5
Digestion and Metabolism of Carbohydrates
Multiple choice questions
Please choose one correct answer
1. A 25 year old man visits his GP complaining of abdominal cramps and diarrhoea after drinking
milk. What is the most likely cause of his problem?
a)
b)
c)
d)
e)
Bacterial and yeast overgrowth in the large intestine
Infection with the intestinal parasite Giardia lamblia
Lack of pancreatic amylase
Lack of small intestinal lactase
Lack of small intestinal sucrase-isomaltase
2. Which one of following statements about glycolysis and gluconeogenesis is correct?
a) All the reactions of glycolysis are freely reversible for gluconeogenesis.
b) Fructose cannot be used for gluconeogenesis in the liver because it cannot be phosphorylated to
fructose 6-phosphate.
c) Glycolysis can proceed in the absence of oxygen only if pyruvate is formed from lactate in
muscle.
d) Red blood cells only metabolize glucose by anaerobic glycolysis (and the pentose phosphate
pathway).
e) The reverse of glycolysis is the pathway for gluconeogenesis in skeletal muscle.
3. Which one of the following statements about glucose metabolism in maximum exertion is correct?
a)
b)
c)
d)
Gluconeogenesis from lactate requires less ATP than is formed during anaerobic glycolysis.
In maximum exertion pyruvate is oxidized to lactate in muscle.
Oxygen debt is caused by the need to exhale carbon dioxide produced in response to acidosis.
Oxygen debt reflects the need to replace oxygen that has been used in muscle during vigorous
exercise.
e) There is metabolic acidosis as a result of vigorous exercise.
CH3
C
CH3
O
C
COOH
pyruvate
enzyme-bound
thiamin diphosphate
CO2
O
SCoA
acetyl CoA
CoASH
CH3
CH3
CHOH
C
O
S
thiamin diphosphate
SH
hydroxethyl thiamin diphosphate
S
S
SH
SH
acetyl lipoamide
lipoamide
NADH
NAD+
reduced
lipoamide
4. The above diagram shows the reaction of the pyruvate dehydrogenase multienzyme complex.
Which one of following statements is correct?
a) In thiamin (vitamin B1) deficiency pyruvate formed in muscle cannot be transaminated to
alanine.
b) In thiamin (vitamin B1) deficiency pyruvate formed in muscle cannot be carboxylated to
oxaloacetate.
c) The reaction of pyruvate dehydrogenase involves decarboxylation and oxidation of pyruvate,
then formation of acetyl CoA.
d) The reaction of pyruvate dehydrogenase is readily reversible, so that acetyl CoA can be used for
the synthesis of pyruvate, and hence glucose.
e) The reaction of pyruvate dehydrogenase leads to the oxidation of NADH to NAD+, and hence
the formation of ~2.5 x ATP per mol of pyruvate oxidized.
5. Which one of following statements about favism (lack of glucose 6-phosphate dehydrogenase) and
the pentose phosphate pathway is correct?
a) In favism red blood cells are more susceptible to oxidative stress because of a lack of NADPH
for fatty acid synthesis.
b) People who lack glucose 6-phosphate dehydrogenase cannot synthesize fatty acids because of a
lack of NADPH in liver and adipose tissue.
c) The pentose phosphate pathway is especially important in tissues that are synthesizing fatty
acids.
d) The pentose phosphate pathway is the only source of NADPH for fatty acid synthesis.
e) The pentose phosphate pathway provides an alternative to glycolysis only in the fasting state.
6. Which one of following statements about glycogen synthesis and utilisation is correct?
a) Glycogen is synthesized in the liver in the fed state, and then exported to other tissues in low
density lipoproteins.
b) Glycogen reserves in liver and muscle will meet energy requirements for several days in
prolonged fasting.
c) Liver synthesizes more glycogen when the hepatic portal blood concentration of glucose is high
because of the activity of glucokinase in the liver.
d) Muscle synthesizes glycogen in the fed state because glycogen phosphorylase is activated in
response to insulin.
e) The plasma concentration of glycogen increases in the fed state.
7. Which one of following statements about glycogen metabolism is correct?
a) Glycogen synthase activity is increased by glucagon.
b) Glycogen phosphorylase is an enzyme that can be activated by phosphorylation of its own serine
residues.
c) Glycogen phosphorylase cannot be activated by calcium ions.
d) cAMP activates glycogen synthesis.
e) Glycogen phosphorylase breaks the α1-4 glycosidic bonds by hydrolysis.
8. In glycolysis, the conversion of 1 mol of fructose 1,6-bisphosphate to 2 mol of pyruvate results in
the formation of:
a)
b)
c)
d)
e)
1 mol NAD+ and 2 mol of ATP
1 mol NADH and 1 mol of ATP
2 mol NAD+ and 4 mol of ATP
2 mol NADH and 2 mol of ATP
2 mol NADH and 4 mol of ATP
The Nutrition Society Textbook Series, Introduction to Human Nutrition, Second Edition
© 2009, 2002 by The Nutrition Society.
Answers
1. D.
2. D.
3. E.
4. C.
5. C.
6. C.
7. B.
8. E.