Photosynthesis - Molecular production part 2

Oxygen and photosynthesis

With respect to oxygen and photosynthesis, there are two important concepts.

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      Plant and algal cells also use oxygen for cellular respiration, although they have a net output of oxygen since much more is produced during photosynthesis.

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      Oxygen is a product of the photolysis reaction not the fixation of carbon dioxide, during the light-independent reactions. Consequently, the source of oxygen during photosynthesis is water, not carbon dioxide.


Bacterial variations

The concept that oxygen production is not directly associated with the fixation of carbon dioxide was first proposed by Cornelis Bernadus van Neil in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including sulfide or hydrogen, so for most of these bacteria oxygen is not produced.

The 'Z-scheme' of electron flow in light-dependent reactions.
The 'Z-scheme' of electron flow in light-dependent reactions.
 

Others, such as the halophiles (an Archeae) produced so called purple membranes where the bacteriorhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmotic theory: light hit the membranes and the pH of the solution that contained the purple membranes dropped as protons were pumping out of the membrane.


Carbon fixation

The fixation of carbon dioxide is a light-independent process in which carbon dioxide combines with a five-carbon sugar, ribulose bisphosphate (RuBP), to give two molecules of a three-carbon compound, glycerate 3-phosphate (GP). This compound is also sometimes known as 3-phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from the light-dependent stages, is reduced to glyceraldehyde 3-phosphate (G3P). This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or even as triose phosphate (a three-carbon sugar). This is the point at which carbohydrates are produced during photosynthesis. Some of the triose phosphates condense to form hexose phosphates, sucrose, starch and cellulose or are converted to acetylcoenzyme A to make amino acids and lipids. Others go on to regenerate RuBP so the process can continue (see Calvin cycle).



source : hydroponicarticle