Chapter 3 Flashcards
- Which of the following statements best explains how living cells do not violate the second law of thermodynamics?
A. They are isolated systems that do not exchange energy with their surroundings.
B. They generate order within themselves but release heat, increasing overall entropy in their surroundings.
C. They convert all absorbed energy into stored chemical bonds without releasing any heat.
D. They strictly follow the principle that total disorder inside the cell must always decrease.
Answer: B
Explanation: The second law of thermodynamics states that the total entropy of a system plus its surroundings must increase. Living cells are open systems that take in energy (e.g., from food or light) and use it to build ordered structures. However, in doing so, they also release heat to their environment. This heat increases the overall disorder (entropy) in the surroundings, ensuring that the total entropy (cell + environment) still goes up.
- Which of the following best describes why living cells can increase their own order without defying thermodynamic principles?
A. They continuously add energy to the environment, thus decreasing total entropy.
B. They utilize external energy sources (food, light) and release heat, raising the net entropy of the system.
C. They function as perfectly efficient energy converters that do not produce heat.
D. They bypass entropy by creating new molecules without any energy cost.
Correct Answer: B
Explanation: Cells maintain and increase their internal order by importing energy from external sources. This process is not 100% efficient, so cells inevitably release heat into their surroundings. This release of heat increases the entropy of the environment, more than compensating for the decrease in entropy within the cell itself. Consequently, the total entropy of the entire system (the cell plus its surroundings) continues to increase, aligning with the second law of thermodynamics.
- Which statement best describes the relationship between catabolic and anabolic pathways in cellular metabolism?
A. Catabolic pathways rely exclusively on ATP, while anabolic pathways rely exclusively on heat.
B. Catabolic pathways synthesize complex molecules, while anabolic pathways break them down.
C. Catabolic pathways release energy from food molecules, which can be harnessed to drive anabolic reactions.
D. Catabolic and anabolic pathways operate independently without influencing one another.
Correct Answer: C
Explanation: Catabolic pathways break down food molecules, releasing energy. Although some of this energy is lost as heat, a portion is converted into useful forms (e.g., ATP), which anabolic pathways use to synthesize new, more complex molecules.
- During catabolism, which of the following best describes the fate of the energy stored in the chemical bonds of food molecules?
A. A substantial portion is dissipated as heat, while some is converted into useful energy forms for biosynthesis.
B. All the energy is perfectly conserved, with no heat loss.
C. Most of the energy is captured directly as macromolecules, bypassing the production of heat.
D. The energy is not utilized by the cell but is stored exclusively as heat in cellular compartments
Correct Answer: A
Explanation: Catabolic reactions break down larger molecules, releasing a significant amount of energy. A major part of this energy is lost as heat, but some is converted into forms (e.g., ATP, NADPH) that drive anabolic pathways to build new cellular components.
- Which of the following processes best demonstrates how cells use the energy transformed during photosynthesis?
A. They store all absorbed light energy indefinitely, never releasing any as heat.
B. They create energy from food molecules and then destroy it during biosynthesis.
C. They lose all captured energy immediately as thermal motion, leading to no net energy gain.
D. They utilize the chemical bond energy formed in photosynthesis to drive metabolic pathways, with some energy inevitably released as heat.
Correct Answer: D
Explanation: After photosynthesis converts light energy into chemical bond energy (e.g., in sugars), cells can tap into that stored energy to power various metabolic pathways. During these processes, some energy is invariably released as heat, but the overall energy is neither created nor destroyed, merely converted from one form to another.
- Which of the following statements best describes how photosynthesis and cell respiration are complementary processes in the carbon cycle?
A. Both processes exclusively produce oxygen as a byproduct.
B. Photosynthesis releases carbon atoms into the atmosphere, while respiration removes them.
C. Photosynthesis incorporates carbon from CO₂ into organic molecules, whereas respiration releases carbon back into the atmosphere as CO₂.
D. Neither process relies on enzymes to facilitate energy transformations.
Correct Answer: C
Explanation: In photosynthesis, organisms such as plants and photosynthetic microbes use sunlight to convert CO₂ into organic molecules (e.g., sugars). During cell respiration, these organic molecules are broken down in the presence of O₂, releasing CO₂ back into the atmosphere. Thus, the carbon cycle is maintained through these complementary processes.
- Which of the following best explains the role of enzymes in metabolic pathways such as photosynthesis and respiration?
A. They slow down the rate of reactions to conserve energy.
B. They convert heat directly into stored chemical energy.
C. They ensure that no energy is lost as heat during metabolism.
D. They enable reactions to occur in small steps, capturing energy in usable forms rather than releasing it all as heat.
Correct Answer: D
Explanation: Enzymes lower the activation energy for each step in metabolic pathways. By breaking down or synthesizing molecules in multiple small steps, cells can harness much of the released energy in the form of ATP or other energy carriers, rather than losing it immediately as heat.
- Which of the following best describes the “transition state” in a chemical reaction?
A. It is the lowest-energy form of the reactants.
B. It has less free energy than either the reactants or the products.
C. It is a higher-energy, unstable configuration that reactants must achieve for the reaction to proceed.
D. It can only be reached if no activation energy is required.
Correct Answer: C
Explanation: The transition state is a temporary, high-energy condition that reactants must attain before being converted into products. It requires an input of activation energy to be reached.
- Why do even energetically favorable reactions in cells often depend on enzymes?
A. Enzymes provide extra energy to the reactants so no activation energy is needed.
B. Enzymes lower the activation energy barrier, allowing the transition state to be reached more easily.
C. Enzymes eliminate the transition state, making reactions proceed spontaneously without energy input.
D. Enzymes reverse the thermodynamic favorability, ensuring no heat is lost.
Correct Answer: B
Explanation: Although a reaction might be thermodynamically favorable (having a negative ΔG), it still needs an initial energy “boost” (activation energy) to reach the transition state. Enzymes reduce this activation energy requirement, enabling the reaction to occur rapidly under physiological conditions.
- Which of the following best explains how lowering the activation energy increases the likelihood that a reaction will occur?
A. It completely removes the energy threshold required for a reaction.
B. It changes the overall ΔG of the reaction from positive to negative.
C. It guarantees that all molecules in the population will react.
D. It increases the fraction of substrate molecules that can reach the transition state at a given temperature.
Correct Answer: D
Explanation: By lowering the activation energy, more substrate molecules in a typical energy distribution have enough energy to cross the barrier. This raises the proportion of molecules that can transition to products, thus increasing the reaction rate.
- At any given instant, why do substrate molecules have a range of energies in the cell?
A. They gain or lose kinetic energy through collisions with surrounding molecules.
B. They receive uniform energy levels from a central cellular energy reservoir.
C. They only gain energy directly from enzyme active sites.
D. They all possess the same baseline energy, but the enzyme imposes different levels.
Correct Answer: A
Explanation: Substrate molecules continuously collide with other molecules, such as water or solutes, which transfers kinetic energy in random amounts. This results in a broad distribution of molecular energies within the cell.
- Which statement best explains how an enzyme selectively binds its substrate?
A. It relies on multiple, complementary noncovalent interactions that collectively stabilize binding.
B. It forms permanent covalent bonds with any molecule that collides with it.
C. It binds substrates only if they have the same overall size, regardless of chemical properties.
D. It attaches to every molecule in the cytosol equally and randomly.
Correct Answer: A
Explanation: Enzymes typically achieve specificity through numerous noncovalent interactions (e.g., hydrogen bonds, ionic bonds, van der Waals forces). Each individual interaction is relatively weak, but when a substrate’s shape and chemical groups fit well, the sum of these interactions can be strong enough to hold the substrate in place.
- Why do substrates that form only a few noncovalent interactions with an enzyme tend to dissociate quickly?
A. Their shape is permanently altered, preventing strong binding.
B. They are stabilized by covalent bonds, which do not withstand thermal motion.
C. Thermal motion can easily break the small number of weak bonds that hold the enzyme-substrate complex together.
D. They immediately convert to products without needing enzyme assistance.
Correct Answer: C
Explanation: A poor fit between enzyme and substrate produces only a few weak interactions, which cannot withstand the random jostling (thermal motion) in the cell. As a result, the substrate rapidly dissociates, preventing stable binding.
- Which parameter is most directly used to predict whether a particular chemical reaction will occur spontaneously in a cell?
A. The change in pH across the membrane
B. The total change in free energy (ΔG)
C. The pressure-volume work (PΔV)
D. The rate of enzyme catalysis
Correct Answer: B
Explanation: Whether a reaction is spontaneous depends on the sign of its Gibbs free energy change, ΔG. If ΔG is negative, the reaction can proceed spontaneously in that direction under the given conditions.
- Which of the following best describes how the direction of a multi-step metabolic pathway is determined?
A. Each individual step must have ΔG = 0.
B. The sum of the ΔG values for all steps must be negative for the overall pathway to proceed spontaneously.
C. The direction is random, as thermodynamics does not apply to multi-step processes.
D. Only the first step needs to have a negative ΔG; subsequent steps are irrelevant.
Correct Answer: B
Explanation: The overall spontaneity of a pathway depends on the total (summed) free energy changes of all its steps. If the combined ΔG is negative, the pathway can proceed in that direction spontaneously.
- Why is a cell considered “dead” if it reaches metabolic equilibrium (ΔG = 0)?
A. Because it has too much free energy to be stable
B. Because it becomes immune to external energy sources
C. Because it can no longer perform work or drive essential processes
D. Because it has permanently lost all of its molecules
Correct Answer: C
Explanation: At metabolic equilibrium, there is no net change in free energy (ΔG = 0). A cell at this state cannot carry out the energy-requiring processes essential for life, effectively rendering it nonfunctional.
- Which statement best explains why metabolic disequilibrium is critical for living systems?
A. It ensures cells remain at maximum stability.
B. It prevents all chemical reactions from occurring spontaneously.
C. It provides a continuous supply of free energy to drive cellular processes.
D. It guarantees that ΔG values for all reactions remain zero.
Correct Answer: C
Explanation: Living cells maintain a steady state of metabolic disequilibrium so that there is always a driving force (negative ΔG) for vital biochemical reactions. This constant “pull” away from equilibrium allows cells to perform work and sustain life.
- Which of the following statements correctly describes the relationship between the equilibrium constant (K) and the standard free energy change (ΔG°)?
A. A reaction with K > 1 has a positive ΔG°.
B. A reaction with K = 1 always has a large negative ΔG°.
C. A reaction with K > 1 has a negative ΔG°.
D. The value of K does not affect ΔG°.
Correct Answer: C
- If the equilibrium constant (K) for the reaction
𝑌
→
𝑋
Y→X is 10 at 37°C, what is the approximate value of ΔG° (in kJ/mol)?
A. +5.94 kJ/mol
B. –5.94 kJ/mol
C. –2.58 kJ/mol
D. +2.58 kJ/mol
Correct Answer: B
-5.94 kJ/mol.
- Which statement best explains how an energetically unfavorable reaction (X → Y) can still proceed in a cell?
A. It is catalyzed by enzymes that eliminate the positive ΔG.
B. It is coupled to a subsequent reaction (Y → Z) that has a sufficiently large negative ΔG to drive the overall process forward.
C. It does not require any energy input if the substrate concentration is high enough.
D. It proceeds spontaneously without any coupling, due to random thermal collisions.
Correct Answer: B
Explanation: When two reactions are linked (sequential reactions), their free energy changes (
ΔG) are additive. A highly favorable (negative
ΔG) reaction can pull the preceding unfavorable reaction forward by effectively lowering its overall free energy cost.
- Which statement best describes the role of activated carriers such as ATP, NADH, and NADPH in metabolism?
A. They decrease the overall energy content of the cell by dispersing heat.
B. They irreversibly bind to enzymes and prevent metabolic pathways from proceeding.
C. They store energy in transferable high-energy bonds or electrons, which can be used to drive biosynthetic reactions.
D. They require no energy to be synthesized from their precursor molecules.
Correct Answer: C
Explanation: Activated carriers like ATP, NADH, and NADPH temporarily store energy in the form of high-energy bonds or electrons. This stored energy can then be transferred to other molecules, helping drive energetically unfavorable reactions forward.
- How is the formation of an activated carrier (e.g., ATP from ADP + Pi) typically coupled to an energetically favorable reaction?
A. The energy released by a favorable reaction is harnessed to drive the synthesis of the activated carrier.
B. The formation of the carrier occurs spontaneously without any energy input.
C. The carrier is only generated after the cell reaches metabolic equilibrium.
D. The carrier must first bind irreversibly to its substrate to become energized.
Correct Answer: A
Explanation: The cell couples the production of activated carriers to exergonic (energy-releasing) reactions. The energy liberated in these favorable reactions is directly used to form the high-energy bonds of the carrier, ensuring the overall process remains thermodynamically feasible.
- Which of the following best explains why ATP hydrolysis releases a significant amount of energy?
A. The base adenine has unusually high energy.
B. The ribose sugar spontaneously degrades in the cell.
C. The phosphate groups repel each other only when bound to proteins.
D. The phosphoanhydride bonds between the outer phosphates are high-energy bonds that, when broken, release usable free energy.
Correct Answer: D
Explanation: ATP’s two terminal phosphate groups are connected by phosphoanhydride bonds. Breaking these bonds (ATP → ADP + Pi) liberates substantial free energy (roughly 46–54 kJ/mol) that the cell can harness to power various processes.
