O2
O2
O2
PHOTORESPIRATION
Definition 1:
O2
O2
The aberrant use of oxygen by chloroplasts
Definition 2:
O2
An interference with carboxylation caused by
the deviant interaction of RUBISCO with oxygen
A process that leads to only one 3PGA being
produced in the dark reaction in
chloroplasts
CH2OP
C=O
C-OH
C-OH
CH2OP
RuBP
Phosphoglycolate
Peroxisomes
CH2OP
CH2OP
CH2OP
C-OH O HO- C-O-OH HO- C-O-OH
2
C-OH
C=O H2O HO-C-OH
C-OH
C-OH
C-OH
CH2OP
CH2OP
CH2OP
CH2OP
COO
COO
3PGA C-OH
PO4
CH2OP
CH2OH
COO Glycolate
Calvin Cycle
Photoefficiency is cut in half
Peroxisome
between 2
chloroplasts
Chloroplast
From
Chloroplasts
Glyoxylate
CH2OH O2
CHO
COO
COO
Glycolate
Glycerate
ATP
NH3+
CH2OH
CH2NH3
CHOH
COO
COO
3PGA
Hydroxy
pyruvate
COO
CO2
NH3+
NADH
C=O
COO
2 CH2NH3
Glycine
NAD+
CH2OH
Mitochondria
CH2OH
CHNH3
COO
Peroxisomes
CH2OH
CHNH3
COO
Serine
Why Photorespiration?
1. O2 (21% of air) is more prevalent than CO2 (0.03%)
2. RUBISCO reacts with O2 (Km =200 M)
as well as CO2 (Km=20 M)
RUBISCO IS BOTH AN OXYGENASE and a CARBOXYLASE
3. Phosphoglycolate can be salvaged.
Glycine  Serine (mitochondria)releases CO2
Glycolate  Glyoxylate (peroxisomes) consumes O2
O2 in CO2 out is respiration
Strategy for Preventing
Photorespiration
PLAN
Avoid RUBISCO
Fix CO2 in an environment shielded from O2
Use an enzyme that does not react with O2
Strategy (cont.)
SOLUTION
CO2 fixation occurs in Mesophyll cells
CO2 fixing enzyme is not RUBISCO
CO2 fixing enzyme is PEP carboxylase
PEP carboxylase will not react with O2
Take
Home
RUBISCO never changed
Instead plant anatomy changed
Anatomy of C3 vs C4 Plants
Calvin Cycle Cells
Mesophyll cell
Oxaloacetate
PPi +
CO2
Bundle-sheath cell
Malate
Malate
Calvin
cycle
Pi +
CO2
AMP
ATP
CO2
Phosphoenolpyruvate
pyruvate
pyruvate
ATP energy is required to concentrate CO2
in bundle-sheath cells
Hatch-Slack C4 Pathway for concentrating CO2
PEP
Carboxylase
2Pi
ATP + Pi AMP + PPi
Pyruvate
phosphate
dikinase
COOPi
C~OPO=3
CH2
PEP CO2
Mesophyll cell
COOCOOC=O Malate
dehydrogenase HO-C-H
CH2
CH2
COO- NADPH NADP+ COOOAA C4
COOC=O
CH3
Pyruvate
Calvin Cycle
Malate C4
Malic enzyme
CO2
NADPH
RUBISCO
2, 3PGA
NADP+
Bundle-sheath cell
Key Points
1. Mesophyll cells have no RUBISCO; Bundle Sheath do
2. Mesophyll cells condense CO2 with PEP-C4 cycle
3. OAA and malate are the C4 components
4. In bundle sheath cells malate is oxidatively
decarboxylated to pyruvate to release the CO2
5. CO2 released enters the Calvin Cycle-C3 cycle
6. In mesophyll cells, pyruvate is converted to PEP by the
enzyme pyruvate-phosphate dikinase
7. Pyruvate-phosphate dikinase uses the equivalent of
2 ATPs to synthesize PEP
C3 vs C4 Plants
A Lesson in Photoefficiency
C3
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CO2 directly
RuBP recipient
RUBISCO open
O2 can interfere
Photorespiration likely
C4
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CO2 indirectly
PEP recipient
RUBISCO shielded
O2 cannot interfere
No photorespriation
C3 and C4 Plants
C3
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soybean
wheat
rice
sugar beet
alfalfa
spinach
tobacco
sunflower
C4
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corn
sorghum
sugar cane
millet
crab grass
Bermuda grass
pigweed
What did you Learn?
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Photorespiration is to be avoided
RUBISCO is an oxidase/carboxylase
Oxygen cuts photoefficiency in half
C3 and C4 plants differ in anatomy
Mesophyll cells fix CO2 to PEP
Bundle sheath cells have Calvin cycle
C4 plants grow more ferociously