Biology 1107 Chapter 7

(e) none of the above

1. Which of the following can do work in a cell? (a) entropy (b) heat (c) heat energy (d) all of the above (e) none of the above

(b) the substrate and enzyme undergo conformational changes

10. "Induced fi t" means that when a substrate binds to an enzyme's active site (a) it fi ts perfectly, like a key in a lock (b) the substrate and enzyme undergo conformational changes (c) a site other than the active site undergoes a conformational change (d) the substrate and the enzyme become irreversibly bound to each other (e) c and d

(b) drive a sequence of reactions in a particular direction

11. Th e function of a biochemical pathway is to (a) supply energy to reactions (b) drive a sequence of reactions in a particular direction (c) maintain chemical equilibrium (d) make energy available to endergonic reactions (e) any of the above, depending on the pathway

(a) enzyme

12. In the following reaction series, which enzyme(s) is/are most likely to have an allosteric site to which the end product E binds? (a) enzyme 1 (b) enzyme 2 (c) enzyme 3 (d) enzyme 4 (e) enzymes 3 and 4

(d) potential energy

2. In a chemical reaction occurring in a cell, free energy is equivalent to (a) heat energy (b) heat (c) disorder (d) potential energy (e) more than one of the above is true

(b) have mechanisms that transform energy from the environment into useful forms

3. Cells are able to function because they (a) are subject to the laws of thermodynamics (b) have mechanisms that transform energy from the environment into useful forms (c) can use enzymes to convert endergonic reactions into spontaneous reactions (d) all of the above

(c) exergonic process because entropy increases

4. Diff usion is an (a) endergonic process because free energy increases (b) endergonic process because free energy decreases (c) exergonic process because entropy increases (d) exergonic process because entropy decreases (e) more than one of the above

(b) negative

5. A spontaneous reaction is one in which the change in free energy (ΔG) has a _____ value. (a) positive (b) negative (c) positive or negative (d) none of these (ΔG has no measurable value)

(c) an ATP/ADP ratio of at least 10:1

6. Healthy living cells maintain (a) ATP and ADP at equilibrium (b) equal concentrations of ATP and ADP (c) an ATP/ADP ratio of at least 10:1 (d) an ATP/ADP ratio of no more than 1:10 (e) most of the cell's stored energy in the form of ATP

(e) I ¡ J, ΔG = −5.91 kJ/mol

7. Which of the following reactions could be coupled to an endergonic reaction with ΔG = +3.56 kJ/mol? (a) A ¡ B, ΔG = +6.08 kJ/mol (b) C ¡ D, ΔG = +3.56 kJ/mol (c) E ¡ F, ΔG = 0 kJ/mol (d) G ¡ H, ΔG = −1.22 kJ/mol (e) I ¡ J, ΔG = −5.91 kJ/mol

(e) the reaction must be coupled to an exergonic reaction

8. Consider this reaction: glucose + 6 O2 ¡ 6 CO2 + 6 H2O (ΔG = −2880 kJ/mol). Which of the following statements about this reaction is not true? (a) the reaction is spontaneous in a thermodynamic sense (b) a small amount of energy (activation energy) must be supplied to start the reaction, which then proceeds with a release of energy (c) the reaction is exergonic (d) the reaction can be coupled to an endergonic reaction (e) the reaction must be coupled to an exergonic reaction

(b) can be lowered by a specifi c enzyme (c) can be raised by a specifi c enzyme

9. Th e required energy of activation of a reaction (a) is fi xed, and cannot be altered (b) can be lowered by a specifi c enzyme (c) can be raised by a specifi c enzyme (d) b or c, depending on the enzyme (e) none of the above

■ When a chemical reaction is in a state of dynamic equilibrium, the rate of change in one direction is exactly the same as the rate of change in the opposite direction; the system can do no work because the free-energy diff erence between the reactants and products is zero. ■ When the concentration of reactant molecules is increased, the reaction shifts to the right and more product molecules are formed until equilibrium is re-established.

Compare the energy dynamics of a reaction at equilibrium with the dynamics of a reaction not at equilibrium.

Energy is the capacity to do work (expressed in kilojoules, kJ). Energy can be conveniently measured as heat energy, thermal energy that flows from an object with a higher temperature to an object with a lower temperature; the unit of heat energy is the kilocalorie (kcal), which is equal to 4.184 kJ. Heat energy cannot do cell work.

Define energy, emphasizing how it is related to work and to heat.

■ Enzymes work best at specifi c temperature and pH conditions. ■ A cell can regulate enzymatic activity by controlling the amount of enzyme produced and by regulating metabolic conditions that influence the shape of the enzyme. ■ Some enzymes have allosteric sites, noncatalytic sites to which an allosteric regulator binds, changing the enzyme's activity. Some allosteric enzymes are subject to feedback inhibition, in which the formation of an end product inhibits an earlier reaction in the metabolic pathway. ■ Reversible inhibition occurs when an inhibitor forms weak chemical bonds with the enzyme. Reversible inhibition may be competitive, in which the inhibitor competes with the substrate for the active site, or noncompetitive, in which the inhibitor binds with the enzyme at a site other than the active site. Irreversible inhibition occurs when an inhibitor combines with an enzyme and permanently inactivates it.

Describe specifi c ways enzymes are regulated.

■ As entropy increases, the amount of free energy decreases, as shown in the equation G = H − TS, in which G is the free energy, H is the enthalpy (total potential energy of the system), T is the absolute temperature (expressed in Kelvin units), and S is entropy. ■ The equation ΔG = ΔH − TΔS indicates that the change in free energy (ΔG) during a chemical reaction is equal to the change in enthalpy (ΔH) minus the product of the absolute temperature (T) multiplied by the change in entropy (ΔS).

Discuss how changes in free energy in a reaction are related to changes in entropy and enthalpy.

■ An exergonic reaction has a negative value of ΔG; that is, free energy decreases. Such a reaction is spontaneous; it releases free energy that can perform work. ■ Free energy increases in an endergonic reaction. Such a reaction has a positive value of ΔG and is nonspontaneous. In a coupled reaction, the input of free energy required to drive an endergonic reaction is supplied by an exergonic reaction.

Distinguish between exergonic and endergonic reactions, and give examples of how they may be coupled.

■ An enzyme is a biological catalyst; it greatly increases the speed of a chemical reaction without being consumed. ■ An enzyme works by lowering the activation energy (EA), the energy necessary to get a reaction going. The active site of an enzyme is a 3-D region where substrates come into close contact and thereby react more readily. When a substrate binds to an active site, an enzyme-substrate complex forms in which the shapes of the enzyme and substrate change slightly. This induced fi t facilitates the breaking of bonds and formation of new ones.

Explain how an enzyme lowers the required energy of activation for a reaction.

■ Adenosine triphosphate (ATP) is the immediate energy currency of the cell. It donates energy by means of its terminal phosphate group, which is easily transferred to an acceptor molecule. ATP is formed by the phosphorylation of adenosine diphosphate (ADP), an endergonic process that requires an input of energy. ■ ATP is the common link between exergonic and endergonic reactions and between catabolism (degradation of large complex molecules into smaller, simpler molecules) and anabolism (synthesis of complex molecules from simpler molecules).

Explain how the chemical structure of ATP allows it to transfer a phosphate group and discuss the central role of ATP in the overall energy metabolism of the cell.

■ Energy is transferred in oxidation-reduction (redox) reactions. A substance becomes oxidized as it gives up one or more electrons to another substance, which becomes reduced in the process. Electrons are commonly transferred as part of hydrogen atoms. ■ NAD+ and NADP+ accept electrons as part of hydrogen atoms and become reduced to form NADH and NADPH, respectively. These electrons (along with some of their energy) can be transferred to other acceptors.

Relate the transfer of electrons (or hydrogen atoms) to the transfer of energy.

■ A closed system does not exchange energy with its surroundings. Organisms are open systems that do exchange energy with their surroundings. ■ The first law of thermodynamics states that energy cannot be created or destroyed but can be transferred and changed in form. The first law explains why organisms cannot produce energy; but as open systems, they continuously capture it from the surroundings. ■ The second law of thermodynamics states that disorder (entropy) in the universe, a closed system, is continuously increasing. No energy transfer is 100% efficient; some energy is dissipated as heat, random motion that contributes to entropy (S), or disorder. As open systems, organisms maintain their ordered states at the expense of their surroundings.

State the first and second laws of thermodynamics, and discuss the implications of these laws as they relate to organisms.

■ Potential energy is stored energy; kinetic energy is energy of motion. ■ All forms of energy are interconvertible. For example, photosynthetic organisms capture radiant energy and convert some of it to chemical energy, a form of potential energy that powers many life processes, such as muscle contraction.

Use examples to contrast potential energy and kinetic energy.