Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 781: Which glycosaminoglycan is present in the cornea?

A. Dermatan sulfate
B. Chondroitin sulfate
C. Hyaluronic acid
D. Keratan sulfate (Correct Answer)

Explanation: ***Keratan sulfate*** - **Keratan sulfate** is a prominent **glycosaminoglycan (GAG)** found in the corneal stroma, playing a crucial role in maintaining corneal transparency. - Its unique structure and interaction with collagen fibrils contribute to the precise spacing and organization of the **corneal lamellae**. *Dermatan sulfate* - **Dermatan sulfate** is primarily found in the skin, blood vessels, and heart valves, but it is not a major GAG component of the cornea. - It is involved in regulating collagen fibril assembly and stability in these specific tissues. *Chondroitin sulfate* - **Chondroitin sulfate** is abundant in cartilage, bone, and connective tissues, providing resistance to compression. - While trace amounts might be present, it is not the primary GAG responsible for the unique properties of the cornea. *Hyaluronic acid* - **Hyaluronic acid**, a non-sulfated GAG, is found in the vitreous humor and synovial fluid, contributing to lubrication and hydration. - It is generally absent or present in very small quantities in the adult cornea under normal physiological conditions.

Question 782: Gluconeogenesis in the fasting state is indicated by:

A. Citrate activation by acetyl-CoA carboxylase
B. Fructose 1,6 bisphosphate activates Pyruvate Kinase
C. Fructose 2,6 bisphosphate activates PFK-1
D. Pyruvate Carboxylase activation by Acetyl CoA (Correct Answer)

Explanation: ***Pyruvate Carboxylase activation by Acetyl CoA*** - In the **fasting state**, Acetyl CoA activates **pyruvate carboxylase**, shunting pyruvate away from glycolysis towards gluconeogenesis. - This activation is crucial for the synthesis of **oxaloacetate** from pyruvate, a key step in glucose production. *Citrate activation by acetyl-CoA carboxylase* - **Citrate** is an allosteric activator of **acetyl-CoA carboxylase**, which is involved in **fatty acid synthesis**, not gluconeogenesis. - This enzyme's activity is favored during states of **energy surplus**, not fasting. *Fructose 1,6 bisphosphate activates Pyruvate Kinase* - **Fructose 1,6-bisphosphate** activates **pyruvate kinase** as part of **glycolysis**, promoting glucose breakdown. - This regulatory mechanism operates during **fed states** to increase glucose utilization, which is opposite to the fasting state. *Fructose 2,6 bisphosphate activates PFK-1* - **Fructose 2,6-bisphosphate** activates **phosphofructokinase-1 (PFK-1)**, a key regulatory enzyme in **glycolysis**. - High levels of fructose 2,6-bisphosphate promote **glucose breakdown**, which is inhibited during the fasting state to conserve glucose.

Question 783: What happens to glucose consumed in a soft drink by a 19-year-old athlete during a track event?

A. Hexokinase helps convert glucose to energy quickly during exercise.
B. Muscles will primarily use glucose for immediate energy production. (Correct Answer)
C. The glucose from the soda will be stored as glycogen in the liver.
D. Glucose will be used by both muscles and liver.

Explanation: ***Correct: Muscles will primarily use glucose for immediate energy production.*** - During intense exercise like a track event, **skeletal muscles** are the primary consumers of circulating glucose for immediate ATP production through glycolysis and oxidative phosphorylation. - The glucose from the soft drink provides readily available fuel for the working muscles, meeting their acute energy demands. - This direct utilization of exogenous glucose **spares muscle and liver glycogen stores**, which is metabolically advantageous during prolonged exercise. - The high rate of glucose uptake by exercising muscle is facilitated by **GLUT4 translocation** to the cell membrane, which occurs independent of insulin during muscle contraction. *Incorrect: Hexokinase helps convert glucose to energy quickly during exercise.* - While hexokinase does phosphorylate glucose to glucose-6-phosphate (the first step of glycolysis), this statement is too mechanistic and doesn't describe the **physiologic fate** of consumed glucose. - The question asks "what happens" in the context of exercise, requiring an answer about tissue-level glucose utilization, not just enzymatic steps. *Incorrect: The glucose from the soda will be stored as glycogen in the liver.* - Glycogen storage occurs in the **fed, resting state** when energy demands are low and insulin levels are high. - During a track event, the body's immediate energy requirements are elevated, and glucose is preferentially oxidized for fuel rather than stored. - Glycogen synthesis would occur during the **recovery phase** after exercise, not during the event itself. *Incorrect: Glucose will be used by both muscles and liver.* - While technically true that both tissues can utilize glucose, this answer lacks precision about the **primary metabolic fate** during exercise. - The liver's main role during exercise is to **maintain blood glucose homeostasis** through glycogenolysis and gluconeogenesis, supplying glucose to the blood rather than consuming it. - During intense exercise, **skeletal muscle glucose uptake can increase 20-50 fold**, making it the predominant consumer of circulating glucose. - This option is too vague and doesn't capture the physiologic priority of muscle glucose utilization during a track event.

Question 784: Facilitative glucose transporters (GLUTs) comprise a large family. Which of the following is a glucose transporter in myocytes?

A. GLUT 1
B. GLUT 2
C. GLUT 3
D. GLUT 4 (Correct Answer)

Explanation: ***GLUT 4*** - **GLUT 4** is the primary **insulin-dependent glucose transporter** found in **adipose tissue** and **striated muscle (skeletal and cardiac muscle)**, including myocytes. - Its translocation to the cell surface is stimulated by **insulin** and muscle contraction, facilitating glucose uptake into these cells. *GLUT 1* - **GLUT 1** is ubiquitously expressed in nearly all mammalian cells, providing a **basal level of glucose uptake**. - It is particularly abundant in **red blood cells** and the **blood-brain barrier**. *GLUT 2* - **GLUT 2** is a **low-affinity, high-capacity glucose transporter** found in the **liver, pancreatic beta cells, kidney, and small intestine**. - It plays a crucial role in **glucose sensing** and the release of insulin. *GLUT 3* - **GLUT 3** is primarily found in **neurons** and the **placenta**, where it is responsible for **high-affinity glucose uptake**. - It ensures a constant supply of glucose to these tissues, even at low blood glucose concentrations.

Question 785: What type of carbohydrate is inulin classified as?

A. Glucosan
B. Fructosan (Correct Answer)
C. Galactosan
D. Mannosan

Explanation: ***Fructosan*** - **Inulin** is a naturally occurring **polysaccharide** composed primarily of **fructose** units. - As such, it is classified as a **fructosan**, a type of **fructan**, meaning its main monosaccharide component is fructose. *Glucosan* - A **glucosan** is a polysaccharide primarily made up of **glucose** units, such as **starch** or **glycogen**. - Inulin's monomeric units are predominantly fructose, not glucose. *Galactosan* - A **galactosan** is a polysaccharide primarily composed of **galactose** units. - Inulin does not primarily consist of galactose units. *Mannosan* - A **mannosan** is a polysaccharide primarily composed of **mannose** units. - Inulin's structure is based on fructose, not mannose.

Question 786: Source of ATP in RBCs is

A. Glucose (Correct Answer)
B. Fatty acid
C. Amino acid
D. Ketone body

Explanation: ***Glucose*** - Red blood cells (RBCs) lack mitochondria, so they cannot perform **aerobic respiration**, **fatty acid oxidation**, or utilize **ketone bodies** for ATP production. - Their sole source of ATP is through **anaerobic glycolysis**, which metabolizes glucose to produce a small amount of ATP. *Fatty acid* - Fatty acid oxidation, or **beta-oxidation**, occurs in the mitochondria. - Since mature **RBCs lack mitochondria**, they cannot metabolize fatty acids to produce ATP. *Amino acid* - Amino acid metabolism for energy primarily involves the **Krebs cycle** and **oxidative phosphorylation**, which also take place in the mitochondria. - **RBCs lack the necessary enzymatic machinery** and organelles for this process. *Ketone body* - Ketone bodies are metabolized for energy in mitochondria, particularly in tissues like the brain and muscle, through the **Krebs cycle**. - **RBCs do not have mitochondria** and thus cannot utilize ketone bodies as an energy source.

Question 787: Both the liver and muscle contain glycogen, yet, unlike the liver, muscle is not capable of contributing glucose to the circulation. What is the reason for this?

A. Glycolytic activity consumes all of the glucose it generates, preventing release into circulation.
B. Does not have the enzyme glucose-1-phosphatase.
C. Does not have the enzyme glycogen phosphorylase.
D. Does not have the enzyme glucose-6-phosphatase (Correct Answer)

Explanation: ***Does not have the enzyme glucose-6-phosphatase*** - **Glucose-6-phosphatase** is the enzyme responsible for dephosphorylating **glucose-6-phosphate** to glucose, allowing it to exit the cell and enter the bloodstream. - Since muscle cells lack this enzyme, the glucose-6-phosphate produced from glycogenolysis is trapped within the muscle cell and used for its own energy needs. *Glycolytic activity consumes all of the glucose it generates, preventing release into circulation.* - While muscle does utilize the glucose it generates for its own energy via glycolysis, the fundamental reason for trapping glucose within the cell is the absence of **glucose-6-phosphatase**, not just the consumption itself. - If **glucose-6-phosphatase** were present, the muscle could still release glucose even if some was used for glycolysis, especially under conditions of high glycogenolysis. *Does not have the enzyme glucose-1-phosphatase.* - **Glucose-1-phosphatase** is not a commonly recognized enzyme in glucose metabolism; the conversion between glucose-1-phosphate and glucose-6-phosphate is catalyzed by **phosphoglucomutase**. - Therefore, the absence of an enzyme with this specific name is not the reason muscle cannot release glucose into circulation. *Does not have the enzyme glycogen phosphorylase.* - Muscle tissue readily expresses **glycogen phosphorylase**, which is the enzyme responsible for breaking down glycogen into **glucose-1-phosphate** during glycogenolysis. - If muscle lacked **glycogen phosphorylase**, it would not be able to break down glycogen at all, which is contrary to its role as an energy reserve.

Question 788: In glycolysis, which of the following enzymes is not involved?

A. Pyruvate dehydrogenase (Correct Answer)
B. Phosphofructokinase
C. Glucokinase
D. Pyruvate kinase

Explanation: ***Pyruvate dehydrogenase*** - **Pyruvate dehydrogenase** is a mitochondrial enzyme complex that converts **pyruvate** to **acetyl-CoA** in the link reaction, which occurs after glycolysis and prepares for the citric acid cycle. - It is not directly involved in the ten-step glycolytic pathway itself, which converts glucose to pyruvate. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is a key regulatory enzyme in glycolysis, catalyzing the phosphorylation of **fructose-6-phosphate** to **fructose-1,6-bisphosphate**. - This step is often considered the **rate-limiting step** of glycolysis. *Glucokinase* - **Glucokinase**, located primarily in the liver and pancreatic beta cells, phosphorylates glucose to **glucose-6-phosphate** in the first step of glycolysis. - It has a high **Km** (low affinity) for glucose, allowing it to respond to high glucose concentrations. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of glycolysis, transferring a phosphate group from **phosphoenolpyruvate (PEP)** to ADP to form **ATP** and **pyruvate**. - This is one of the **irreversible** steps in glycolysis and a point of regulation.

Question 789: Which of the following metabolic pathways in carbohydrate metabolism is required for the synthesis of nucleic acids?

A. Gluconeogenesis
B. Glycolysis
C. HMP shunt (Correct Answer)
D. Glycogenesis

Explanation: ***HMP shunt*** - The **hexose monophosphate (HMP) shunt**, also known as the **pentose phosphate pathway**, is crucial for producing **ribose-5-phosphate**. - **Ribose-5-phosphate** is a direct precursor for the synthesis of **nucleotides** and subsequently **nucleic acids** (DNA and RNA). *Gluconeogenesis* - This pathway is responsible for the synthesis of **glucose from non-carbohydrate precursors**, primarily to maintain blood glucose levels during fasting. - It does not directly produce components needed for nucleic acid synthesis. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down **glucose into pyruvate**, generating ATP. - While it produces intermediates, it is not directly involved in creating the specific pentose sugars required for nucleic acids. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen from glucose** for storage in the liver and muscles. - This pathway is focused on glucose storage and has no direct role in nucleic acid synthesis.

Question 790: Which of the following statements is true regarding the pentose phosphate pathway?

A. Glucose is the only substrate that can enter this pathway
B. The pentose phosphate pathway is a direct oxidative pathway of glucose metabolism (Correct Answer)
C. The pathway has only monophosphates as intermediates
D. None of the options

Explanation: ***The pentose phosphate pathway is a direct oxidative pathway of glucose metabolism*** - The pathway **oxidizes glucose-6-phosphate** to generate **NADPH** and **pentose phosphates**, notably ribose-5-phosphate. - It is considered "direct oxidative" because it bypasses the initial steps of glycolysis to produce these crucial products. - The oxidative phase directly converts glucose-6-phosphate through two irreversible NADPH-generating steps. *Glucose is the only substrate that can enter this pathway* - While **glucose-6-phosphate** is the primary entry point, other intermediates from glycolysis can be channeled into the non-oxidative phase. - For example, **fructose-6-phosphate** and **glyceraldehyde-3-phosphate** can be interconverted with pentose phosphates, allowing their entry. *The pathway has only monophosphates as intermediates* - The pathway involves various intermediates, including **sugar phosphates** like ribulose-5-phosphate, xylulose-5-phosphate, and sedoheptulose-7-phosphate. - Many of these are **phosphates of different sugar lengths**, not exclusively monophosphates in the strict sense. *None of the options* - This statement is incorrect because the option regarding the pentose phosphate pathway being a **direct oxidative pathway of glucose metabolism** is accurate. - The pathway's central role involves both the oxidative production of **NADPH** and the non-oxidative interconversion of sugars.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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