The Calvin Cycle
Photosynthesis
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:
RIBULOSE BISPHOSPHATE
CARBOXYLASE/OXYGENASE
(RUBISCO)
• 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 
MOST ABUNDANT
PROTEIN ON EARTH!
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
reactions.

RuBP becomes a ‘reactant’ of the
process.
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
___ NADPH
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.
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
chloroplasts.

◦ Which environmental conditions will cause ‘plants’ to
store this starch?
_______________________________________
______________________________________
◦ Which environmental conditions will cause ‘plants’ to
use G3P immediately? _____________________
◦ ______________________________________.
Possible Paths of G3P
Plants....

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.
Homework/Seatwork
Page 166
# 9, 10, 11
