Transcript “categories” of the Calvin Cycle

The Light-Independent Reactions / Calvin
Cycle / Calvin-Benson Cycle / Dark Reaction of
Photosynthesis
Biochem.
Animation
• The ATP and NADPH made in the lightdependent reaction combined with CO2
begins the process of making glucose (and
other “good stuff”)
• Glucose is actually synthesized outside the
thylakoid membrane in the stroma.
• 6CO2 + 12NADPH + 12H+ + 18ATP ----> C6H12O6 + 12NADP +
18ADP + 18Pi + 6H2O
• All of this happens in a circular pathway
called the Calvin cycle
• Biochem. Animation
Main “categories” of the Calvin Cycle (see Fig.
7.15 in text
1. Carboxylation/Carbon fixation
• CO2 is added to ribulose bisphosphate (a 5carbon molecule) by the enzyme ribulose
bisphosphate carboxylase (rubisco) to make
two phosphoglycerates (a 3 carbon molecule)
2. Phosphorylation
• The two phosphoglycerates are
phosphorylated by ATP to make two
biphosphoglycerates
3. Reduction
• The two biphosphoglycerates are reduced by
NADPH to form two glyceraldehyde-3phosphates
4. Regeneration or Yield
• Yield - Some (only 1 out of 6) of the
glyceraldehyde-3-phosphates leave the
chloroplast and enter the cytoplasm to form
glucose (or other pathways leading to starches,
fatty acids, glycerol, or amino acids)
• Regeneration – most (5 out of 6) glyceraldehyde3-phosphates are phosphorylated by ATP to
make the original ribulose bisphosphate (a 5carbon molecule)
6
12
enzyme
6
6
12
Limiting Factors in Photosynthesis
• The factor/step that is lowest to its
minimum and prevents “the system” from
working is called the limiting factor or
rate-limiting step
• Three factors that can determine the rate
of photosynthesis--
Light intensity
• at low light there is a shortage of
products made by the light-dependent
reactions (NADPH and ATP) and
therefore the Calvin cycle is limited
• not the limiting factor very often
CO2 concentration
• at low CO2 concentrations, the fixation
process in the Calvin cycle is limited and
rubisco and NADPH accumulate
• this is often the limiting factor because
normally only .04% of earth’s
atmosphere is CO2
• this can cause photorespiration to occur
where oxygen binds with the enzyme
rubisco instead of CO2
Temperature
• at low temperatures, all of the enzymes in
the Calvin cycle work very slowly and
NADPH accumulates
• at high temperatures, ribulose
bisphosphate carboxylase (rubisco) does
not work effectively and NADPH
accumulates.