notes: ch 10, calvin cycle (10.3) & alternative mechanisms ... · rubp, so that the first...
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NOTES: CH 10,
part 3 – Calvin
Cycle (10.3) &
Alternative
Mechanisms of
C-Fixation
(10.4)
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10.3 - The Calvin cycle uses ATP
and NADPH to convert CO2 to
sugar
● The Calvin cycle, like the citric acid cycle,
regenerates its starting material after
molecules enter and leave the cycle
● The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
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● Carbon enters the cycle in the form of
CO2 and leaves in the form of sugar
(C6H12O6)
● ATP and NADPH are consumed
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● the carbohydrate produced is actually a 3-
C sugar: G3P
● to make 1 molecule of G3P the cycle
occurs 3X (consumes 3CO2)
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The Calvin Cycle can be
divided into 3 phases:
1) Carbon Fixation
2) Reduction
3) Regeneration of CO2 acceptor
(RuBP)
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Phase 1:
Carbon
Fixation
● each CO2 is attached to a 5-C sugar,
RuBP, forming an unstable 6-C sugar
which immediately splits into two 3-C
molecules of 3-phosphoglycerate (this
is done by the enzyme: RUBISCO)
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Phase 1: Carbon Fixation
6 3-C fragments
3
3 3
splits into…
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[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHT
REACTIONS
CALVIN
CYCLE
LightInput
3
CO2
(Entering one
at a time)
Rubisco
3 P P
Short-lived
intermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVIN
CYCLE
3 P P
Ribulose bisphosphate
(RuBP)
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Phase 2: Reduction● each 3-phosphoglycerate receives an additional
Pi from ATP, forming 1,3-bisphosphoglycerate
● Next, a pair of electrons from NADPH reduces
the 1,3-bisphosphoblycerate to G3P
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[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHT
REACTIONS
CALVIN
CYCLE
LightInput
CO2
(Entering one
at a time)
Rubisco
3 P P
Short-lived
intermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVIN
CYCLE
3
P P
Ribulose bisphosphate
(RuBP)
3
6 NADP+
6
6 NADPH
P i
6 P
1,3-Bisphosphoglycerate
P
6 P
Glyceraldehyde-3-phosphate
(G3P)
P1
G3P(a sugar)
Output
Phase 2:Reduction
Glucose and
other organic
compounds
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3CO2 + 6 G3P 1 G3P (a sugar
3 5-C RuBP to be incorporated into
glucose, etc.)
5 G3P (returned to cycle)
P
P
P
P
P
P
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unstable
intermediate
NADPH
NADP+
CO2
RuBP
G3P
-1 to glucose
-5 recycled
6
6
(3)
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Phase 3: Regeneration of RuBP
● the 5 molecules of G3P are rearranged
through a series of reactions to regenerate
RuBP (this uses an additional 3 ATP)
**Reason for Cyclic electron flow!!
Calvin cycle consumes:
– 9 ATP
– 6 NADPH
Leads to negative
feedback!
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unstable
intermediate
NADPH
NADP+
CO2
RuBP
G3P
-1 to glucose
-5 recycled
3 (9)ATP’s go in
for every 2 (6)
NADPH
6
6
(3)
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[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHT
REACTIONS
CALVIN
CYCLE
LightInput
CO2
(Entering one
at a time)
Rubisco
3 P P
Short-lived
intermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVIN
CYCLE
3
P P
Ribulose bisphosphate
(RuBP)
3
6 NADP+
6
6 NADPH
P i
6 P
1,3-Bisphosphoglycerate
P
6 P
Glyceraldehyde-3-phosphate
(G3P)
P1
G3P(a sugar)
Output
Phase 2:Reduction
Glucose and
other organic
compounds
3
3 ADP
ATP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
P5
G3P
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Thank you,
RUBISCO!
P.S. my #6
most
favorite
term in bio!
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Net Equation for Photosynthesis :
6CO2 + 6H2O + light C6H12O6 + 6O2
energy
chloroplast
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10.4 - Alternative mechanisms of carbon
fixation have evolved in hot, arid climates
● Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis
● On hot, dry days, plants close stomata, which
conserves water but also limits photosynthesis
● The closing of stomata reduces access to CO2 and
causes O2 to build up
● These conditions favor a seemingly wasteful
process called PHOTORESPIRATION
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● PHOTORESPIRATION: consumes O2,
releases CO2 into peroxisomes,
generates no ATP, decreases
photosynthetic output
O2 replaces CO2 in a non-productive, wasteful
reaction;
Oxygen OUTCOMPETES CO2 for the enzyme’s
active site!
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Mechanisms of C-Fixation
● C3 Plants: plants which combine CO2 to RuBP, so that the first organic product of the cycle is the 3-C molecule 3-PGA
(3-phosphoglycerate)
what we’ve just discussed!
-examples: rice, wheat, soybeans
-produce less food when their stomata close on hot, dry days; this deprives the Calvin cycle of CO2 and photosynthesis slows down
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Photorespiration:
An Evolutionary Relic?
● In most plants (C3 plants), initial fixation
of CO2, via rubisco, forms a three-carbon
compound
● In photorespiration, rubisco adds O2 to
the Calvin cycle instead of CO2
● Photorespiration consumes O2 and
organic fuel and releases CO2 without
producing ATP or sugar
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● Photorespiration may be an evolutionary
relic because rubisco first evolved at a
time when the atmosphere had far less O2
and more CO2
● In many plants, photorespiration is a
problem because on a hot, dry day it can
drain as much as 50% of the carbon fixed
by the Calvin cycle
Photorespiration:
An Evolutionary Relic?
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1. C4 Plants: use an alternate mode of C-
fixation which forms a 4-C compound as its
first product
– examples: sugarcane, corn, grasses
– the 4-C product is produced in mesophyll cells
– it is then exported (through plasmodesmata! )
to bundle sheath cells (where the Calvin cycle
occurs);
Has a higher
affinity for CO2
than RuBP does
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– once here, the 4-C product releases CO2 which
then enters the Calvin cycle to combine with
RuBP (the enzyme rubisco catalyzes this step)
– this process keeps the levels of CO2 sufficiently
high inside the leaf cells, even on hot, dry days
when the stomata close
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Photosynthetic
cells of C4 plant
leaf
Mesophyll cell
Bundle-
sheath
cell
Vein
(vascular tissue)
C4 leaf anatomy
Stoma
Bundle-
sheath
cell
Pyruvate (3 C)
CO2
Sugar
Vascular
tissue
CALVIN
CYCLE
PEP (3 C)
ATP
ADP
Malate (4 C)
Oxaloacetate (4 C)
The C4 pathway
CO2PEP carboxylase
Mesophyll
cell
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2. CAM Plants: stomata are open
at night, closed during day (this
helps prevent water loss in hot, dry
climates)
– at night, plants take up CO2 and
incorporate it into organic acids which
are stored in the vacuoles until day;
– then CO2 is released once the light
reactions have begun again
– examples: cactus plants, pineapples
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Bundle-
sheath
cell
Mesophyll
cell Organic acid
C4
CO2
CO2
CALVIN
CYCLE
Sugarcane Pineapple
Organic acids
release CO2 to
Calvin cycle
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Organic acid
CAM
CO2
CO2
CALVIN
CYCLE
Sugar
Spatial separation of steps Temporal separation of steps
Sugar
Day
Night
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C4 and CAM Pathways are:
● Similar: in that CO2 is first incorporated
into organic intermediates before Calvin
cycle
● Different:
-in C4 plants the first and second steps are
separated spatially (different locations)
-in CAM plants the first and second steps
are separated temporally (occur at
different times)
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What happens to the products of
photosynthesis?
● O2: consumed during cellular respiration
● 50% of SUGAR made by plant is consumed by
plant in cellular respiration
● some is incorporated into polysaccharides:
-cellulose (cell walls)
-starch (storage; in fruits,
roots, tubers, seeds)
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The Importance of
Photosynthesis: A Review
● The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
● Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
● In addition to food production, photosynthesis produces the oxygen in our atmosphere!!
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The Importance of
Photosynthesis: A Review
**no process is more important than
photosynthesis to the welfare of life on
Earth!...Thank you photosynthesis (&
RUBISCO!).
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Light
CO2H2O
Light reactions Calvin cycle
NADP+
RuBP
G3PATP
Photosystem II
Electron transport
chain
Photosystem I
O2
Chloroplast
NADPH
ADP
+ P i
3-Phosphoglycerate
Starch
(storage)
Amino acids
Fatty acids
Sucrose (export)