Photosynthesis, Environment & Adaptation

C3 vs. C4 vs. CAM Photosynthesis

How does matter move in and out of plant cells? Think about the process

that matter must go through to move in or out of a plant cell.

Diffusion

What two molecules must enter a plant

cell in order for photosynthesis to

take place?

Carbon dioxide & Water

What one molecule must leave a plant cell in order for photosynthesis to take place?

Oxygen

While molecules move in and out of a plant cell via diffusion, they actually enter and exit the plant through tiny openings in the leaf called stomata.

Stomata in Duck Weed

When the stomata close, what two molecules can’t

exit the leaf?

When the stomata close, what molecule (think about

photosynthesis) can’t enter the leaf?

So, what happens to the rate of photosynthesis when the stomata close?

C3 Photosynthesis

C3 plants evolved in a high CO2 environment

Examples: Wheat, rice, tomatoes, oak trees

Photosynthesis in the Presence of CO2

Photorespiration in the Presence of O2

When C3 Plants close their stomata, they begin a process called photorespiration•The concentration of O2 builds up inside the leaf.

•The enzyme rubisco attaches O2 to RuBP instead of CO2.

•The intermediate product breaks down to one PGA and one glycolate.

What are the implications of photorespiration?

•Only one molecule of PGA enters the Calvin Cycle

•Glycolate is broken down and releases a CO2

•Photorespiration is inefficient for plants

C4 Photosynthesis

C4 plants evolved independently in a hot, dry environment where their stomata MUST be partially closed during the day

Examples: Corn, sugar cane and grasses

What happens in C4 Photosynthesis?

•C4 plants have a different cell structure than C3 plants.•The Calvin Cycle only takes place in bundle sheath cells

What happens when the stomata close & O2 builds up?

PEP carboxylase only bonds with CO2, forming oxaloacetate.

The intermediate product malate diffuses to adjacent bundle sheath cells where malate breaks down to pyruvate & CO2.

The CO2 then enters the Calvin Cycle and rubisco binds to the CO2 and the Calvin Cycle proceeds as usual.

ATP is used in the process, but photorespiration is minimized and sugar production is increased.

What is the impact on Carbon fixation?

Photorespiration: An Evolutionary Relic?

• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound

• In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle

• Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

• Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle

• In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

C4 Plants

• C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

• This step requires the enzyme PEP carboxylase

• PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low

• These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 10-19

C4 leaf anatomy

Mesophyll cellPhotosyntheticcells of C4

plant leafBundle-sheathcell

Vein(vascular tissue)

Stoma

The C4 pathway

Mesophyllcell CO2PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C)ADP

ATP

Pyruvate (3C)

CO2

Bundle-sheathcell

CalvinCycle

Sugar

Vasculartissue

Fig. 10-19a

Stoma

C4 leaf anatomy

Photosyntheticcells of C4

plant leaf

Vein(vascular tissue)

Bundle-sheathcell

Mesophyll cell

Fig. 10-19b

Sugar

CO2

Bundle-sheathcell

ATP

ADP

Oxaloacetate (4C) PEP (3C)

PEP carboxylase

Malate (4C)

Mesophyllcell

CO2

CalvinCycle

Pyruvate (3C)

Vasculartissue

The C4

pathway

CAM Plants

• Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon

• CAM plants open their stomata at night, incorporating CO2 into organic acids

• Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings