspondingly, carbon dioxide the byproduct of respiration. Trees are often credited as the major oxygen generator for the planet, but that would be false. Most of the planet is covered with water and the collective photosynthesis of lowly algae is the true oxygen machine.

Nevertheless, trees and forests are, indeed, significant oxygen producers. However, if oxygen was the only benefit of trees and forests, we could easily live without them. And some forests actually produce more carbon dioxide than oxygen. Fortunately, the benefits of both trees and forests extend far beyond something as narrow as oxygen production.

Much of the basic structural material of plants and wood is cellulose, which is an especially complex sugar. The constituent molecules of carbon, hydrogen and oxygen can be recombined to form lots of useful chemicals such as ethanol, perfumes, bioplastics, clothing fabrics and a range of industrial ingredients. It’s generally agreed that sources from within renewable living ecosystems have distinct advantages over using the ancient materials that make up fossil fuels.

Plants and photosynthesis are the basis of fossil fuels, too, but from millions and millions of years ago. Bringing huge volumes of those molecules back into living ecosystems has a few drawbacks that science has gotten pretty good at measuring and describing.

Trees, forests, forest soils and forest products are mighty important in the cycling of carbon and the relative size of various carbon pools. There are other elements that also cycle through forests. Science has a pretty good handle on these relationships, too. Michigan residents might do well to place a bit more weight on these service benefits of trees, forests, and forest management.

As for photosynthesis itself, maybe it’s better if we think more about the energy capture and less about the oxygen production.