Q: A reaction has $$\Delta H = -50\ \mathrm{kJ/mol}$$ and $$\Delta S = +0.1\ \mathrm{kJ/(mol\cdot K)}. At$$ $$T = 300\ \mathrm{K}$$, what is $$\Delta G$$ for this reaction, and is it spontaneous?

$$\Delta G = -50\ \mathrm{kJ/mol} - (300\ \mathrm{K} \times 0.1\ \mathrm{kJ/(mol\cdot K)}) = -80\ \mathrm{kJ/mol}$$; the reaction is spontaneous.

Question 1

Which of the following best explains why ATP hydrolysis is often coupled to endergonic reactions in cells?

Accepted Answer

The energy released from ATP hydrolysis ($$\mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i}) is $$greater than the energy required for the endergonic reaction, making the overall process exergonic.

Question 2

A scientist observes that a certain metabolic pathway is shut down when the final product accumulates. Which regulatory mechanism is most likely responsible for this observation?

Accepted Answer

Feedback inhibition, where the final product acts as an allosteric inhibitor of an earlier enzyme.

Question 3

Which of the following statements about enzymes is correct?

Accepted Answer

Enzymes lower the activation energy ($$E_A) $$required for a reaction to proceed.

Question 4

A metabolic pathway involves the breakdown of glycogen into glucose molecules, releasing energy. This process is best described as:

Accepted Answer

Catabolic, because it breaks down complex molecules and releases energy.

Question 5

Which of the following best describes the difference between G protein-coupled receptors (GPCRs) and ligand-gated ion channels?

Accepted Answer

GPCRs activate G proteins and secondary messengers, leading to signal amplification and prolonged responses, while ligand-gated ion channels open to allow ions to flow, resulting in rapid, short-term changes.

Question 6

According to the second law of thermodynamics, which of the following statements is true regarding biological systems?

Accepted Answer

Biological processes tend to increase the disorder (entropy) of the universe, and some energy is always lost as heat.

Question 7

A reaction has $$\Delta H = -50\ \mathrm{kJ/mol}$$ and $$\Delta S = +0.1\ \mathrm{kJ/(mol\cdot K)}. At$$ $$T = 300\ \mathrm{K}$$, what is $$\Delta G$$ for this reaction, and is it spontaneous?

Accepted Answer

$$\Delta G = -50\ \mathrm{kJ/mol} - (300\ \mathrm{K} 	imes 0.1\ \mathrm{kJ/(mol\cdot K)}) = -80\ \mathrm{kJ/mol}$$; the reaction is spontaneous.

Question 8

Which of the following best explains why enzymes are highly specific for their substrates?

Accepted Answer

Enzymes have active sites with unique three-dimensional structures that only fit specific substrates.

Question 9

A patient is given a drug that acts as a noncompetitive inhibitor of an enzyme. Which of the following is most likely to occur?

Accepted Answer

The maximum reaction rate ($$V_{max}) $$decreases, but the substrate can still bind to the enzyme.

Question 10

Which of the following scenarios best illustrates the concept of signal amplification in a signal transduction pathway?

Accepted Answer

A single hormone molecule binds to a receptor, leading to the activation of many secondary messenger molecules and a large cellular response.

Question 11

Which of the following best describes the role of cofactors in enzyme function?

Accepted Answer

Cofactors are non-protein molecules that assist enzymes, sometimes by stabilizing the substrate or participating directly in the reaction.

Question 12

A researcher measures the rate of an enzyme-catalyzed reaction and finds that increasing substrate concentration no longer increases the reaction rate. What is the most likely explanation?

Accepted Answer

The enzyme is saturated; all active sites are occupied by substrate.

Question 13

Which of the following statements about the coupling of catabolic and anabolic reactions is correct?

Accepted Answer

Catabolic reactions release energy that can be used to drive anabolic reactions, often through the intermediate formation and hydrolysis of ATP.

Question 14

Which of the following is a correct interpretation of the equation $$\Delta G = \Delta H - T \cdot \Delta S$$?

Accepted Answer

A reaction with a large negative $$\Delta H$$ and a large positive $$\Delta S$$ will have a more negative $$\Delta G$$, favoring spontaneity.

Question 15

Which of the following best explains why most of the energy from food is lost as heat during catabolic processes?

Accepted Answer

The second law of thermodynamics states that energy transformations increase entropy, and some energy is always lost as heat.

Question 16

A mutation causes an enzyme's active site to change shape so that it no longer binds its substrate efficiently. Which of the following is the most likely consequence?

Accepted Answer

The reaction rate will decrease because the substrate cannot bind effectively.

Question 17

Which of the following is an example of allosteric regulation of an enzyme?

Accepted Answer

A molecule binds to a site other than the active site, changing the enzyme's shape and activity.

Question 18

In a metabolic pathway, the binding of one substrate molecule to an enzyme increases the affinity of additional substrate molecules to bind. This phenomenon is known as:

Accepted Answer

Cooperativity

Question 19

Which of the following best describes the effect of a competitive inhibitor on enzyme kinetics?

Accepted Answer

It increases the $$K_m ($$apparent affinity for substrate) by competing with the substrate for the active site, but does not change $$V_{max}.$$

Energy, Enzymes, and Metabolic Pathways in General Biology

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