Unit 8 Question Bank

Free energy · coupling · high-energy phosphates · the ATP/ADP cycle

25 MCQ · five options6 Definitions2 Written answersHarper's verified

Format note: the TMU Biochemistry paper gives five suggested answers (A–E), not four — these MCQs match that. Items tagged TMU 2019 or TMU 2020/21 come from the real papers. Answers are verified against Harper's Illustrated Biochemistry; the "marking schemes" in the source folder are other students' answer sheets, not official, so they are never used as the authority.

0 / 25 answered

1Gibbs free energy (ΔG) is best defined as ( ).

A. the total heat released by a reaction

B. the energy required to reach the transition state

C. the portion of the energy change available for doing work

D. the increase in entropy accompanying a reaction

E. the energy stored in the terminal phosphate bond of ATP

Answer: C

ΔG is the useful energy, also called the chemical potential. Option A is enthalpy (ΔH), option B is activation energy, option D is ΔS. The three are related by ΔG = ΔH − TΔS.Harper's ch.11, p.114 · TMU Lecture 8 Slide 5

2The second law of thermodynamics states that ( ).

A. the total energy of a system including its surroundings remains constant

B. heat cannot be converted into work at constant temperature

C. every reaction proceeds towards minimum enthalpy

D. the total entropy of a system must increase if a process is to occur spontaneously

E. the free energy of a system is always positive

Answer: D

Entropy is the extent of disorder or randomness of a system, and becomes maximal as equilibrium is approached. Option A is the first law.Harper's ch.11, p.114

3A reaction with a large NEGATIVE ΔG is ( ).

A. endergonic and essentially irreversible

B. exergonic but readily reversible

C. at equilibrium

D. endergonic and requires coupling

E. exergonic and essentially irreversible

Answer: E

Negative ΔG = exergonic = proceeds spontaneously with loss of free energy; and if ΔG is of great magnitude the reaction goes virtually to completion and is essentially irreversible. This is why the three irreversible steps of glycolysis are irreversible — and why gluconeogenesis needs separate enzymes to bypass them.Harper's ch.11, p.114

4ΔG⁰′ differs from ΔG⁰ in that it is measured at ( ).

A. pH 7.0, the biochemical standard state

B. 0 °C

C. 1 atmosphere pressure

D. the physiological concentration of each reactant

E. equilibrium

Answer: A

ΔG⁰ is the standard free-energy change with reactants at 1.0 mol/L; for biochemical reactions the standard state additionally specifies pH 7.0, denoted ΔG⁰′. Note also that the actual ΔG may be larger or smaller than ΔG⁰′ depending on the concentrations actually present.Harper's ch.11, p.114

5An enzyme added to a biochemical reaction ( ).

A. converts an endergonic reaction into an exergonic one

B. hastens equilibrium but never changes its final position

C. increases the equilibrium constant of the reaction

D. lowers the free energy of the reaction products

E. raises the free energy of the substrates

Answer: B

Harper's states it in exactly those words. An enzyme lowers the activation energy, not ΔG — so it changes the rate at which equilibrium is reached, never the position of that equilibrium. This is Unit 5's rule stated thermodynamically.Harper's ch.11, p.114

6An endergonic process in a living cell ( ).

A. occurs spontaneously provided a catalyst is present

B. releases free energy into the surroundings

C. must be coupled to an exergonic reaction to proceed at all

D. has a negative ΔG under all cellular conditions

E. occurs only when the system is at equilibrium

Answer: C

Vital processes — synthetic reactions, muscular contraction, nerve impulse conduction and active transport — are all endergonic and obtain energy by chemical linkage, or coupling, to oxidative reactions.Harper's ch.11, p.114

7The chief biological advantage of using a high-energy intermediate such as ATP to couple reactions is that ( ).

A. it releases more energy than any other cellular compound

B. it crosses all cellular membranes without a transporter

C. it does not require an enzyme in order to react

D. it need not resemble the reactants, so it links any two pathways

E. it is never consumed in the course of the reaction

Answer: D

This is the whole reason ATP exists. Because the carrier is chemically unrelated to the reactants, it can serve as a transducer of energy from a wide range of exergonic reactions to an equally wide range of endergonic ones — biosynthesis, muscular contraction, nervous excitation, active transport. It is money rather than barter.Harper's ch.11, p.115

8In its reactions within the cell, ATP functions as ( ).

A. the free tetra-anion

B. a Ca²⁺ complex

C. the protonated acid

D. a complex with coenzyme A

E. the Mg²⁺ complex

Answer: E

The triphosphate chain carries a dense cluster of negative charges; Mg²⁺ neutralises part of that charge and holds the phosphates in the geometry kinases recognise. ADP forms a similar complex — which is why so many kinases are listed as magnesium-requiring enzymes.Harper's ch.11, p.115 · TMU Lecture 8 Slide 9

9How many high-energy phosphate groups do ATP, ADP and AMP contain respectively?

A. Two, one and none

B. Three, two and one

C. Two, two and one

D. Three, one and none

E. One, one and one

Answer: A

ATP has two anhydride bonds; ADP has one; AMP's single phosphate is an ordinary ester bond and carries no high-energy phosphate at all.Harper's ch.11, p.116, Figure 11–5

10The standard free energy of hydrolysis of ATP to ADP + P_i is approximately ( ).

A. −61.9 kJ/mol

B. −30.5 kJ/mol

C. −13.8 kJ/mol

D. −43.1 kJ/mol

E. −19.2 kJ/mol

Answer: B

−30.5 kJ/mol (−7.3 kcal/mol). The distractors are all real values from the same table: −61.9 is phosphoenolpyruvate, −43.1 creatine phosphate, −19.2 PP_i, and −13.8 glucose-6-phosphate.Harper's ch.11, p.116, Table 11–1

11Which compound has the HIGHEST standard free energy of hydrolysis?

A. Creatine phosphate

B. 1,3-Bisphosphoglycerate

C. Phosphoenolpyruvate

D. ATP

E. Glucose-6-phosphate

Answer: C

Phosphoenolpyruvate at −61.9 kJ/mol tops the table. Note which compounds sit above ATP: phosphoenolpyruvate and 1,3-bisphosphoglycerate — and those are precisely the two glycolytic intermediates that can make ATP, at pyruvate kinase and phosphoglycerate kinase respectively.Harper's ch.11, p.116, Table 11–1

12The bioenergetic significance of ATP's INTERMEDIATE position in the table of free energies of hydrolysis is that ( ).

A. it is the most stable phosphate compound in the cell

B. it prevents any other compound from acting as a phosphate donor

C. it makes ATP hydrolysis reversible under all conditions

D. it can be regenerated from above and donate phosphate below

E. it can be hydrolysed without an enzyme

Answer: D

If ATP were at the top of the table nothing could recharge it; at the bottom it would be useless to spend. Sitting in the middle is what makes it a currency. Harper's: “The intermediate position of ATP allows it to play an important role in energy transfer.”Harper's ch.11, p.116

13High-energy phosphates include anhydrides, enolphosphates and ( ).

A. ester phosphates

B. phosphodiesters

C. phospholipids

D. phosphoproteins

E. phosphoguanidines

Answer: E

The three classes of high-energy phosphate are anhydrides (e.g. 1,3-bisphosphoglycerate), enolphosphates (phosphoenolpyruvate) and phosphoguanidines (creatine phosphate, arginine phosphate). Ester phosphates — the glycolytic intermediates — are the LOW-energy group.Harper's ch.11, p.115

14The large free-energy change on hydrolysis of ATP is due to ( ).

A. relief of charge repulsion, and resonance stabilisation of the phosphate

B. the unusually strong covalent bond between the β and γ phosphates

C. the release of a large quantity of heat from the bond itself

D. the binding of magnesium ions to the reaction products

E. an increase in the entropy of the adenosine moiety

Answer: A

Two reasons, and most students give only one. Note that neither is about the bond containing energy — energy is released because the products are more stable. This is exactly why “group transfer potential” is preferred to “high-energy bond”.Harper's ch.11, p.116

15The ATP/ADP cycle must turn over extremely rapidly because ( ).

A. ATP is unstable and hydrolyses spontaneously within seconds

B. the ATP/ADP pool would last an active tissue only seconds

C. mitochondria can store only a limited amount of oxygen

D. the concentration of inorganic phosphate is very low

E. ADP is toxic to the cell if it accumulates

Answer: B

ATP is not a store of energy but a flow — made continuously at the rate it is spent, like electricity in a city rather than fuel in a tank. This is why interrupting the respiratory chain kills within minutes, and why phosphagens exist to buffer the seconds in between.Harper's ch.11, p.116

16The GREATEST quantitative source of high-energy phosphate in aerobic organisms is ( ).

A. glycolysis

B. the citric acid cycle

C. oxidative phosphorylation

D. the pentose phosphate pathway

E. creatine phosphate breakdown

Answer: C

The three sources of ~℗ are oxidative phosphorylation (the greatest, generated in the mitochondrial matrix as O₂ is reduced to H₂O), glycolysis (a net 2), and the citric acid cycle (1, directly, at succinate thiokinase).Harper's ch.11, pp.116–117 · TMU Lecture 8 Slide 14

17The net formation of two high-energy phosphates in glycolysis is catalysed by ( ).

A. hexokinase and phosphofructokinase

B. aldolase and enolase

C. glucokinase and glucose-6-phosphatase

D. phosphoglycerate kinase and pyruvate kinase

E. succinate thiokinase and pyruvate carboxylase

Answer: D

Those are the two ATP-generating steps. Note the logic: they are the steps immediately following 1,3-bisphosphoglycerate and phosphoenolpyruvate — the only two glycolytic intermediates ABOVE ATP in the free-energy table. Hexokinase and phosphofructokinase consume ATP.Harper's ch.11, p.116 · TMU Lecture 8 Slide 14

18In the citric acid cycle, high-energy phosphate is generated directly at the step catalysed by ( ).

A. citrate synthase

B. isocitrate dehydrogenase

C. fumarase

D. malate dehydrogenase

E. succinate thiokinase

Answer: E

One ~℗ is generated directly in the cycle at the succinate thiokinase step (your slide calls it succinyl thiokinase — the same enzyme). This is substrate-level phosphorylation; the cycle's much larger contribution is the reduced coenzymes it feeds to the respiratory chain.Harper's ch.11, p.117 · TMU Lecture 8 Slide 14

19The phosphorylation of glucose to glucose-6-phosphate has ΔG⁰′ = +13.8 kJ/mol. Coupled by hexokinase to ATP hydrolysis (ΔG⁰′ = −30.5 kJ/mol), the overall ΔG⁰′ becomes ( ).

A. −16.7 kJ/mol

B. +44.3 kJ/mol

C. −44.3 kJ/mol

D. +16.7 kJ/mol

E. −30.5 kJ/mol

Answer: A

+13.8 + (−30.5) = −16.7 kJ/mol. The coupled reaction is therefore highly exergonic and, under physiological conditions, irreversible — which is exactly why gluconeogenesis cannot simply reverse it and needs glucose-6-phosphatase instead. Harper's adds: “Many activation reactions follow this pattern.”Harper's ch.11, p.117

20Phosphagens are best described as ( ).

A. enzymes that transfer phosphate from ATP onto glucose

B. storage forms of high-energy phosphate such as creatine phosphate

C. the low-energy phosphate esters of the glycolytic pathway

D. inorganic phosphate stores within the mitochondria

E. phospholipids of the inner mitochondrial membrane

Answer: B

Creatine phosphate occurs in vertebrate skeletal muscle, heart, spermatozoa and brain; arginine phosphate in invertebrate muscle. They maintain ATP concentration during rapid utilisation, and accumulate when the ATP/ADP ratio is high.Harper's ch.11, p.117

21Creatine phosphate can regenerate ATP from ADP because its free energy of hydrolysis (−43.1 kJ/mol) is ( ).

A. smaller than that of ATP, placing it below ATP in the table

B. identical to that of ATP

C. greater than that of ATP, placing it above ATP in the table

D. positive rather than negative

E. independent of pH

Answer: C

Being above ATP is the whole design — it can donate phosphate to ADP instantly, without waiting for any metabolic pathway. Hence the three energy systems of exercise: creatine phosphate for the first seconds, then glycolysis, then oxidative phosphorylation. Three positions in one table.Harper's ch.11, pp.116–117

22Adenylate kinase (myokinase) catalyses ( ).

A. ATP + H₂O → ADP + P_i

B. ADP + P_i → ATP

C. ATP + creatine → creatine phosphate + ADP

D. ATP + AMP ⇌ 2 ADP

E. AMP + P_i → ADP

Answer: D

It allows high-energy phosphate in ADP to be used to synthesise ATP, allows AMP formed in activation reactions to be recovered, and — most importantly — allows AMP to rise when ATP is depleted, acting as an allosteric signal to increase catabolism.Harper's ch.11, p.117

23AMP makes a better metabolic alarm signal than ATP because ( ).

A. AMP is the only nucleotide that can bind allosteric sites

B. AMP cannot be metabolised any further by the cell

C. the ATP concentration does not vary at all

D. AMP is confined to the mitochondrial matrix

E. the AMP pool is small, so a small ATP fall raises AMP sharply

Answer: E

Amplification. The ATP pool is large relative to the AMP pool, so AMP is a far more sensitive indicator of energy deficit than ATP is a gauge. You will meet AMP as an allosteric activator repeatedly through the metabolic units.Harper's ch.11, p.117

24When ATP is hydrolysed directly to AMP + PP_i, the reaction is driven to completion partly by ( ).

A. hydrolysis of PPi by inorganic pyrophosphatase

B. the immediate rephosphorylation of AMP by adenylate kinase

C. the reaction being catalysed by two enzymes simultaneously

D. removal of the AMP by rapid deamination

E. binding of Mg²⁺ to the reaction products

Answer: A

By destroying one of its own products, the cell makes the reaction effectively irreversible — the same logic as coupling, applied in reverse. The reaction is also accompanied by loss of free energy as heat. Watch for this pattern in fatty acid activation, nucleotide synthesis and DNA replication.Harper's ch.11, p.117

25Which of the following is NOT one of the “high-energy compounds” listed by Harper's outside the phosphates?

A. Acetyl-CoA

B. Glucose-6-phosphate

C. S-adenosylmethionine

D. UDP-glucose

E. PRPP

Answer: B

Glucose-6-phosphate is a LOW-energy phosphate (−13.8 kJ/mol), well below ATP. The genuine non-phosphate high-energy compounds are the thiol esters involving coenzyme A (acetyl-CoA), acyl carrier protein, the amino acid esters of protein synthesis, S-adenosylmethionine, UDPGlc and PRPP — every one of which you will meet later in the course.Harper's ch.11, p.116

1 Bioenergetics — 2′+

The study of the energy changes accompanying biochemical reactions.

Biological systems are essentially isothermic — they operate at constant temperature — and therefore use chemical energy, not heat, to power living processes. Because there is no temperature gradient to exploit, energy must be moved around chemically, which is why a carrier molecule such as ATP is necessary at all.Harper's ch.11, p.113 · TMU Lecture 8 Slide 4

2 Free energy (ΔG) — 3′+

That portion of the total energy change in a system that is available for doing work — the useful energy, also known as the chemical potential.

It combines the two laws of thermodynamics: ΔG = ΔH − TΔS, where ΔH is the change in enthalpy, T the absolute temperature and ΔS the change in entropy.

If ΔG is negative the reaction is exergonic and proceeds spontaneously; if of great magnitude it is essentially irreversible. If positive it is endergonic and proceeds only if free energy is supplied. If zero, the system is at equilibrium.Harper's ch.11, p.114

3 High-energy phosphate — 3′+

A phosphate compound whose standard free energy of hydrolysis is greater than that of ATP.

The value for hydrolysis of the terminal phosphate of ATP (−30.5 kJ/mol) divides biochemical phosphates into two groups: low-energy phosphates, exemplified by the ester phosphates of the glycolytic intermediates, and high-energy phosphates, comprising anhydrides (1,3-bisphosphoglycerate), enolphosphates (phosphoenolpyruvate) and phosphoguanidines (creatine phosphate, arginine phosphate).

The symbol ~℗ indicates that transfer of the attached group to a suitable acceptor results in transfer of the larger quantity of free energy; the term group transfer potential is preferred by some to “high-energy bond”.Harper's ch.11, p.115

4 The ATP/ADP cycle — 3′ · TMU study question+

The continuous consumption and regeneration of ATP that connects those processes which generate high-energy phosphate to those which utilise it.

ATP donates high-energy phosphate to form the compounds below it in the table of free energies of hydrolysis; with the necessary enzymes, ADP accepts high-energy phosphate from the compounds above ATP to re-form it.

The cycle turns over very rapidly, because the total ATP/ADP pool is extremely small — sufficient to maintain an active tissue for only a few seconds.

The three sources of ~℗: oxidative phosphorylation (the greatest quantitative source), glycolysis (net 2, via phosphoglycerate kinase and pyruvate kinase) and the citric acid cycle (1, at succinate thiokinase).Harper's ch.11, p.116

5 Phosphagen — 2′+

A storage form of high-energy phosphate.

Phosphagens include creatine phosphate, occurring in vertebrate skeletal muscle, heart, spermatozoa and brain, and arginine phosphate, occurring in invertebrate muscle.

When ATP is rapidly being utilised — as in muscular contraction — phosphagens permit its concentration to be maintained; when the ATP/ADP ratio is high their concentration increases, acting as a store. Creatine phosphate lies above ATP in the free-energy table (−43.1 kJ/mol), which is precisely what allows it to rephosphorylate ADP.Harper's ch.11, p.117

6 Exergonic and endergonic reactions — 3′+

An exergonic reaction has a negative ΔG and proceeds spontaneously with loss of free energy; if ΔG is of great magnitude the reaction goes virtually to completion and is essentially irreversible.

An endergonic reaction has a positive ΔG and proceeds only if free energy can be gained. In practice an endergonic process cannot exist independently, but must be a component of a coupled exergonic-endergonic system in which the overall net change is exergonic.

The exergonic reactions constitute catabolism, the endergonic synthetic reactions anabolism; together they constitute metabolism.Harper's ch.11, p.114

1 What is the ATP/ADP cycle? Write the major sources and the usage of ATP. 8′ — TMU study question

Definition of the cycle

ATP is able to act as a donor of high-energy phosphate to form those compounds below it in the table of standard free energies of hydrolysis; likewise, with the necessary enzymes, ADP can accept high-energy phosphate from those compounds above ATP to form ATP. In effect, the ATP/ADP cycle connects those processes that generate ~℗ to those processes that utilise ~℗, continuously consuming and regenerating ATP.

This occurs at a very rapid rate, since the total ATP/ADP pool is extremely small — sufficient to maintain an active tissue for only a few seconds. ATP is therefore not a store of energy but a flow, made continuously at the rate at which it is spent.

Why ATP can occupy this role

The free energy of hydrolysis of the terminal phosphate of ATP (−30.5 kJ/mol) places it in an intermediate position in the table of biochemical phosphates. Compounds above it — phosphoenolpyruvate (−61.9), 1,3-bisphosphoglycerate (−49.3), creatine phosphate (−43.1) — can regenerate ATP; compounds below it — glucose-6-phosphate (−13.8), glycerol-3-phosphate (−9.2) — can be formed from it. Were ATP at the top of the table nothing could recharge it; at the bottom it would be useless as a donor.

The three major sources of ~℗

Source	Detail
1 · Oxidative phosphorylation	The greatest quantitative source in aerobic organisms. ATP is generated in the mitochondrial matrix as O₂ is reduced to H₂O by electrons passing down the respiratory chain
2 · Glycolysis	A net formation of two ~℗ per molecule of glucose converted to lactate, in the two reactions catalysed by phosphoglycerate kinase and pyruvate kinase
3 · The citric acid cycle	One ~℗ generated directly, at the succinate thiokinase step

Sources 2 and 3 are substrate-level phosphorylation; source 1 dominates quantitatively.

Additional stores and interconversions

Phosphagens — creatine phosphate in vertebrate skeletal muscle, heart, spermatozoa and brain; arginine phosphate in invertebrate muscle — act as storage forms, maintaining ATP concentration when it is rapidly utilised and accumulating when the ATP/ADP ratio is high.

Adenylate kinase (myokinase) catalyses ATP + AMP ⇌ 2 ADP, which allows high-energy phosphate in ADP to be used for ATP synthesis, allows AMP formed in activation reactions to be recovered, and allows AMP to rise when ATP is depleted and act as an allosteric signal increasing the rate of catabolic reactions.

The usage of ATP

Biosynthesis — the endergonic synthetic reactions of anabolism.
Muscular contraction.
Nervous excitation — nerve impulse conduction.
Active transport across membranes.
Activation reactions, in which ATP may be hydrolysed to AMP + PP_i; hydrolysis of the PP_i by inorganic pyrophosphatase (ΔG⁰′ −19.2 kJ/mol) helps drive these to completion.

When ATP acts as a phosphate donor to form compounds of lower free energy of hydrolysis, the phosphate group is invariably converted to one of low energy.

An example of coupling

The phosphorylation of glucose (ΔG⁰′ = +13.8 kJ/mol) is highly endergonic and cannot proceed physiologically. Coupled by hexokinase to ATP hydrolysis (−30.5 kJ/mol), the overall change becomes −16.7 kJ/mol — highly exergonic and, under physiological conditions, irreversible.

Marking guide: cycle defined as connecting generation to utilisation 1.5 · rapid turnover with the few-seconds pool 1 · ATP's intermediate position explained 1 · three sources named with detail 2.5 · at least four uses of ATP 1.5 · a worked coupling example or the role of phosphagens/adenylate kinase 0.5.

2 Outline briefly how endergonic processes are made to proceed in the cell. 5′ — 'outline briefly'

The problem

Vital processes — synthetic reactions, muscular contraction, nerve impulse conduction and active transport — have a positive ΔG and are therefore endergonic: they proceed only if free energy is supplied. An endergonic process cannot exist independently; it must be a component of a coupled exergonic-endergonic system in which the overall net change is exergonic.

Simple coupling through a shared intermediate

In its simplest form, the conversion of metabolite A to B occurs with release of free energy and is chemically linked to a second reaction in which free energy is required to convert C to D. The limitation is that the two reactions must be chemically related, since they share an intermediate.

Coupling through a high-energy intermediate

The superior alternative is to synthesise a compound of high-energy potential in the exergonic reaction and incorporate it into the endergonic reaction, thereby transferring free energy between the two pathways.

The biological advantage is decisive: the high-potential compound need not be structurally related to any of the reactants, and can therefore serve as a transducer of energy from a wide range of exergonic reactions to an equally wide range of endergonic ones. In the living cell that carrier is ATP — the “energy currency”.

Why ATP works

ATP occupies an intermediate position in the table of free energies of hydrolysis, so it can be regenerated by compounds above it and donate phosphate to compounds below it. Its large free energy of hydrolysis arises from relief of charge repulsion between adjacent negatively charged oxygens and stabilisation of the released phosphate as a resonance hybrid. In the cell it functions as the Mg²⁺ complex.

The worked example

Phosphorylation of glucose to glucose-6-phosphate has ΔG⁰′ = +13.8 kJ/mol and cannot proceed under physiological conditions. Coupled by hexokinase to the hydrolysis of the terminal phosphate of ATP (ΔG⁰′ = −30.5 kJ/mol), the overall reaction has ΔG⁰′ = −16.7 kJ/mol and proceeds readily, irreversibly under physiological conditions. Many activation reactions follow this pattern.

A further device

Where ATP is hydrolysed to AMP + PP_i, as in the activation of long-chain fatty acids, the reaction is driven further to the right by hydrolysis of the PP_i by inorganic pyrophosphatase, itself with a large ΔG⁰′ of −19.2 kJ/mol — the cell destroying one of its own products to make the reaction irreversible.

Marking guide: endergonic processes cannot proceed alone and must be coupled 1 · the shared-intermediate mechanism and its limitation 1 · the high-energy intermediate mechanism and its advantage 1.5 · ATP identified as the carrier 0.5 · a worked numerical example 1.