Cellular Energy — Hard Practice Quiz

A Biology cheat sheet for Cellular Energy — every key formula with its symbols defined — plus a hard-level practice quiz to test recall.

Practice quiz

1. A cell synthesizes a complex protein from amino acids, an anabolic process. Which of the following statements accurately describes the energy transformations and thermodynamic implications of this process within the cell?

   • The synthesis is exergonic, increasing the cell's overall free energy and decreasing entropy in the universe.
   • The energy for synthesis is directly created by the cell, violating the First Law of Thermodynamics, but overall entropy increases.
   • The endergonic protein synthesis is coupled with an exergonic reaction like ATP hydrolysis, leading to a net increase in the entropy of the universe, consistent with the Second Law.
   • The process is spontaneous because it decreases the cell's internal disorder, thereby decreasing the entropy of the universe.

   Answer: The endergonic protein synthesis is coupled with an exergonic reaction like ATP hydrolysis, leading to a net increase in the entropy of the universe, consistent with the Second Law.

2. A eukaryotic cell is actively performing aerobic cellular respiration. If oxygen suddenly becomes unavailable, which of the following is the most immediate and direct consequence for the cell's energy production pathways?

   • The Citric Acid Cycle will immediately cease due to lack of oxygen as a direct reactant.
   • Glycolysis will halt because $NADH$ cannot be converted back to $NAD^+$ in the absence of the electron transport chain.
   • Oxidative phosphorylation will cease, leading to an accumulation of $NADH$ and $FADH_2$, which will subsequently inhibit the Citric Acid Cycle and promote fermentation.
   • The cell will switch entirely to producing ATP via the Citric Acid Cycle, as it does not directly require oxygen.

   Answer: Oxidative phosphorylation will cease, leading to an accumulation of $NADH$ and $FADH_2$, which will subsequently inhibit the Citric Acid Cycle and promote fermentation.

3. An enzyme catalyzes a specific biochemical reaction within a cell. Which of the following statements accurately describes the effect of the enzyme on the reaction's thermodynamics and kinetics?

   • The enzyme converts an endergonic reaction into an exergonic one, making it spontaneous by lowering the activation energy.
   • The enzyme increases the rate of reaction by lowering the activation energy, but it does not alter the overall free energy change ($\Delta G$) of the reaction.
   • The enzyme provides the necessary activation energy for the reaction to proceed, thereby making a non-spontaneous reaction spontaneous.
   • The enzyme shifts the equilibrium of the reaction towards product formation by decreasing the $\Delta G$ of the products.

   Answer: The enzyme increases the rate of reaction by lowering the activation energy, but it does not alter the overall free energy change ($\Delta G$) of the reaction.

4. Consider the cycle of ATP and ADP within a cell. When a cell performs an anabolic process, such as synthesizing a polypeptide chain, which of the following best describes the energy transformation and the role of ATP?

   • Chemical potential energy stored in the polypeptide bonds is converted into kinetic energy, regenerating ADP into ATP.
   • The chemical potential energy stored in ATP's phosphate bonds is released as kinetic energy, driving the endergonic synthesis and forming ADP.
   • The exergonic hydrolysis of ATP releases chemical potential energy, which is then used to increase the chemical potential energy stored in the newly formed bonds of the polypeptide, converting ATP to ADP.
   • ADP is directly converted into ATP by the addition of a phosphate group, providing the kinetic energy needed for the anabolic reaction.

   Answer: The exergonic hydrolysis of ATP releases chemical potential energy, which is then used to increase the chemical potential energy stored in the newly formed bonds of the polypeptide, converting ATP to ADP.

5. A heterotrophic organism consumes an autotroph. Which statement accurately describes the energy flow and its adherence to the laws of thermodynamics in this interaction?

   • The heterotroph creates new energy by breaking down the autotroph's organic molecules, violating the First Law of Thermodynamics.
   • The chemical potential energy stored in the autotroph's organic molecules is transformed by the heterotroph into ATP, with some energy inevitably lost as heat, increasing the entropy of the universe.
   • The heterotroph efficiently converts all the autotroph's stored energy into ATP, demonstrating a perfect energy transfer with no entropy increase.
   • The autotroph's energy is destroyed during consumption, and the heterotroph then generates its own energy from scratch.

   Answer: The chemical potential energy stored in the autotroph's organic molecules is transformed by the heterotroph into ATP, with some energy inevitably lost as heat, increasing the entropy of the universe.

6. A muscle cell is undergoing intense exercise, leading to anaerobic conditions. Compared to a cell performing complete aerobic respiration, what is the net ATP yield from one glucose molecule and the primary purpose of fermentation in the muscle cell?

   • $30-32 \text{ ATP}$; to produce additional ATP beyond glycolysis.
   • $2 \text{ ATP}$; to regenerate $FAD^+$ for the Citric Acid Cycle.
   • $2 \text{ ATP}$; to regenerate $NAD^+$ so that glycolysis can continue to produce ATP.
   • $0 \text{ ATP}$; to detoxify pyruvate into lactate.

   Answer: $2 \text{ ATP}$; to regenerate $NAD^+$ so that glycolysis can continue to produce ATP.

7. In oxidative phosphorylation, a proton gradient is established across the inner mitochondrial membrane. If a chemical agent were to make this membrane freely permeable to protons ($H^+$ ions), what would be the most direct and significant consequence for ATP production?

   • The electron transport chain would halt due to a lack of electron acceptors.
   • ATP synthase would reverse its function, pumping protons out of the matrix and consuming ATP.
   • The proton-motive force would dissipate, severely reducing ATP synthesis by chemiosmosis, even if the electron transport chain continued to function.
   • Oxygen would no longer be able to accept electrons, leading to a buildup of $NADH$ and $FADH_2$.

   Answer: The proton-motive force would dissipate, severely reducing ATP synthesis by chemiosmosis, even if the electron transport chain continued to function.

8. A cell needs to synthesize a complex lipid from simpler precursors, an endergonic process. Which of the following strategies is employed by the cell to achieve this synthesis, considering the principles of free energy and enzyme function?

   • The cell uses an enzyme to lower the $\Delta G$ of the lipid synthesis, making the reaction exergonic and spontaneous.
   • The endergonic lipid synthesis is coupled with an exergonic reaction, often ATP hydrolysis, and facilitated by enzymes that lower the activation energy of the coupled reactions.
   • The cell increases the concentration of lipid precursors to shift the equilibrium towards product formation, bypassing the need for energy input.
   • Enzymes directly provide the energy required for the endergonic reaction, acting as energy sources rather than catalysts.

   Answer: The endergonic lipid synthesis is coupled with an exergonic reaction, often ATP hydrolysis, and facilitated by enzymes that lower the activation energy of the coupled reactions.

9. A molecule of glucose is completely oxidized during aerobic cellular respiration. At which stage(s) are carbon atoms fully oxidized and released as $CO_2$, and which stage(s) produce the majority of the ATP via substrate-level phosphorylation versus oxidative phosphorylation?

   • $CO_2$ is released during glycolysis and the Citric Acid Cycle; most ATP is from substrate-level phosphorylation in glycolysis.
   • $CO_2$ is released during the conversion of pyruvate to Acetyl-CoA and the Citric Acid Cycle; the majority of ATP is generated by oxidative phosphorylation.
   • $CO_2$ is released only during oxidative phosphorylation; ATP is primarily produced by substrate-level phosphorylation in the Citric Acid Cycle.
   • $CO_2$ is released during glycolysis and oxidative phosphorylation; ATP is equally divided between substrate-level and oxidative phosphorylation.

   Answer: $CO_2$ is released during the conversion of pyruvate to Acetyl-CoA and the Citric Acid Cycle; the majority of ATP is generated by oxidative phosphorylation.

10. During cellular respiration, glucose (a molecule with high chemical potential energy) is broken down to produce ATP. Which statement best describes the energy transformations and thermodynamic principles involved?

    • The chemical potential energy in glucose is entirely converted into the chemical potential energy of ATP, demonstrating perfect energy conservation.
    • The chemical potential energy in glucose is transformed into the chemical potential energy of ATP and thermal energy (heat), with the total energy conserved, but the entropy of the system plus surroundings increases.
    • The process creates new chemical energy in the form of ATP from the breakdown of glucose, violating the First Law of Thermodynamics.
    • The breakdown of glucose decreases the overall entropy of the cell by organizing energy into ATP molecules, which is the primary goal of respiration.

    Answer: The chemical potential energy in glucose is transformed into the chemical potential energy of ATP and thermal energy (heat), with the total energy conserved, but the entropy of the system plus surroundings increases.