Download DARK REACTIONS energy utilization The Calvin Cycle

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
Lecture 17
Photosynthesis
Oct 10, 2005
II. Calvin Cycle
1
2
The purpose of the Carbon-fixation (Calvin Cycle) Reactions
CO2 + NADPH + H+ + ATP
DARK REACTIONS
energy utilization
C6H12O6 + NADP+ + ADP + Pi
carbohydrate
The Calvin Cycle
3
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
Overview of Carbon-Fixation Reactions
24
CO2
– Carbon fixation
– Reduction (energy input, reducing equiv input)
– Regeneration of the CO2 acceptor
(energy input – “priming step”)
+
Carbon Acid
2 3with
one phosphate on it
5 Carbon Sugar
with 2 phosphates on it
ATP
ADP + Pi
ADP + Pi
NADPH
ATP
NADP+
12
with one phosphate on it
24
O
3 Carbon Aldehyde R-C-H
=
20/24
5 Carbon Sugar
with one phosphate on it
4/24
6 Carbon Sugar - Glucose 2
5
R-C-OH
Reduction
The Calvin cycle has three phases
O
=
12
Higher
Energy
Compound
6
with NO phosphate on it
Carbon
Fixation
Carbon Fixation
Priming
Step
Acid
Rubisco
-COOH
Input of
energy
Reduction
Aldehyde
-C=O
H
Carried out by the enzyme “rubisco”
(ribulose 1,5 bisphosphate carboxylase oxygenase)
Do need to know this enzyme7
Key regulatory enzyme
Regenerate
What started
with
8
Regulation of Rubisco
Enzyme in Calvin Cycle
Integration of Light-Dependent
and
Light-Independent Reactions
1st
Substrate/Product availability
Allosterically regulated by NADPH and ATP
Very Narrow pH optimum
They generally occur AT THE SAME TIME
pH 8 / pH7
Light reaction
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+P1
pH 8 or above, inactive at 7
RuBP
Photosystem II
Electron transport chain
Photosystem I
Enzyme must be in
reduced form
ATP
3-Phosphoglycerate
G3P
Starch
(storage)
NADPH
9
Figure 10.21
Chloroplast
Amino acids
Fatty acids
O2
PhotoRespiration - “ the OXYGEN PROBLEM”
Some Plants Deal with this problem by a
Oxygen is a competing substrate for the 1st enzyme
DIVISION OF LABOR BETWEEN CELLS
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
11
of glucose
10
Sucrose (export)
C4 Plants
Mesophyll cells perform “usual”
noncyclic LightLight-Dependent Reactions
make oxygen,
oxygen, ATP and NADPH
Do NOT perform the C3 (Calvin cycle) reactions
Bundle Sheath cells perform “UN
usual””
“UNusual
cyclic LightLight-Dependent Reactions
a lot of ATP but
very little NADPH
and very little O2
12
PERFORM the usual C3 (Calvin Cycle) reactions
C4 leaf anatomy and the C4 pathway
Photosynthetic
cells of C4 plant
leaf
Mesophyll
cell
Mesophyll
cell
BundleBundlesheath
cell
PEP (3 C)
NADPH
used
ADP
Malate (4 C)
ATP
Malate (4 C)
Oxaloacetate (4 C) Pyruate (3 C)
NADPH
regenerated
CO2
C4 leaf anatomy
Mesophyll
CO
CO
2 2
PEP carboxylase
Oxaloacetate (4 C)
Vein
(vascular tissue)
Mesophyll Cell C4 Metabolism
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
13
Figure 10.19
Bundle Sheath - Cell C4 Metabolism
OH
NADPH
/
+
C=O
H+
CH2
H-C-O-H
C=O
\
NADP+
OH
“carries”
carries”
malate
Reducing
4C acid
equivalents
OH
/
C=O
CH2
C=O
C=O
\
OH
Oxaloacetate
4C acid
Very low O2
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
CO2
Calvin
Cycle
Needs ATP Only
Cyclic electron flow
CH3
C=O
C=O
\
O-H
pyruvate
P
PE ase
CO2
xyl
NADPH
o
b
OH
car
+
OH
/
CH3
H+
/
C=O
C=O
C=O
CH2
C=O
CH2
C=O
\
H-C-O-H
C=O + Pi
O-Pi
C=O
\
NADP+
“PEP”
\
OH
OH
Oxaloacetate
ADP
malate
4C acid
4C acid
CH3
ATP
“carries”
carries”
C=O
Reducing
C=O
equivalents Needs ATP and NAPDPH + H+ \
O-H
NonNon-cyclic electron flow pyruvate 14
NADPH
ATP
ATP
glucose
15
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
?
16
ATP is not limiting
CAM Plants
Cacti, pineapple
Generally
Slow
growing
– 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
17
sugars and starch
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
19
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.