Bioenergetics Flashcards
True or False: If a reaction has a standard free energy change that is positive, then the free energy change in vivo will also be positive.
False Reactions that do not occur spontaneously under standard conditions always have a standard free energy (ΔGº) that is greater than zero. BUT under non standard conditions in vivo, the equilibrium and temp can impact free energy change.
Suppose that the standard change in free energy for the reaction between two substrates A and B, which yields product C, is equal to 5.3 kJ/mol. However, this reaction is found to occur spontaneously inside cells. Would the removal of product C be able to explain this?
Yes-Reducing the concentration of product C shifts the reaction’s equilibrium toward products because substrates are in much higher abundance. Under certain circumstances, this can even cause the reaction’s ΔG to become negative, allowing it to occur spontaneously.
Suppose that the standard change in free energy for the reaction between two substrates A and B, which yields product C, is equal to 5.3 kJ/mol. However, this reaction is found to occur spontaneously inside cells. Would coupling this reaction to ATP hydrolysis help explain this?
Yes-ATP hydrolysis provides roughly -30.5 kJ of free energy per mol under standard conditions. Although cellular conditions differ from standard conditions, ATP still provides a lot of energy. The energy released during ATP hydrolysis can be used to power the normally nonspontaneous reaction between substrates A and B.
True or false: In vivo, reactions that have positive ΔGº values cannot proceed forward without an external source of energy.
This statement is false. Reactions with positive ΔGº values are nonspontaneous under standard conditions. However, they can sometimes occur spontaneously inside cells (e.g., under nonstandard conditions) when the ratio of products to reactants is sufficiently small. This is an important strategy that cells use to drive nonspontaneous reactions forward.
Which of the following reaction conditions is most likely to give rise to a negative ΔG for a reaction that does not occur spontaneously under standard conditions? QK
Q
True or False: Exergonic reactions are spontaneous
Because exergonic reactions release energy, the reactants always have more free energy than the products. This is the reason that they are assigned negative ΔG values.
How can the net free energy of a spontaneous, coupled reaction be calculated from the ΔG values of its constituent steps?
Reaction coupling is a strategy for driving endergonic reactions in the forward reaction. The basic idea is that the exergonic steps provide energy to fuel the endergonic steps, resulting in a net negative ΔG value. To calculate it, we simply add the -ΔG values for the exergonic steps to the +ΔG values for the endergonic steps.
The standard Gibbs free energies of Cu2S decomposition and SO2 formation are provided in the table below. Suppose that these reactions are allowed to occur simultaneously in the same environment. What will the ΔG of Cu2S decomposition be?
Less than 86.2 kJ because of the rapid removal of sulfur from the reaction coordinate
The solid sulfur generated in the first reaction can react with O2 to form SO2. The net result is that solid sulfur is quickly removed from the reaction environment, which disturbs the equilibrium of reaction 1. To reestablish equilibrium, reaction 1 begins to generate more products. This is the basis of reaction coupling. Reaction 2, which is highly exergonic, provides energy to fuel reaction 1.
Suppose that a biochemical redox reaction has a standard free energy of +3.7 kJ/mol. How might the ratio of products/reactants impact the spontaneity of this reaction in nonstandard conditions?
It could occur spntaneously if the ratio is small.
Consider the equation ΔG = ΔGº + RTlnQ and the simple reaction: Reactant <=> Product. Since Q is the ratio of product to reactant, Q will be less than 1 when the concentration of product is lower than the concentration of reactant. This causes RTlnQ to be negative. If Q is sufficiently small, the negative RTlnQ term will outweigh the positive ΔGº value, giving rise to a reaction with a negative ΔG, which will take place spontaneously.
Viewing the first step of glycolysis as a coupled process and given the following information, what is the free energy change of the net reaction for production of glucose-6-phosphate?
Glucose + Pi → glucose 6-phosphate + H2O ΔGº = +13.9 kJ/mol
ADP + Pi → ATP + H2O ΔGº = +30.5 kJ/mol
-16.6kj/mol
ATP hydrolysis is generally coupled to energetically unfavorable biochemical reactions, which have positive ΔG values. Therefore, the first reaction provided in the question stem likely proceeds in the forward direction, whereas the second reaction must proceed in reverse. If we add -30.5 kJ/mol and +13.9 kJ/mol, we obtain a net ΔG of -16.6 kJ/mol.
