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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

9

(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

13

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