Photosynthesis: Bioenergetics

 

Energy consumption of photosynthesis

• 2 quanta of light (1 quantum for PSI and PSII each) are required for transfer of 1 electron from H2O to NADP+.

• 4 electrons are required for reduction of H2O and evolution of 1 molecule of O2.

• Therefore, 8 quanta of light are required for evolution of 1 molecule of O2 and fixation of 1 molecule of CO2.

• This means that 48 quanta (8 x 6) of light are required to fix 6 molecules of CO2 and generate 1 molecule of glucose (6C).

Quantum yield = 48 quanta of red light

• 1 quantum = 42 Kcal, 48 quanta = 2016 Kcal

• The standard Gibbs free energy change for the synthesis of one mole of glucose from CO2 and water is approximately 673 kcal/mol. This is the chemical energy that plants store.

Photosynthetic efficiency = 673/2016 x 100 = 33%

C3 cycle

Fixation of 1 molecule of CO2 requires: 3 ATP and 2 NADPH

               1 ATP = 7.3 Kcal, 1 NADPH = 52.6 Kcal

Energy requirement to fix 6 molecules of CO2: 

18 ATP (3 x 6) = 18 x 7.3 Kcal = 131.4 Kcal 

                                

12 NADPH (2 x 6) = 12 x 52.6 Kcal = 631.2 Kcal

Total: 131.4 Kcal + 631.2 Kcal = 762.6 Kcal

Internal efficiency = 673/762.6 x 100 = 88%

C4 cycle

Fixation of 1 molecule of CO2 requires: 5 ATP and 2 NADPH

            1 ATP = 7.3 Kcal, 1 NADPH = 52.6 Kcal

Energy requirement to fix 6 molecules of CO2:

30 ATP (5 x 6) = 30 x 7.3 Kcal = 219 Kcal 

12 NADPH (2 x 6) = 12 x 52.6 Kcal = 631.2 Kcal

Total: 219 Kcal + 631.2 Kcal = 850.2 Kcal

Internal efficiency = 673/850.2 x 100 = 79%