Transcript Photorespiration

Photorespiration
 Photorespiration occurs when the CO2 levels
inside a leaf become low. This happens on hot dry
days
 On hot dry days the plant is forced to close its
stomata to prevent excess water loss.
 The plant continues fix CO2 when its stomata are
closed, the CO2 will get used up and the O2 ratio in
the leaf will increase relative to CO2 concentrations.
 When the CO2 levels inside the leaf drop to around
50 ppm, Rubisco starts to combine O2 with RuBP
instead of CO2
 The net result of this is that instead of producing 2
3C PGA molecules, only one molecule of PGA is
produced and a toxic 2C molecule called
phosphoglycolateis produced.
phosphoglycolate
 The plant must get rid of the phosphoglycolate
since it is highly toxic.
 It converts the molecule to glycolic acid.
 The glycolic acid is then transported to the
peroxisome and there converted to glycine.
phosphoglycolate
Glycolic acid
In
peroxisome
s
Glycine
In
mitochondri
a
Serine
• The serine is then used to make other
organic molecules.
• All these conversions cost the plant
energy and results in the net loss of CO2
from the plant
• To prevent this process, two specialized
biochemical additions have been evolved
in the plant world: C4 and CAM
metabolism.
The C4 Pathway
 The C4 pathway is designed to efficiently fix CO2 at
low concentrations and plants that use this
pathway are known as C4 plants.
 These plants fix CO2 into a four carbon compound
(C4) called oxaloacetate. This occurs in cells called
mesophyll cells.
1. CO2 is fixed to a three-carbon compound called
phosphoenolpyruvate to produce the fourcarbon compound oxaloacetate.
The enzyme catalyzing this reaction, PEP
carboxylase, fixes CO2 very efficiently so the C4
plants don't need to to have their stomata open as
much.
The oxaloacetate is then converted to another
four-carbon compound called malate in a step
requiring the reducing power of NADPH
2. The malate then exits the mesophyll cells and
enters the chloroplasts of specialized cells called
bundle sheath cells.
Here the four-carbon malate is decarboxylated to
produce CO2, a three-carbon compound called
pyruvate, and NADPH.
The CO2 combines with ribulose bisphosphate and
goes through the Calvin cycle.
3.The pyruvate re-enters the mesophyll cells,
reacts with ATP, and is converted back to
phosphoenolpyruvate, the starting compound of
the C4 cycle.
The CAM pathway
 CAM plants live in very dry condition and,
unlike other plants, open their stomata to fix
CO2 only at night.
 Like C4 plants, the use PEP carboxylase to fix
CO2, forming oxaloacetate.
 The oxaloacetate is converted to malate which is
stored in cell vacuoles. During the day when the
stomata are closed, CO2 is removed from the stored
malate and enters the Calvin cycle
Differences between calvin (C3) and C4
C4
C3
 Temp




15-250
C
Absence of malate
One carboxylation
reaction
CO2 is the substrate
Usual leaf structures
Temp 30-350 C
Presence of malate
2 carboxylation reactions
HCO3 is the substrate
Closed stomata to reduce
water loss and
concentrating CO2 in the
bundle sheet cells
 Additional ATP is
required





Comparison between C3, C4, and CAM
C3
C4
CAM
product
G3P
Day
&night
Malate
Day &night
Malate
Night only
Anatomy
No bundle Bundle
sheet cell sheet cell
No bundle
sheet cell
No of stomata 200031000
1000016000
100-800
Photorespirati Up to
on
40%
Not
detectable
Not
detectable
Species
Sugar cane
Pineapple,
vanilla, cacti
Wheat,
rice,
potato
Factors affecting photorespiration
 O2: CO2Ratio
 If Cells Have Low O2 but Higher CO2, Normal
photosynthesis i.e. Calvin Cycle Dominates
 C4Plants Have Little Photorespiration because They
Carry the CO2to the bundle Sheath Cells and can
Build up High [CO2]
• Calvin Cycle Reactions always Favored over
Photorespiration
• If Cells Have Higher O2and Lower CO2,
Photorespiration Dominates
• Temperature
Photorespiration Increases with
Temperature
Regulation of Carbon Dioxide Fixation
 Plant cells have chloroplasts that carry out􀂄
photosynthesis: CO2
glucose
 Plant cells also have mitochondria and carry out
glycolysis, TCA, and oxidative phosphorylation:
Glucose
CO2
 To prevent futile cycling of carbohydrate, cells must
regulate the activities of key Calvin cycle enzymes .
 These enzymes respond indirectly to light activation.
 light energy is available
the Calvin cycle
proceeds.
 If no light available, no fixation of CO2occur
 Among the key changes that regulate Calvin cycle
versus respiration are:Environment Factors: Light
intensity, temperature, & availability of H2O, CO2,
O2
 Cellular factors: cell state of key metabolites
(NADPH, ATP, inhibitors, reducing power, etc )