Concept 9.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules

Question 1

Glycolysis releases less than a quarter of the chemical energy in glucose that can be harvested by cells; most of the energy remains stockpiled in the

Answer 1

two molecules of pyruvate.

Question 2

When O2 is present, the pyruvate in eukaryotic cells enters a mitochondrion, where the

Answer 2

oxidation of glucose is completed.

Question 3

In aerobically respiring prokaryotic cells, this process occurs in the

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cytosol.

Question 4

Upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called

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acetyl coenzyme A, or acetyl CoA

Question 5

This step, linking glycolysis and the citric acid cycle, is carried out by a multienzyme complex that catalyzes three reactions:

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1 Pyruvate’s carboxyl group , the remaining two-carbon fragment is oxidized and the electrons transferred to , storing energy in the form of NADH, 3 Finally, coenzyme A (CoA), a sulfur-containing compound derived from a B vitamin, is attached via its sulfur atom to the two-carbon intermediate, forming acetyl CoA. Acetyl CoA has a high potential energy,

Question 6

as the molecule enters the mitochondrion one

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carbon is removed

Question 7

forming carbon-dioxide as a

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by-product

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electrons are stripped forming

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NADH

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coenzyme-A attaches to the two carbon fragment forming

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acetyl-CoA

Question 10

The citric acid cycle functions as a metabolic furnace that further oxidizes organic fuel derived from

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pyruvate

Question 11

Figure 9.11 summarizes the inputs and outputs as pyruvate is broken down to three CO2 molecules, including the molecule of CO2 released during the conversion of pyruvate to

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acetyl CoA

Question 12

The cycle generates 1 ATP per turn by substrate-level phosphorylation, but most of the chemical energy is transferred to

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NAD+ and FAD during the redox reactions.

Question 13

The reduced coenzymes, NADH and FADH2, shuttle their cargo of high-energy electrons into the

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electron transport chain

Question 14

The citric acid cycle is also called the _______________________________, the latter honoring Hans Krebs, the German-British scientist who was largely responsible for working out the pathway in the 1930s.

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tricarboxylic acid cycle or the Krebs cycle

Question 15

the oxidation of glucose continues in the

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citric acid cycle

Question 16

pyruvate molecules formed during glycolysis are transported from the

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cytosol into the mitochondrion

Question 17

but pyruvate itself does not enter the

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citric acid cycle

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a reaction occurs that removes a carbon atom, releasing it in

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carbon dioxide

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electrons are transferred to an

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NADH molecule, storing energy

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Coenzyme A, or CoA joins with the

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2 carbon fragments forming acetyl CoA

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one molecule of acetyl CoA enters the

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citric acid cycle

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the 2 carbon fragments of acetyl CoA attaches to the

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four carbon molecule oxaloacetate in the first reaction of the cycle

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this forms

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citrate

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in a series of steps, bonds

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break and reform

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two carbon atoms are released, one at a time, in molecules of

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carbon dioxide

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electrons are carried off by molecules of

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NADH and FADH2

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one step produces an ATP molecule by

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substrate-level phosphorylation

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a four carbon oxaloacetate molecule is

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regenerated

Question 29

since two acetyl CoA molecules are produced for each glucose molecule broken down, a second acetyl CoA enters the

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citric acid cycle

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the same series of reactions occur, releasing

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carbon dioxide and producing more NADH, FADH, and ATP

Question 31

the cell has gained

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two ATPs that can be used directly

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however most of the energy originally contained in the bonds of glucose is now carried by the

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NADH and FADH2 molecules

Question 33

citric acid cycle has eight steps, each catalyzed by a specific

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enzyme

Question 34

for each turn of the citric acid cycle, two carbons (red) enter in the relatively reduced form of an

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acetyl group (step 1)

Question 35

and two different carbons (blue) leave in the completely oxidized form of

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molecules (steps 3 and 4).

Question 36

The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate, forming

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citrate (step 1).

Question 37

Citrate is the ________________ of citric acid, for which the cycle is named.

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 ionized form

Question 38

The next seven steps decompose the citrate back to

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oxaloacetate.

Question 39

For each acetyl group entering the cycle, 3 are reduced to.

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 NADH (steps 3, 4, and 8)

Question 40

n step 6, electrons are transferred not to , but to FAD, which accepts 2 electrons and 2 protons to become

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FADH2

Question 41

In many animal tissue cells, the reaction in step 5 produces a guanosine triphosphate (GTP) molecule by

Answer 41

substrate-level phosphorylation

Question 42

This GTP may be used to make an ATP molecule (as shown) or directly power work in the

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cell.

Question 43

In the cells of plants, bacteria, and some animal tissues, step 5 forms an ATP molecule directly by

Answer 43

substrate-level phosphorylation