Cyclic Electron Flow
Only involves photosystem I
A photon of light excites an electron from photosystem I
The electron is passed to ferredoxin (Fd), which passes the
electrons back through the Q cycle, the b6-f cytochrome
complex, and back to chlorophyll P700.
This cyclic pathway generates a proton gradient for
chemiosmotic ATP synthesis
Note: This pathway
does not release electrons
to generate NADPH.
Without NADPH, the
reactions of carbon
fixation cannot occur
Calvin Cycle
What is the function of cyclic electron flow?
Noncyclic electron flow produces ATP and NADPH in roughly equal
quantities, but the Calvin cycle consumes more ATP than NADPH.
Cyclic electron flow makes up the difference (more ATP).
The concentration of NADPH in the chloroplast may help regulate
which pathway --cyclic versus noncyclic-- electrons take through the
light reactions. If the chloroplast runs low on ATP for the Calvin
cycle, NADPH will begin to accumulate as the Calvin cycle slows down.
The rise in NADPH may stimulate a temporary shift from noncyclic to
cyclic electron flow until ATP supply catches up with demand.
Phase 1: Carbon Fixation
CO
adds to a 5-carbon molecule, ribulose 1,5-bisphosphate, RuBP
2
This forms an unstable 6 carbon intermediate, which instantly splits into two 3
carbon molecules called 3-phosphoglycerate (PGA).
The name C3 Photosynthesis is given to the Calvin cycle because of PGA (3Carbon)
This is catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase
(RUBISCO)
Rubisco is a slow acting enzyme. It processes three substrates per second. SO,
many copies of Rubisco are required to complete all the reactions
This reaction is exergonic, RuBP has a higher potential chemical energy than PGA,
so when PGA is created energy is released.
Three CO2 molecules are
reacted, how many PGA
are created?
1
Phase 2: Reduction Reactions
Phase 3: RuBP Regeneration
The remaining 5 molecules of G3P (5x3C=15C) are rearranged to regenerate three
molecules of RuBP (3x5C=15C)
This requires 3 molecules of ATP
Producing more RuBP allows more Carbon dioxide to be fixed.
G3P molecules that leave are used to synthesize larger sugar molecules such as
glucose and other carbohydrates
3 carbon dioxide molecules are required to
produce 1 G3P
6 Turns of the cycle are required to fix enough
CO2 to make one 6-carbon glucose.
Each of the 6 molecules of PGA is
phosphorylated by an ATP to form 6
molecules of 1,3 bisphosphoglycerate
(1,3-BPG)
NADPH is then reduced to NADP+
and Pi releasing two electrons
6 NADPH molecules give off(are
oxidized)two electrons used to
reduce 6 molecules of 1,3-BPG
to give 6 molecules of glyceraldehyde
3-phosphate (G3P or PGAL), a sugar
3 CO2 + 6 NADPH + 5 H2O + 9 ATP
→ glyceraldehyde-3-phosphate (G3P) + 2 H+ +
6 NADP+ + 9 ADP + 8 Pi
G3P EXITS the Calvin Cycle as a
final product!
Video
G3P may be converted to Glucose in either the Stroma or the Cytoplasm of the
plant cell.
When there is more Glucose produced than needed it is stored as Starch in the
chloroplasts. (approx 1/3 of the glucose produced from G3P)
Starch can be converted back into glucose when environmental conditions do
not promote photosynthesis such as in a time of low light.
Glucose
2