Download Photosynthesis: dark reactions

5/27/08
Photosynthesis: Calvin Cycle and other things
animation of electron transport in thylakoid membrane
Dark Reactions:
Synthesis of glucose from CO2
The fixation and reduction of CO2 is endergonic
and coupled to these exergonic reactions:
• Hydrolysis of ATP to ADP and Pi
• Oxidation of NADH to NAD+
1
2
Fixation of Carbon Dioxide
Rubisco = ribulose-1,5-bisphosphate carboxylase:
enzyme catalyzing fixation of CO2
• CO2 fixation means addition to an organic
molecule
• abundant protein comprising more than 15% of the
total chloroplast protein
• Rubisco maybe the most abundant protein on earth
3
Stage 1
• Adding CO2 to a five carbon compound RuBP (ribulosebisphosphate)
• this results in an unstable 6-C compound that splits into 2 3-C
compounds (2 3-phosphoglycerates)
4
Stage 2
• Reduction of 3-phosphoglycerate to glyceraldehyde 3phosphate
• this is reduction of a carboxyl to an aldehyde
Stage 3
• Regeneration of RuBP (ribulose-bisphosphate) from triose
phosphates
Good animation of the calvin cycle -- for the nonchemists
http://www.science.smith.edu/departments/Biology/Bio111/calvin.html
5
For the biochemists
6
What happens to the products of photosynthesis
(“photosynthate”) ?
• much of the photosynthate is used as fuel for cellular
respiration
• some 3PGA (phosphoglyceric acid -- product of first
step in Calvin Cycle) is transported into the cytosol
and used to make amino acids
• G-3-P (glyceraldehyde 3-P) is used to make fructose
with is in turn used to make other sugars and starch
• some fructose is converted into glucose; molecular of
glucose are smaller and store more energy than ATP
• fructose and glucose are used to make sucrose which
is shipped throughout the plant
• much of the photosynthate is used to make cellulose
which is the most abundant organic compound on
earth
• some of the photosynthate is used to make secondary
metabolites such as latex
7