Presumably, after plants make sugars from photosynthesis they oxidize them to produce energy just like humans and yeasts, etc., which in the latter organisms produces carbon dioxide as a waste product. Is it just that photosynthesis produces vastly more oxygen, or that much more carbon is fixed in new forms? I realize this is a silly question that betrays my complete ignorance. But thanks for answering.
Photosynthetic organisms produce their constituent material by combining dissociated Water (H[sub]2[/sub]O) and Carbon DiOxide (CO[sub]2[/sub]), and trace elements (taken up in plants by the root system) and, using the power of sunlight, forming CarboHydrates, used for energy and as raw material to manufacture Cellulose, and other components of their cytoplasm.
In the chemical reactions, there is more Oxygen in the reactants than is needed for the products, so Oxygen is given off as a waste gas.
As the organism goes about its life, it metabolizes some of the Carbohydrates formed to extract the stored energy and produce whatever the organism requires. At the end of the metabolic pathway, Carbohydrates are oxidized to Water (H[sub]2[/sub]O) and Carbon DiOxide (CO[sub]2[/sub]), which are respired, just as they would be in a non-photosynthetic organism.
As the plant grows, its mass increases and, overall, more Carbon is incorporated into its tissues than is released in respiration. The net equation is, as you suspected, vastly more Oxygen is released than Carbon.
Thank you for you response. Simply seeing that more oxygen is taken up than needed suits my understanding of the subject. Recalling that cellulose is a sugar or saccharide and therefore carbon-based, and is structural rather than metabolic, helped me put it all together.
ISTR that the process is on a day-long cycle, as well: since light is needed for photosynthesis, most photosynthesis takes place during the daytime. What I don’t know is how much does the amount of CO[sub]2[/sub] released vary throughout the daily cycle, whether it’s pretty much the same at 3am and at 3pm (but at 3pm the plant is releasing a lot more O[sub]2[/sub] than CO[sub]2[/sub]) or it also varies.
To a certain extent, this is true. Photosynthesis and metabolism are completely seperate processes going on at the same time. While photosynthesis—by definition—needs light to proceed, and therefore fluctuates greatly depending on the incident light strength, metabolism is roughly constant throughout the day. Since a lot of the plant’s growth occurs during daylight hours, the rate of metabolism also fluctuates, because growth requires energy.
Obviously, during the night, respiration of water and carbon dioxide are the dominant releases from the plant, even though it may be that there is less than during the day, when active metabolic processes are demanding more energy.
Which begs the question of why the bacterial precursors to plants spent so much energy production so much extra oxygen as pure waste. Although many hypotheses have been advanced asserting oxygen as a protective mechanism against anaerobic bacteria. However, the ecological effects of elevated atmospheric oxygen levels are wide ranging, including creating new mineral strata that may have been a more suitable habitat, or releasing water from the local environment, or any number of alternatives or combinations thereof. The production of oxygen allowed the development of more energetic life that eventually became kingdom Animalia.
Stranger
Note that plant life, while it’s in a steady state, is carbon-neutral. In, say, a mature forest, at any given time you’ll have some trees that are growing and taking in net carbon, and some that are dead and rotting and releasing carbon back into the environment, and the two are in balance. However, planting a new forest where there wasn’t one before will take carbon out of the environment, and cutting down a mature forest will put carbon into the environment.