Alternative Mechanisms of

Carbon FixationC3 plants

C4 plants

CAM plants

The story so far….

Rubisco

• Catalyzes 2 reactions:

▫ Addition of CO2 to RuBP(carboxylation)

Photosynthesis

▫ Addition of O2 to RuBP(oxidation)

Photorespiration

BOTH happening inside plants!

What Weather Has To Do With It…

• Stomata gas exchange & temperature control

• When it is hot, stomata openings shrink to conserve water

▫ CO2 concentrations decline inside the leaf

• Cells continue to undergo photosynthesis

▫ oxygen levels increase – can’t exit via stomata

• Oxygen competes with CO2 for Rubisco’s active site

▫ Oxygen has a higher affinity ∴ less CO2 binds

Photorespiration

• “Photo” – needs products from light reactions

• Removes PGA from Calvin Cycle…slowing carbohydrate production.

• C3 plants lose up to ¼ of the carbon they fix to photorespiration.

Don’t’ Copy…Just Look

Photosynthesis Photorespiration

Rubisco Substrate: Rubisco Substrate:

Products: Products:

What Happens? What Happens?

Optimal Temperature: Optimal Temperature:

CO2

GlucoseO2

Calvin cycle proceeds “normally”

15-25°C

O2

PGACO2

- No ATP produced- No G3P produced- Loses C

30-47°C

Photorespiration

• As C3 photosynthesis declines, photorespiration increases … could kill the plant due to lack of food.

• Then why???

▫ Rubisco a remnant of earlier mechanisms better suited to high atmospheric CO2

C4 Plants (4-Carbon molecule)

Enzyme:

Intermediary Molecule:

Types of Plants:

Types of Cells:

Purpose:

Energy Comparison to C3 Plants:

Phosphoenol pyruvate Carboxylase (PEP carboxylase)

Oxaloacetate (OAA) & Malate

Drought resistant

Keep [CO2] high to compete with O2 for Rubisco

Ex. corn, sugarcane

Bundle sheath: surround veinsMesophyll – around bundle sheath

ATP required to convert pyruvate into PEP

Requires more energy

C4 Plant Photosynthetic Cells• Bundle Sheath: site of Calvin cycle

• Mesophyll

▫ No mesophyll cells are more than 3 cells away from any bundle sheath cell at anytime.

How Do C4 Plants Reduce

Photorespiration?

• Continually move CO2 into bundle sheath cells, via malate

• Keeps CO2 concentrations high so that it may outcompete oxygen.• 10-120x higher than normal

• Using more ATP is still advantageous in a HOTclimate!

CAM PlantsCrassulacean Acid Metabolism

Stomata Action: Why?

NIGHT:Enzyme: Intermediary:

Storage Location:

DAY:What happens?

Night – open Day – closed

Conserve water

PEP carboxylase Malic acid

Vacuole

- Malic acid broken down- CO2 released- Calvin cycle occurs

In Summary…

Classwork/Homework