How do plants and cyanobacteria produce net positive oxygen if they need to also metabolize oxygen for energy production?


As I understand, plants use 6 carbon dioxide molecules to make 6 Oxygen molecules and 1 glucose molecule. However, to metabolize that glucose to use as an energy source requires using up another 6 Oxygen molecules, with no net production of oxygen. How did the Great Oxidation Event happen if oxygen produced by photosynthesis needs to also be burned when metabolizing the glucose?

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Plants and phytoplankton produce sugars from CO2. Some of those sugars, they later burn for energy, but some of them are used to grow the plant or propagate the plankton.

So, the sugars that

For your question, you have to assume that they are producing glucose and consuming glucose for energy at the same rate. They are not. One reason is the normal day/night cycle. Photosynthetic organisms have to produce enough glucose during the day to sustain them through the night when they cannot produce glucose. So, during the day they will net produce oxygen, while at night they will net consume oxygen.

A similar cycle happens across the seasons. During the summer, the direct sunlight allows for more production of glucose. During the winter, the indirect sunlight and shorter daylight hours mean less production. For deciduous tress, that becomes *zero* production. You can see that in [graphs like this one]( showing CO2 levels. The zig-zagging of the line is showing the seasonal change in CO2 because the northern hemisphere contains more trees, so when it’s in the winter season the overall CO2 level of the Earth goes up as those trees fail to consume CO2 and produce oxygen.

Not all of the glucose is consumed for energy. Plant cell walls are made of cellulose, which is made of long chains of glucose stuck together. As the plant grows, it *sequesters* the carbon – locking it into the cellulose, which is a net consumption of CO2 and net release of O2. Similarly, whenever the organism is consumed as food by something else, some of that carbon is going to get incorporated into the structures of the body and not released as CO2.

The structures that they create are net oxygen positive, if you then burn the entire structure like trees or historically speaking oil and coal then you are releasing the carbon dioxide that the plant locked up and use up the oxygen.