Carbohydrates: physiology and metabolism

10 ASCP MLS practice questions on Carbohydrates: physiology and metabolism, part of Chemistry. The first 10 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   Liver glycogen can supply blood glucose during fasting, but muscle glycogen cannot. Why?

   1. AMuscle lacks insulin receptors
   2. BMuscle lacks glycogen phosphorylase
   3. CMuscle lacks hexokinase
   4. DMuscle lacks glucose-6-phosphatase

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   Answer: D · Muscle lacks glucose-6-phosphatase

   Glycogen breakdown releases glucose-1-phosphate, which is converted to glucose-6-phosphate. Only cells with glucose-6-phosphatase, mainly liver and kidney, can remove the phosphate and release free glucose into the blood. Muscle keeps its glucose-6-phosphate and uses it in glycolysis for its own energy. Phosphorylated glucose cannot cross the cell membrane, so it stays trapped in the muscle cell.

   • A: Muscle has many insulin receptors and takes up glucose in response to insulin.
   • B: Muscle has glycogen phosphorylase; McArdle disease is its deficiency.
   • C: Muscle has hexokinase, which traps glucose as glucose-6-phosphate.

2. Question 2 · Easy

   Inside the beta cell, proinsulin is cut into insulin and which other peptide?

   1. AC-peptide
   2. BSomatostatin
   3. CAmylin
   4. DGlucagon

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   Answer: A · C-peptide

   Proinsulin is a single chain that is folded and cut into insulin (A and B chains joined by disulfide bonds) and the connecting peptide, C-peptide. The two are released in equal molar amounts. C-peptide therefore reflects the patient's own insulin output, even when insulin is injected. Pharmaceutical insulin contains no C-peptide.

   • B: Somatostatin is made by delta cells.
   • C: Amylin is released by beta cells but comes from a separate gene.
   • D: Glucagon is made by alpha cells from proglucagon.

3. Question 3 · Medium

   After about two days without food, liver glycogen is nearly used up. Blood glucose is then kept up mainly by:

   1. AConversion of fatty acids to glucose
   2. BGlycogen breakdown in muscle
   3. CGluconeogenesis in liver and kidney
   4. DRelease of glucose from red cells

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   Answer: C · Gluconeogenesis in liver and kidney

   Once glycogen runs low, gluconeogenesis makes new glucose from lactate, glycerol and amino acids, driven by glucagon and cortisol while insulin is low. Fatty acids supply energy and ketones, but even-chain fatty acids cannot be made into glucose, because acetyl-CoA cannot be turned back into pyruvate. The kidney provides a growing share of gluconeogenesis as fasting continues. Ketones then spare glucose by fueling the brain in part.

   • A: Acetyl-CoA from fatty acids cannot be made into glucose.
   • B: Muscle glycogen cannot release free glucose into the blood.
   • D: Red cells consume glucose; they do not release it.

4. Question 4 · Easy

   When oxygen is short, as in hard-working muscle, glycolysis ends with pyruvate being converted to:

   1. AAcetyl-CoA
   2. BAcetoacetate
   3. CGlycogen
   4. DLactate

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   Answer: D · Lactate

   Without enough oxygen, lactate dehydrogenase reduces pyruvate to lactate. This regenerates NAD+, so glycolysis can keep making ATP. The lactate travels to the liver, where it is made back into glucose (the Cori cycle). Tissue hypoxia, as in shock, therefore raises blood lactate.

   • A: Acetyl-CoA forms from pyruvate in mitochondria when oxygen is available.
   • B: Acetoacetate is a ketone body made from fatty acids in the liver.
   • C: Glycogen is a storage form of glucose, not a product of pyruvate.

5. Question 5 · Easy

   Insulin lowers blood glucose mainly by:

   1. AIncreasing glucose loss in the urine
   2. BStimulating liver glycogen breakdown
   3. CStimulating gluconeogenesis in the liver
   4. DMoving glucose into muscle and fat cells

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   Answer: D · Moving glucose into muscle and fat cells

   Insulin binds its receptor on muscle and fat cells and moves GLUT4 transporters to the cell surface, so glucose enters the cells. It also promotes glycogen storage and suppresses glucose output by the liver. Insulin is the main hormone that lowers glucose, while glucagon, cortisol, epinephrine and growth hormone raise it. Lack of insulin, or resistance to it, leads to hyperglycemia.

   • A: Insulin does not work by making the kidney lose glucose.
   • B: Insulin inhibits glycogen breakdown and promotes glycogen synthesis.
   • C: Insulin suppresses gluconeogenesis; glucagon and cortisol stimulate it.

6. Question 6 · Easy

   After a carbohydrate meal, the rise in blood glucose triggers insulin release from which pancreatic islet cells?

   1. AAlpha cells
   2. BAcinar cells
   3. CDelta cells
   4. DBeta cells

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7. Question 7 · Hard

   A 5-month-old has a large liver, a fasting glucose of 35 mg/dL (1.9 mmol/L), and raised lactate, uric acid and triglycerides. Glucagon given after a fast raises lactate but not glucose. Which enzyme is most likely deficient?

   1. AMuscle glycogen phosphorylase
   2. BGlucose-6-phosphatase
   3. CGalactose-1-phosphate uridyltransferase
   4. DLysosomal acid alpha-glucosidase

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8. Question 8 · Easy

   Cortisol raises blood glucose mainly by promoting:

   1. AInsulin release from beta cells
   2. BGlucose uptake by skeletal muscle
   3. CGluconeogenesis from amino acids
   4. DGlycogen breakdown in muscle

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9. Question 9 · Easy

   During an overnight fast, glucagon keeps blood glucose up mainly by stimulating:

   1. AFat storage in adipose tissue
   2. BLiver glycogen synthesis
   3. CMuscle glucose uptake
   4. DLiver glycogen breakdown

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10. Question 10 · Medium

    Why do ketone bodies build up in untreated type 1 diabetes?

    1. ALow insulin allows unchecked fat breakdown
    2. BMuscle converts excess glucose to ketones
    3. CHigh insulin drives fatty acid synthesis
    4. DThe kidneys stop excreting ketone bodies

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