Test: Principles of Bioenergetics - 1 - MCAT MCQ

An exothermic reaction produces heat as a product.
If a product of a reaction is added to an equilibrium mixture of elements of the reaction, the equilibrium will shift towards reactants.
The concentration of reactants will increase if an equilibrium mixture of A, B, and C is heated slowly.

The value of ΔG for an endothermic reaction depends on the specific reaction and the concentrations of reactants and products. Without additional information, we cannot determine the exact sign of ΔG. It could be positive, negative, or even zero, depending on the specific circumstances.

ATP contains high-energy phosphoanhydride bonds between its phosphate groups. During hydrolysis, the bond between the γ phosphate and the β phosphate is cleaved, resulting in the release of inorganic phosphate (Pi) and ADP (adenosine diphosphate). This hydrolysis reaction releases energy that can be utilized by cells for various biological processes.

Spontaneous reactions are those that occur without requiring an input of energy. One of the key characteristics of spontaneous reactions is that they tend to decrease the Gibbs free energy of the system. Gibbs free energy (ΔG) is a measure of the energy available to do work in a system, and for a spontaneous reaction, the ΔG value is negative. This means that in a spontaneous reaction, the free energy of the system decreases, indicating a more stable and energetically favorable state.

An exergonic reaction is a reaction that releases energy, while an endothermic reaction is a reaction that absorbs heat from its surroundings. In this case, since the reaction is exergonic, it indicates that the overall change in Gibbs free energy (ΔG) is negative, meaning the reaction is energetically favorable. However, since the reaction is endothermic, it absorbs heat, which is why it is not spontaneous under standard conditions.

When an endothermic reaction occurs, it often leads to an increase in entropy (disorder) of the system. This increase in entropy compensates for the energy input required for the reaction to proceed. Therefore, option A is correct.

The standard reduction potential (E∘) is a measure of the tendency of a species to gain electrons and undergo reduction. It provides information about the electron-donating or electron-accepting ability of a molecule.

In the case of NAD, which is an electron carrier involved in oxidation-reduction reactions, the reduced form NADH donates electrons to other molecules. This means that NAD is being oxidized (losing electrons) while acting as an electron donor.

The standard reduction potential of NAD is negative because it has a tendency to donate electrons and undergo oxidation. By convention, the standard reduction potential is measured with respect to a standard hydrogen electrode (SHE), which is assigned a potential of zero. A negative standard reduction potential indicates that NAD has a lower electron-donating ability compared to the standard hydrogen electrode.

A flavoprotein containing one flavin adenine dinucleotide (FAD) moiety can accept two electrons when it takes on its fully reduced form.

FAD is a redox-active molecule that can accept or donate electrons in biological reactions. It has two potential electron-accepting sites within its structure: the flavin mononucleotide (FMN) portion and the adenine nucleotide portion. Each of these sites can accept one electron during reduction.

When FAD accepts two electrons, it undergoes a two-electron reduction, resulting in the formation of FADH2 (flavin adenine dinucleotide in its fully reduced form).

When it is stated that ATP hydrolysis under standard biochemical conditions has ΔG << 0, it implies that the reaction is thermodynamically favorable and spontaneous. A negative ΔG indicates that the reaction releases free energy, making it energetically favorable to proceed in the forward direction.

Option A, "The hydrolysis reaction decreases entropy," is incorrect. ATP hydrolysis actually increases entropy because ATP is a high-energy molecule, and its hydrolysis leads to the production of ADP and inorganic phosphate (Pi), resulting in an increase in the total number of molecules.

Option C, "Cleavage of the phosphoanhydride bond that occurs during hydrolysis does not require the input of energy," is incorrect. ATP hydrolysis requires the input of water and catalysis by enzymes (such as ATPases) to break the phosphoanhydride bond, and it releases free energy in the process.

Option D, "The hydrolysis reaction is exothermic," is also incorrect. ATP hydrolysis is an endothermic reaction, meaning it requires the input of energy (in the form of water hydrolysis) to break the bond and release the stored energy in ATP.

Under standard physiological conditions, ADP (adenosine diphosphate) and AMP (adenosine monophosphate) both store positive free energy. However, the free energy stored by ADP is greater than the free energy stored by AMP. This is because ADP has one more phosphate group than AMP, and the additional phosphate group contains high-energy bonds that contribute to the overall free energy content of the molecule.

ATP hydrolysis is typically a highly spontaneous reaction, releasing energy and driving cellular processes. However, if the concentrations of ATP are significantly lower than the equilibrium concentration and the concentrations of ADP are significantly higher than the equilibrium concentration, the reaction may not proceed spontaneously.

In a non-equilibrium state where ATP concentrations are very low and ADP concentrations are very high, the reaction would not be thermodynamically favorable because the standard free energy change (ΔG°) for ATP hydrolysis assumes equilibrium concentrations. If the concentrations deviate significantly from equilibrium, the reaction may no longer be spontaneous.