Post on 27-Mar-2022
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Photosynthesis 2 Today�s topics:
• Review: how plants capture
light energy – Proton gradients and ATP
synthase – NADPH
• How plants use that energy – “dark” reactions use ATP and
NADPH to fix CO2 and produce sugars
– Calvin Cycle details
• Photorespiration and the C4 pathway Oct 13, 2021
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STROMA
H2O O2 1⁄2
Light
THYLAKOID SPACE
Pq Pc
Fd
2 H+ 2 H+
2 H+ Light
Photosystem II Photosystem I
3
ATP
e– e
–
e–
e–
e–
Photosystem II Photosystem I
e–
e–
NADPH
Linear Electron Flow Photosystem II
- Light Energy used to Form H+ gradient (ATP Synthesis)
Photosystem I - Light Energy used to reduce NADPH
4
Mill makes
ATP
ATP
e–
e– e–
e–
e–
Phot
on
Photosystem II Photosystem I
e–
e–
NADPH
Phot
on
Photosystem I can also make H+ gradient by itself.
Cyclic Electron Flow
5
LIGHT REACTOR
NADP+ ADP
ATP
NADPH
CALVIN CYCLE
[CH2O] (sugar) STROMA (Low H+ concentration)
Photosystem II
LIGHT H2O CO2
Cytochrome complex
O2
H2O O2
1⁄2
Photosystem I Light
THYLAKOID SPACE (High H+ concentration)
STROMA (Low H+ concentration)
Thylakoid membrane
ATP synthase
Pq Pc
Fd
NADP+
reductase
NADPH + H+
NADP+ + 2H+
To Calvin cycle
ADP
P ATP
H+
2 H+ +2 H+
2 H+
Light Dependent Reactions Produce
NADPH
And ATP
to power the
Calvin Cycle
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• The Calvin Cycle (C3)
– Reduce CO2 à Sugars – Requires ATP and NADPH
from light reactions
The Dark Reactions
2
Light Reactions
(energy capture)
�Dark Reactions�
(energy utilization)8
(Entering one at a time) CO2
3
1: Carbon fixation
Rubisco
Short-lived intermediate
3 P P
3 P P
Ribulose bisphosphate (RuBP)
P
3-Phosphoglycerate
P 6 P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 P i
P 6
Glyceraldehyde-3-phosphate (G3P)
2: Reduction
6 ATP
3 ATP
3 ADP CALVIN CYCLE
P 5 Phase 3: Regeneration of the CO2 acceptor (RuBP) P 1
G3P (a sugar) Output
Glucose and other organic compounds
G3P
6 ADP
Light H2O CO2
LIGHT REACTIONS
NADPH
NADP+
[CH2O] (sugar)
CALVIN CYCLE
Input
ATP
ADP
O2
6
Phase 1: Carbon fixation
“RUBISCO” is the enzyme that fixes carbon dioxide
Phase 2: Reduction
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(Entering one at a time) CO2
3
Rubisco
Short-lived intermediate
3 P P
3 P P
Ribulose bisphosphate (RuBP)
P
3-Phosphoglycerate
P 6 P
1,3-Bisphosphoglycerate 6 NADPH
6 NADP+
6 P i
P 6
Glyceraldehyde-3-phosphate (G3P)
6 ATP
3 ATP
3 ADP CALVIN CYCLE
P 5
P 1
G3P (a sugar) Output
Glucose and other organic compounds
G3P
6 ADP
Light H2O CO2
LIGHT REACTIONS
NADPH
NADP+
[CH2O] (sugar)
CALVIN CYCLE
Input
ATP
ADP
O2
6
9
NADH + 2 H+
P i
2 P C
CHOH
O
P
O
CH2 O
2 O–
1, 3-Bisphosphoglycerate
ATP
CH2 O P
2
C
CHOH
3-Phosphoglycerate
O–
C
C
CH2OH
H O P
2-Phosphoglycerate
2 H2O
2 O–
Enolase
C
C
O
P O
CH2 Phosphoenolpyruvate
ATP O–
C
C
O
O
CH3
2
6
8
7
9
10
O
Pyruvate
Glyceraldehyde-3-P
O
Gly
coly
sis,
pay
off p
hase
10
5 sec
30 sec
Melvin Calvin
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Photorespiration
Oxygen is a competing substrate for Rubisco
Rubisco
CO2
O2
5 Carbons
3 C Acid 3 C Acid
2 C Acid 3 C Acid
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Solutions to the Oxygen Problem: C4 Pathway
e.g. Corn
• Division of Labor between Cells
• Keep O2 and Rubisco separated
3
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CO2
PEP carboxylase
Oxaloacetate (4 C) PEP (3 C)
Malate (4 C)
ADP
ATP
CO2
Pyruate (3 C)
CALVIN CYCLE
Sugar
Vein
CO2
NADPH used
NADPH regenerated
Cyclic e- flow Little O2
Oxaloacetate (4 C)
C4 pathway
In Mesophyll Cell • Produce
NADPH & ATP • Capture CO2
In Bundle Sheath Cell
• Produce Sugar (Calvin Cycle)
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CAM Plants
DAY • light reactions • mostly cyclic e- flow to produce
ATP (low O2) • decarboxylate malate to yield
CO2 and NADPH • Use C3 Calvin Cycle to
produce sugars and starch
NIGHT • CO2 Fixation PEP carboxylase • Accumulate malate in vacuole • Get energy from sugar oxidation
(NADH and ATP)
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Pineapple Sugarcane
Spatial separation
Bundle-sheath cell
Mesophyll Cell Organic acid
CALVIN CYCLE
Sugar
CO2 CO2
Organic acid
CALVIN CYCLE
Sugar
C4 CAM
Temporal separation .
Night
Day
Figure 10.20
Sunlight Powers Both
phases
Sugar Oxidation
Powers One
Phase
Compare C4 and CAM