Question

You will have the opportunity to practice converting ΔE to ΔG using data from actual electron transport chains that power many of the organisms in your Winogradsky columns. When calculating theoretical ATP yields, please report fractional numbers (i.e., 3.33 rather than 3). Even though a cell cannot produce 0.33 ATPs, cells run their electron transport chains repeatedly so over time those bits of ATPs do add up to enough energy to produce actual ATP molecules. When calculating actual ATP yields, assume that 3 H+ passing through an ATP synthase is enough to produce one ATP.

Anoxygenic Photosynthesis – Anoxygenic photosynthesis is also not theoretically possible based on the identity of the electron donor and electron acceptor. The electron donor is H2S, which is converted to elemental sulfur (S0). The electron acceptor is NAD+.
a. Calculate ΔG. (0.9 pts)

b. How many ATP can theoretically be produced for each run of the electron transport chain? (0.8 pts)

c. 2 H+ are pumped by this electron transport chain. How many ATP are actually produced by each run of the electron transport chain?

Answer 1

a.

The actual Delta G's will be influenced by the drain of products, so that the second term in the equation for Delta G is being brought into play here.

Delta G = Delta Go + RTln([products]/[reactants]),    where RT~0.6 (in kcal/mole)

For instance, if the products are drained such that the ratio of P/R reaches down to 0.00001, this produces ~ 7 kcal/mole of negative Delta G , enough to balance out the unfavorable Delta Go

This indirect affect on the Delta G is the second method the cell utilizes to carry out an individual reaction that has an unfavorable Delta Go.

So we have 2 methods:

1.) DIRECT COUPLING of the unfavorable reaction to an energetically favorable one to produce a new coupled reaction with a net negative (favorable) Delta Go (as in the hexokinase reaction ).

2.) INDIRECTLY, via the WITHDRAWAL OF PRODUCTS or buildup of substrates

SO, we have our overall negative Delta Go , and we have generated net ATP

b.Calculations giving 36-38 ATP per glucose are based on the assumption that oxidation of NADH produces 3 ATP and oxidation of UQH2 (FADH2, Succinate) produces 2 ATP. ... They translocate protons outward across the inner mitochondrial membrane, and the resulting proton gradient is used by the ATP synthase to produce ATP.

c.

34 ATP

Electron transport chain

This stage produces most of the energy ( 34 ATP molecules, compared to only 2 ATP for glycolysis and 2 ATP for Krebs cycle). The electron transport chain takes place in the mitochondria. This stage converts the NADH into ATP.