DHN 101 – HUMAN NUTRITION AND WELLNESS EXAM 4 – ADVANCED
CONCEPTS & CLINICAL APPLICATIONS 250 EXPERT-VERIFIED
QUESTIONS WITH DETAILED RATIONALES 100% CORRECT ANSWERS |
ALREADY GRADED A+
SECTION 1: ADVANCED MACRONUTRIENT METABOLISM & BIOCHEMICAL
PATHWAYS (Q1-25)
Q1. What is the primary rate-limiting enzyme in glycolysis?
A) Hexokinase
B) Phosphofructokinase-1 (PFK-1)
C) Pyruvate kinase
D) Aldolase
Correct Answer: B
Rationale: Phosphofructokinase-1 (PFK-1) is the primary rate-limiting enzyme in
glycolysis. It catalyzes the conversion of fructose-6-phosphate to
fructose-1,6-bisphosphate and is regulated by ATP, AMP, and citrate. When ATP
levels are high, PFK-1 is inhibited; when AMP levels are high (indicating low
energy), PFK-1 is activated. This enzyme is a critical control point for
glucose metabolism, ensuring that glycolysis proceeds only when the cell
requires energy. Hexokinase is also regulated but is not the primary
rate-limiting enzyme; pyruvate kinase is the third regulatory enzyme in
glycolysis.
Q2. What is the primary function of the pentose phosphate pathway?
A) To produce ATP
B) To produce NADPH and ribose-5-phosphate
C) To produce acetyl-CoA
D) To produce glucose
Correct Answer: B
Rationale: The pentose phosphate pathway (PPP) has two main functions: (1)
production of NADPH, which is essential for reductive biosynthesis (fatty acid
synthesis) and antioxidant defense (regeneration of glutathione), and (2)
production of ribose-5-phosphate, which is essential for nucleotide and nucleic
acid synthesis. The PPP is particularly active in tissues that require rapid
1
,cell division (bone marrow, skin) and tissues that require NADPH for
biosynthesis (liver, adipose tissue). It does not produce ATP directly; rather,
it provides reducing power for biosynthetic reactions and generates pentose
sugars for DNA and RNA synthesis.
Q3. What is the primary fuel source for the brain during prolonged fasting?
A) Glucose
B) Fatty acids
C) Ketone bodies
D) Amino acids
Correct Answer: C
Rationale: During prolonged fasting (after 2-3 days), the brain adapts to using
ketone bodies (acetoacetate and beta-hydroxybutyrate) as its primary fuel
source. This adaptation spares glucose for tissues that require it, such as red
blood cells. The brain cannot use fatty acids directly because they cannot
cross the blood-brain barrier. However, ketone bodies are water-soluble and can
cross the barrier, providing an alternative fuel source. This metabolic
adaptation allows humans to survive extended periods without food by
preserving
muscle protein and utilizing stored fat for energy.
Q4. What is the primary purpose of the Cori cycle?
A) To transport lactate from muscles to the liver for gluconeogenesis
B) To transport glucose from the liver to muscles
C) To synthesize glycogen
D) To break down glycogen
Correct Answer: A
Rationale: The Cori cycle is a metabolic pathway that transports lactate
produced by anaerobic glycolysis in skeletal muscles to the liver, where it is
converted back to glucose through gluconeogenesis. The newly synthesized
glucose can then be released into the bloodstream and transported back to the
muscles for energy. This cycle is particularly important during intense
exercise when muscles produce lactate faster than they can oxidize it. It
prevents the accumulation of lactate in muscles and provides a mechanism for
recycling lactate into glucose. The Cori cycle is energetically costly, as it
requires 6 ATP molecules to convert two lactates into one glucose molecule.
2
,Q5. What is the primary function of the glucose-alanine cycle?
A) To transport alanine from muscles to the liver for gluconeogenesis
B) To transport glucose from the liver to muscles
C) To synthesize glycogen
D) To break down glycogen
Correct Answer: A
Rationale: The glucose-alanine cycle is a mechanism for removing amino groups
from muscles and transporting them to the liver for disposal. When muscles
break down amino acids for energy during fasting or exercise, the amino group
is transferred to pyruvate to form alanine. Alanine is then transported in the
blood to the liver, where it is converted back to pyruvate, and the amino group
is converted to urea for excretion. The carbon skeleton of pyruvate is then
used for gluconeogenesis. This cycle serves two purposes: (1) it removes excess
nitrogen from muscles and (2) it provides a carbon source for hepatic
gluconeogenesis.
Q6. What is the primary function of carnitine in fatty acid metabolism?
A) To transport fatty acids into the mitochondria for beta-oxidation
B) To synthesize fatty acids
C) To break down ketone bodies
D) To synthesize cholesterol
Correct Answer: A
Rationale: Carnitine is a molecule that facilitates the transport of long-chain
fatty acids across the inner mitochondrial membrane into the mitochondrial
matrix, where beta-oxidation occurs. Carnitine combines with fatty acyl-CoA to
form acylcarnitine, which is transported into the mitochondria by the carnitine
palmitoyltransferase (CPT) system. Without carnitine, long-chain fatty acids
cannot enter the mitochondria and cannot be used for energy production. The
CPT
system is rate-limiting for fatty acid oxidation and is regulated by malonyl-CoA
(the first intermediate of fatty acid synthesis), which inhibits CPT-1 to
prevent simultaneous synthesis and oxidation of fatty acids.
Q7. What is the primary cause of ketosis?
A) High carbohydrate intake
3
, B) Low carbohydrate intake or prolonged fasting
C) High protein intake
D) Excessive fat intake
Correct Answer: B
Rationale: Ketosis occurs when the body's carbohydrate stores are depleted and
fatty acid oxidation is accelerated. This typically happens during low-carbohydrate
diets, fasting, or starvation. When oxaloacetate (a Krebs cycle intermediate)
is depleted due to lack of carbohydrate-derived pyruvate, acetyl-CoA cannot
enter the Krebs cycle and is instead diverted to ketone body synthesis in the
liver. The three ketone bodies produced are acetoacetate, beta-hydroxybutyrate,
and acetone. Ketone bodies provide an alternative fuel source for the brain and
other tissues. Ketosis can be physiological (during fasting or ketogenic diets)
or pathological (in uncontrolled type 1 diabetes, called diabetic ketoacidosis).
Q8. What is the primary function of the electron transport chain?
A) To produce NADH
B) To produce ATP through oxidative phosphorylation
C) To break down glucose
D) To synthesize fatty acids
Correct Answer: B
Rationale: The electron transport chain (ETC) is a series of protein complexes
located in the inner mitochondrial membrane that transfers electrons from NADH
and FADH2 to oxygen. As electrons flow through the ETC, protons are pumped
from the mitochondrial matrix to the intermembrane space, creating a proton
gradient. The flow of protons back into the matrix through ATP synthase drives
the synthesis of ATP, a process called oxidative phosphorylation. The ETC is
responsible for producing the vast majority of ATP (approximately 28-30 ATP
molecules) from the complete oxidation of one glucose molecule. Oxygen serves
as the final electron acceptor, combining with electrons and protons to form
water.
Q9. What is the primary function of the Krebs cycle (citric acid cycle)?
A) To produce ATP directly
B) To generate NADH and FADH2 for the electron transport chain
C) To break down fatty acids
D) To produce glucose
4
CONCEPTS & CLINICAL APPLICATIONS 250 EXPERT-VERIFIED
QUESTIONS WITH DETAILED RATIONALES 100% CORRECT ANSWERS |
ALREADY GRADED A+
SECTION 1: ADVANCED MACRONUTRIENT METABOLISM & BIOCHEMICAL
PATHWAYS (Q1-25)
Q1. What is the primary rate-limiting enzyme in glycolysis?
A) Hexokinase
B) Phosphofructokinase-1 (PFK-1)
C) Pyruvate kinase
D) Aldolase
Correct Answer: B
Rationale: Phosphofructokinase-1 (PFK-1) is the primary rate-limiting enzyme in
glycolysis. It catalyzes the conversion of fructose-6-phosphate to
fructose-1,6-bisphosphate and is regulated by ATP, AMP, and citrate. When ATP
levels are high, PFK-1 is inhibited; when AMP levels are high (indicating low
energy), PFK-1 is activated. This enzyme is a critical control point for
glucose metabolism, ensuring that glycolysis proceeds only when the cell
requires energy. Hexokinase is also regulated but is not the primary
rate-limiting enzyme; pyruvate kinase is the third regulatory enzyme in
glycolysis.
Q2. What is the primary function of the pentose phosphate pathway?
A) To produce ATP
B) To produce NADPH and ribose-5-phosphate
C) To produce acetyl-CoA
D) To produce glucose
Correct Answer: B
Rationale: The pentose phosphate pathway (PPP) has two main functions: (1)
production of NADPH, which is essential for reductive biosynthesis (fatty acid
synthesis) and antioxidant defense (regeneration of glutathione), and (2)
production of ribose-5-phosphate, which is essential for nucleotide and nucleic
acid synthesis. The PPP is particularly active in tissues that require rapid
1
,cell division (bone marrow, skin) and tissues that require NADPH for
biosynthesis (liver, adipose tissue). It does not produce ATP directly; rather,
it provides reducing power for biosynthetic reactions and generates pentose
sugars for DNA and RNA synthesis.
Q3. What is the primary fuel source for the brain during prolonged fasting?
A) Glucose
B) Fatty acids
C) Ketone bodies
D) Amino acids
Correct Answer: C
Rationale: During prolonged fasting (after 2-3 days), the brain adapts to using
ketone bodies (acetoacetate and beta-hydroxybutyrate) as its primary fuel
source. This adaptation spares glucose for tissues that require it, such as red
blood cells. The brain cannot use fatty acids directly because they cannot
cross the blood-brain barrier. However, ketone bodies are water-soluble and can
cross the barrier, providing an alternative fuel source. This metabolic
adaptation allows humans to survive extended periods without food by
preserving
muscle protein and utilizing stored fat for energy.
Q4. What is the primary purpose of the Cori cycle?
A) To transport lactate from muscles to the liver for gluconeogenesis
B) To transport glucose from the liver to muscles
C) To synthesize glycogen
D) To break down glycogen
Correct Answer: A
Rationale: The Cori cycle is a metabolic pathway that transports lactate
produced by anaerobic glycolysis in skeletal muscles to the liver, where it is
converted back to glucose through gluconeogenesis. The newly synthesized
glucose can then be released into the bloodstream and transported back to the
muscles for energy. This cycle is particularly important during intense
exercise when muscles produce lactate faster than they can oxidize it. It
prevents the accumulation of lactate in muscles and provides a mechanism for
recycling lactate into glucose. The Cori cycle is energetically costly, as it
requires 6 ATP molecules to convert two lactates into one glucose molecule.
2
,Q5. What is the primary function of the glucose-alanine cycle?
A) To transport alanine from muscles to the liver for gluconeogenesis
B) To transport glucose from the liver to muscles
C) To synthesize glycogen
D) To break down glycogen
Correct Answer: A
Rationale: The glucose-alanine cycle is a mechanism for removing amino groups
from muscles and transporting them to the liver for disposal. When muscles
break down amino acids for energy during fasting or exercise, the amino group
is transferred to pyruvate to form alanine. Alanine is then transported in the
blood to the liver, where it is converted back to pyruvate, and the amino group
is converted to urea for excretion. The carbon skeleton of pyruvate is then
used for gluconeogenesis. This cycle serves two purposes: (1) it removes excess
nitrogen from muscles and (2) it provides a carbon source for hepatic
gluconeogenesis.
Q6. What is the primary function of carnitine in fatty acid metabolism?
A) To transport fatty acids into the mitochondria for beta-oxidation
B) To synthesize fatty acids
C) To break down ketone bodies
D) To synthesize cholesterol
Correct Answer: A
Rationale: Carnitine is a molecule that facilitates the transport of long-chain
fatty acids across the inner mitochondrial membrane into the mitochondrial
matrix, where beta-oxidation occurs. Carnitine combines with fatty acyl-CoA to
form acylcarnitine, which is transported into the mitochondria by the carnitine
palmitoyltransferase (CPT) system. Without carnitine, long-chain fatty acids
cannot enter the mitochondria and cannot be used for energy production. The
CPT
system is rate-limiting for fatty acid oxidation and is regulated by malonyl-CoA
(the first intermediate of fatty acid synthesis), which inhibits CPT-1 to
prevent simultaneous synthesis and oxidation of fatty acids.
Q7. What is the primary cause of ketosis?
A) High carbohydrate intake
3
, B) Low carbohydrate intake or prolonged fasting
C) High protein intake
D) Excessive fat intake
Correct Answer: B
Rationale: Ketosis occurs when the body's carbohydrate stores are depleted and
fatty acid oxidation is accelerated. This typically happens during low-carbohydrate
diets, fasting, or starvation. When oxaloacetate (a Krebs cycle intermediate)
is depleted due to lack of carbohydrate-derived pyruvate, acetyl-CoA cannot
enter the Krebs cycle and is instead diverted to ketone body synthesis in the
liver. The three ketone bodies produced are acetoacetate, beta-hydroxybutyrate,
and acetone. Ketone bodies provide an alternative fuel source for the brain and
other tissues. Ketosis can be physiological (during fasting or ketogenic diets)
or pathological (in uncontrolled type 1 diabetes, called diabetic ketoacidosis).
Q8. What is the primary function of the electron transport chain?
A) To produce NADH
B) To produce ATP through oxidative phosphorylation
C) To break down glucose
D) To synthesize fatty acids
Correct Answer: B
Rationale: The electron transport chain (ETC) is a series of protein complexes
located in the inner mitochondrial membrane that transfers electrons from NADH
and FADH2 to oxygen. As electrons flow through the ETC, protons are pumped
from the mitochondrial matrix to the intermembrane space, creating a proton
gradient. The flow of protons back into the matrix through ATP synthase drives
the synthesis of ATP, a process called oxidative phosphorylation. The ETC is
responsible for producing the vast majority of ATP (approximately 28-30 ATP
molecules) from the complete oxidation of one glucose molecule. Oxygen serves
as the final electron acceptor, combining with electrons and protons to form
water.
Q9. What is the primary function of the Krebs cycle (citric acid cycle)?
A) To produce ATP directly
B) To generate NADH and FADH2 for the electron transport chain
C) To break down fatty acids
D) To produce glucose
4
