Fig. 8.2

The Calvin Cycle(reductivepentose phosphatecycle)

3 Stages•Carboxylation•Reduction•Regeneration

A 3 carbon molecule

An outline of C3 photosynthesis

Carboxylation•The key initial step in C3 photosynthesis•RUBP + CO2 ---> 3-PGA •Catalyzed by “Rubisco”: ribulose 1,5-bisphosphate carboxylase-oxygenase• binds the 5C RUBP molecule and 1C CO2, 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

RegenerationThe regeneration steps of the Calvin Cycleuse ATP to regenerate RUBP from some ofthe glyceraldehyde-3-P so the cyclecan continue.

Some of the carbohydrate is converted backinto ribulose 1,5 bisphosphate, the initial CO2

receptor molecule.

Fig. 8.3 (partial) - the regeneration steps

RUBP3-PGA

Fig. 8.43 carbon molecules,

hence “C3” photosynthesis

1. Carboxylation. 1 CO2 binds to 1 RuBP (5C) producingtwo 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 CO2) 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 CO2 fixed (remember quantum yield?)

•Red light (680nm) = 175kJ/mol photons (from E = h

•6 CO2/hexose x 8photons/CO2 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 CO2/8 photons2) higher energy light (< 680nm) is used

Fig 9.8Typical light response of photosynthesis for a C3 plantQuantum yield

=CO2 fixed/photon absorbed

In standardair, 21% O2.

In lowO2 air, 2%.

Why does decreasing the O2 concentration around a C3 leaf increase the uptake of CO2?

Why is this effect not seen in some plants such as corn, sugar cane, and many grasses common in warm environments?

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 PEPcase activity is not affected by O2.

PEPcase uses HCO3-, not CO2.

[HCO3-] > [CO2]

C4 leaf anatomy model (Fig 8.8d)

C4 leaf anatomy (Fig. 8.9a)C4 leaf anatomy (Fig. 8.9a)

C4 leaf anatomy differs from C3Primary carboxylation is spatially separated from the Calvin cycle.

The C4 system concentrates CO2 at Rubisco.

This is particularly useful in warm environments because

1) the solubility of CO2 decreases more with temperaturethan the solubility of O2.

2) Allows C4 plants to operate with lower stomatalaperture (conductance), thereby losing less water.

Extra ATP cost of regenerating PEP means that C4 CO2 fixation requires more light energy.

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