Chapter 10: Photosynthesis
Concept 10.4: Alternative
mechanisms of carbon fixation
have evolved in hot, arid
climates.
Dehydration & Tradeoffs with Photosynthesis
 On hot, dry days, plants conserve water by closing
or partially closing their stomata.
 Unfortunately, this limits CO2 uptake for making
sugars through photosynthesis.
 The closing of stomata also causes O2 released
from the light reactions to build up in leaves.
 These conditions favor a seemingly wasteful
process called “photorespiration.”
Photorespiration
 In most plants, initial fixation of CO2 occurs via
rubisco, forming a 3-carbon compound that is used
to make sugar. (For this reason, these plants are
called “C3” plants.)
 In photorespiration, rubisco adds O2 to the Calvin
cycle instead of CO2, forming a two-carbon
compound that must exit the chloroplast to be
rearranged in peroxisomes and mitochondria.
Photorespiration:
Using oxygen and fuel and
producing carbon dioxide
in the presence of light.
Photorespiration Is Wasteful!
 Photorespiration consumes O2 and RuBP (fuel)
while releasing CO2. However, unlike cellular
respiration, it does not produce ATP.
 Photorespiration inputs into the Calvin cycle only
three PGA (3-phosphoglycerate) molecules for
each O2 instead of 6 PGA for each CO2.
Photorespiration Is Wasteful!
 On a hot, dry day photorespiration can drain
as much as 50% of the carbon fixed by the
Calvin cycle (three PGA instead of six PGA).
 From our heterotrophic perspective, this is
needlessly wasteful. Crop yields would be
much higher if photorespiration did not occur.
 Scientists are trying to bioengineer C3 crops
that can minimize photorespiration.
Photorespiration: Evolutionary Baggage?
 Photorespiration occurs because rubisco has an
affinity for both CO2 and O2.
 Rubisco first evolved at a time when the
atmosphere had far less O2 and more CO2.
 Under these conditions, the inability of rubisco’s
active site to exclude O2 made little difference.
 Modern rubisco retains some of its affinity for O2,
making photorespiration inevitable.
C4 and CAM plants have adaptations that minimize photorespiration.
Sugarcane
Pineapple
CAM
C4
CO2
Mesophyll
cell
Organic acid
C4 Plants
Spatial
Separation
Bundlesheath
cell
CO2
CO2 incorporated
into four-carbon Organic acid
organic acids
(carbon fixation)
CO2
CALVIN
CYCLE
Sugar
Spatial separation of steps
CO2
Organic acids
release CO2 to
Calvin cycle
Night
Day
CAM Plants
Temporal
Separation
CALVIN
CYCLE
Sugar
Temporal separation of steps
C4 Plants Spatially Separate
Carbon Fixation and the Calvin Cycle
 C4 plants minimize photorespiration
by incorporating CO2 into 4-carbon
compounds in their mesophyll cells.
 These 4-carbon compounds are then
transferred to bundle-sheath cells,
where they release CO2 to be fixed
by rubisco in the Calvin cycle.
Mesophyll
cell
Mesophyll cell
Photosynthetic
cells of C4 plant
leaf
CO2
PEP carboxylase
Bundlesheath
cell
The C4 pathway
Oxaloacetate (4 C)
PEP (3 C)
Vein
(vascular tissue)
ADP
Malate (4 C)
ATP
C4 leaf anatomy
Stoma
Bundlesheath
cell
Pyruvate (3 C)
CO2
CALVIN
CYCLE
Sugar
Vascular
tissue
How do C4 plants minimize photorespiration?
 In C4 mesophyll cells, PEP carboxylase fixes CO2.
PEP carboxylase has no affinity for O2.
 C4 plants effectively “pump” CO2 into bundle
sheath cells, keeping the CO2 level high enough for
rubisco to bind CO2 rather than O2.
 This cyclic series of reactions is essentially a
CO2-concentrating pump powered by ATP.
 Even with stomata partially closed, C4 plants can
still produce sugar and avoid protorespiration.
CAM Plants Temporally Separate
Carbon Fixation and the Calvin Cycle
 CAM plants close their
stomata during the day and
open them at night.
 At night, CAM plants fix CO2
into organic acids which are
stored in their vacuoles.
 CO2 is then released during
the day to be fixed by
rubisco in the Calvin cycle.
How do CAM plants minimize photorespiration?
 In CAM plants, water loss is minimized by
having the stomata closed throughout the day.
 The release of CO2 from organic acids during
the day keeps the CO2 levels in the mesophyll
cells high, minimizing the fixation of O2 by
rubisco.
 Even with stomata closed during the day, CAM
plants can still produce sugar and avoid
photorespiration.
Comparison of Alternate Methods of CO2 Fixation