Question

ENERGENTICS OF PHOTOSYNTHESIS (6 pts): One of the common sugars that plants produce is sucrose (12-carbon sugar). The chemical bonds in sucrose contain 5643 kJ of energy. We know from Web Topic 8.4 that the Calvin-Benson Cycle uses about 3648 kJ of energy for every six carbons fixed.

What proportion of the energy fixed is converted to energy in the bonds of sugar? This gives us an idea of how efficient the Calvin-Benson Cycle is (2 pts).

Nine moles of photons are used to fix 2 NADPH and 3 ATP. How many moles of photons are required to supply the necessary products to supply enough of these molecules to allow for the production on sucrose (3 pts)?

Answer 1

Sucrose molecular formula = C₁₂H₂₂O₁₁
Energy stored in the chemical bonds of sucrose = 5643 kJ
Energy used by C3 cycle to fix 6 carbons = 3648 kJ
Energy used by C3 cycle to fix 12 carbons = 3648 X 2 = 7296 kJ

Energy efficiency of C3 cycle = 5643/7296 = 0.773 = 77.3%

Nine moles of photons are required to fix 2 NADH and 3 ATP.
1 glucose molecule requires 18 ATP and 12 NADH.
1 sucrose molecule requires 36 ATP and 24 NADH

So, number of moles of photons required = 12 moles

(Here, the energy required for the formation of a glycosidic bond between two glucose molecules to produce sucrose is not considered.)