Chapter 8

If the entropy of a living organism is decreasing, which of the following is most likely to be occurring simultaneously? In this situation, the second law of thermodynamics must not apply. The entropy of the organism's environment must also be decreasing. Energy input into the organism must be occurring to drive the decrease in entropy. Heat is being used by the organism as a source of energy. The first law of thermodynamics is being violated.

Energy input into the organism must be occurring to drive the decrease in entropy.

Which of the following statements about enzyme function is correct?

Enzymes can lower the activation energy of reactions, but they cannot change the equilibrium point because they cannot change the net energy output.

What best characterizes the role of ATP in cellular metabolism?

The free energy released by ATP hydrolysis that may be coupled to an endergonic process via the formation of a phosphorylated intermediate.

Which of the following correctly states the relationship between anabolic and catabolic pathways?

Anabolic pathways synthesize more complex organic molecules using the energy derived from catabolic pathways.

At low temperatures, a particular enzyme catalyzes a reaction, but at a slow rate. At high temperatures, the enzyme is completely inactive. What statement best explains the difference in how temperature affects the function of this enzyme?

At low temperatures, there is not enough free energy for the enzyme to function at a high rate, and at high temperatures, the enzyme is denatured, leaving it nonfunctional.

Which of the following statements correctly describes cofactors and coenzymes?

Both are nonprotein enzyme helpers; but most cofactors are metal ions, and coenzymes are organic molecules that are a specific type of cofactor.

Which of the following states the relevance of the first law of thermodynamics to biology? Photosynthetic organisms produce energy in sugars from sunlight. Energy can be freely transformed among different forms as long as the total energy is conserved. Energy is destroyed as glucose is broken down during cellular respiration. The total energy taken in by an organism must be greater than the total energy stored or released by the organism. Living organisms must increase the entropy of their surroundings.

Energy can be freely transformed among different forms as long as the total energy is conserved.

Which of the following statements about enzymes is true? All of the listed responses are correct. The more heat that is added to a reaction, the faster the enzymes will function. Enzymes react with their substrate (form chemical bonds), forming an enzyme-substrate complex, which irreversibly alters the enzyme. Enzymes increase the rate of a reaction by raising the activation energy for reactions. Enzymes speed up the rate of the reaction without changing the ΔG for the reaction.

Enzymes speed up the rate of the reaction without changing the ΔG for the reaction. An enzyme is a macromolecule that acts as a catalyst, a chemical agent that speeds up a reaction without being consumed by the reaction. An enzyme catalyzes a reaction by lowering the activation energy EA barrier, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures. An enzyme cannot change the ΔG for a reaction; it cannot make an endergonic reaction exergonic. Enzymes can only hasten reactions that would eventually occur anyway, but this function makes it possible for the cell to have a dynamic metabolism, routing chemicals smoothly through the cell's metabolic pathways.

The primary manner in which cells manage their energy resources in order to do work is called energy coupling. Which of the following statements accurately defines energy coupling?

Exergonic reactions drive endergonic reactions. Based on their free-energy changes, chemical reactions can be classified as either exergonic ("energy outward") or endergonic ("energy inward"). An exergonic reaction proceeds with a net release of free energy. An endergonic reaction absorbs free energy from its surroundings. A key feature in the way cells manage their energy resources in order to do this work is energy coupling, the use of an exergonic process to drive an endergonic one. ATP is responsible for mediating most energy coupling in cells, and in most cases it acts as the immediate source of energy that powers cellular work.

Which of the following reactions would be endergonic? Glucose + fructose → sucrose All of the listed responses are correct. YOU WERE SURE AND INCORRECT ATP → ADP + Pi HCl → H+ + Cl C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

Glucose + fructose → sucrose An endergonic reaction is one that absorbs free energy from its surroundings. Because this kind of reaction essentially stores free energy in molecules (G increases), ΔG is positive. Such reactions are nonspontaneous, and the magnitude of ΔG in the equation ΔG = ΔH - TΔS is the quantity of energy required to drive the reaction. Combining glucose and fructose to produce sucrose is an example of an energy storing endergonic reaction with the product more complex (lower entropy) than the reactants (glucose and fructose). previous nex

Which of the following is true regarding metabolic pathways? Metabolic pathways consist of a single chemical reaction. Metabolic pathways are not important to a cell's ability to function. Each reaction in the pathway is catalyzed by the same enzyme. Metabolic pathways consist of a series of reactions, each catalyzed by a different enzyme. Metabolic pathways consist of only anabolic pathways.

Metabolic pathways consist of a series of reactions, each catalyzed by a different enzyme.

Which of these statements about enzyme inhibitors is true?

The action of competitive inhibitors may be reversible or irreversible. Certain chemicals selectively inhibit the action of specific enzymes, and we have learned a lot about enzyme function by studying the effects of these molecules. If the inhibitor attaches to the enzyme by covalent bonds, inhibition is usually irreversible. Many enzyme inhibitors, however, bind to the enzyme by weak interactions, in which case inhibition is reversible. Some reversible inhibitors resemble the normal substrate molecule and compete for admission into the active site. These mimics, called competitive inhibitors, reduce the productivity of enzymes by blocking substrates from entering active sites. This kind of inhibition can be overcome by increasing the concentration of substrate so that as active sites become available, more substrate molecules than inhibitor molecules are around to gain entry to the sites. In contrast, noncompetitive inhibitors do not directly compete with the substrate to bind to the enzyme at the active site. Instead, they impede enzymatic reactions by binding to another part of the enzyme. This interaction causes the enzyme molecule to change its shape in such a way that the active site becomes less effective at catalyzing the conversion of substrate to product.

Which of the following is changed by the presence of an enzyme in a reaction? The magnitude of ΔG The G value for the reactants The sign of ΔG The activation energy The G value for the products

The activation energy

Which of the following is an example of the second law of thermodynamics as it applies to biological reactions?

The aerobic respiration of one molecule of glucose produces six molecules each of carbon dioxide and water. The second law of thermodynamics states that spontaneous processes, those requiring no outside input of energy, increase the entropy (disorder) of the universe. In this example, carbon dioxide and water are more disordered than glucose. Cellular respiration, in which the sugar glucose and other organic fuels are broken down in the presence of oxygen to release energy and produce carbon dioxide and water, is a major pathway of catabolism. Energy that was stored in the organic molecules becomes available to do the work of the cell, such as ciliary beating or membrane transport.

Which of the following statements is correct regarding ATP? ATP molecules do not release free energy when hydrolyzed. The energy in an ATP molecule is released through hydrolysis of one of the phosphate groups. The energy in an ATP molecule is released from the adenine group. ATP cannot transfer energy to other molecules. The energy in an ATP molecule is released from the ribose group.

The energy in an ATP molecule is released through hydrolysis of one of the phosphate groups.

ATP allosterically inhibits enzymes in ATP-producing pathways. The result of this is called __________.

feedback inhibition When ATP allosterically inhibits an enzyme in an ATP-generating pathway, the result is feedback inhibition, a common mode of metabolic control. In feedback inhibition, a metabolic pathway is halted by the inhibitory binding of its end product to an enzyme that acts early in the pathway. Feedback inhibition thereby prevents the cell from making more product than is necessary and thus wasting chemical resources.

What would the value of ΔS be for a chemical reaction in which a molecule is broken down into smaller components?

positive

An exergonic (spontaneous) reaction is a chemical reaction that __________.

releases energy when proceeding in the forward direction

An exergonic reaction __________ free energy, and an endergonic reaction __________ free energy.

releases, absorbs

In general, the hydrolysis of ATP drives cellular work by __________.

releasing free energy that can be coupled to other reactions

As ATP begins to build up in a cell, metabolism slows down. How does this happen?

ATP acts as an allosteric inhibitor to many of the enzymes involved in metabolism, thus slowing their function. ATP acts as an inhibitor, not an activator. ATP is an allosteric inhibitor. It does not bind to the active site of the enzymes it regulates. Excess ATP acts as an allosteric inhibitor. It deactivates enzymes by reversibly changing their shape.

Cells use ATP constantly, but ATP is considered a renewable resource. What process makes this possible?

ATP can be regenerated by the addition of a phosphate group to ADP.

How does ATP drive transport work inside a cell?

By binding to motor proteins Transport and mechanical work in the cell are also nearly always powered by the hydrolysis of ATP. In these cases, ATP hydrolysis leads to changes in a protein's shape and often its ability to bind to another molecule. Sometimes this occurs via a phosphorylated intermediate. ATP hydrolysis causes changes in the shapes and binding affinities of proteins. This can occur directly, by phosphorylation, for a membrane protein carrying out active transport of a solute or indirectly, via noncovalent binding of ATP and its hydrolytic products, as is the case for motor proteins that move vesicles (and other organelles) along cytoskeletal "tracks" in the cell.

Which of the following statements is correct regarding competitive and noncompetitive enzyme inhibitors?

Competitive inhibitors bind to the active site of an enzyme while noncompetitive inhibitors bind to an enzyme away from the active site.

Which of the following statements is correct regarding kinetic and potential energy?

Kinetic energy is associated with the relative motion of objects, and potential energy is the energy that matter possesses because of its location or structure.

mechanical work vs. transport work

Mechanical work is driven by ATP binding to motor proteins. transport work: ATP phosphorylates transport proteins

Which of the following statements about the active site of an enzyme is correct?

The active site may resemble a groove or pocket in the surface of a protein into which the substrate fits.

The process of stabilizing the structure of an enzyme in its active form by the binding of a molecule is an example of __________.

allosteric regulation

Enzyme activity is affected by pH because __________.

high or low pH may disrupt hydrogen bonding or ionic interactions and thus change the shape of the active site