Fig. 8.2 The Calvin Cycle (reductive pentose phosphate cycle) 3 Stages Carboxylation Reduction Regeneration A 3 carbon molecule An outline of C3 photosynthesis

Carboxylation The key initial step in C3 photosynthesis RUBP + CO 2 ---> 3-PGA Catalyzed by “Rubisco”: ribulose 1,5-bisphosphate carboxylase-oxygenase binds the 5C RUBP molecule and 1C CO 2, making two 3C molecules. 5 C + 1 C -----> 2 x 3C molecules Fig. 8.3 (partial)

Fig. 8.2 Carboxylation Reduction Regeneration

Reduction steps of the Calvin Cycle use ATP and NADPH to produce a carbohydrate, glyceraldehyde 3 phosphate. 3PGA + ATP + NADPH --> G3P G3P can be used to make sucrose or starch Reduction

Fig. 8.3 (partial) - the reduction steps

Fig. 8.2 Carboxylation Reduction Regeneration

The regeneration steps of the Calvin Cycle use ATP to regenerate RUBP from some of the glyceraldehyde-3-P so the cycle can continue. Some of the carbohydrate is converted back into ribulose 1,5 bisphosphate, the initial CO 2 receptor molecule.

Fig. 8.3 (partial) - the regeneration steps

RUBP 3-PGA Fig carbon molecules, hence “C3” photosynthesis

1.Carboxylation. 1 CO 2 binds to 1 RuBP (5C) producing two molecules of 3-PGA (total of 6 C). 2. Reduction. The two 3-PGA (3 C each) are reduced to two glyceraldehyde 3 phosphate (G3P, 3 C each) using ATP and NADPH produced by the light reactions (still 6 C). 3. Regeneration. 5 of the 6 C in the 2 molecules of G3P are used to regenerate one RuBP (5C) using ATP. A total of 6 turns of the Calvin cycle are required to make one hexose (6C). This requires 18 ATP + 12 NADPH. Reviewing the Calvin cycle and counting carbon (C) atoms associated with one carboxylation.

6 turns (6 CO 2 ) of the Calvin cycle are required to make one hexose (6C). This requires 18 ATP + 12 NADPH. How much light energy is required to produce hexose? Minimum of 8 (often 9 to 10) photons required per CO 2 fixed (remember quantum yield?) Red light (680nm) = 175kJ/mol photons (from E = h  6 CO 2 /hexose x 8photons/CO 2 x 175 kJ/photon = 8400 kJ/mole hexose

What is the energy efficiency of hexose production? 8400 kJ/mole hexose (for the red light example!) One mole of hexose (e.g. glucose or fructose) yields about 2800 kJ when it’s oxidized. (The heat of combustion) Efficiency = energy output/energy input = 2800kJ/8400kJ = 33% This is the maximum overall thermodynamic efficiency of photosynthesis. Actual efficiency is much lower because: 1) quantum yield is < 1 CO 2 /8 photons 2) higher energy light (  < 680nm) is used

Fig 9.8 Typical light response of photosynthesis for a C3 plant Quantum yield =CO 2 fixed/photon absorbed

In standard air, 21% O 2. In low O 2 air, 2%.

Why does decreasing the O 2 concentration around a C3 leaf increase the uptake of CO 2 ? Why is this effect not seen in some plants such as corn, sugar cane, and many grasses common in warm environments?

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 PEPcase activity is not affected by O 2. PEPcase uses HCO 3 -, not CO 2. [HCO 3 - ] > [CO 2 ]

C4 leaf anatomy model (Fig 8.8d)

C4 leaf anatomy (Fig. 8.9a)

C4 leaf anatomy differs from C3 Primary carboxylation is spatially separated from the Calvin cycle.

The C4 system concentrates CO 2 at Rubisco. This is particularly useful in warm environments because 1) the solubility of CO 2 decreases more with temperature than the solubility of O 2. 2) Allows C4 plants to operate with lower stomatal aperture (conductance), thereby losing less water.

Extra ATP cost of regenerating PEP means that C4 CO 2 fixation requires more light energy. 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