MCAT Biochemistry Review

Chapter 12: Bioenergetics and Regulation of Metabolism

Practice Questions

·  Adding heat to a closed biological system will do all of the following EXCEPT:

·  increase the internal energy of the system.

·  increase the average of the vibrational, rotational, and translational energies.

·  cause the system to do work to maintain a fixed internal energy.

·  increase the enthalpy of the system.

·  At 25°C the ΔG° for a certain reaction A → B + 2 C is 0. If the concentration of A, B, and C in the cell at 25°C are all 10 mM, how does the ΔG compare to the measurement taken with 1 M concentrations?

·  ΔG is greater than ΔG°, thus the reaction is spontaneous.

·  ΔG is less than ΔG°, thus the reaction is spontaneous.

·  ΔG is greater than ΔG°, thus the reaction is nonspontaneous.

·  ΔG is less than ΔG°, thus the reaction is nonspontaneous.

·  Which of the following statements is true about the hydrolysis of ATP?

·  The free energy of ATP hydrolysis is independent of pH.

·  One mole of creatine phosphate can phosphorylate two moles of ADP.

·  The free energy of hydrolysis of ATP is nearly the same as for ADP.

·  ATP yields cyclic AMP after two hydrolysis reactions.

·  The reduction half-reaction in the last step of the electron transport chain is:

·  O2 + 4 e + 4 H+ → 2 H2O

·  NADPH → NADP+ + e + H+

·  NADP+ + e + H+ → NADPH

·  Ubiquinone (Q) → Ubiquinol (QH2)

·  The ability to exist in both an oxidized and a reduced state is characteristic of:

·  adenosine triphosphate (ATP).

·  electron carriers.

·  regulatory enzymes.

·  peptide hormones.

·  What energy state was described in the introduction to this chapter?

·  Absorptive

·  Postabsorptive

·  Starvation

·  Vegetative

·  With prolonged fasting, the brain can turn to which alternative fuel for energy?

·  Fructose

·  Lactate

·  Ketone bodies

·  Fatty acids

·  How do hormonal controls of glycogen metabolism differ from allosteric controls?

·  Hormonal control is systemic and covalent.

·  Hormonal control is local and covalent.

·  Hormonal control is systemic and noncovalent.

·  Hormonal control is local and noncovalent.

·  Which of the following tissues is most dependent on insulin?

·  Active skeletal muscle

·  Resting skeletal muscle

·  Cardiac muscle

·  Smooth muscle

·  Glucocorticoids have been implicated in stress-related weight gain because:

·  they increase appetite and decrease satiety signals.

·  they increase the activity of catabolic hormones.

·  they increase glucose levels, which causes insulin secretion.

·  they interfere with activity of the leptin receptor.

·  In the absence of oxygen, which tissue will experience damage most rapidly?

·  Skin

·  Brain

·  Red blood cells

·  Liver

·  A respiratory quotient approaching 0.7 indicates metabolism primarily of which macromolecule?

·  Carbohydrates

·  Lipids

·  Nucleic acids

·  Amino acids

·  Which of the following side effects would be anticipated in someone taking leptin to promote weight loss?

·  Drowsiness

·  Increased appetite

·  Irritability

·  Fever

·  Which of the following statements is FALSE?

·  Growth hormone participates in glucose counter-regulation.

·  T4 acts more slowly than T3.

·  ATP stores are turned over more than 10,000 times daily.

·  Catecholamines stimulate the sympathetic nervous system.

·  Which process is expected to begin earliest in a prolonged fast?

·  Ketone bodies are used by the brain.

·  Glycogen storage is halted.

·  Proteins are broken down.

·  Enzyme phosphorylation and dephosphorylation.


Answers and Explanations

·  CIn a closed biological system, enthalpy, heat, and internal energy are all directly related because there is no change in pressure or volume. Because pressure and volume are fixed, work cannot be done, thus choice (C) is correct.

·  B

To solve this question, we can use the equation ΔG = ΔG° + RT ln QQ, the reaction quotient, is  for this reaction. Plugging in the variables, we get ΔG = 0 + RT

Because both R and T are positive, we know that ΔG must be negative and therefore lower than the original value. A negative ΔG corresponds to a spontaneous reaction.

·  CThe hydrolysis of ATP is energetically favorable because there are repulsive negative charges that are relieved when hydrolyzed, and the new compounds are stabilized by resonance. This is true of both ATP and ADP. Some of the other answer choices are tempting, though. Inchoice (A), ATP hydrolysis relies on pH because a protonated ATP molecule experiences less repulsive force. For choice (B), the energy released by one mole of creatine phosphate upon hydrolysis is not sufficient to phosphorylate two moles of ADP according to Table 12.1. Forchoice (D), the removal of two phosphate groups from ATP yields AMP, not cyclic AMP.

·  AReduction is a gain of electrons, which eliminates choice (B) because it is an oxidation reaction. NADPH, choice (C), is a product of the pentose phosphate pathway. Ubiquinone, choice (D), transfers electrons during the course of the electron transport chain, but is not the final electron acceptor. This title belongs to oxygen.

·  BIn order to transport electrons, electron carriers like flavoproteins must be able to exist in a stable oxidized state and a stable reduced form. ATP can be dephosphorylated but is generally not oxidized or reduced.

·  BSkipping a single meal is not a prolonged fast. However, the increase in hormones that promote gluconeogenesis and glycogenolysis indicates that the absorptive phase has ended.

·  CThe brain is almost exclusively dependent on glucose for energy; however, in a prolonged fast, ketone bodies can be used for up to  of the brain's energy requirement.

·  AHormonal controls are coordinated to regulate the metabolic activity of the entire organism, while allosteric controls can be local or systemic. The modification of the enzymes of glycogen metabolism by insulin and glucagon is either through phosphorylation or dephosphorylation, both of which modify covalent bonds.

·  BAdipose tissue and resting skeletal muscle require insulin for glucose uptake. Active skeletal muscle, choice (A), uses creatine phosphate and glycogen (regulated by epinephrine and AMP) to maintain its energy requirements.

·  CShort-term glucocorticoid exposure causes a release of glucose and the hydrolysis of fats from adipocytes. However, if this glucose is not used for metabolism, it causes an increase in glucose level which promotes fat storage. The net result is the release of glucose from the liver to be converted into lipids in the adipose tissue under insulin stimulation.

·  BThe brain uses aerobic metabolism of glucose exclusively and therefore is very sensitive to oxygen levels. The extremely high oxygen requirement of the brain (20% of the body's oxygen content) relative to its size (2% of total body weight) implies that brain is the most sensitive organ to oxygen deprivation.

·  BThe respiratory quotient (RQ) gives an indication of the primary fuel being utilized. An RQ around 0.7 indicates lipid metabolism, 0.8–0.9 indicates amino acid metabolism, choice (C), and 1.0 indicates carbohydrate metabolism, choice (A). Nucleic acids do not contribute significantly to the respiratory quotient.

·  ALeptin acts to decrease appetite by inhibiting the production of orexin. Orexin is also associated with alertness, so decreasing the level of orexin in the body is expected to cause drowsiness. Even without this information, the answer should be apparent because the body tends to maintain an energy balance. If consumption decreases, energy expenditures are expected to decrease as well.

·  CATP stores are turned over about 1,000 times per day, not 10,000.

·  DA prolonged fast is characterized by an increase in glucagon, which accomplishes its cellular activity by phosphorylating and dephosphorylating metabolic enzymes. Glycogen storage, choice (B), is then halted, but this requires enzyme regulation by glucagon to occur. Later in the postabsorptive state, protein breakdown, choice (C), begins. Eventually, in starvation, ketone bodies, choice (A), are used by the brain for its main energy source.