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Photosynthesis:Variations on the Theme

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Remember what plants need… Photosynthesis

light reactions light H2O

Calvin cycle CO2

What structures have plants evolved to

supply these needs?

sun

ground

air

O

OC

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Leaf Structure

H2O

CO2

O2 H2O

phloem (sugar)xylem (water)

stomate guardcell

palisadeslayer

spongylayer

cuticleepidermis

O2 CO2

Transpiration

vascular bundle

Gas exchange

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Controlling water loss from leaves Hot or dry days

stomates close to conserve water guard cells

excess H2O = stomates open

conserve H2O = stomates close

adaptation to living on land, but…

creates PROBLEMS!

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When stomata close…

xylem (water)

phloem (sugars)

H2OO2 CO2

Closed stomata lead to… O2 build up inside the plant from light reactions

CO2 is depleted /can’t enter for the Calvin cycle causes problems in Calvin Cycle

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RuBisCo

Carbon Fixation enzyme in the Calvin Cycle

Fixes Carbon Dioxide onto RuBP

• When the concentration of oxygen is high/ CO2 is low, RuBisCo will fix Oxygen onto RuBP in a

process called photorespiration.

•The products of fixed O2 are not particularly healthy for the plant, and most assuredly don’t

lead to the formation of glucose, like the fixation of CO2 does.

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6Cunstable

intermediate

1C CO2

Calvin cycle when CO2 is abundant

5CRuBP

3CPGA

ADP

ATP

3C

NADP

NADPH

ADP

ATP

Leaveto make glucose

3C

2 PGAL

5C

RuBisCo

C3 plants

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Calvin cycle when O2 is high

5CRuBP

3C2C

to mitochondria

–––––––without

making ATP

photorespiration

O2

RuBisCo

1 PGA

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Impact of Photorespiration Oxidation of RuBP

short circuit of Calvin cycle reduces production of photosynthesis

no C6H12O6 (food) produced

if photorespiration could be reduced, plant would become 50% more efficient pressure to evolve alternative carbon fixation systems

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Reducing photorespiration Separate carbon fixation from Calvin cycle

C4 plants PHYSICALLY separate carbon fixation from Calvin cycle

different cells to fix carbon vs. where Calvin cycle occurs store carbon in 4C compounds

different enzyme to capture CO2 (fix carbon) PEP carboxylase

CAM plants separate carbon fixation from Calvin cycle by TIME OF

DAY fix carbon during night

store carbon in 4C compounds

perform Calvin cycle during day

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C4 plants are so named because instead of forming PGA’s in the beginning of the Calvin

cycle, they form a four-carbon compound OAA (Oxaloacetate)

Several thousand species use the C4 pathway.

Instead of being fixed by rubisco, CO2 combines with a 3Carbon molecule PEP to form OAA, using the fixing enzyme PEP carboxylase.

• OAA is then converted to malate, and that is shuttled to the bundle sheath cells.

• Here, malate is converted to pyruvate and CO2.

• The pyruvate moves back to the mesophyll cells where one ATP is broken down to form AMP (not ADP) which is required to convert the pyruvate back to PEP (to help continue the cycle)

• The overall effect of this process is to move CO2 from mesophyll cells to the bundle sheath cells, in order to make photosynthesis more efficient.

AMP

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C4 plants A better way to capture CO2

1st step before Calvin cycle, fix carbon with enzymePEP carboxylase store as 4C compound

adaptation to hot, dry climates have to close stomates a lot

sugar cane, corn, other grasses…

sugar cane

corn

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C4 leaf anatomy

PEP (3C) + CO2 oxaloacetate (4C)

CO2

CO2

O2

light reactions

C4 anatomy

C3 anatomy

PEP carboxylase enzyme higher attraction for CO2 than O2

better than RuBisCo

fixes CO2 in 4C compounds

regenerates CO2 in inner cells for RuBisCo keeping O2 away from RuBisCo

bundlesheath

cell RuBisCo

PEPcarboxylase

stomate

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CAM (Crassulacean Acid Metabolism) plants

Adaptation to hot, dry climates separate carbon fixation from Calvin cycle by TIME

close stomates during day

open stomates during night

at night: open stomates & fix carbonin 4C “storage” compounds

in day: release CO2 from 4C acids into normal Calvin cycle increases concentration of CO2 in cells

succulents, some cacti, pineapple

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CAM plants

succulents

cacti

pineapple

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C4 vs CAM Summary

C4 plants separate 2 steps of C fixation anatomically in 2 different cells

CAM plants separate 2 steps of C fixation temporally =2 different times

night vs. day

solves CO2 / O2 gas exchange vs. H2O loss challenge

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Characteristics of Photosynthesis in C3, C4 and CAM plants

Characteristic C3 Plant C4 Plant CAM Plant

Photorespiration Yes Little none

Rubisco present Yes Yes Yes

PEP Carboxylase present No Yes Yes

Initial CO2 fixation directly into Calvin Cycle via Rubisco

into OAA via PEP carboxylase, then to

malic acid which moves from mesophyll cell to bundle sheath cell and then releases

CO2.

into OAA via PEP carboxylase, then to malic acid which

moves into vacuole (during night). CO2 released during the

day.

Secondary CO2 fixation ---------------- In bundle sheath cell using Rubisco

In “mesophyll”* cell using Rubisco – in morning

Site of Calvin cycle mesophyll cells bundle sheath cells Mesophyll cells