Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 751: Gluconeogenesis occurs in all except:

A. Muscle (Correct Answer)
B. Kidney
C. Gut
D. Liver

Explanation: ***Muscle*** - **Muscle tissue** lacks the enzyme **glucose-6-phosphatase**, which is essential for releasing free glucose into the bloodstream during gluconeogenesis. - While muscle can store glycogen, it primarily uses glucose for its own energy needs and does not contribute significantly to systemic glucose homeostasis through gluconeogenesis. *Liver* - The **liver** is the primary site of **gluconeogenesis**, producing glucose to maintain blood glucose levels during fasting and starvation. - It contains all the necessary enzymes, including **glucose-6-phosphatase**, to convert precursors like lactate, amino acids, and glycerol into glucose. *Kidney* - The **kidney** becomes a significant site of **gluconeogenesis** during prolonged fasting, contributing up to 10-20% of the body's glucose production. - Renal gluconeogenesis primarily utilizes **lactate** and **glutamine** as substrates. *Gut* - The **small intestine (gut)** has been identified as a site of **gluconeogenesis**, particularly following a meal rich in protein. - Its contribution is relatively smaller compared to the liver but plays a role in **postprandial glucose homeostasis**.

Question 752: Glucagon stimulates

A. Gluconeogenesis (Correct Answer)
B. Glycogenesis
C. Fatty acid synthesis
D. Glycolysis

Explanation: ***Gluconeogenesis*** - **Glucagon** is a hormone that primarily acts to raise **blood glucose levels** by stimulating the production of glucose from non-carbohydrate sources. - This process, **gluconeogenesis**, occurs mainly in the liver and is initiated by glucagon to counteract hypoglycemia. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen** from glucose and is primarily stimulated by insulin when blood glucose levels are high. - Glucagon's role is to *inhibit* glycogen synthesis and instead promote glycogen breakdown. *Fatty acid synthesis* - **Fatty acid synthesis** is an anabolic process that primarily occurs when there is an excess of energy and glucose, often stimulated by **insulin**. - Glucagon generally has an **inhibitory effect** on fatty acid synthesis, as its main goal is to mobilize energy stores, not create them. *Glycolysis* - **Glycolysis** is the breakdown of glucose to produce energy, and it is stimulated when glucose is abundant and energy is needed. - Glucagon primarily acts to *inhibit* glycolysis in the liver, thereby conserving glucose for use by other tissues and promoting its release into the bloodstream.

Question 753: Which element is required by phosphofructokinase?

A. Magnesium (Correct Answer)
B. Inorganic phosphate
C. Manganese
D. Copper

Explanation: **Magnesium** - **Phosphofructokinase** (PFK) is an enzyme in **glycolysis** that catalyzes the phosphorylation of fructose-6-phosphate. - This reaction requires **ATP**, and like many enzymes that utilize ATP, PFK requires **magnesium ions (Mg²⁺)** as a cofactor, typically forming a complex with ATP (MgATP²⁻). *Inorganic phosphate* - **Inorganic phosphate** is a substrate for some kinase reactions, but not a direct cofactor requirement for the *activation* of phosphofructokinase itself. - While phosphate is incorporated into molecules during phosphorylation, it does not act as a metal ion cofactor to facilitate the enzyme's activity. *Manganese* - While **manganese (Mn²⁺)** can sometimes substitute for magnesium in certain enzyme reactions, it is not the primary or required cofactor for phosphofructokinase under normal physiological conditions. - Many enzymes have a preference for specific metal ions based on their active site structure and coordination chemistry. *Copper* - **Copper (Cu²⁺)** is a cofactor for a variety of enzymes, particularly those involved in **redox reactions** (e.g., cytochrome c oxidase, superoxide dismutase). - However, copper is not a required metallic cofactor for the activity of **phosphofructokinase** in glycolysis.

Question 754: Which metabolic pathway provides instant energy to muscles?

A. Embden-Meyerhof pathway (Correct Answer)
B. HMP shunt
C. Cori cycle
D. TCA cycle

Explanation: ***Embden-Meyerhof pathway*** - This pathway, also known as **glycolysis**, rapidly breaks down glucose into pyruvate to produce **ATP without oxygen**, providing instant energy to muscles during high-intensity activity. - Generates a net of **two ATP molecules** per glucose molecule, which is crucial for quick bursts of energy. *HMP shunt* - The **hexose monophosphate shunt** primarily produces **NADPH** for reductive biosynthesis and **ribose-5-phosphate** for nucleotide synthesis, not immediate large-scale ATP for muscle contraction. - Plays a role in protecting cells from **oxidative stress** and synthesizing precursors for DNA and RNA. *Cori cycle* - The **Cori cycle** involves the recycling of **lactate** produced in muscles back to glucose in the liver, which is a slower process for maintaining glucose homeostasis rather than providing instant muscle energy. - It helps prevent **lactic acidosis** during strenuous activity but is not a primary pathway for rapid ATP generation. *TCA cycle* - The **TCA cycle (Krebs cycle)** is part of **aerobic respiration** and produces a significant amount of ATP, but it is a slower, more sustained energy production pathway that requires oxygen. - Primarily active during **lower-intensity**, longer-duration activities when oxygen supply is adequate.

Question 755: What is the main enzyme involved in glycogen breakdown (glycogenolysis)?

A. Glycogen phosphorylase (Correct Answer)
B. Glycogen synthase
C. Glucose-6-phosphatase
D. Hexokinase

Explanation: ***Glycogen phosphorylase*** - This is the **rate-limiting and primary enzyme** for **glycogenolysis**, the breakdown of glycogen into glucose units. - It cleaves **α-1,4-glycosidic bonds** in glycogen, releasing **glucose-1-phosphate** units. - Regulated by both **allosteric mechanisms** and **hormonal control** (epinephrine, glucagon). - Works until it reaches 4 glucose residues from a branch point, where debranching enzyme takes over. *Glycogen synthase* - This is the main enzyme for **glycogenesis** (glycogen synthesis), not breakdown. - It catalyzes formation of α-1,4-glycosidic bonds to build glycogen chains. - This is the opposite direction of metabolism from what the question asks about. *Glucose-6-phosphatase* - This enzyme is involved in **gluconeogenesis** and the final step of converting **glucose-6-phosphate to free glucose**. - It is NOT directly involved in glycogen breakdown itself, but rather in the subsequent conversion pathway. - Found primarily in **liver and kidney** to release free glucose into blood. *Hexokinase* - This enzyme phosphorylates free glucose to **glucose-6-phosphate** (opposite direction). - It is involved in **glucose utilization**, not glycogen breakdown. - It traps glucose inside cells for metabolism or glycogen synthesis.

Question 756: Which of the following is the major glycosaminoglycan of synovial fluid?

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

Explanation: ***Hyaluronic acid*** - **Hyaluronic acid** is the primary glycosaminoglycan in **synovial fluid**, providing its characteristic **viscosity** and **lubricating properties**. - It plays a crucial role in maintaining **joint health** by reducing friction and acting as a shock absorber. *Chondroitin sulfate* - **Chondroitin sulfate** is abundant in **cartilage**, contributing to its **compressive strength**. - While present in connective tissues, it is not the major glycosaminoglycan of synovial fluid. *Dermatan sulfate* - **Dermatan sulfate** is primarily found in **skin**, **blood vessels**, and **heart valves**. - Its main roles involve tissue structure and repair, not lubrication of synovial fluid. *Heparan sulfate* - **Heparan sulfate** is found on **cell surfaces** and in the **extracellular matrix**, especially in the **basement membranes**. - It regulates cell growth, adhesion, and signaling, and is not a major component of synovial fluid viscosity.

Question 757: Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?

A. 2,3-Bisphosphoglycerate (2,3-BPG)
B. Fructose 2,6-bisphosphate (Correct Answer)
C. Glucokinase
D. Phosphoenolpyruvate (PEP)

Explanation: ***Fructose 2,6-bisphosphate*** - **Fructose 2,6-bisphosphate** is a potent **allosteric activator** of **phosphofructokinase-1 (PFK-1)**, increasing its affinity for fructose 6-phosphate and overcoming ATP inhibition. - Its synthesis is regulated by **insulin** (stimulating) and **glucagon** (inhibiting), linking glucose availability to glycolytic flux. *2,3-Bisphosphoglycerate (2,3-BPG)* - **2,3-BPG** is an important regulator of **hemoglobin oxygen affinity** in red blood cells. - It is not an allosteric activator of **PFK-1**; its primary role is in oxygen delivery. *Glucokinase* - **Glucokinase** is an **enzyme** in glycolysis, specifically catalyzing the phosphorylation of glucose to glucose 6-phosphate in the liver and pancreatic beta cells. - It is not an allosteric activator of **PFK-1** but rather an upstream enzyme in the pathway. *Phosphoenolpyruvate (PEP)* - **PEP** is an intermediate in glycolysis, formed from 2-phosphoglycerate and converted to pyruvate by pyruvate kinase. - It acts as an **allosteric inhibitor** of phosphofructokinase-1, signaling high energy status and slowing down glycolysis.

Question 758: Which of the following tests is most commonly used to detect glucose in urine?

A. a) Benedicts test
B. c) Glucose-oxidase test (Correct Answer)
C. b) Fehling solution
D. d) None of the above

Explanation: ***Glucose-oxidase test*** - The **glucose-oxidase test** is a highly specific and sensitive enzymatic test used to detect **glucose** in urine. - It uses the enzyme glucose oxidase which specifically catalyzes the oxidation of glucose to gluconic acid and hydrogen peroxide, which then produces a color change. - This is the **most commonly used method** in modern clinical practice for detecting glucosuria due to its **high specificity for glucose** and ease of use (dipstick method). - It is the preferred test for **monitoring diabetes** and screening for hyperglycemia. *Benedict's test* - **Benedict's test** is a general chemical test for **all reducing sugars** (glucose, fructose, galactose, lactose, maltose), not specifically glucose. - It works by reducing copper sulfate (Cu²⁺) to copper oxide (Cu⁺) in an alkaline solution, forming a colored precipitate (green, yellow, orange, or brick-red depending on sugar concentration). - While it can detect glucose, it **lacks specificity** and can give false positives with other reducing substances (vitamin C, certain drugs), making it less suitable for routine clinical testing. *Fehling's solution* - **Fehling's solution** is also a general chemical test for **reducing sugars** based on copper reduction, similar to Benedict's test. - It consists of two solutions mixed before use and detects various reducing sugars, not just glucose. - It is **not commonly used in clinical urine analysis** due to lack of specificity and the need for heating and mixing two solutions, making it impractical compared to the simple glucose-oxidase dipstick. *None of the above* - This option is incorrect because the **glucose-oxidase test** is indeed the most commonly used test for detecting glucose in urine in modern clinical practice.

Question 759: What is the maximum value on the glycemic index scale that classifies a food as low glycemic index?

A. 25
B. 45
C. 55 (Correct Answer)
D. 65

Explanation: ***55*** - A food is classified as having a **low glycemic index (GI)** if its GI value is **55 or less**. - The GI scale classifies foods as: **low GI (≤55), medium GI (56-69), and high GI (≥70)**. - This classification indicates that the food causes a slower and lower rise in blood glucose levels compared to high or medium GI foods. *25* - This value is well below the threshold for a low GI food and is not the maximum value for this classification. - While a food with a GI of 25 would indeed be considered low GI, the question asks for the **maximum value** that still falls within this category. *45* - This value is still within the low GI range, but it is not the maximum value for this classification. - Foods with a GI up to 55 are considered low GI. *65* - A GI value of 65 falls into the **medium glycemic index** category (GI 56-69). - Therefore, this value classifies a food as medium GI, not low GI.

Question 760: ATP is consumed at which of the following steps of glycolysis?

A. Pyruvate kinase
B. Isomerase
C. Hexokinase (Correct Answer)
D. Enolase

Explanation: ***Hexokinase*** - This enzyme catalyzes the **first step of glycolysis**, the phosphorylation of glucose to **glucose-6-phosphate**, which requires the consumption of one molecule of **ATP**. - ATP is hydrolyzed to **ADP**, providing the necessary phosphate group and energy for this irreversible reaction. - Note: Hexokinase is one of **two ATP-consuming steps** in glycolysis (the other being phosphofructokinase in step 3). *Pyruvate kinase* - This enzyme catalyzes the **final step of glycolysis**, converting **phosphoenolpyruvate (PEP)** to pyruvate. - This reaction involves the **production of ATP** from ADP, not its consumption, as it's one of the substrate-level phosphorylation steps. *Isomerase* - Isomerase enzymes, like phosphoglucose isomerase, convert one isomer to another (e.g., glucose-6-phosphate to fructose-6-phosphate). - These reactions generally involve an **internal rearrangement of atoms** and do not directly consume or produce ATP. *Enolase* - Enolase catalyzes the reversible conversion of **2-phosphoglycerate to phosphoenolpyruvate (PEP)**, releasing a molecule of water. - This step occurs before the ATP-generating step catalyzed by pyruvate kinase and **does not consume or produce ATP**.

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

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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