Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 741: Which of the following tissues relies EXCLUSIVELY on anaerobic glycolysis for ATP production?

A. Skeletal muscle during exercise (anaerobic)
B. Liver hepatocytes (primarily aerobic)
C. Cardiac muscle (primarily aerobic)
D. Mature RBCs (exclusively anaerobic) (Correct Answer)

Explanation: ***Mature RBCs (exclusively anaerobic)*** - **Mature red blood cells** lack mitochondria, making them incapable of **oxidative phosphorylation** and thus relying entirely on **anaerobic glycolysis** for ATP. - This pathway produces **2 net ATP** molecules per glucose molecule, which is sufficient for their metabolic needs like maintaining ion gradients. *Skeletal muscle during exercise (anaerobic)* - While skeletal muscle can perform **anaerobic glycolysis** during intense exercise when oxygen supply is limited, it is not an exclusive reliance. - Skeletal muscle also utilizes **aerobic respiration** and **creatine phosphate** for ATP production depending on activity level and oxygen availability. *Cardiac muscle (primarily aerobic)* - **Cardiac muscle** has a very high metabolic demand and is rich in **mitochondria**, relying almost exclusively on **aerobic respiration** for ATP production. - It uses fatty acids, glucose, and lactate as fuel sources, producing a large amount of ATP efficiently with oxygen. *Liver hepatocytes (primarily aerobic)* - **Liver hepatocytes** are highly metabolically active and primarily rely on **aerobic respiration** for ATP production, performing diverse functions such as gluconeogenesis, glycogenolysis, and detoxification. - Although the liver can perform some anaerobic glycolysis under hypoxic conditions, it is not its exclusive or primary mode of ATP synthesis.

Question 742: 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 743: Which of the following is an aldose?

A. Fructose
B. Erythrulose
C. Glucose (Correct Answer)
D. None of the options

Explanation: ***Glucose*** - An **aldose** is a monosaccharide containing an **aldehyde group** (—CHO) in its open-chain form. - **Glucose** possesses an aldehyde group at carbon-1 and is therefore classified as an aldose. *Fructose* - **Fructose** is a **ketose**, meaning it contains a **ketone group** (C=O) in its open-chain structure, typically at carbon-2. - While it is a monosaccharide, its functional group differentiates it from aldoses. *Erythrulose* - **Erythrulose** is a **ketotetrose**, meaning it is a four-carbon sugar with a **ketone group**. - Unlike aldoses, which have an aldehyde group, erythrulose's defining characteristic is its ketone functional group. *None of the options* - This option is incorrect because **Glucose** is indeed an aldose, fitting the definition of having an aldehyde functional group. - Therefore, there is a correct option provided among the choices.

Question 744: 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 745: Hexokinase is inhibited by?

A. Glucose-6-phosphate (G6P) (Correct Answer)
B. Glucose
C. Insulin
D. Glucagon

Explanation: ***Glucose-6-phosphate (G6P)*** - Hexokinase is subject to **feedback inhibition** by its product, **glucose-6-phosphate**, preventing the accumulation of high levels of G6P inside the cell. - This regulatory mechanism ensures that glycolysis does not proceed unchecked when energy needs are met or when G6P levels are already sufficient. *Glucagon* - **Glucagon** is a hormone that generally promotes **glucose production** and release, primarily by stimulating gluconeogenesis and glycogenolysis, rather than directly inhibiting hexokinase. - Its effects on glucose metabolism are more about increasing blood glucose levels than directly regulating the initial step of glycolysis in most tissues. *Glucose* - **Glucose** is the **substrate** for hexokinase, meaning it is the molecule that hexokinase acts upon to convert it into glucose-6-phosphate. - Therefore, glucose does not inhibit hexokinase; instead, its presence is necessary for the enzyme's activity. *Insulin* - **Insulin** is a hormone that promotes **glucose uptake** and utilization by cells, often by increasing the number of glucose transporters on cell surfaces. - While insulin can indirectly influence glycolysis by increasing glucose availability, it does not directly inhibit hexokinase; rather, it generally supports cellular glucose metabolism.

Question 746: 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 747: Which of the following statements about gluconeogenesis is correct?

A. Occurs mainly in the liver (Correct Answer)
B. It uses exactly the same enzymes as glycolysis in reverse
C. It only occurs during fed state when insulin levels are high
D. Fatty acids are the primary substrate for gluconeogenesis

Explanation: ***Occurs mainly in the liver*** - The **liver** is the primary site for **gluconeogenesis**, responsible for maintaining blood glucose levels during fasting. - The kidneys also contribute, especially during prolonged fasting, but to a lesser extent. *It uses exactly the same enzymes as glycolysis in reverse* - While gluconeogenesis shares some enzymes with glycolysis, there are **three irreversible steps in glycolysis** that require different enzymes in gluconeogenesis to bypass them. - Key bypass enzymes include **pyruvate carboxylase**, **phosphoenolpyruvate carboxykinase (PEPCK)**, **fructose-1,6-bisphosphatase**, and **glucose-6-phosphatase**. *It only occurs during fed state when insulin levels are high* - **Gluconeogenesis is activated during fasting or starvation** when blood glucose levels are low, and it is largely **inhibited by high insulin levels**. - Its purpose is to produce new glucose to prevent hypoglycemia, not to store excess glucose. *Fatty acids are the primary substrate for gluconeogenesis* - **Fatty acids cannot be directly converted to glucose** in significant amounts in humans because they are broken down into acetyl-CoA, which cannot be used for net glucose synthesis. - Primary substrates include **lactate**, **amino acids** (from protein breakdown), and **glycerol** (from triglyceride breakdown).

Question 748: Which of the following is NOT required for gluconeogenesis from lactate?

A. Transamination of pyruvate to alanine (Correct Answer)
B. Transport of lactate from muscle to liver
C. Conversion of lactate to pyruvate
D. None of the above

Explanation: ***Transamination of pyruvate to alanine*** - While **alanine** can be a substrate for gluconeogenesis, **lactate** is directly converted to pyruvate, which then enters the gluconeogenesis pathway. **Transamination to alanine** is not a required intermediate step for lactate-derived glucose production. - The direct conversion of **lactate to pyruvate** by **lactate dehydrogenase** is the key initial step, not its conversion to alanine. *Transport of lactate from muscle to liver* - **Lactate** produced in muscles (e.g., during intense exercise) must be transported to the **liver** via the bloodstream to be used for **gluconeogenesis** in the **Cori cycle**. - This transport is essential for clearing lactate from the periphery and supplying the liver with a gluconeogenic precursor. *Conversion of lactate to pyruvate* - **Lactate dehydrogenase** catalyzes the reversible conversion of **lactate to pyruvate**, which is the critical first step in converting lactate into a gluconeogenic substrate. - This reaction regenerates **NAD+** (not NADH), which is necessary for glycolysis to continue in muscle tissue. *None of the above* - This option is incorrect because there IS a step listed above that is not required: **transamination of pyruvate to alanine** is indeed not necessary for gluconeogenesis from lactate, making Option A the correct answer to this "NOT required" question.

Question 749: Which of the following statements about glycolysis is incorrect?

A. Provide nutrition to cancer cells
B. Two carbon end product is formed (Correct Answer)
C. Substrate level phosphorylation at pyruvate kinase
D. NADPH is formed by glyceraldehyde-3-phosphate dehydrogenase

Explanation: ***Two carbon end product is formed*** - Glycolysis breaks down one molecule of **glucose (a 6-carbon sugar)** into two molecules of **pyruvate**, which is a **3-carbon compound**. - Therefore, the end product of glycolysis is a **3-carbon molecule**, not a 2-carbon molecule. *Provide nutrition to cancer cells* - Many cancer cells exhibit increased rates of glycolysis, even in the presence of oxygen, a phenomenon known as the **Warburg effect**. - This increased glycolysis provides necessary **ATP and metabolic intermediates** for rapid cell proliferation. *Substrate level phosphorylation at pyruvate kinase* - **Pyruvate kinase** catalyzes the transfer of a phosphate group from **phosphoenolpyruvate (PEP)** to ADP, forming ATP and pyruvate. - This is a classic example of **substrate-level phosphorylation** within glycolysis. *NADPH is formed by glyceraldhyde-3-phosphate dehydrogenase* - During the oxidation of **glyceraldehyde-3-phosphate** to **1,3-bisphosphoglycerate** by glyceraldehyde-3-phosphate dehydrogenase, **NAD+ is reduced to NADH**, not NADPH. - **NADPH** is primarily generated in the **pentose phosphate pathway** and is used for reductive biosynthesis, while NADH is used in the electron transport chain for ATP production.

Question 750: Which transporter is responsible for the transport of glucose in the pancreas?

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

Explanation: ***GLUT 2*** - **GLUT2** is a **low-affinity, high-capacity** glucose transporter primarily found in the **pancreatic beta cells**, liver, small intestine, and kidneys. - In pancreatic beta cells, GLUT2 allows rapid entry of glucose for metabolism, leading to **insulin secretion** in response to elevated blood glucose levels. *GLUT 1* - **GLUT1** is a **ubiquitous glucose transporter** found in most tissues, including red blood cells and the blood-brain barrier. - It has a high affinity for glucose, ensuring **basal glucose uptake** even at low concentrations. *GLUT 3* - **GLUT3** is a **high-affinity glucose transporter** concentrated in **neurons** and the brain. - Its efficient glucose uptake is critical for the constant and high energy demands of the central nervous system. *GLUT 4* - **GLUT4** is an **insulin-dependent glucose transporter** primarily found in **adipose tissue** and **striated muscle (skeletal and cardiac muscle)**. - Insulin stimulates the translocation of GLUT4 to the cell membrane, facilitating glucose uptake from the blood after a meal.

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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