Download 3. DarkReaction

The Dark Reaction
-
light-independent
-
energy stored in ATP and NADPH
(from light reaction) is used to
reduce CO2 to sugar
Three independent ways to reduce CO2 to
make sugar:
1. the Calvin cycle (C3),
2.
C4 photosynthesis,
3.
crassulacean acid metabolism (CAM).
The Calvin Cycle
1.
CO2 is fixed by rubisco
1.
2.
CO2 + RuBP  unstable C6  2 PGA
Reduction of CO2 to make G3P
-
3.
Uses ATP and NADPH
G3P is exported to cytoplasm to make starch,
sucrose, oils
Regenerating RuBP
1.
- for enery 12 molecules of G3P made in
the Calvin cycle two are “released”
2.
- the Calvin cycle needs to “turn” 6 times
to make one glucose!!!
G3P
-
one-third forms starch
-
two-thirds are converted to sucrose
and then hydrolyzed in other parts of
plant into glucose and fructose
-
Ultimately used as a source of C for
nucleic acids, amino acids, fats…
Rubisco
●
●
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Ribulose bisphosphate
carboxylase oxygenase
fixes CO2 & O2
Enzyme in Calvin Cycle (1st step)
Most abundant protein on Earth
–
About 50% total plant protein!
Stomata - lungs


openings on the surface
of the leaf that allow the
exchange of gases
between air spaces in the
leaf and the atmosphere
Guard cells
– control the size of the
stoma in response to
environmental conditions
●
●
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The size of the guard cell
changes when water moves
into or out of the cell
K+ ions are actively pumped
into the guard cell and water
follows by osmosis
Light, and CO2
concentration affect the
movement of K+ ions into
the cells
Generally stomata are open
during the day and closed at
night
Photorespiration
●
the reaction of RuBP with oxygen, reduces
the efficiency of photosynthesis
●
●
●
rubisco is inefficient: “fixes” O2, as well
as CO2
C3 plants lose 20% of their energy to fix
one CO2
this gets worse with heat!
-
Under hot and dry conditions
(daytime) plants will close their
stomata to prevent water loss
-
This causes a build up of oxygen
since CO2 can’t enter…so MORE
photorespiration
Avoiding Photorespiration
●
●
●
C3
– The majority of plants
C4
– CO2 temporarily stored as 4-C organic acids resulting in
more more efficient C exchange rate
– Advantage in high light, high temperature, low CO2
– Many grasses and crops (e.g., corn, sorghum, millet,
sugar cane)
CAM
– Stomata open during night
– Advantage in arid climates
– Many succulents (e.g. cacti)
Fig. 10.21
Comparison of Photosynthesis in C3 and C4 Plants
VARIABLE
C3 PLANTS
C4 PLANTS
Photorespirati
on
Extensive
Minimal
Perform Calvin
cycle?
Yes
Yes
Primary CO2
acceptor
RuBP
PEP
CO2-fixing
enzyme
Rubisco (RuBP
carboxylase/oxyge
nase)
PEP carboxylase
and rubisco
First product
of CO2 fixation
3PG (3-carbon
compound)
Oxaloacetate (4carbon
compound)
Affinity of
carboxylase
for CO2
Moderate
High
Photosynthetic Mesophyll
cells of leaf
Classes of
chloroplasts
One
Mesophyll +
bundle sheath
Two
-
-
-
C3 photosynthesis: about 3.5 billion
years ago,
C4 plants appeared about 12 million
years ago.
A possible factor in the emergence
of the C4 pathway is the decline in
atmospheric CO2
CAM PLANTS
-
CAM is similar to C4
-
CO2 is fixed to a 4-carbon compound
-
Separated by time rather than space:
At night
 cooler and water loss is minimized
 stomata open and CO2 is fixed in mesophyll cells to
form the 4- carbon oxaloacetate, which is converted
into malic acid.
During the day
 when the stomata close to reduce water loss, the
accumulated malic acid is shipped to the chloroplasts
to form sugars
Fig. 10.22
CAM Plants
Global Environmental Change &
Photosynthesis:
C3 vs. C4 vs. CAM
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Increasing CO2
Increasing chronic and acute temperatures
Changes in water
*At high CO2, C3 more efficient than C4 at all temps.
(photosynthesis only, not other processes)