Biochemistry : Biological Energetics

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Biological Energetics

A biochemist is studying two metabolic reactions at constant temperature and pressure. Reaction 1 is found to have a G value of . Reaction 2 has a G value of . Which statement is true about these reactions?

Possible Answers:

Reaction 1 and reaction 2 proceed at equally fast rates

None of the other answers are true

Reaction 1 and reaction 2 proceed at equally slow rates

Reaction 1 proceeds at a faster rate than reaction 2

Reaction 1 proceeds at a slower rate than reaction 2

Correct answer:

None of the other answers are true

Explanation:

The G values given in the question relate to the reactions' thermodynamics. A negative G value means that a reaction is thermodynamically spontaneous. A spontaneous reaction can occur without further energy input. This does not tell us anything about the reaction's rate (kinetics). A spontaneous reaction may be slow or it may be fast. 

Example Question #2 : Biological Energetics

The rate of a slow metabolic reaction is sped up by the addition of a catalyst. Which of the following is a possible function of the catalyst?

Possible Answers:

Decrease the free energy required to reach the transition state

Increase the energy of the transition state

Increase the activation energy

Decrease the G value

Increase the G value

Correct answer:

Decrease the free energy required to reach the transition state

Explanation:

A catalyst lowers the activation energy of a reaction. This could be accomplished through stabilization of the transition state by lowering its energy. A catalyst does not change the free energy difference between products and reactants, so no change in G of the reaction is observed.

Example Question #2 : Biological Energetics

Consider the hydrolysis of a molecule of ATP, as shown by the reaction below:

If the pH of the solution in which this reaction is occurring were to be lowered, what effect would this have on the above reaction?

Possible Answers:

The reaction will be pushed to the left

The equilibrium constant of the reaction will increase

The reaction will be pushed to the right

The equilibrium constant of the reaction will decrease

A change in pH will have no effect on the reaction

Correct answer:

The reaction will be pushed to the left

Explanation:

This question is asking us to determine what effect a pH change would have on the reaction in which ATP is hydrolyzed into ADP. First, we have to realize that if pH were lowered, that means we are dealing with a more acidic solution. A more acidic solution, in turn, means that we have an increased concentration of . So, in essence, the question is asking what effect an increased  concentration will have on the reaction. In the reaction, we can see that  is on the product side of the reaction (on the right side). Therefore, if we drive the  concentration up, the reaction will be pushed toward the left according to Le Chatelier's principle. Furthermore, it's important to note that changing the concentration of any of the reactants or products will not have an effect on the equilibrium constant, . In fact, it is precisely because the equilibrium constant doesn't change that the reaction will shift to the left, so that the constant will remain just that, constant. Moreover, the only thing that can change an equilibrium constant is the temperature at which the reaction takes place.

Example Question #3 : Biological Energetics

The standard free energy change for any chemical reaction becomes less favorable when __________.

Possible Answers:

 decreases and  decreases.

 increases and  increases.

 increases and  decreases

 decreases and  increases

temperature increases

Correct answer:

 increases and  decreases

Explanation:

The Gibbs free energy  becomes less favorable, or more positive, as enthalpy  increases and entropy  decreases.

Example Question #4 : Biological Energetics

For this reaction,  at given reaction conditions.

Which of the following statements concerning the reaction is true?

Possible Answers:

The mixture contains more reactants than products

The mixture contains an equal amount of reactants and products

More than one of these

Entropy increases as the reaction proceeds to the left

The mixture contains more products than reactants

Correct answer:

The mixture contains more products than reactants

Explanation:

Because  for the reaction is negative, it is spontaneous and proceeds favorably to the right. Equilibrium does not occur when the concentrations of reactants and products are equal; it occurs when the rates of forward and reverse reactions are equal. At equilibrium  thus there is no net change in the concentration of either the reactants or the products.

Example Question #5 : Biological Energetics

Regarding formation of double stranded DNA from two complementary single strands, which of the following is true?

Possible Answers:

None of these

The formation of dsDNA is entropically favorable, but is enthapically unfavorable.

The formation of dsDNA is entropically and enthalpically favorable.

The formation of dsDNA is entropically and enthalpically unfavorable.

The formation of dsDNA is entropically unfavorable, but is enthalpically favorable.

Correct answer:

The formation of dsDNA is entropically unfavorable, but is enthalpically favorable.

Explanation:

If two complementary single strands of DNA are put into a solution, they will spontaneously form dsDNA.  This process results in a loss of heat from the system - demonstrating that it is an enthalpically favorable process.  However, it is entropically unfavorable given the formation of a more ordered structure.

Example Question #6 : Biological Energetics

A person just eats a very large meal filled with starches and sugars. What is expected to happen to their insulin to glucagon ratio as a result? Assume the person is not diabetic. 

Possible Answers:

Insulin will levels will increase and glucagon levels will drop to zero

Insulin will increase relative to glucagon

The ratio will remain more or less constant

Glucagon will increase relative to insulin

Glucagon will levels will increase and insulin levels will drop to zero

Correct answer:

Insulin will increase relative to glucagon

Explanation:

Insulin is produced by pancreatic beta cells in response to a rise in blood sugar, which occurs after eating a meal rich in carbohydrates (sugars and starches). It causes other cells of the body to take up the blood sugar (primarily glucose) and use it for energy production, and it prompts the liver to store excess glucose as glycogen. Glucagon is produced when blood sugar is too low, and it blocks glycolysis and prompts liver cells to convert stored glycogen back to glucose and release it and also produce glucose through gluconeogenesis. This is always determined by the ratio of the levels of two hormones - if there is far more insulin than glucagon, its physiological effects will dominate. There is never absolutely zero of either hormone unless the individual is diabetic to such a degree that their cells cannot produce insulin. 

Example Question #7 : Biological Energetics

Under what conditions will a reaction with a positive change in entropy and a positive change in enthalpy take place spontaneously?

Possible Answers:

When temperature is very high

This reaction will always take place spontaneously

When temperature is very low

When pressure is low

This reaction will never take place spontaneously

Correct answer:

When temperature is very high

Explanation:

A negative change in Gibbs free energy means that a reaction will take place spontaneously. By using the equation:

We can see that if both entropy and enthalpy are positive,  will only be negative when  is sufficiently high.

Example Question #8 : Biological Energetics

What is reaction coupling?

Possible Answers:

Pairing two unfavorable reactions together so that they both become favorable

Combining two reactions in order to double the amount of product that will be created

Driving an unfavorable reaction forward by pairing it with a very favorable reaction

Stopping a favorable reaction from occurring by pairing it with an unfavorable reaction

Pairing two reactions together in order to alter their chemical equilibriums

Correct answer:

Driving an unfavorable reaction forward by pairing it with a very favorable reaction

Explanation:

Reaction coupling is the pairing of one unfavorable reaction to another reaction that is favorable. The energetics of the favorable reaction drive the unfavorable one forward.

Example Question #9 : Biological Energetics

Which of the following is true of a reaction that has reached equilibrium?

Possible Answers:

There is no net movement between the products and reactants

The reaction quotient (Q) is less than the equilibrium constant (Keq)

The rate of the reaction is at its maximum

There is no movement at all between products and reactants

The reaction quotient (Q) is greater than the equilibrium constant (Keq)

Correct answer:

There is no net movement between the products and reactants

Explanation:

It is a common misconception that at chemical equilibrium, movement between the product and reactant sides of the equation has stopped. In reality, reactants and products are still converting back and forth to one another, however there is no longer any net movement from one side of the equation to the other. Also, Q=Keq at equilibrium.

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