Photosynthesis:
Variations on the Theme
AP Biology
Remember what plants need…
 Photosynthesis

light reactions
 light  sun
 H2O  ground

Calvin cycle
 CO2  air
What structures have
plants evolved to
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these needs?
O
C
O
vascular bundle
xylem (water)
Leaf Structure
phloem (sugar)
cuticle
epidermis
palisades
layer
spongy
layer
O2 H O
2
CO2
stomate
Transpiration
Gas exchange
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O2
H 2O
CO2
guard
cell
Controlling water loss from leaves
 Hot or dry days
stomates close to conserve water
 guard cells

 excess H2O = stomates open
 conserve H2O = stomates close

adaptation to
living on land,
but…
creates PROBLEMS!
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When stomata close…
 Closed stomata lead to…


O2 build up inside the plant  from light reactions
CO2 is depleted /can’t enter for the Calvin cycle
 causes problems in Calvin Cycle
xylem
(water)
phloem
(sugars)
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O2
CO2
H2O
RuBisCo
Carbon Fixation enzyme in the Calvin Cycle
Fixes Carbon Dioxide onto RuBP
• When the concentration of oxygen is high/ CO2
is low, RuBisCo will fix Oxygen onto RuBP in a
process called photorespiration.
•The products of fixed O2 are not particularly
healthy for the plant, and most assuredly don’t
lead to the formation of glucose, like the
fixation of CO2 does.
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Calvin cycle when CO2 is abundant
1C
RuBP
ATP
CO2
5C RuBisCo
ADP
6C
unstable
intermediate
5C
NADPH
NADP
3C
C3 plants
ATP
Leave
to make
glucose
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3C
PGA
2 PGAL
3C
ADP
Calvin cycle when O2 is high
O2
RuBP
5C
RuBisCo
2C
3C
1 PGA
photorespiration
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to
mitochondria
–––––––
without
making ATP
Impact of Photorespiration
 Oxidation of RuBP
short circuit of Calvin cycle
 reduces production of photosynthesis

 no C6H12O6 (food) produced

if photorespiration could be reduced,
plant would become 50% more efficient
 pressure to evolve
 alternative carbon fixation systems
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Reducing photorespiration
 Separate carbon fixation from Calvin cycle

C4 plants
 PHYSICALLY separate carbon fixation from Calvin cycle
 different cells to fix carbon vs. where Calvin cycle occurs
 store carbon in 4C compounds
 different enzyme to capture CO2 (fix carbon)
 PEP carboxylase

CAM plants
 separate carbon fixation from Calvin cycle by TIME OF DAY
 fix carbon during night
 store carbon in 4C compounds
 perform Calvin cycle during day
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C4 plants are so named because instead of forming PGA’s in the beginning of the Calvin
cycle, they form a four-carbon compound OAA (Oxaloacetate)
Several thousand species use the C4 pathway.
AMP
Instead of being fixed by rubisco, CO2 combines with
a 3Carbon molecule PEP to form OAA, using the
fixing enzyme PEP carboxylase.
• OAA is then converted to malate, and that is
shuttled to the bundle sheath cells.
• Here, malate is converted to pyruvate and CO2.
• The pyruvate moves back to the mesophyll cells
where one ATP is broken down to form AMP (not ADP)
which is required to convert the pyruvate back to PEP
(to help continue the cycle)
• The overall effect of this process is to move CO2 from mesophyll cells to the bundle sheath cells,
in order to make photosynthesis more efficient.
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C4 plants
 A better way to capture CO2

1st step before Calvin cycle,
fix carbon with enzyme
PEP carboxylase
corn
 store as 4C compound

adaptation to hot,
dry climates
 have to close stomates a lot

sugar cane, corn,
other grasses…
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sugar cane
PEP (3C) + CO2  oxaloacetate (4C)
C4 leaf anatomy
O2
light reactions
CO2
PEP
carboxylase
C3 anatomy
stomate
 PEP carboxylase enzyme

bundle
sheath
cell
CO2
RuBisCo
higher attraction for CO2 than O2
 better than RuBisCo


fixes CO2 in 4C compounds
regenerates CO2 in inner cells for
RuBisCo
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 keeping
O2 away from RuBisCo
C4 anatomy
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CAM (Crassulacean Acid Metabolism) plants
 Adaptation to hot, dry climates

separate carbon fixation from Calvin cycle by TIME
 close stomates during day
 open stomates during night
at night: open stomates & fix carbon
in 4C “storage” compounds
 in day: release CO2 from 4C acids
into normal Calvin cycle

 increases concentration of CO2 in cells

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succulents, some cacti, pineapple
CAM plants
cacti
succulents
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pineapple
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C4 vs CAM Summary
solves CO2 / O2 gas exchange vs. H2O loss challenge
C4 plants
CAM plants
separate 2 steps
of C fixation
anatomically in 2
different cells
separate 2 steps
of C fixation
temporally =
2 different times
night vs. day
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Characteristics of Photosynthesis in C3, C4 and CAM plants
Characteristic
C3 Plant
C4 Plant
CAM Plant
Photorespiration
Yes
Little
none
Rubisco present
Yes
Yes
Yes
PEP Carboxylase present
No
Yes
Yes
into OAA via PEP
carboxylase, then to
malic acid which
moves from mesophyll
cell to bundle sheath
cell and then releases
CO2.
into OAA via PEP carboxylase,
then to malic acid which
moves into vacuole (during
night). CO2 released during the
day.
Initial CO2 fixation
directly into
Calvin Cycle
via Rubisco
Secondary CO2 fixation
----------------
In bundle sheath cell
using Rubisco
In “mesophyll”* cell using
Rubisco – in morning
mesophyll cells
bundle sheath cells
Mesophyll cells
Site of Calvin cycle
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