MadSci Network: Botany

Re: What type of sugar does green algae produce as a product of photosynthesis?

Date: Mon Mar 26 11:56:11 2001
Posted By: Mark Schneegurt, Assistant Professor, Biological Sciences, Wichita State University
Area of science: Botany
ID: 975331519.Bt

Green algae perform photosynthesis in the same way that higher plants do, 
and have a similar set of dark reactions, called the Calvin-Benson Cycle.  
The carboxylation phase of the Calvin cycle is where carbon dioxide is 
"fixed", being converted from the inorganic form to an organic form by 
combination with ribulose 1,5-bisphosphate to form two molecules of 3-
phosphoglycerate.  The reduction phase of the Calvin cycle then produces 
glyceraldehyde 3-phosphate (GAP), by reducing the organic acid to an 
aldehyde.  It is here that the regeneration phase starts, and it is here 
that your question arises.

In actuality, glucose is not part of the Calvin cycle.  Glucose can be made 
through gluconeogenesis from a host of precursors, including pyruvate, 3-
phosphoglycerate, GAP, and fructose 6-phosphate.  We've seen that 3-
phosphoglycerate and GAP are products of the Calvin cycle.  GAP can also be 
converted into dihydroxyacetone phosphate (DHAP).  When GAP and DHAP 
condense, the product is the sugar fructose 1,6-biphosphate.  This can be 
converted into glucose.  It can also be converted into fructose 6-phosphate, 
which can also be converted into glucose.  

Fructose 6-phosphate is central to the regeneration phase of the Calvin 
cycle.  What is being regenerated?  Ribulose 1,5-bisphosphate, the starting 
material of the carboxylation phase of the Calvin cycle.  There are a number 
of sugar conversion involved in converting fructose 6-phosphate to ribulose 
1,5-bisphosphate.  GAP can condense with fructose 6-phosphate to produce 
xylulose 5-phosphate and erythrose 4-phosphate.  DHAP then condenses with 
the erythrose to produce sedoheptulose, which then condenses with another 
molecule of GAP to make xylulose and ribose.  Both the these later compounds 
can be converted to ribulose.  (I've left off some of the phosphorylation 
steps here).  

So, the simple answer to your question is that glucose is not directly 
involved in the Calvin cycle or carbon fixation.  PGAL is what I am calling 
GAP here.  GAP contributes to the formation of fructose, erythrose, 
xylulose, sedoheptulose, and ribose.  Note however that these processes do 
not occur in isolation.  Just as the intermediates of the Krebs cycle are 
used in biosynthetic reactions, giving rise to amino acids and nucleic acids 
and not just an energy-producing cycle, so too the intermediates in the 
Calvin cycle are integrated into biosynthetic pathways.  Could the GAP 
generated in the Calvin cycle contribute to glycolysis?  Yes.  Could the GAP 
generated in the Calvin cycle contribute to gluconeogenesis?  Yes.  Could 
the carbon fixed into GAP via the Calvin cycle eventually be respired 
through the Krebs cycle and re-released as carbon dioxide?  Yes, it happens 
all the time.  Metabolism is an integrated whole.  Pathways only help our 
understanding of these complex systems.  They are artificial constructs and 
do not adequately express the true nature of living organisms.

Hope this helps.


Dr. Mark Schneegurt

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