Plants convert light energy into chemical potential energy stored in the covalen
ID: 101536 • Letter: P
Question
Plants convert light energy into chemical potential energy stored in the covalent bonds of glucose. The second law of thermodynamics governs this conversion, so it is less than 100% efficient.
Things you need to know to answer this question.
Light Reaction
Energy in calories (cal) of one mole of photons is calculated by:
2 moles of light photons (one mole captured by Photosystem I and another mole captured by Photosystem II) are required to energize each mole of electrons that is removed from water.
Four moles of electrons are used to make 3 moles of ATP and 2 moles of NADPH.
Dark Reaction
3 moles of ATP and 2 moles NADPH are required to fix one mole of carbon into glucose.
Energy Yield
The fixation of one mole of carbon into glucose requires 114,000 cal of energy.
Calculate the energy efficiency of carbon fixation in percent when a plant uses 602 nm (red) light
Explanation / Answer
Energy associated with hydrolysis of ATP
DGo' = -7.3 kcal. mol-1
Energy associated with hydrolysis of 3 moles of ATP
DGo' = -7.3 kcal. mol-1 x 3
21.9 kcal
For NADPH the free energy change is:
G° = 218 jK mol1
52.1032505 kcal. x 2
104.206 kcal
Thus total energy released = 21.9 kcal + 104.206 kcal
126.106 kcal
The fixation of one mole of carbon into glucose requires 114,000 cal of energy
Therefore the energy efficiency will be:
Total energy consumed/ Total energy released
114 kcal / 126.106 kcal
0.90400139565
= 90.4 %