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MCAT Pentose Phosphate Pathway and Glycogen Metabolism Practice Questions

By StudyStore · Published 2026-11-04 · 8 original questions · about 12 min

Eight free MCAT-style practice questions on the pentose phosphate pathway, NADPH, glycogen synthesis and breakdown, and their hormonal control, with explanations.

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Quick concept primer

The pentose phosphate pathway (PPP) runs in the cytoplasm and makes NADPH and ribose 5-phosphate without producing ATP. Its oxidative phase starts with glucose 6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme, which is activated by NADP+ and inhibited by NADPH. NADPH supplies reductive biosynthesis (fatty acids, steroids), keeps glutathione reduced in red blood cells, and powers the oxidative burst in phagocytes. Ribose 5-phosphate is the sugar for nucleotides.

Glycogen is stored mainly in liver and muscle. Glycogen synthase adds glucose from UDP-glucose; glycogen phosphorylase removes glucose as glucose 1-phosphate, and a debranching enzyme handles branches. Glucagon (liver) and epinephrine (liver and muscle) promote breakdown; insulin promotes synthesis. Only the liver has glucose 6-phosphatase, so only the liver releases free glucose into the blood.

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

Which enzyme catalyzes the rate-limiting step of the pentose phosphate pathway?

  1. Transketolase
  2. Glucose 6-phosphate dehydrogenase
  3. Phosphofructokinase-1
  4. Glycogen phosphorylase
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Answer: B. Glucose 6-phosphate dehydrogenase

G6PD converts glucose 6-phosphate to 6-phosphogluconolactone and produces NADPH, and it is the committed, regulated step of the oxidative phase. Phosphofructokinase-1 is the key control point of glycolysis, and glycogen phosphorylase controls glycogen breakdown.

Question 2

What is the main metabolic product of the oxidative phase of the pentose phosphate pathway that is used for reductive biosynthesis?

  1. NADH
  2. FADH2
  3. ATP
  4. NADPH
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Answer: D. NADPH

The oxidative phase generates two NADPH per glucose 6-phosphate, which reduces substrates in fatty acid and steroid synthesis and regenerates glutathione. It does not make ATP. NADH is used mainly for oxidative phosphorylation, and the cell keeps NADPH and NADH pools distinct.

Question 3

A cell has a high NADPH to NADP+ ratio. What is the expected effect on the pentose phosphate pathway?

  1. The oxidative phase is inhibited because NADPH inhibits G6PD
  2. The oxidative phase is stimulated because NADPH activates G6PD
  3. The pathway is unaffected because it does not use NADP+
  4. Glycolysis is blocked at pyruvate kinase
Show answer and explanation

Answer: A. The oxidative phase is inhibited because NADPH inhibits G6PD

NADP+ is the substrate and activator of G6PD, and NADPH is a competitive product inhibitor. When NADPH is abundant, there is little NADP+ to accept electrons, so flux through the oxidative phase falls. Demand for NADPH therefore sets the rate of the pathway.

Question 4

Red blood cells depend heavily on the pentose phosphate pathway. What is the most direct reason?

  1. They need ribose 5-phosphate for rapid DNA synthesis
  2. They need NADPH to keep glutathione reduced and protect against oxidative damage
  3. They need the pathway to produce ATP because they lack mitochondria
  4. They use it to convert fatty acids into glucose
Show answer and explanation

Answer: B. They need NADPH to keep glutathione reduced and protect against oxidative damage

Red cells lack mitochondria and are exposed to oxygen, so they rely on NADPH from the pentose phosphate pathway to regenerate reduced glutathione. In G6PD deficiency, oxidative stress can cause hemolysis. The pathway does not make ATP, and mature red cells do not replicate DNA.

Question 5

Which enzyme adds glucose units to a growing glycogen chain?

  1. Glycogen phosphorylase
  2. Glucose 6-phosphatase
  3. Glycogen synthase
  4. Hexokinase
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Answer: C. Glycogen synthase

Glycogen synthase transfers glucose from UDP-glucose onto the nonreducing end of the chain through alpha-1,4 linkages. Glycogen phosphorylase does the opposite, releasing glucose 1-phosphate. A branching enzyme then creates alpha-1,6 branch points.

Question 6

During fasting, which statement correctly describes the effect of glucagon on liver glycogen metabolism?

  1. Glycogen phosphorylase is activated and glycogen synthase is inhibited
  2. Both glycogen phosphorylase and glycogen synthase are activated
  3. Glycogen synthase is activated and glycogen phosphorylase is inhibited
  4. Both enzymes are inhibited
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Answer: A. Glycogen phosphorylase is activated and glycogen synthase is inhibited

Glucagon raises cAMP and activates protein kinase A, which phosphorylates and activates glycogen phosphorylase (through phosphorylase kinase) and phosphorylates glycogen synthase to inactivate it. This reciprocal regulation prevents a futile cycle and releases glucose for the blood.

Question 7

Why can liver glycogen raise blood glucose but muscle glycogen cannot?

  1. Muscle lacks glycogen phosphorylase
  2. Muscle lacks glucose 6-phosphatase
  3. Liver has no glycogen branching enzyme
  4. Muscle glycogen uses a different type of glycosidic bond
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Answer: B. Muscle lacks glucose 6-phosphatase

Glucose 6-phosphate must be dephosphorylated to cross the plasma membrane through GLUT transporters. Only the liver (and kidney) expresses glucose 6-phosphatase, so muscle keeps the glucose 6-phosphate for its own glycolysis. Muscle does have glycogen phosphorylase.

Question 8

Epinephrine released during exercise acts on muscle. What is its expected effect on glycogen metabolism?

  1. Stimulates glycogen synthesis to build reserves
  2. Stimulates glycogen breakdown to supply glucose 6-phosphate for glycolysis
  3. Stimulates the release of free glucose into the blood from muscle
  4. Has no effect because muscle lacks adrenergic receptors
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Answer: B. Stimulates glycogen breakdown to supply glucose 6-phosphate for glycolysis

Epinephrine binds adrenergic receptors in muscle, raises cAMP and activates glycogen phosphorylase. The glucose 6-phosphate produced enters glycolysis to supply ATP for contraction. Because muscle lacks glucose 6-phosphatase, it does not export free glucose.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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