Light--Independent Reactions
Light
Chapter 10.4
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The Problem with Rubisco
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Rubisco will also bind with oxygen gas
instead of carbon dioxide
Catalyzes reaction between RuBP & O2
Ability to catalyze reaction involving either
O2 or CO2 is why rubisco is called
ribulose-1,5-bisphosphate carboxylase
oxygenase
yg
O2 and CO2 compete for the same active
site on rubisco enzyme
Recall…carbon is fixed in the Calvin cycle
(in the stroma of chloroplasts) by
undergoing a chemical reaction with RuBP
Carbon fixation requires the enzyme
rubisco to form 3PG (3-phosphoglycerate)
3PG is a 3-carbon molecule and thus C3
plants are those that fix carbon into 3PG
p
via the Calvin cycle
C3 plants undergo C3 photosynthesis
Photorespiration
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Photorespiration is the reaction of O2
with RuBP (ribulose-1,5-bisphosphate) in
a process that reverses carbon fixation
and reduces efficiency of
photosynthesis
Products are a 2-C compound called
phosphoglycolate and only one 3-C
3PG
Phosphoglycolate is considered a waste
product and is not useful to the cell
All the energy used to regenerate RuBP is
wasted (less 3PG being formed, hence
less sugars)
1
Photorespiration
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Under normal conditions (25oC), C3 plants
lose 20% of energy
Biologist estimate the maximum
possible efficiency of photosynthesis in
C3 plants is 25%
In nature, photosynthetic efficiency
ranges from 0.1 – 3%
Rubisco has a greater attraction for CO2
and binds with CO2 80 times faster than
O2
Photorespiration
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However, atmosphere contains 21% O2
and approx. 0.04% CO2
Under normal conditions, the rate of
carbon fixation by Rubisco is four times
that of oxidation
Drains cell resources but plant can still fix
enough carbon for production of energyrich carbohydrates
If supply of CO2 in cell is significantly
reduced, photorespiration becomes a
serious concern
Photorespiration
Problem:
 Plants in hot,
hot dry
dry, sunny environments
 Leaves begin to lose water through
stomata
 Stomata will close to prevent water loss
 Oxygen formed in light-dependent
reactions accumulates in leaves
 Carbon dioxide cannot enter leaf
 Photorespiration increases with higher
ratio of oxygen to carbon dioxide
2
Why the C3 problem?
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Possibly evolutionary baggage
 Rubisco evolved in high CO2 atmosphere
 there wasn’t strong selection against active site of Rubisco
accepting both CO2 & O2
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Today it makes a difference
 21% O2 vs. 0.03% CO2
 photorespiration can drain away 50% of carbon
fixed by Calvin cycle on a hot, dry day
 strong selection pressure to evolve better way
to fix carbon & minimize photorespiration
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BUT! W/O it, GM plants are prone to light
damage.  protective role?
C4 Plants & C4 Photosynthesis
 Alternate
form of carbon fixation
that some plants use
use, particularly in
hot/dry climates to increase
concentration of CO2 available for
Calvin cycle reactions
 Tropical plants and temperate crop
(corn sugarcane)
species (corn,
 Florida – 70% of native species are
C4 plants
 Manitoba – 0% C4 species
Reducing Photorespiration
 Due
to photorespiration, C3 plants are
extremely
y inefficient in hot, dry
y
climates
 Plants that thrive in these
environments have evolved two
mechanisms to reduce
photorespiration and increase
efficiency
 C4 plants
 CAM plants
Leaf Anatomy
 Internal
leaf structure
minimizes photorespiration
 Reactions of Calvin cycle performed
by bundle-sheath cells which
surround leaf veins
 Bundle-sheath cells are surrounded
by mesophyll cells that separate
them from air spaces within leaf
 Reduces exposure of rubisco (in
bundle-sheath cells) to O2
3
C4 Plants
Mesophyll cells
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C4
Photosynthesis
1st step before Calvin cycle
CO2 is fixed by addition to 3-C compound PEP (phosphoenolpyruvate)
Uses enzyme called PEP carboxylase
Product is 4-C compound - oxaloacetate
Hence name C4 plants!
Oxaloacetate is converted into 4
4-C
C
compound – malate
Malate is transported into bundle-sheath
cells
Malate is decarboxylated to pyruvate
C4 Plants
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3-C compound pyruvate is transported
back to mesophyll cells and converted into
PEP
Bundle-sheath cells are impermeable to
CO2
CO2 is concentrated in bundle-sheath cells
where Calvin cycle
y
takes place
p
High CO2 concentration makes Calvin
cycle much more efficient than in C3
plants
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PEP Carboxylase
Comparative anatomy
 Separate reactions in different cells
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light reactions
carbon fixation
Calvin cycle
C3
C4
 PEP carboxylase enzyme
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higher
hi
h
affinity
ffi it for
f
CO2 than
th
rubisco (no affinity for O2)
fixes CO2 in 4C compounds
CO2 is regenerated in
bundle-sheath cells for
rubisco
PEP (3C) + CO2  oxaloacetate (4C)
CAM Plants
C4 Photosynthesis
Physically separated C fixation from Calvin
cycle
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ce s
 Oute
Outer cells
light reaction &
carbon fixation
 pumps CO2 to
inner cells
 keeps O2 away
from inner cells
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 away from
Rubisco
CO2
O2
O2
CO2
 Inner cells
Calvin cycle
 glucose to veins
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C4 plants run Calvin cycle and carbon
fixation simultaneously but in different
locations
CAM plants run Calvin cycle and carbon
fixation in same cell but at different
times
CAM – crassulacean acid metabolism
Include succulent (water-storing)
(water storing) plants
(cacti, pineapples)
Biochemical pathway is identical to C4
plants
Thrive in hot, arid desert conditions
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CAM Plants
•
Separate carbon fixation from Calvin cycle
by time of day
• stomata close during day, prevent
CAM
Pl t
Plant
water loss
• stomata open at night
Opposite from most other plants!
Carbon dioxide is fixed at night when
stomata are opened
Calvin cycle produces carbohydrates
during the day when stomata are closed
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•
•
CAM Plants
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p
g
CO2 is incorporated
into 4C organic
acid
(malic acid, isocitric acid)
The organic acid is stored in the large
vacuole until daytime, when stomata close
Organic acid exits vacuole and is
decarboxylated,
y
, freeing
g CO2 for Calvin
cycle
CO2 is fixed again by rubisco
C4 vs CAM Summary
solves CO2 / O2 gas exchange vs. H2O loss challenge
C4 plants
separate 2
steps of C
fixation
anatomically
in 2 different
cells
CAM plants
separate 2
steps of C
fixation
temporally at
2 different
times
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