BIOLOGY CH 8 Practice Test

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Introduction to Metabolism
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. Correct. The synthesis of complex molecules in anabolic pathways requires an input of energy from catabolic pathways.

Organisms are described as thermodynamically open systems. Which of the following statements is consistent with this description?

Organisms acquire energy from, and lose energy to, their surroundings. Correct. Open systems are those in which energy and materials can be exchanged between the system and its surroundings.

Consider the growth of a farmer’s crop over a season. Which of the following correctly states a limitation imposed by the first or second law of thermodynamics?

To obey the first law, the crops must represent an open system. Correct. The first law states that energy cannot be created. The growth of plants stores much energy in the body of the plant. That energy must have been obtained from the plant’s environment; thus, the plant must be an open system.

Which of the following states the relevance of the first law of thermodynamics to biology?

Energy can be freely transformed among different forms as long as the total energy is conserved. Correct. The first law of thermodynamics states that energy can be transformed but can be neither created nor destroyed.

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. Correct. The second law of thermodynamics states that every energy transformation makes the universe more disordered—carbon dioxide and water are more disordered than glucose.

According to the second law of thermodynamics, which of the following is true?

The decrease in entropy associated with life must be compensated for by increased entropy in the environment in which life exists. Correct. The second law of thermodynamics demands that total entropy increase with any reaction.

If the entropy of a living organism is decreasing, which of the following is most likely to be occurring simultaneously?

Energy input into the organism must be occurring to drive the decrease in entropy. Correct. If the entropy is decreasing, this would tend to make the free energy change associated with this positive. Thus, an input of energy would be required to make this decrease in entropy occur.

Which of the following has the most free energy per molecule?

a starch molecule Correct. Starch is a large polymer of sugar and thus contains many covalent bonds.

Which part of the equation Delta G = Delta H – T Delta S tells you if a process is spontaneous?

Delta G Correct. In any spontaneous process, the free energy of a system decreases. The change in free energy equals the change in total energy minus the change in entropy times the temperature.

If, during a process, the system becomes more ordered, then

Delta S is negative Correct. In an endergonic reaction, in which order is increased, the change in entropy, symbolized by Delta S, is negative.

When one molecule is broken down into six component molecules, which of the following will always be true?

Delta S is positive. Correct. The large increase in disorder associated with this reaction means that entropy Delta S increases.

From the equation Delta G = Delta H – T Delta S it is clear that

a decrease in the system’s total energy will increase the probability of spontaneous change, increasing the entropy of a system will increase the probability of spontaneous change, and increasing the temperature of a system will increase the probability of spontaneous change Correct. This is the best response.

An exergonic (spontaneous) reaction is a chemical reaction that

releases energy when proceeding in the forward direction Correct. Exergonic reactions proceed with a net release of free energy, and they occur spontaneously.

Which of the following reactions would be endergonic?

glucose + fructose → sucrose Correct. In this case, the product is more complex, lower entropy, than the reactants, and like many anabolic reactions, this one requires an input of energy.

Molecules A and B contain 110 kcal/mol of free energy, and molecules B and C contain 150 kcal/mol of energy. A and B are converted to C and D. What can be

The reaction that proceeds to convert A and B to C and D is endergonic; the products are more organized than the reactants. Correct. C and D contain more energy than do A and B; therefore they are more organized and their construction required an input of energy.

Which of the following determines the sign of Delta G for a reaction?

the free energy of the reactants and the free energy of the products Correct. By subtracting the free energy of the reactants from the free energy of the products, the Delta G can be calculated and the difference in these values determines the sign of the difference.

Metabolic pathways in cells are typically far from equilibrium. Which of the following processes tend(s) to keep these pathways away from equilibrium?

The first and second listed responses are correct. Correct. Pathways can be displaced from equilibrium either by adding free energy or by removal of the products of the pathway by other reactions.

The free energy derived from the hydrolysis of ATP can be used to perform many kinds of cellular work. Which of the following is an example of the cellular work involved in the production of electrochemical gradients?

proton movement against a gradient of protons Correct. Protein pumps that hydrolyze ATP can generate electrochemical gradients.

In general, the hydrolysis of ATP drives cellular work by

releasing free energy that can be coupled to other reactions Correct. With the help of specific enzymes, the cell can couple the energy of ATP hydrolysis directly to endergonic processes.

Which of the following statements correctly describes some aspect of ATP hydrolysis being used to drive the active transport of an ion into the cell AGAINST the ion’s
concentration gradient?

This is an example of energy coupling. Correct. The free energy released from the hydrolysis of ATP is coupled to the energy-requiring active transport of the ion across a membrane.

Much of the suitability of ATP as an energy intermediary is related to the instability of the bonds between the phosphate groups. These bonds are unstable because

the negatively charged phosphate groups vigorously repel one another and the terminal phosphate group is more stable in water than it is in ATP Correct. Negative charges repel each other. Loss of the terminal phosphate removes some of the repulsion.

When 1 mole of ATP is hydrolyzed in a test tube without an enzyme, about twice as much heat is given off as when 1 mole of ATP is hydrolyzed in a cell. Which of the
following best explains these observations?

In the cell, the hydrolysis of ATP is coupled to other endergonic reactions. Correct. The coupling of ATP to other endergonic processes in cells means that less of the free energy is released as heat. When ATP is hydrolyzed without this coupling, all of the energy is released as heat.

Which of the following best characterizes the role of ATP in cellular metabolism?

The free energy released by ATP hydrolysis may be coupled to an endergonic process via the formation of a phosphorylated intermediate. Correct. This is one way a cell can use an exergonic reaction to drive an endergonic reaction.

The formation of glucose-6-phosphate from glucose is an endergonic reaction and is coupled to which of the following reactions or pathways?

the hydrolysis of ATP Correct. With the help of specific enzymes, the cell is able to couple the energy of ATP hydrolysis directly to endergonic processes by transferring a phosphate group from ATP to some other molecule, such as glucose.

A chemical reaction is designated as exergonic rather than endergonic when

the potential energy of the products is less than the potential energy of the reactants Correct. If a reaction is exergonic, the formation of new bonds releases more energy than was invested in breaking the old bonds.

Which of the following is changed by the presence of an enzyme in a reaction?

the activation energy Correct. An enzyme lowers the amount of energy required to get the reactants to the transition state.

What do the sign and magnitude of the Delta G of a reaction tell us about the speed of the reaction?

Neither the sign nor the magnitude of Delta G has anything to do with the speed of a reaction. Correct. The speed of a reaction is determined by the activation energy barrier of the reaction and the temperature, which determines how many reactants have the energy to overcome the barrier.

How do enzymes lower activation energy?

by locally concentrating the reactants Correct. One of the ways enzymes work is to increase the concentrations of reactants at a single place.

Which of the following statements about enzymes is/are true?

Enzymes speed up the rate of the reaction without changing the DG for the reaction. Correct. Enzymes speed up reactions by lowering the activation energy barrier for the reaction. Enzymes cannot change the DG for the reaction.

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. Correct. Enzymes lower the activation energy barrier of a reaction but do not change the free energy of the products.

A plot of reaction rate, velocity, against temperature for an enzyme indicates little activity at 10°C and 45°C, with peak activity at 35°C. The most reasonable explanation for the low velocity at 10°C is that

there is too little activation energy available Correct. The environment usually supplies activation energy in the form of heat. The lower the temperature, the less energy that is available to overcome the activation energy barrier.

Which of the following statements about enzymes is INCORRECT?

An enzyme is consumed during the reaction it catalyzes. Correct. If enzymes were consumed during the reaction they catalyzed, they would not be able to act as a catalyst. This statement is incorrect.

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. Correct. Only a restricted region of an enzyme molecule (the active site) binds to the substrate. Usually, the active site is formed by only a few of the amino acids, with the rest of the protein molecule providing a framework that reinforces the configuration of the active site.

What is meant by the "induced fit" of an enzyme?

The enzyme changes its shape slightly as the substrate binds to it. Correct. The enzyme changes slightly to bind to the substrate and catalyze the reaction.

Which of the following statements correctly describe(s) the role or roles of heat in biological reactions?

The first and second choices are correct. Correct. As the heat energy in a system increases, so does the kinetic energy of the reactants. As the kinetic energy of the reactants increases, the reactants are more likely to interact, with each other directly, or with the active site of an enzyme. Subsequently, the reaction rate would increase.

Above a certain substrate concentration, the rate of an enzyme-catalyzed reaction drops as the enzymes become saturated. Which of the following would lead to a faster conversion of substrate into product under these saturated conditions?

The first and second listed responses are correct. Correct. Either increasing the enzyme concentration or slightly increasing the temperature will increase the rate of product formation.

Which of the following environments or actions would NOT affect the rate of an enzyme reaction?

None of the listed responses is correct. Correct. Changes in temperature, substrate concentration, and pH are all likely to affect enzyme activity.

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 Correct. Each enzyme has an optimal pH at which it is most active, and variations in pH can alter the enzyme’s structure, changing activity.

Which of these statements about enzyme inhibitors is true?

The action of competitive inhibitors may be reversible or irreversible. Correct. Competitive inhibitors that bind covalently to the enzyme would be irreversible, and those that bind weakly would be reversible.

Succinylcholine is structurally almost identical to acetylcholine. If succinylcholine is added to a mixture that contains acetylcholine and the enzyme that hydrolyzes
acetylcholine (but not succinylcholine), the rate of acetylcholine hydrolysis is decreased. Subsequent addition of more acetylcholine restores the original rate of
acetylcholine hydrolysis. Which of the following correctly explains this observation?

Succinylcholine must be a competitive inhibitor with acetylcholine. Correct. Competitive inhibition occurs when a molecule mimics the substrate by competing with it at the active site.

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

allosteric activation Correct. The molecule in this example would be termed an allosteric activator.

Which of the following statements about allosteric proteins is/are true?

All of the first three listed responses are correct.

The binding of an allosteric inhibitor to an enzyme causes the rate of product formation by the enzyme to decrease. Which of the following best explains why this decrease occurs?

The allosteric inhibitor causes a structural change in the enzyme that prevents the substrate from binding at the active site. Correct. In general, any allosteric regulator functions by changing the structure of the enzyme to change the ability of the active site to bind the substrate.

Under most conditions, the supply of energy by catabolic pathways is regulated by the demand for energy by anabolic pathways. Considering the role of ATP formation
and hydrolysis in energy coupling of anabolic and catabolic pathways, which of the following statements is most likely to be true?

High levels of ADP result in allosteric activation of catabolic pathways. Correct. In this case, high demand for energy by anabolic pathways uses ATP, increasing the level of ADP, which in turn increases the supply of ATP from catabolic pathways.

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