I. Photorespiration

II. CO2 concentrating mechanisms - variation on the “C3” photosynthetic metabolism.

Plant of the day, Zea mays (Poaceae)

How does the photosynthetic response to light compare in corn and beans?

Corn

Bean

Corn vs. beanCorn has:1. Lower QY

2. Higher max.photosynthesis

3. Higher lightsaturation

4. O2 insensitive

The first step in the Calvin cycle is the carboxylation of RUBP by Rubisco.

Remember Rubisco’s full name?

Ribulose 1,5 bisphosphate carboxylase-oxygenase

Rubisco

Rubisco can catalyze the oxygenation (O2) of RuBP and the carboxylation (CO2) of RuBP.

Fig. 8.8

The set of reactions that begins with Rubiscooxygenation of RUBP is called photorespiration.

When Rubisco oxygenates RUBP, a CO2 is lostfrom the leaf, reducing the net uptake of CO2.

CO2 Carbon gain

+ RuBP

+O2 Carbon loss, photorespiration

What determines the rate of carboxylation vs. oxygenation?

What determines the reaction rates for any two competingsubstrates in an enzyme-catalyzed reaction?

Chloroplast stroma

RubiscoDeterminants of carboxylation vs. oxygenation.1. Concentration of CO2 & O2

2. Rubisco specificity for CO2 vs. O2

Concentration of O2 >> CO2, but Rubisco specificity favors CO2 binding.

CO2 O2

In standardair, 21% O2.

In lowO2 air, 2%.

Oxygenation of RuBP causes a loss of CO2 and reduces CO2 uptake.

So why does Rubisco have this inefficient property?

Consider Earth’s atmosphere 3 billion years ago.High CO2/low O2

20% CO2

no O2

Oxygenation was not a problem

CO2/O2 ratio has decreased greatly over Earth’s history

0.04% CO2 (and rising)21% O2

The O2 inhibition of CO2 uptake represents a huge selectivepressure for plant characteristics to prevent carboxylation.

How to avoid oxygenation?

1. Develop new Rubisco that’s insensitive to O2

2. Reduce O2 concentration in chloroplast

3. Increase CO2 concentration in chloroplast

Plants like corn show no effect of O2 concentration; apparently no oxygenation by Rubisco.

They also have different initial products; 14C label shows up firstin 4 carbon organic acids - malic acid, aspartic acid. These are called “C4” plants.

C4 plants have Rubisco, so how do they avoid oxygenation?

a) Initial carboxylation is not by Rubisco in C4 plants

b) C4 leaf anatomy differs

How does C4 biochemistry differ from C3?

• Primary carbon fixation step uses different substrates and enzymes.

HCO3- + PEP --------> 4 carbon organic

acidsPEP

carboxylase

Phosphenol pyruvate = PEPPhosphenol pyruvate carboxylase = PEPcase

Two important differences between PEPcase and Rubisco1. PEPcase activity is not affected by O2.

2. PEPcase uses HCO3-, not CO2.

[HCO3-] > [CO2]

C4 leaf anatomy model (Fig 8.8d)

Two distinct cell types:

1. Mesophyll (PEPcase)

2. Bundle sheath(Rubisco)

C4 leaf anatomy (Fig. 8.9a)

C4 leaf anatomy relates to its biochemistry

Initial carboxylation is in mesophll cells and is spatially separated from the Calvin cycle in the bundle sheath cells

The C4 biochemistry and anatomy concentrates CO2 in the b.s. cells at Rubisco.

This is advantageous in warm environments because:

1) the solubility of CO2 decreases more with temperaturethan the solubility of O2, so photorespiration is a biggerproblem in warmer environments.

2) C4 plants can operate with lower stomatal aperture (conductance), thereby losing less water.

Web Topic 8.3

Temp..

CO2/O2

CO2 and O2 solubilities

There’s no energetically free biochemical lunch!!The CO2 concentrating mechanism requires extra energy.

2

Extra ATP is needed to regenerate PEP, meaning that CO2 fixation by C4 plants requires more light energy than

C3 photosynthesis.

Symptoms of this added cost:

1. Quantum yield of C4 < C3

Extra ATP (light) cost is not a problem in high lightenvironments, but is in low light environments.

Few C4 “shade” plants.

Corn, a C4 plant

Bean, a C3 plant

Corn vs. bean1. Lower QY

2. Higher max.photosynthesis

3. Higher lightsaturation

4. O2 insensitive

C4 has two features that are advantages in warm, dryenvironments.1. Suppression of photorespiration (more C gain)2. Lower stomatal conductance (less water loss)

• C4 plants can achieve high photosynthetic rates at lower stomatal conductance than C3 plants. How?

Stomatal conductance

Photo-synthesis

C3C4

C4 plantsBecause of the CO2 concentrating mechanism, the [CO2] at Rubisco is much higher than in the leaf internal air spaces. A saturating level of [CO2] at Rubisco can be achieved at low stomatal aperture and current atmospheric [CO2]

C3 plantsThe [CO2] decreases from the leaf internal air spaces to the chloroplast, and photosynthesis is not saturated at current CO2 levels.