I. Photorespiration II. CO 2 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. bean Corn has: 1.Lower QY 2. Higher max. photosynthesis 3. Higher light saturation 4. O 2 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 (O 2 ) of RuBP and the carboxylation (CO 2 ) of RuBP. Fig. 8.8

The set of reactions that begins with Rubisco oxygenation of RUBP is called photorespiration. When Rubisco oxygenates RUBP, a CO 2 is lost from the leaf, reducing the net uptake of CO 2.

CO 2 Carbon gain + RuBP + O 2 Carbon loss, photorespiration What determines the rate of carboxylation vs. oxygenation? What determines the reaction rates for any two competing substrates in an enzyme-catalyzed reaction?

Chloroplast stroma Rubisco Determinants of carboxylation vs. oxygenation. 1. Concentration of CO 2 & O 2 2. Rubisco specificity for CO 2 vs. O 2 Concentration of O 2 >> CO 2, but Rubisco specificity favors CO 2 binding. CO 2 O2O2

In standard air, 21% O 2. In low O 2 air, 2%. Oxygenation of RuBP causes a loss of CO 2 and reduces CO 2 uptake.

So why does Rubisco have this inefficient property? Consider Earth’s atmosphere 3 billion years ago. High CO 2 /low O 2 20% CO 2 no O 2 Oxygenation was not a problem CO 2 /O 2 ratio has decreased greatly over Earth’s history 0.04% CO 2 (and rising) 21% O 2

The O 2 inhibition of CO 2 uptake represents a huge selective pressure for plant characteristics to prevent carboxylation. How to avoid oxygenation? 1. Develop new Rubisco that’s insensitive to O 2 2. Reduce O 2 concentration in chloroplast 3. Increase CO 2 concentration in chloroplast

Plants like corn show no effect of O 2 concentration; apparently no oxygenation by Rubisco. They also have different initial products; 14 C label shows up first in 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. HCO PEP >4 carbon organic acids PEP carboxylase Phosphenol pyruvate = PEP Phosphenol pyruvate carboxylase = PEPcase Two important differences between PEPcase and Rubisco 1. PEPcase activity is not affected by O PEPcase uses HCO 3 -, not CO 2. [HCO 3 - ] > [CO 2 ]

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 CO 2 in the b.s. cells at Rubisco. This is advantageous in warm environments because: 1) the solubility of CO 2 decreases more with temperature than the solubility of O 2, so photorespiration is a bigger problem in warmer environments. 2) C4 plants can operate with lower stomatal aperture (conductance), thereby losing less water.

Web Topic 8.3 Temp.. CO 2 /O 2 CO 2 and O 2 solubilities

There’s no energetically free biochemical lunch!! The CO 2 concentrating mechanism requires extra energy.

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Extra ATP is needed to regenerate PEP, meaning that CO 2 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 light environments, but is in low light environments. Few C4 “shade” plants.

Corn, a C4 plant Bean, a C3 plant Corn vs. bean 1.Lower QY 2. Higher max. photosynthesis 3. Higher light saturation 4. O 2 insensitive

C4 has two features that are advantages in warm, dry environments. 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 C3 C4

C4 plants Because of the CO 2 concentrating mechanism, the [CO 2 ] at Rubisco is much higher than in the leaf internal air spaces. A saturating level of [CO 2 ] at Rubisco can be achieved at low stomatal aperture and current atmospheric [CO 2 ] C3 plants The [CO 2 ] decreases from the leaf internal air spaces to the chloroplast, and photosynthesis is not saturated at current CO 2 levels.