The Calvin Cycle

The Calvin Cycle
The Big Picture
The Calvin cycle
◦ Occurs in the stroma
◦ Cyclic: ‘final’ product is the ‘initial’ reactant
◦ Details determined by Melvin Calvin, 1960. He
received the Nobel Prize in 1961.
◦ Can be divided into three phases
 Carbon fixation
 Reduction Reactions
 Ribulose 1,5-bisphosphate (RuBP) regeneration
Phase 1: Carbon Fixation
The Calvin cycle is also known
as C3 photosynthesis.
Most plants are C3 plants.
3CO2 + 3 ribulose 1,5-bisphosphate (RuBP)
(5C)  3 unstable 6C intermediates
3 6C splits into 6 ____ molecules: PGA
INITIAL REACTANT(S) __________________
FINAL PRODUCT(S):____________________
ATP USED: ________________________
NADPH USED: ______________________
Reaction catalyzed by the
• enzyme works very slowly
• typical plant enzymes process
1000 molecules/s.
• rubisco: 3 molecules/s.
• rubisco makes up about ½ all
protein in a typical leaf 
Phase 2: Reduction Reactions
Each of the 6 3C PGA molecules
is phosphorylated by an ATP to
form ___ molecules of 1,3-BPG.
 Each 1,3-BPG reduced to
glyceraldehyde 3-phosphate, G3P.
◦ 6 NADPH  6 NADP+ + 6 Pi
1 molecule of G3P exits the cycle
as a final product.
INITIAL REACTANT(S) __________________
FINAL PRODUCT(S):____________________
ATP USED: ____________________________
NADPH USED: ________________________
Phase 3: RuBP Regeneration
5 G3P (___C)  3 RuBP (__ C)
Series of enzyme-catalyzed
RuBP becomes a ‘reactant’ of the
INITIAL REACTANT(S) __________________
FINAL PRODUCT(S):____________________
ATP USED: ____________________________
NADPH USED: ________________________
Why is the Calvin cycle x 3?
3 CO2 used at first.
◦ Three CO2 molecules must be fixed before
one 3C molecule of G3P can be removed.
◦ Why?: need to maintain pool of
intermediate molecules to sustain the cycle.
◦ 6 ‘turns’ of cycle fix enough to produce the
equivalent of 1 glucose.
Overall Reaction:
For the NETsynthesis of ONE
G3P molecule
___ ATP
are used.
These molecules come from:
G3P will become:
Which will be used in:
The Importance of G3P (1)
When is that made, again?
◦ Recall that 6 G3P are formed
during the reduction reactions
of the Calvin cycle (Phase 2)
◦ One of the 6 G3P molecules is
used to make glucose, starch, or
sucrose... used in
◦ G3P can be stored (as one of
the former molecules) or act as
a reactant in the Calvin cycle.
How many ‘turns’ of the Calvin cycle does
it take to produce ONE G3P molecule?
 How many carbons does one G3P
molecule contain?
 How many carbons does one glucose
molecule contain?
 How many G3P molecules are needed to
produce one glucose molecule?
 How many turns of the Calvin cycle are
required to produce one glucose?
The Importance of G3P (II)
Higher plants generally fix more CO2 into sugars
than they require for immediate energy needs.
When conditions are optimal, and photosynthesis
produces more glucose molecules than needed
 glucose is polymerized into amylose and
amylopectin  starch granules within
◦ Which environmental conditions will cause ‘plants’ to
store this starch?
◦ Which environmental conditions will cause ‘plants’ to
use G3P immediately? _____________________
◦ ______________________________________.
Possible Paths of G3P
Possess many nonphotosynthetic tissues
◦ Roots
◦ Some stems
◦ Some flowers
 These tissues need energy to meet their needs.
So how do they function?
◦ Plants have a ‘circulatory system’ just like we do.
◦ Xylem vessels move water and nutrients from the
root to the stoma (via __________________)
◦ Phloem moves glucose (made by the ________)
around the plant.
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