The Calvin Cycle
Part II of Photosynthesis
Calvin
 Named after American
biochemist Melvin Calvin
 Most commonly used pathway by most
plants
 Calvin cycle is used by plants that are called
C3 because of the 3-Carbon molecules that
are made
Photosynthesis
 Light-independent reaction
(Dark Reaction)
 Does not require light
 Calvin Cycle
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Occurs in stroma of chloroplast
Requires CO2
Uses ATP and NADPH as fuel to run
Makes glucose sugar from CO2 and
Hydrogen
Calvin Cycle
 The Calvin cycle uses products from the
light reactions + CO2 to make sugars and
other compounds
 What are the products of the light reactions?
 Where does the CO2 come from?
Calvin Cycle
 CO2 is changed to sugar in a series of chemical
reactions:
 CO2 + RuBP  6-Carbon molecule
 RuBP = 5 Carbon sugar
 The Enzyme that catalyzes this reaction is
rubisco = the most abundant protein in
nature(25%)
Step 1
 CO2 is diffused into the stroma of the
chloroplast
 A 5-Carbon molecule named RuBP combines
to the CO2
 This becomes a 6-Carbon molecule that is very
unstable
 Split to become two 3-Carbon molecules called 3phosphoglycerate (3-PGA)
 6-Carbon Sugar  3-PGA + 3-PGA
Step 2
 3-PGA is still unstable
 3-PGA  glyceraldehyde 3-phosphate (G3P)
 For this to occur, each3-PGA molecule gets a
phosphate from ATP and a proton from
NADPH
 Once the molecule receives the P and proton
it converts into G3P
3-PGA + P + H  G3P
Where did the P and H come
from?
*ATP ADP + P
*NADPH  NADP+ + H
Step 3
 One G3P molecule leaves the Calvin cycle
 This will be used to make a carbohydrate later
Step 4
 Other G3P molecule gets converted BACK to RuBP
due to an addition of another phosphate from ATP
 G3P + P  RuBP
 This RuBP goes back to the Calvin cycle to be fixed
again
Photosynthesis Formula
Light energy
6CO2 + 6H2O  C6H12O6 + 6O2
Redox Reactions
Reduction
Oxidation
 Gain electrons
 Loses electrons
Oxidation Reaction
Loss of electrons from a
substance in a chemical
reaction
What molecules were
oxidized during
photosynthesis?
Reduction Reaction
The gain of electrons by a
substance in a chemical
reaction
Which substances were
reduced during
photosynthesis?
Stoma
This opening how plants exchange gases!
Why are they
stomata located
on the bottom of
leaves?
Stoma
 Stoma Open
 CO2 can
increase
 O2 will
decrease and
leave cells
 Stoma Closed
 CO2 decrease
 O2 increases
C4 Pathway
 Plants that use this are called C4 plants and
have stomata closed during hot part of day
 Enzyme fixes CO2 to a 4-carbon compound
when CO2 is low and O2 is high
 Corn, sugar cane, & crab grass
 Usually tropical climates
CAM Pathway
 Water conserving pathway
 Hot dry climates
 Stoma closed during day & open at night
 Opposite of ordinary plants
 Pineapples & cactuses
CAM Pathway
 During the day
 Stoma are closed
 CO2 is released from compounds and
enters Calvin cycle
 During the night
 Stoma are open
 Take in CO2 and fix into carbon
compounds
PHOTOSYNTHESIS
 What affects photosynthesis?
 Light intensity: as light increases, rate of photosynthesis
increases
PHOTOSYNTHESIS
 What affects photosynthesis?
 Carbon Dioxide: As CO2 increases, rate of photosynthesis
increases
PHOTOSYNTHESIS
 What affects photosynthesis?
 Temperature:
 Temperature Low = Rate of
photosynthesis low
 Temperature Increases = Rate
of photosynthesis increases
 If temperature too hot, rate
drops