Question

Calculate the energy cost of the Calvin cycle combined with glycolysis and oxidative phosphorylation, that is, the ratio of the energy spent synthesizing starch from $C{o}_2$and photosynthetically produced NADPH and ATP to the energy generated by the complete oxidation of starch. Assume that eachNADPH is energetically equivalent to 2.5 ATP and that starch biosynthesis and breakdown are mechanistically identical to glycogen synthesis and breakdown.

Short answer

The ratio of energy spent by synthesizing starch from 6$C{O}_2$ and produced NADPH and ATP generated photosynthetically to the energy generated by the complete oxidation of starch is 1.5.

Step-by-step solution

Photosynthesis

The biological process by which food's chemical energy is liberated and transformed to ATP is known as cellular respiration.

Carbohydrates, lipids, and proteins can all be used as fuel in cellular respiration. Glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation are the four major stages. The Calvin cycle is a set of metabolic events that take place within the stroma of chloroplasts and transform atmospheric carbon dioxide into sugar molecules.

The production in the Calvin cycle

⦁ The first step of Calvin cycle involves the production of$2-glyceraldehyde-3-phosphate\left(GAP\right)$ from 6$C{O}_2$and consumes 18ATP and 12NADPH which is equivalent to30ATP .

⦁ During gluconeogenesis, is converted into glucose-6-phospate(G6P) and this reaction, does not require energy input nor the isomerization of G6P to glucose-1-phosphate(GIP)

⦁ The activation of GIP to its nucleotide derivative uses 2ATP equivalents, but ADP is released when the glucose residue is incorporated into starch.

Therefore, the overall energy investment

18+30+1$=$49ATP

The production in starch breakdown

⦁ The breakdown of the starch with the help of phosphorolysis gives glucose-1-phospate(GIP). .The degradation of GIP by glycolysis produces 3ATP,2NADH , (which is equivalent to 5ATP ), and 2 pyruvate.

⦁ The complete oxidation of 2 pyruvate to6$C{O}_2$ (with the help of pyruvate dehydrogenase reaction and the citric acid cycle) produces 8NADH (equivalent to20ATP ), 2FAD$H_2$ (equivalent to 3ATP), and 2GTP (equivalent to 2ATP ).

Therefore, the overall is estimated to be;

$3+5+20+3+2=33ATP$

Therefore, the ratio of energy spent to energy recovered is $49/33=1.5$ ⦁