The Calvin Cycle
Also known as the dark
reaction
Carbon Fixation by the Calvin Cycle
 From
the light reaction,
NADPH, ATP and oxygen
was produced.
 Carbon fixation: C is bonded,
or fixed, into an organic
compound
 Calvin cycle has 3 major steps
occurring in the stroma
Step One
 Carbon dioxide diffuses into
the stroma from the
surrounding cytosol
 An
enzyme combines CO2
with RuBP, a 5 carbon
molecule
 The 6 carbon molecules splits
into two 3-C molecules called
PGA
Step Two
 PGA is converted into another 3C molecule called PGAL
 Each PGA gains a phosphate
from a molecule of ATP
 This compound receives a
proton (H+) from NADPH, then
releases the phosphate group.

The resulting NADP+, ADP and
phosphate go back to the light reaction
Step Three
 Most
of the PGAL is
converted back to RuBP by
using up a phosphate from
ATP, making ADP
 So RuBP actually begins
and ends the Calvin Cycle.
Where’d that other Carbon go
 Some
PGAL and other molecules
made in the Calvin cycle are used
to make amino acids, lipids, and
carbohydrates.
 Carbohydrates include glucose,
fructose, sucrose, glycogen,
starch, and cellulose.
How Many?
 PGAL
is a 3-C molecule, and
one turn of the Calvin cycle
fixes one carbon, so it takes 3
turns of the cycle to make 1
PGAL.
 For 3 turns, 9 molecules of
ATP and 6 molecules of
NADPH are used.
Equations
+ H2O + light energy 
(CH2O) + O2
 CO2
 6CO2
+ 6H2O + light energy
C6H12O6 + 6O2
light-independent reactions
in stroma of chloroplast
light-dependent
reactions at
thylakoids of
chloroplast
light
12H2O
LIGHT-DEPENDENT
REACTIONS
ADP + ATP
Pi
6CO2
6O
CO2 into leaf
O2 out
2
NADP NADPH
+
PGA CALVIN- PGAL
BENSON
CYCLE
RuBP
6H2O
P
C6H12O
6
(phosphorylated glucose)
end product (e.g. sucrose, starch, cellulose)
Fig. 6.17, p. 127
Rate of Photosynthesis
 Rate is affected by the
environment
 As light increases, the rate
increases then levels off.
 As temperature increases, the
rate increases, hits a peak, then
declines.
 As CO2 levels increase, the rate
increases then levels off
Conclusion and Dark Reaction Chart
Location – Stroma, fluid in the
chloroplast surrounding the Thylakoid
membranes
 Function – produce carbohydrates,
(glucose)
 Reactants – Carbon Dioxide, RuBP,
ATP and NADPH+
 Products – Glucose, ADP and
NADP