Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 741: Which of the following statements about GLUT 2 transporters is correct?

A. Insulin dependent
B. Insulin independent (Correct Answer)
C. Found in cardiac muscle
D. Found in brain

Explanation: ***Insulin independent*** - GLUT2 transporters facilitate glucose transport into cells **regardless of insulin levels**, making them crucial for basal glucose sensing and transport functions. - This **insulin independence** is vital for organs like the liver and pancreatic beta cells to respond to varying glucose concentrations. *Insulin dependent* - **Insulin-dependent** transporters, such as **GLUT4**, respond to insulin by relocating to the cell membrane to increase glucose uptake. - This characteristic applies to tissues like **skeletal muscle** and **adipose tissue**, not GLUT2. *Found in cardiac muscle* - **Cardiac muscle** primarily utilizes **GLUT4** for glucose uptake, which is insulin-dependent. - While other GLUT transporters might be present in cardiac tissue, **GLUT2** is not the primary mechanism for glucose transport here. *Found in brain* - The **brain** predominantly uses **GLUT1** and **GLUT3** for glucose transport, which have **high affinity** for glucose to ensure constant supply. - **GLUT2** is not a primary transporter of glucose in the brain.

Question 742: The energy for glycogenesis is provided by -

A. GTP
B. GDP
C. UTP (Correct Answer)
D. AMP

Explanation: ***UTP*** - **Uridine triphosphate (UTP)** is essential for **glycogenesis** as it activates glucose by forming **UDP-glucose** from glucose-1-phosphate. - The reaction (Glucose-1-P + UTP → UDP-glucose + PPi) creates a **high-energy intermediate** that drives glycogen synthesis. - The subsequent hydrolysis of pyrophosphate (PPi) makes this activation step **irreversible**, and the energy stored in UDP-glucose is used for **glycosidic bond formation** when glucose is added to the growing glycogen chain. *GTP* - **Guanosine triphosphate (GTP)** is primarily involved in **protein synthesis**, G-protein signaling, and the citric acid cycle. - It is not used for glucose activation in glycogenesis; that role is specific to **UTP**. *GDP* - **Guanosine diphosphate (GDP)** is a product of GTP hydrolysis and functions in regulatory processes. - It does not serve as an energy donor for glycogen synthesis. *AMP* - **Adenosine monophosphate (AMP)** is a low-energy signal molecule that indicates cellular energy depletion. - High AMP levels **inhibit glycogenesis** and activate glycogenolysis through allosteric regulation of key enzymes. - It does not provide energy for anabolic pathways like glycogen synthesis.

Question 743: 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 744: 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 745: 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 746: 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 747: 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 748: Fructose intolerance is due to deficiency of which enzyme?

A. Aldolase B (Correct Answer)
B. Aldolase A
C. Fructokinase
D. Triokinase

Explanation: ***Aldolase B*** - **Hereditary fructose intolerance** is a genetic disorder caused by a deficiency in the enzyme **aldolase B**. - This deficiency leads to an accumulation of **fructose-1-phosphate** in the liver, kidneys, and small intestine, causing **hypoglycemia**, **vomiting**, and **liver damage** upon exposure to fructose. *Fructokinase* - A deficiency in **fructokinase** causes **essential fructosuria**, a benign metabolic disorder. - This condition is asymptomatic because **fructose** simply accumulates in the blood and urine without causing significant clinical problems. *Triokinase* - **Triokinase**, also known as **glycerol kinase**, is involved in glycerol metabolism, converting glycerol to **glycerol-3-phosphate**. - Its deficiency is not directly linked to fructose intolerance and typically presents with **hyperglycerolemia**. *Aldolase A* - **Aldolase A** is one of the three aldolase isoenzymes (A, B, and C) and is primarily involved in **glycolysis**, specifically in the breakdown of **fructose-1,6-bisphosphate**. - A deficiency in aldolase A can lead to **hemolytic anemia** and **myopathy**, not directly fructose intolerance.

Question 749: 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.

Question 750: 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.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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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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