Organisms capture and store free
energy for use in biological processes
Calvin Cycle
Where does the Calvin Cycle take
place?
 Stroma of the chloroplast – the fluid filled area outside of the
thylakoid membrane
How does CO2 enter the Calvin Cycle?
 CO2 enters through the stomata – microscopic pores in
leaves
 Once in the leaf the CO2 diffuses into mesophyll cells where
it can enter the chloroplast
 Within the chloroplast carbon fixation takes place
Fig. 10-3a
Leaf cross section
Vein
Mesophyll
Stomata
Chloroplast
CO2
O2
Mesophyll cell
5 µm
What occurs during carbon fixation?
 Carbon dioxide joins a five-carbon molecule called ribulose
bisphophate (RuBP)
 This reactions is catalyzed by RuBP carboxylase, aka Ribisco
 Ribisco – the most abundant enzyme in nature
 This enzyme often takes up 50% of the total chloroplast protein
content
 Ribisco is a slow – only catalyzing 3 molecules of substrate per
second (compared to 1,000 per second)
 Unstable 6 carbon compound is formed which splits to form
2 three carbon molecules of PGA (phosphoglycerate)
How is PGA turned into sugar?
 Each molecule of PGA is systematically reduced by enzyme




action.
NADPH provides the hydrogen atoms and ATP provides the
energy for these reactions to occur. (NADPH and ATP from
Light Reactions)
PGAL (phosphoglyceraldehyde), also called G3P
(glyceraldehyde-3-phosphate) is the final product of the
Calvin Cycle
G3P can be exported to the cytoplasm and combined to
form fructose-6-phosphate and glucose 1-phosphate.
Fructose and glucose can join to form sucrose
How does the Calvin Cycle get back to
5-C RuBP?
 For every 3 molecules of carbon dioxide fixed, 6 molecules
of G3P are formed
 Only 1 of the G3P exits the cycle
 The other five G3P (3C) molecules are used to regenerate 3
molecules of RuPB (5C) using ATP from the Light Reactions
Fig. 10-18-3
Input
3
CO2
(Entering one
at a time)
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
3 P
Ribulose bisphosphate
(RuBP)
P
6
P
3-Phosphoglycerate
P
6
ATP
6 ADP
3 ADP
3
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
ATP
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
Output
P
G3P
(a sugar)
Glucose and
other organic
compounds
Phase 2:
Reduction
Organisms use feedback mechanisms to
maintain their internal environments and
respond to external environmental changes
Alternative Carbon Fixation Mechanisms
Why do plants need alternative
mechanisms for carbon fixation?
 Dehydration is a problem for plants, sometimes requiring
trade-offs with other metabolic processes, especially
photosynthesis
 On hot, dry days, plants close stomata, which conserves H2O
but also limits photosynthesis
 The closing of stomata reduces access to CO2 and causes O2
to build up
 These conditions favor a seemingly wasteful process called
photorespiration
What is photorespiration?
 In most plants (C3 plants), initial fixation of CO2, via
rubisco, forms a three-carbon compound
 In photorespiration, rubisco adds O2 instead of CO2 in
the Calvin cycle
 Photorespiration consumes O2 and organic fuel and releases
CO2 without producing ATP or sugar
How do C4 plants avoid photorespiration?
 C4 plants minimize the cost of photorespiration by incorporating
CO2 into four-carbon compounds in mesophyll cells
 This step requires the enzyme PEP carboxylase
 PEP carboxylase has a higher affinity for CO2 than rubisco does; it
can fix CO2 even when CO2 concentrations are low
 These four-carbon compounds are exported to bundle-sheath
cells, where they release CO2 that is then used in the Calvin cycle
Fig. 10-19
The C4 pathway
C4 leaf anatomy
Photosynthetic
cells of C4
plant leaf
Mesophyll
cell
Mesophyll cell
CO2
PEP carboxylase
Bundlesheath
cell
Oxaloacetate (4C)
Vein
(vascular tissue)
PEP (3C)
ADP
Malate (4C)
Stoma
Bundlesheath
cell
ATP
Pyruvate (3C)
CO2
Calvin
Cycle
Sugar
Vascular
tissue
How do CAM plants avoid photorespiration?
 Some plants, including succulents, use crassulacean acid
metabolism (CAM) to fix carbon
 CAM plants open their stomata at night, incorporating
CO2 into organic acids
 Stomata close during the day, and CO2 is released from
organic acids and used in the Calvin cycle
Fig. 10-20
Sugarcane
Pineapple
C4
CAM
CO2
Mesophyll
cell
Bundlesheath
cell
Organic acid
CO2
1 CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
CO2
Calvin
Cycle
Night
Organic acid
CO2
2 Organic acids
release CO2 to
Calvin cycle
Day
Calvin
Cycle
Sugar
Sugar
(a) Spatial separation of steps
(b) Temporal separation of steps
Review
 The energy entering chloroplasts as sunlight gets stored as
chemical energy in organic compounds
 Sugar made in the chloroplasts supplies chemical energy and
carbon skeletons to synthesize the organic molecules of cells
 Plants store excess sugar as starch in structures such as roots,
tubers, seeds, and fruits
 In addition to food production, photosynthesis produces the
O2 in our atmosphere
Fig. 10-21
H2O
Light
CO2
NADP+
ADP
+ P
i
Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
Electron transport chain
RuBP
ATP
NADPH
3-Phosphoglycerate
Calvin
Cycle
G3P
Starch
(storage)
Chloroplast
O2
Sucrose (export)