What role does oxygen play in photosynthesis?

Most life on Earth depends on photosynthesis.The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O2) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

The process

During photosynthesis, plants take in carbon dioxide (CO2) and water (H2O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts, which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll, which is responsible for giving the plant its green color. During photosynthesis, chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent reactions vs. light-independent reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light-dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH. The light-independent stage, also known as the Calvin Cycle, takes place in the stroma, the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light-independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide.

C3 and C4 photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water.

Photosynthesis is not just about oxygen production it is also about energy production.

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.

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