The Citric Acid Cycle: Harvesting Electrons from the Cycle Part 1

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The citric acid cycle consists of ___ stages.

oxidizes, energy rich

Stage one ___ two carbon atoms to gather ___ electrons.

oxaloacetate, harvests

Stage two regenerates ___ and ___ energy-rich electrons.


The citric acid cycle is ___.


The glyoxylate cycle enables plants and bacteria to convert fats into ___.

carbon, oxidized

Fuel molecules are ___ compounds that are capable of being ___; of losing electrons.

oxidation-reduction, oxidation, CO2

The citric acid cycle includes a series of ___ reactions that ultimately result in the ___ of the acetyl group to two molecules of ___.

high-transfer-potential, high-energy, ATP

Citric acid cycle oxidations generate ___ or ___ electrons that will be used to power the synthesis of ___.

harvesting, carbon

The function of the citric acid cycle is the ___ of high-energy electrons from ___ fuels.

oxidizes, oxaloacetate

The cycle consists of two parts: one ___ carbon atoms to CO2 and the other regenerates ___.

oxidizes, CO2, ATP, NADH, FADH2

The citric acid cycle ___ two-carbon units, producing two molecules of ___, one molecule of ___, and high-energy electrons in the form of ___ and ___.

acetyl, four, oxaloacetate, tricarboxylic

In the citric acid cycle, a two-carbon ___ unit condenses with a ___-carbon component of the citric acid cycle, ___, to yield a six-carbon ___ acid.

four, oxaloacetate

The six-carbon compound releases CO2 twice, which yields high-energy electrons. A ___-carbon compound remains. This compound is further oxidized to regenerate ___, which can initiate another round fo the cycle.

acetyl, CO2

Two carbon atoms enter the cycle as an ___ unit and two carbon atom leave the cycle in the form of two molecules of ___.

ATP, oxygen

Note that the citric acid cycle neither generates a large amount of ___ nor includes ___ as a reactant.

removes, citrate, NADH, FADH2, nine, oxidative phosphorylation

The citric acid cycle ___ electrons from ___ and uses these electrons to form ___ and ___. These electron carriers yield ___ molecules of ATP when they are oxidized by O2 in ___.

NADH, FADH2, electron-transport, proton, ADP, inorganic phosphate

Electrons released in the reoxidation of ___ and ___ flow through a series of membrane proteins, known as the ___ chain, to generate a ___ gradient across the membrane. This gradient is used to generate ATP from ___ and ___.

oxaloacetate, citrate, CO2, citrate, four

In the first stage of the citric acid cycle, two carbon atoms are introduced to the cycle by coupling with ___ to form ___, and two carbon atoms are released as ___ as ___ is metabolized to a ___-carbon molecule.

four, oxaloacetate

In the second stage of the citric acid cycle, the resulting ___-carbon molecule is metabolized to regenerate ___, allowing the continued functioning of the cycle.

cellular, removal, O2, ATP

The citric acid cycle constitutes the first stage in ___ respiration, the ___ of high-energy electrons from carbon fuels. These electrons reduce ___ to generate a proton gradient, which is used to synthesize ___, which constitues oxidative phosphorylation.

oxaloacetate, acetyl, tricarboxylic

In the first part of the citric acid cycle, the four-carbon molecule ___ condenses with a two-carbon ___ unit to yield citrate, a six-carbon ___ acid.

CO2, oxidative decarboxylation, four, two

As citrate moves through the first part of the cycle, two carbon atoms are lost as ___ in a process called ___, which yields a ___-carbon molecule and high-transfer-potential electrons captured as ___ molecules of NADH.

synthase, acetyl CoA

Citrate ___ forms citrate from oxaloacetate and ___.

condensation, H2O, citrate

The citric acid cycle begins with the ___ of a four-carbon unit, oxaloacetate, and a two-carbon unit, the acetyl group of acetyl CoA. Oxaloacetate reacts with acetyl CoA and ___ to yield ___ and CoA.


___ + acetyl CoA ----> citryl CoA --H2O--CoA--> citrate

acetyl CoA

oxaloacetate + ___ ----> citryl CoA --H2O--CoA--> citrate

citryl CoA

oxaloacetate + acetyl CoA ----> ___ --H2O--CoA--> citrate


oxaloacetate + acetyl CoA ----> citryl CoA --___--CoA--> citrate


oxaloacetate + acetyl CoA ----> citryl CoA --H2O--___--> citrate


oxaloacetate + acetyl CoA ----> citryl CoA --H2O--CoA--> ___

citryl CoA, thioester

Oxaloacetate first condenses with acetyl CoA to from ___, a molecule that is energy rich because it contains a ___ bond that originated in acetyl CoA.

hydrolysis, thioester, two

The ___ of the citryl CoA ___ linkage to citrate and CoA drives the overall reaction far in the direction of the synthesis of citrate, powering the synthesis of a new molecule from ___ precursors.


The mechanism of citrate synthase prevents ___ reactions.

dimer, active site, adjacent, conformational

Mammalian citrate synthase is a ___ of identical 49-kd subunits. Each ___ is located in a cleft between the large and small domains of a subunit, ___ to the subunit interface. X-ray crystallographic studies revealed that the enzyme undergoes large ___ changes in the course of catalysis.

oxaloacetate, acetyl CoA, oxaloacetate, open, closed, acetyl CoA

The reason for the ordered binding is that ___ induces a major structural rearrangement leading to the creation of a binding site for ___. The binding of ___ converts the ___ form of the enzyme to a ___ form. These structural changes create a binding site for ___.

condensation, citryl CoA, enclosed, citryl CoA, hydrolysis, open

Citrate synthase first catalyzes the ___ of citrate and acetyl CoA to form ___, which induces additional structural changes in the enzyme, causing the active site to become completely ___. The enzyme then cleaves the ___ thioester by ___. CoA leaves the enzyme, followed by citrate, and the enzyme returns to the initial ___ conformation.

hydrolysis, citrate synthase, oxaloacetate, thioester, Induced

The wasteful ___ of acetyl CoA is prevented because ___ is well suited to the hydrolysis of citryl CoA but not acetyl CoA. First, acetyl CoA does not bind to the enzyme until ___ is bound and ready for condensation. Second, the catalytic residues crucial for the hydrolysis of the ___ linkage are not appropriately positioned until citryl CoA is formed. ___ fit prevents an undesirable side reaction.


Citrate is isomerized into ___.

oxidation, high-transfer-potential

A major purpose of the citric acid cycle is the ___ of carbon atoms leading to the capture of ___ electrons.

citrate, hydroxyl, decarboxylations, citrate, isocitrate, decarboxylation

The newly formed ___ molecule is not optimally structured for the required oxidation reactions. In particular, the ___ group is not properly located in the molecule for the oxidative ___ that follow. Thus, ___ is isomerized into ___ to enable the six-carbon unit to undergo oxidative ___.

dehydration, hydration, H, OH, aconitase, cis-aconitate

The isomerization of citrate is accomplished by a ___ step followed by a ___ step. The result is an interchange of an ___ and an ___. The enzyme catalyzing both steps is called ___ because ___ is an intermediate.


Isocitrate is oxidized and decarboxylated to ___.

isocitrate dehydrogenase, oxalosuccinate, α-ketoacid, CO2, NADH

The oxidative decarboxylation of isocitrate to α-ketoglutarate is catalyzed by ___. The intermediate in this reaction is ___, an unstable ___. While bound to the enzyme, it loses ___ to form α-ketoglutarate. This oxidation generates the first high-transfer-potential electron carrier in the cycle, ___.


___ + NAD^+ ----> α-ketoglutarate + CO2 + NADH


isocitrate + ___ ----> α-ketoglutarate + CO2 + NADH


isocitrate + NAD^+ ----> ___ + CO2 + NADH


isocitrate + NAD^+ ----> α-ketoglutarate + ___ + NADH


isocitrate + NAD^+ ----> α-ketoglutarate + CO2 + ___

Succinyl CoA

___ is formed by the oxidative decarboxylation of α-ketoglutarate.

CO2, five, four, α-ketoglutarate dehydrogenase complex, three, α-ketoacid

Removing ___ from the α-ketoglutarate, ___ carbon atoms, to form succinyl CoA, ___ carbon atoms. This reaction is catalyzed by the ___, an organized assembly of ___ kinds of enzymes that is structurally similar to the PDH complex. In fact, the oxidative decarboxylation of α-ketoglutarate closely resembles that of pyruvate, also an ___.


___ + NAD^+ + CoA ----> succinyl CoA + CO2 + NADH


α-ketoglutarate + ___ + CoA ----> succinyl CoA + CO2 + NADH


α-ketoglutarate + NAD^+ + ___ ----> succinyl CoA + CO2 + NADH

succinyl CoA

α-ketoglutarate + NAD^+ + CoA ----> ___ + CO2 + NADH


α-ketoglutarate + NAD^+ + CoA ----> succinyl CoA + ___ + NADH


α-ketoglutarate + NAD^+ + CoA ----> succinyl CoA + CO2 + ___

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