Transcript 17 photosynth 2 10 10 05

Lecture 17
Photosynthesis
Oct 10, 2005
II. Calvin Cycle
1
Lecture Outline
1. The Calvin Cycle fixes carbon
makes reduced carbon compounds
2. Reactions of the Calvin Cycle – anabolic pathway
input of NADPH + H+, input of ATP
3. Regulation of the Calvin Cycle
4. The problem with oxygen – Photorespiration
5. Tricks some plants use to limit photorespiration
- C4 anatomy, C4 metabolism – division of labor
- CAM plants, the difference is night and day
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DARK REACTIONS
energy utilization
The Calvin Cycle
3
The purpose of the Carbon-fixation (Calvin Cycle) Reactions
CO2 + NADPH + H+ + ATP
C6H12O6 + NADP+ + ADP + Pi
carbohydrate
Note: synthesis of carbohydrate from CO2 is favorable only
because coupled to very favorable reactions
NADPH to NADP+
and
ATP to ADP + Pi
energy released is greater than
it costs to make carbohydrate
4
The Calvin cycle has three phases
– Carbon fixation
– Reduction (energy input, reducing equiv input)
– Regeneration of the CO2 acceptor
(energy input – “priming step”)
5
O
=
12
Overview of Carbon-Fixation Reactions
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CO2
+
R-C-OH
5 Carbon Sugar
with 2 phosphates on it
Reduction
Carbon Acid
2 3with
one phosphate on it
ATP
ADP + Pi
ADP + Pi
NADPH
ATP
NADP+
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5 Carbon Sugar
with one phosphate on it
O
3 Carbon Aldehyde R-C-H
=
20/24
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with one phosphate on it
4/24
6 Carbon Sugar - Glucose
with NO phosphate on it
2
Higher
Energy
Compound
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Carbon Fixation
Carried out by the enzyme “rubisco”
(ribulose 1,5 bisphosphate carboxylase oxygenase)
Do need to know this enzyme7
Key regulatory enzyme
Carbon
Fixation
Acid
Rubisco
-COOH
Input of
energy
Reduction
Priming
Step
Aldehyde
-C=O
H
Regenerate
What started
with
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Regulation of Rubisco
1st Enzyme in Calvin Cycle
Substrate/Product availability
Allosterically regulated by NADPH and ATP
Very Narrow pH optimum
pH 8 / pH7
pH 8 or above, inactive at 7
Enzyme must be in
reduced form
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Integration of Light-Dependent
and
Light-Independent Reactions
They generally occur AT THE SAME TIME
Light reaction
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+P1
RuBP
3-Phosphoglycerate
Photosystem II
Electron transport chain
Photosystem I
ATP
NADPH
Figure 10.21
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
O2
Sucrose (export)
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PhotoRespiration - “ the OXYGEN PROBLEM”
Oxygen is a competing substrate for the 1st enzyme
in the C3 cycle (Rubisco)
CO2
+
5 Carbon Sugar
with 2 phosphates on it
O2
+
5 Carbon Sugar
with 2 phosphates on it
3 Carbon Acid
with one phosphate on it
3 Carbon Acid
with one phosphate on it
2 Carbon Acid
with one phosphate on it
3 Carbon Acid
with one phosphate on it
Net increase
in material
make
glucose with
“extra”
Energy
Wasted
With NO
synthesis
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of glucose
Some Plants Deal with this problem by a
DIVISION OF LABOR BETWEEN CELLS
C4 Plants
Mesophyll cells perform “usual”
noncyclic Light-Dependent Reactions
make oxygen, ATP and NADPH
Do NOT perform the C3 (Calvin cycle) reactions
Bundle Sheath cells perform “UNusual”
cyclic Light-Dependent Reactions
a lot of ATP but
very little NADPH
and very little O2
PERFORM the usual C3 (Calvin Cycle) reactions
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C4 leaf anatomy and the C4 pathway
Mesophyll
cell
Photosynthetic
cells of C4 plant
leaf
Mesophyll
cell
Bundlesheath
cell
PEP (3 C)
Oxaloacetate (4 C)
Vein
(vascular tissue)
NADPH
used
ADP
Malate (4 C)
ATP
Malate (4 C)
C4 leaf anatomy
Mesophyll
CO
CO
2 2
PEP carboxylase
Oxaloacetate (4 C) Pyruate (3 C)
NADPH
regenerated
CO2
Stomata
BundleSheath
cell
Produce NADPH and ATP
PEP carboxylase insensitive to O2
Malate brings across reducing equivalents
Cyclic
e- flow
Little O2
CALVIN
CYCLE
Sugar
Vascular
tissue
Figure 10.19
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Mesophyll Cell C4 Metabolism
OH
/
C=O
CH2
H-C-O-H
C=O
\
OH
malate
4C acid
“carries”
Reducing
equivalents
NADPH
OH
+
/
+
H
C=O
CH2
C=O
C=O + Pi
\
NADP+
OH
Oxaloacetate
4C acid
CO2
CH3
C=O
C=O
\
O-Pi
“PEP”
ADP
CH3
ATP
C=O
C=O
Needs ATP and NAPDPH + H+ \
O-H 14
Non-cyclic electron flow
pyruvate
Bundle Sheath - Cell C4 Metabolism
OH
NADPH
/
+
C=O
+
H
CH2
H-C-O-H
C=O
\
NADP+
OH
“carries”
malate
Reducing
4C acid
equivalents
OH
/
C=O
CH2
C=O
C=O
\
OH
Oxaloacetate
4C acid
CO2
Calvin
Cycle
Needs ATP Only
Cyclic electron flow
Very low O2
CH3
C=O
C=O
\
O-H
pyruvate
NADPH
ATP
ATP
glucose
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Mesophyll cells provide a means for bundle sheath
cells to acquire NADPH + H+ reducing power
Mesophyll cells provide carbon dioxide to bundle sheath
cells at higher concentration than in air
Bundle Sheath cells not making oxygen, so very little
competitor with C3 reactions
Costs more energy to do business this way…
but has the advantage when CO2 is limiting
(when stomates are closed - like on hot days)
Who cares as long as the sun is shining
?
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ATP is not limiting
CAM Plants
Cacti, pineapple
– Open their stomata only at night, too hot during
day – survive very adverse (dry) conditions
NIGHT
Perform PEP carboxylase reaction at night (CO2 assimilation)
accumulate malate to high concentration in central vacuole
use sugar oxidation/catabolism to power (NADH and ATP)
carbon fixation
DAY
Perform “light” reactions during the day
mostly cyclic e- flow to produce ATP (low O2)
decarboxylate malate to yield CO2 and NADPH + H+
perform C3 reactions (Calvin Cycle) to produce
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sugars and starch
Generally
Slow
growing
Pineapple
Sugarcane
Sunlight
Powers
Both
phases
(a)
Figure 10.20
C4
Mesophyll Cell
Organic acid
Bundlesheath
cell
Spatial
separation
of steps. In C4
plants, carbon fixation
and the Calvin cycle
occur in different
types of cells.
CAM
CO2
CO2
1 CO incorporated Organic acid
2
into four-carbon
organic acids
(carbon fixation)
Night
Day
(b)
CALVIN
CYCLE
Sugar
2 Organic acids
release CO2 to
Calvin cycle
CALVIN
CYCLE
Sugar
Sugar
Oxidation
Powers
One
Phase
Temporal
separation
of steps. In CAM
plants, carbon fixation
and the Calvin cycle
occur in the same cells
at different18
times.
Summary
1. Photosynthetic “light” reactions produce
ATP and reducing potential NADPH + H+
2. Dark reactions use ATP and reducing potential
to synthesize carbohydrates
- powers reduction of 3-carbon acid
to 3-carbon aldehyde
- powers regeneration of starting material
5-carbon di-phosphate (priming step for CO2 fixation)
3. Rubisco enzyme regulated tightly by allosteric modulators
pH, and reducing status of stroma
4. O2 interferes with carbon fixation by Rubisco enzyme
5. Metabolic “tricks” to avoid photorespiration
- C4 metabolism - CAM metabolism
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