One of the most important concepts in modern biochemistry is that standard Gibbs
ID: 970773 • Letter: O
Question
One of the most important concepts in modern biochemistry is that standard Gibbs free energy changes don't tell you a damn thing about whether a reaction is going to proceed inside a cell. That because most biochemical reactions are near-equilibrium reactions where G = 0. When that's not true, as in the case of ATP hydrolysis, you need to know why it's not true and the actual concentrations of products and reactants.What does this mean? Do you agree? If so, why? One of the most important concepts in modern biochemistry is that standard Gibbs free energy changes don't tell you a damn thing about whether a reaction is going to proceed inside a cell. That because most biochemical reactions are near-equilibrium reactions where G = 0. When that's not true, as in the case of ATP hydrolysis, you need to know why it's not true and the actual concentrations of products and reactants.
What does this mean? Do you agree? If so, why?
What does this mean? Do you agree? If so, why?
Explanation / Answer
the standard Gibbs free energy changes aren't very useful when you're dealing with charged molecules and the ATP hydrolysis reactions have charged molecules—even when some of the negative charges are neutralized by Mg2+ ions. That's why it's better to calculate new values of "standard" Gibbs free energy changes in the presence of Mg2+ ions and an ionic strength that's closer to physiological values.
standard Gibbs free energy changes don't mean very much in biochemistry. What really counts is the actual free energy change and that depends on the in vivo concentrations of reactants and products.
we know that the actual Gibbs free energy isn't the same as the standard Gibbs free energy but what is it's actual value? The short answer is that in most cases we don't know. It has to be some large negative value but it's very difficult to measure the concentrations of ATP, ADP, AMP, and inorganic phosphate inside cells.
The situation isn't entirely hopeless since there are some good estimates. Unfortunately the best examples come from rat hepatocytes and erythrocytes1. We don't know if this is typical of all cells (bacteria, plants etc.) or whether mammalian cells are special.
We can calculate the actual Gibbs free energy change for ATP hydrolysis (to ADP) given the known concentrations of reactants and products in rat hepatocytes. The answer is G = -48 kJ mol-1. Thus, the actual Gibbs free energy change is 1½ times the standard Gibbs free energy change. This has important consequences when we try to figure out how ATP is synthesized and where that energy comes from.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.