CH 8- Mastering Biology

"Conservation of energy" refers to the fact that _____.

Energy cannot be created or destroyed but can be converted from one form to another.

Chemical energy is a form of _____ energy.

Potential energy because chemical energy is considered stored energy.

In your body, what process converts the chemical energy found in glucose into the chemical energy found in ATP?

Cellular respiration because this is the name given to the process by which the body converts food energy to energy stored in ATP.

Which of these are by-products of cellular respiration?

Heat, carbon dioxide and water.

The process of cellular respiration, which converts simple sugars such as glucose into CO2 and water, is an example of _____.

A catabolic pathway

Energy is observed in two basic forms: potential and kinetic. Which of the following correctly matches these forms with a source of energy?

The covalent bonds of a sugar molecule: potential energy. Bonds are a form of potential energy because the energy arises from the relative positions of the atoms that form the bond.

Which of the following statements about the combustion of glucose with oxygen to form water and carbon dioxide (C6H12O6 + 6 O2 → 6 CO2 + 6 H2O) is correct?

The entropy of the products is greater than the entropy of the reactants. A large molecule (glucose) has been converted into several smaller molecules (water and carbon dioxide); thus, the products have more disorder (greater entropy) than the reactants.

Which of the following statements about equilibrium of chemical reactions is correct?

A reaction that is at equilibrium is not capable of doing any work. The ΔG for a reaction at equilibrium is zero, which means that there is no free energy available to do any work.

Select the highest energy form of adenosine from the following images.

Adenosine triphosphate (ATP) is the high-energy form of adenosine because it contains the most phosphate groups (three). This molecule fuels many different endergonic (energy-requiring) enzymatic processes in biological organisms. ATP molecules diffuse or are transported to the place where the energy is needed and deliver chemical energy from the breaking of their phosphate bonds.

Which part of the adenosine triphosphate molecule is released when it is hydrolyzed to provide energy for biological reactions?

The γ-phosphate is the primary phosphate group on the ATP molecule that is hydrolyzed when energy is needed to drive anabolic reactions. Located the farthest from the ribose sugar, it has a higher energy than either the α- or β-phosphate.

The following reaction
A –> B + C + heat
is a(n) _____ reaction.

Exergonic

A(n) _____ reaction occurs spontaneously.

Exergonic because in exergonic reactions the products have less potential energy than the reactants.

Which of these reactions requires a net input of energy from its surroundings?

Endergonic because the products of endergonic reactions have more potential energy than the reactants.

In cells, what is usually the immediate source of energy for an endergonic reaction?

ATP because the hydrolysis of ATP provides the energy needed for an endergonic reaction.

The reaction ADP + P –> ATP is a(n) _____ reaction.

Endergonic because energy has been acquired from the surroundings.

The energy for an endergonic reaction comes from a(n) _____ reaction.

Exergonic because the energy released by an exergonic reaction can be used to drive an endergonic reaction.

What is the fate of the phosphate group that is removed when ATP is converted to ADP?

It is acquired by a reactant in an endergonic reaction because by acquiring the phosphate group the reactant acquires energy.

Select the INCORRECT association.

Exergonic….Uphill

What is energy coupling?

The use of energy released from an exergonic reaction to drive an endergonic reaction.

What type of reaction breaks the bonds that join the phosphate groups in an ATP molecule?

Hydrolysis because it involves breaking bonds with the addition of water.

Which of the following statements about ATP (adenosine triphosphate) is correct?

The cycling between ATP and ADP + Pi provides an energy coupling between catabolic and anabolic pathways. Catabolic pathways provide the energy needed to make ATP from ADP and Pi. The hydrolysis of ATP to ADP + Pi releases the same amount of energy.

Consider a situation in which the enzyme is operating at optimum temperature and pH, and has been saturated with substrate. What is your best option for increasing the rate of the reaction?

Increase the enzyme concentration. If an enzyme is saturated with substrate, and it is operating at optimum pH and optimum temperature, there is very little that can be done except to increase the enzyme concentration. Some enzymes can be activated further by allosteric activators, in which case one might add some activator to the reaction. But otherwise, increasing the enzyme concentration is the only option.

When the noncompetitive inhibitor is bonded to the enzyme, the shape of the _____ is distorted.

Enzyme

Insecticides and nerve gases act as irreversible inhibitors of _____ , an enzyme needed for nerve conduction.

Acetylcholinesterase

Usually, an _______ inhibitor forms a covalent bond with an amino acid side group within the active site, which prevents the substrate from entering the active site or prevents catalytic activity.

Irreversible

A _______ inhibitor has a structure that is so similar to the substrate that it can bond to the enzyme just like the substrate.

Competitive

The competitive inhibitor competes with the substrate for the ______ on the enzyme.

Active Site

A _______ inhibitor binds to a site on the enzyme that is not the active site.

Noncompetitive

You have added an irreversible inhibitor to a sample of enzyme and substrate. At this point, the reaction has stopped completely.

The enzyme is inactive at this point. New enzyme must be added to regain enzyme activity. Because they bind directly to the active site by covalent bonds, irreversible inhibitors permanently render an enzyme inactive. Some drugs are irreversible inhibitors, including the antibiotic penicillin (which inhibits an enzyme involved in bacterial cell-wall synthesis) and aspirin (which inhibits cyclooxygenase-2, the enzyme involved in the inflammatory reaction).

You have an enzymatic reaction proceeding at the optimum pH and optimum temperature. You add a competitive inhibitor to the reaction and notice that the reaction slows down.

Add more substrate; it will outcompete the inhibitor and increase the reaction rate. Competitive inhibition can be overcome by adding more substrate to outcompete the inhibitor. Many drugs used to treat different medical conditions, including hypertension, are competitive inhibitors. It is fairly easy to make a molecule that is similar in structure to a particular substrate because the known enzyme’s shape can be used as a model of what the molecule needs to look like. It is more difficult to make a noncompetitive inhibitor because it is less obvious what the noncompetitive inhibitor’s shape and structure should be.

In general, enzymes are what kinds of molecules?

Proteins

Enzymes work by _____.

Reducing activation energy.

An enzyme _____.

is an organic catalyst.

What name is given to the reactants in an enzymatically catalyzed reaction?

Substrate

As a result of its involvement in a reaction, an enzyme _____

Is unchanged

How do cells use ATP to raise the energy level of reaction substrates?

The terminal phosphate of ATP is bound to the substrate. The bond between Pi and the substrate preserves much of the energy that was in ATP.

Why is ATP a good source of energy for biological reactions?

Triphosphate chains are unstable. The instability is associated with high energy, favoring reactions that break the triphosphate chain.

A reaction is said to be unfavorable if …

The free energy change for the reaction is positive and the equilibrium favors the reactants, not the products

The reaction A → B is unfavorable by itself, but through energy-coupling, cells can use ATP to convert A into B. How is this done?

The unfavorable reaction is replaced by two favorable reactions. The first reaction transfers part of ATP to A, making a high-energy product. That product is then converted to B. Both reactions release energy, so both are favorable.

How do cells replace the energy-rich ATP that is destroyed in energy-coupled reactions?

Chloroplasts use light energy to synthesize ATP and the mitochondria synthesizes ATP using energy that’s released by oxidizing sugars and fats.

Select the correct statement about chemical energy, a term used by biologists to refer to potential energy available for release in a chemical reaction.

Light energy is converted to chemical energy during photosynthesis. Photosynthetic cells capture light energy and transform some of it to chemical energy as they synthesize glucose from CO2 and H2O. Read about the laws of energy transformation.

Which of the following statements about the role of ATP in cell metabolism is true?

The energy from the hydrolysis of ATP may be directly coupled to endergonic processes by the transfer of the phosphate group to another molecule. A key feature in the way cells manage their energy resources 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. Read about energy coupling.

What is the free energy change (ΔG) of the hydrolysis of ATP to ADP?

The free-energy change (ΔG) of the hydrolysis of ATP to ADP and Pi may vary considerably with variations in pH, temperature, atmospheric pressure, and concentrations of reactants and products. The free-energy change (ΔG) of the hydrolysis of ATP to ADP and Pi is -7.3 kcal/mole under standard conditions. Standard conditions are defined as a temperature of 298 K (or 250C), 1 atm, pH 7, and equal 1M concentrations present of all reactants and products. In living cells, conditions do not conform to standard conditions, primarily because reactant and product concentrations differ from 1 M. For example, when ATP hydrolysis occurs under cellular conditions, the actual ΔG is about -13 kcal/mol, 78% greater than the energy released by ATP hydrolysis under standard conditions.

How does an enzyme increase the rate of the chemical reaction it catalyzes?

An enzyme catalyzes a reaction by lowering EA, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures. Read about enzymes and activation energy.

Which statement about the binding of enzymes and substrates is correct?

When substrate molecules bind to the active site of the enzyme, the enzyme undergoes a slight change in shape. As the substrate enters the active site, the enzyme changes shape slightly due to interactions between the substrate’s chemical groups and chemical groups on the side chains of the amino acids that form the active site. This shape change makes the active site fit even more snugly around the substrate. This induced fit is like a clasping handshake.

Which of the following statements about feedback regulation of a metabolic pathway is correct?

The final product of a metabolic pathway is usually the compound that regulates the pathway. It is quite common that the end product of the pathway controls the overall rate of the pathway.

Which type of control agent never speeds an enzyme’s action?

Substrate analogs resemble the substrate enough to bind to the active site, but they can’t react. Accordingly, they just block the active site and slow the reaction.

Which type of control agent exerts noncompetitive inhibition?

Protein kinase and allosteric effector. Competitive inhibitors bind to the active site and compete with the substrate for that position. Allosteric effectors don’t do that, nor do the phosphoryl groups that are added by protein kinases.

In cooperativity, …

If one substrate is bound, the next binds more easily. All reactions run faster when the substrate concentration rises, but the cooperative effect causes even more acceleration in response to added substrate.

Which statement is characteristic of allosteric effectors?

They may not resemble the enzyme’s substrates.

When allosteric effector X binds to enzyme #1, the enzyme stops working. Nevertheless, the speed of the reaction can be altered by adjusting the concentration of X. How?

Because X leaves an enzyme easily and lets the enzyme go back to work, there are always some working enzymes. The concentration of X determines how many copies are working at each moment. This wouldn’t be true if the enzymes couldn’t recover.

When a pathway is subject to allosteric feedback inhibition, …

An accumulation of effectors slows the pathway. With this arrangement, feedback inhibition limits the accumulation of effectors.

Which statement is true of enzymes?

There are many copies of each enzyme; they’re usually proteins but sometimes they are RNA; and they only attack substrates that fit the shape and charge of the active site.

What are true about enzymes?

(1) Enzymes may change shape when they bind substrates; (2) Enzymes provide no energy for the reaction, except collision energy; (3) Enzymes may release substrates.

How can "induced fit" influence the specificity of an enzyme?

The active site of the enzyme will change shape to make a better fit with only the appropriate substrate, which can bring the reactive portion of the enzyme closer to the substrate.

Enzymes speed reactions mainly by …

Lowering EA. Enzymes always lower EA, though they may have other effects as well. With a lower EA, more collisions can produce the transition state.

Which fact is most important in explaining how enzymes speed reactions?

High-energy collisions are less common than low-energy collisions. Enzymes provide reaction pathways that have low activation energy requirements. This allows low-energy collisions to cause reactions.

In an experiment with an enzyme, the 58th amino acid seems to form a covalent bond with a substrate molecule as part of the catalytic process. What would you say?

At some point the bond between the amino acid and the substrate must break. To work repeatedly, the enzyme must return to its original state at the end of each catalytic cycle. Some steps may form bonds or break bonds between the enzyme and substrate.

Dr. Haxton thinks a certain enzyme works by the steps shown in the animation below. What would a good student say about the proposed mechanism?

Enzymes do the kinds of things that are shown here; it’s possible. Enzymes may give and take atoms and form temporary covalent bonds with substrates as they catalyze reactions.

Which of the following is NOT a way in which an enzyme can speed up the reaction that it catalyzes?

The active site can provide heat from the environment that raises the energy content of the substrate. An enzyme cannot extract heat from the environment to speed a reaction. It can only lower the activation energy barrier so that more substrates have the energy to react.

The binding of a compound to an enzyme is observed to slow down or stop the rate of the reaction catalyzed by the enzyme. Increasing the substrate concentration reduces the inhibitory effects of this compound. Which of the following could account for this observation?

The compound is a competitive inhibitor. A competitive inhibitor slows down the enzyme by competing with the substrate for binding at the active site. Increasing substrate concentrations will reduce the effectiveness of a competitive inhibitor.

Enzymes are described as catalysts, which means that they _____.

Increase the rate of a reaction without being consumed by the reaction. This permits enzyme molecules to be used repeatedly.