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O 2. O 2. O 2. O 2. PHOTORESPIRATION. Definition 1:. O 2. The aberrant use of oxygen by chloroplasts . Definition 2:. O 2. An interference with carboxylation caused by the deviant interaction of RUBISCO with oxygen. A process that leads to only one 3PGA being

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photorespiration

O2

O2

O2

O2

PHOTORESPIRATION

Definition 1:

O2

The aberrant use of oxygen by chloroplasts

Definition 2:

O2

An interference with carboxylation caused by

the deviant interaction of RUBISCO with oxygen

A process that leads to only one 3PGA being

produced in the dark reaction in chloroplasts

slide2

CH2OP

CH2OP

CH2OP

C-OH

HO-

HO-

C-O-OH

C-O-OH

H2O

CH2OP

C-OH

C-OH

C=O

HO-

C=O

C-OH

C-OH

C-OH

C-OH

CH2OP

CH2OP

CH2OP

C-OH

CH2OP

CH2OP

COO

COO

C-OH

PO4

CH2OP

CH2OH

COO

O2

RuBP

Phospho-

glycolate

3PGA

Peroxisomes

Glycolate

Calvin Cycle

Photoefficiency is cut in half

slide4

Glyoxylate

CHNH3

CH2NH3

CH2NH3

CHNH3

CH2OH

CHO

COO

COO

COO

COO

COO

COO

NH3+

Glycerate

CH2OH

CH2OH

NAD+

NH3+

CH2OH

CH2OH

NADH

CHOH

C=O

COO

COO

Hydroxy

pyruvate

From

Chloroplasts

O2

Mitochondria

Glycolate

2

Glycine

CO2

ATP

3PGA

Serine

Peroxisomes

why photorespiration
Why Photorespiration?

1. O2 (21% of air) is more prevalent than CO2 (0.03%)

2. RUBISCO reacts with O2 (Km =200 M) as well as CO2 (Km=20 M)

RUBISCO IS BOTH AN OXYGENASE and a CARBOXYLASE

3. Phosphoglycolate can be salvaged.

Glycine  Serine (mitochondria)releases CO2

Glycolate  Glyoxylate (peroxisomes) consumes O2

O2 in CO2 out is respiration

strategy for preventing photorespiration
Strategy for Preventing Photorespiration

PLAN

Avoid RUBISCO

Fix CO2 in an environment shielded from O2

Use an enzyme that does not react with O2

strategy cont

Take

Home

Strategy (cont.)

SOLUTION

CO2 fixation occurs in Mesophyll cells

CO2 fixing enzyme is not RUBISCO

CO2 fixing enzyme is PEP carboxylase

PEP carboxylase will not react with O2

RUBISCO never changed

Instead plant anatomy changed

slide8

Anatomy of C3 vs C4 Plants

Calvin Cycle Cells

slide9

Mesophyll cell

Bundle-sheath cell

Oxaloacetate

Malate

Malate

Calvin

cycle

PPi + Pi +

CO2

CO2

ATP

AMP

CO2

Phosphoenol-

pyruvate

pyruvate

pyruvate

ATP energy is required to concentrate CO2

in bundle-sheath cells

slide10

Hatch-Slack C4 Pathway for concentrating CO2

PEP

Carboxylase

COO-

COO-

COO-

ATP + Pi

AMP + PPi

Pi

=

C=O

C-H

HO-

C~OPO3

CH2

CH2

CH2

COO-

COO-

NADPH

NADP+

Malate C4

COO-

C=O

CH3

NADP+

NADPH

Pyruvate

Calvin Cycle

2, 3PGA

Mesophyll cell

2Pi

Malate

dehydrogenase

Pyruvate

phosphate

dikinase

CO2

PEP

OAA C4

Malic enzyme

CO2

RUBISCO

Bundle-sheath cell

key points
Key Points

1. Mesophyll cells have no RUBISCO; Bundle Sheath do

2. Mesophyll cells condense CO2 with PEP-C4 cycle

3. OAA and malate are the C4 components

4. In bundle sheath cells malate is oxidatively decarboxylated to pyruvate to release the CO2

5. CO2 released enters the Calvin Cycle-C3 cycle

6. In mesophyll cells, pyruvate is converted to PEP by the

enzyme pyruvate-phosphate dikinase

7. Pyruvate-phosphate dikinase uses the equivalent of

2 ATPs to synthesize PEP

c 3 vs c 4 plants a lesson in photoefficiency
CO2 directly

RuBP recipient

RUBISCO open

O2 can interfere

Photorespiration likely

CO2 indirectly

PEP recipient

RUBISCO shielded

O2 cannot interfere

No photorespriation

C3 vs C4 PlantsA Lesson in Photoefficiency

C3

C4

c3 and c4 plants
soybean

wheat

rice

sugar beet

alfalfa

spinach

tobacco

sunflower

corn

sorghum

sugar cane

millet

crab grass

Bermuda grass

pigweed

C3 and C4 Plants

C3

C4

what did you learn
What did you Learn?
  • Photorespiration is to be avoided
  • RUBISCO is an oxidase/carboxylase
  • Oxygen cuts photoefficiency in half
  • C3 and C4 plants differ in anatomy
  • Mesophyll cells fix CO2 to PEP
  • Bundle sheath cells have Calvin cycle
  • C4 plants grow more ferociously