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BIOL350

Spring 10

Other CO2 fixation mechanisms

1. C3 pathway to fix CO2 to make sugars C3 reduction: or Calvin cycle (Melvin Calvin 1950s, Nobel prize in 1961) CO2 → C3 → C6

2. Regulation of key enzymes by light

3. Many plants have photorespiration

4. C4 plants maximize CO2 fixation using a C4 pathway that increases CO2 concentration

5. CAM plants minimize water loss by fixing CO2 at night.

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BIOL350

Spring 10

C4 Carbon Cycle - Concepts

PHOTORESPIRATION: O2 is consumed and CO2 is released in the light. Why? O2 consumption by RUBISCO

C4 METABOLISM: - Mechanism to concentrate CO2 in chloroplasts to minimize RUBISCO oxygenase activity- primary carboxylation is catalyzed by phosphoenolpyruvate carboxylase (PEPcase) → C4 acid- release of CO2 from C4 acid for Calvin cycle- regeneration of CO2 acceptor, PEP

C4 Plants: e.g. corn, sugarcane

2 different cell types: MESOPHYLL CELL BUNDLE SHEATH CELL

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BIOL350

Spring 10

C4 Leaf Morphology

Kranz anatomy

Bundle sheath cells:Unstacked thylakoidsLarge starch granules

Mesophyll cells:Stacked thylakoidsLittle or no starch

→ Cell-specific compart-mentalization of C4 cycle enzymes

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BIOL350

Spring 10

C3 vs. C4 Leaf Morphology

C4 Monocot: Sugarcane

C3 Monocot: Barley, Rice

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BIOL350

Spring 10

C4 Leaf Morphology

C4 Dicot: Flaveria australasica Australian Yellow Weed

(Asteraceae)

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BIOL350

Spring 10

Basic C4 photosynthetic carbon cycle

4 Stages:

1) Carboxylation: formation of C4 acid in mesophyll cell

2) Transport of C4 acid into bundle sheath cell

3) Decarboxylation of C4 acid → generation of high [CO2]; Fixation of released CO2 in Calvin cycle

4) Transport of C3 acid and regeneration of PEP acceptor

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BIOL350

Spring 10

C4 Carbon Cycle

4 Stages:

1. Carboxylation: in MESOPHYLL CELL CO2 + PEP (C3) → OAA (C4) → MAL (C4)

2. Transport of MAL into BUNDLE SHEATH

3. Decarboxylation: MAL (C4) → Pyruvate + CO2 → Calvin Cycle

4. Transport of pyruvate back to MESOPHYLL CELL, regeneration of PEP

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BIOL350

Spring 10

C4 Carbon Cycle

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BIOL350

Spring 10

C4 photosynthetic pathway

CO2 concentrates in bundle sheath cells

10

BIOL350

Spring 10

Single Cell C4 photosynthesisBorszczowia Chenopodiaceaearalocaspica (goosefoot family)

Bienertia cycloptera

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BIOL350

Spring 10

Single Cell C4 photosynthesisBorszczowia aralocaspica Bienertia cycloptera

Confocal fluorescence of a chlorenchyma cell of Borszczowia aralocaspica (A) and Bienertia cycloptera (B) illustrating the chloroplasts in the two cytoplasmic compartments. The broken white lines show the outline of a single cell. Scale bars = 20 mm.

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BIOL350

Spring 10

CAM (Crassulacean Acid Metabolism)

CAM plants fix CO2 at night

Amaranthus tricolor (pigweed)

Agave

Also:PineappleOrchidsCacti

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BIOL350

Spring 10

DARK: Stomata opened

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BIOL350

Spring 10

Light: Stomata closed

How do plants regulate this?

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BIOL350

Spring 10

CAM PEPcase is active in the night

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BIOL350

Spring 10

Regulation of CAM

1. PEP Carboxylase: is shut down during the day.

2. MAL product inhibits PEPcase in the day.

Summary:

CAM reduces water loss by separating reactions in time.

CAM plants are adapted to dry habitats.