Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 471: Which of the following statements about the Hexose Monophosphate (HMP) pathway is FALSE?

A. The oxidative phase of the pathway produces NADPH.
B. The pathway does not produce ATP.
C. It occurs in the testes, ovaries, placenta, and adrenal cortex.
D. Ribose-5-phosphate is produced during the oxidative phase of the pathway. (Correct Answer)

Explanation: ### Explanation The Hexose Monophosphate (HMP) Shunt, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol and consists of two distinct phases: oxidative and non-oxidative. **Why Option D is the Correct (False) Statement:** Ribose-5-phosphate is the end product of the **non-oxidative phase**, not the oxidative phase. The oxidative phase starts with Glucose-6-phosphate and ends with **Ribulose-5-phosphate**, producing NADPH in the process. Ribulose-5-phosphate must then be isomerized by *phosphopentose isomerase* into Ribose-5-phosphate during the reversible non-oxidative phase. **Analysis of Other Options:** * **Option A (True):** The oxidative phase involves two key enzymes, *Glucose-6-phosphate dehydrogenase (G6PD)* and *6-phosphogluconate dehydrogenase*, both of which reduce $NADP^+$ to **NADPH**. * **Option B (True):** Unlike glycolysis, the HMP shunt is an alternative pathway for glucose oxidation that **does not consume or produce ATP**. * **Option C (True):** The pathway is highly active in tissues requiring NADPH for reductive biosynthesis (fatty acids, steroids) or protection against free radicals. This includes the **adrenal cortex, testes, ovaries, placenta** (steroidogenesis), liver, mammary glands, and RBCs. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-phosphate dehydrogenase (G6PD). * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to maintain reduced glutathione in RBCs, resulting in **Heinz bodies** and **Bite cells**. * **Transketolase:** An enzyme in the non-oxidative phase that requires **Thiamine (Vitamin B1)** as a cofactor. Measuring erythrocyte transketolase activity is used to diagnose Thiamine deficiency. * **Key Products:** NADPH (for fatty acid/steroid synthesis and respiratory burst in WBCs) and Ribose-5-phosphate (for nucleotide synthesis).

Question 472: Which of the following sugars is not synthesized or utilized in the human body?

A. L-fucose
B. L-fructose (Correct Answer)
C. D-Glucose
D. D-Fructose

Explanation: **Explanation:** The human body primarily utilizes **D-isomers** of sugars (like D-glucose and D-fructose) for metabolism. While most sugars in our body belong to the D-series, there are specific exceptions where L-isomers are synthesized and utilized. **L-fructose**, however, is neither synthesized nor metabolized by human enzymes and has no physiological role in the body. **Analysis of Options:** * **L-fructose (Correct):** This is a synthetic sugar not found in human metabolic pathways. Human hexokinase and other glycolytic enzymes are stereospecific for D-fructose. * **L-fucose:** This is a crucial exception to the "D-sugar rule." It is an L-isomer sugar synthesized in the body and is a vital component of **glycoproteins** and **blood group substances** (H-substance). * **D-Glucose:** The primary metabolic fuel for the human body, especially for the brain and RBCs. * **D-Fructose:** A common dietary monosaccharide (found in fruits and honey) metabolized via the fructokinase pathway in the liver. **High-Yield Clinical Pearls for NEET-PG:** * **The "L-Sugar" Exceptions:** While most human sugars are D-isomers, remember these two L-isomers: **L-fucose** (found in glycoproteins) and **L-xylulose** (excreted in excess in Essential Pentosuria due to deficiency of L-xylulose reductase). * **Stereospecificity:** Enzymes are highly stereospecific; for example, glucose oxidase reacts only with D-glucose, not L-glucose. * **L-fucose Source:** It is derived from GDP-mannose. Deficiency in its transport leads to **Leukocyte Adhesion Deficiency Type II**.

Question 473: When muscle glycogen is used for anaerobic glycolysis, how many ATPs are formed?

A. 2
B. 3 (Correct Answer)
C. 4
D. 7

Explanation: ### Explanation The correct answer is **3 ATPs**. **1. Why Option B is Correct:** The key to this question lies in the starting material: **Muscle Glycogen** versus free Glucose. * When muscle glycogen is broken down (glycogenolysis), it releases **Glucose-1-Phosphate (G-1-P)** via the enzyme *Glycogen Phosphorylase*. * G-1-P is then converted to **Glucose-6-Phosphate (G-6-P)** by *Phosphoglucomutase*. * In regular glycolysis starting from free glucose, 1 ATP is consumed by Hexokinase to create G-6-P. However, when starting from glycogen, this **Hexokinase step is bypassed**, saving 1 ATP. * **Net Calculation:** 4 ATPs are produced during the payoff phase (2 from each Glyceraldehyde-3-phosphate), and only 1 ATP is consumed (at the Phosphofructokinase-1 step). * **Net Yield:** 4 (produced) - 1 (consumed) = **3 ATP**. **2. Why Other Options are Incorrect:** * **Option A (2 ATP):** This is the net yield of anaerobic glycolysis when the starting material is **free Glucose**, as 2 ATPs are consumed in the preparatory phase (Hexokinase and PFK-1). * **Option C (4 ATP):** This is the *gross* yield of ATP in glycolysis before subtracting the energy investment phase. * **Option D (7 ATP):** This refers to the net yield of *aerobic* glycolysis (2 ATP + 5 ATP from 2 NADH) per molecule of glucose, not anaerobic conditions. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Lactate Production:** In anaerobic conditions, pyruvate is converted to lactate by *Lactate Dehydrogenase (LDH)* to regenerate **NAD+**, which is essential for glycolysis to continue. * **Arsenate Poisoning:** Arsenate competes with inorganic phosphate, leading to a net ATP yield of **zero** in glycolysis because the substrate-level phosphorylation step is bypassed. * **Muscle vs. Liver:** Muscle lacks the enzyme *Glucose-6-Phosphatase*; therefore, muscle glycogen cannot contribute to blood glucose and is used exclusively for local energy production.

Question 474: Which glucose transporter (GLUT) is primarily responsible for transporting glucose from the intestine to the liver?

A. GLUT1
B. GLUT2 (Correct Answer)
C. GLUT3
D. GLUT4

Explanation: **Explanation:** The correct answer is **GLUT2**. **Why GLUT2 is correct:** GLUT2 is a high-capacity, low-affinity (high $K_m$) bidirectional transporter. It is primarily located in the **basolateral membrane of intestinal mucosal cells**, the liver, pancreatic beta cells, and the renal tubular cells. After glucose is absorbed from the intestinal lumen via SGLT-1, it exits the enterocyte into the portal circulation via GLUT2. Because of its high $K_m$, GLUT2 acts as a "glucose sensor," ensuring that glucose uptake by the liver is proportional to blood glucose levels, especially in the postprandial (fed) state. **Why other options are incorrect:** * **GLUT1:** This is a high-affinity transporter found in almost all tissues, particularly **RBCs and the Blood-Brain Barrier**. It provides basal glucose uptake. * **GLUT3:** This is the primary transporter in **neurons**. It has a very low $K_m$, ensuring the brain receives glucose even during hypoglycemia. * **GLUT4:** This is the only **insulin-dependent** transporter. It is located in **skeletal muscle and adipose tissue**. In the absence of insulin, it remains sequestered in intracellular vesicles. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 vs. GLUT2:** Remember that SGLT-1 (Sodium-Glucose Co-transporter 1) handles the *active* uptake from the intestinal lumen, while GLUT2 handles the *facilitated* exit into the blood. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a congenital defect in **GLUT2**, leading to hepatomegaly and glucose intolerance. * **Bidirectionality:** GLUT2 is unique because it allows glucose to enter the liver for glycogenesis and exit the liver during gluconeogenesis/glycogenolysis.

Question 475: ATP is an allosteric regulator of which enzymes?

A. Hexokinase and PFK 1
B. PFK 1 and PFK 2
C. PFK 1 and Pyruvate kinase (Correct Answer)
D. Glyceraldehyde 3 phosphate dehydrogenase and PFK 1

Explanation: ### Explanation The correct answer is **PFK-1 and Pyruvate kinase**. This question tests the understanding of the **allosteric regulation of Glycolysis**, specifically how the cell senses its energy status to control metabolic flux. #### 1. Why the Correct Answer is Right Glycolysis is regulated primarily at three irreversible steps. ATP acts as a "high-energy signal." When ATP levels are high, the cell does not need to oxidize more glucose for energy. * **Phosphofructokinase-1 (PFK-1):** This is the rate-limiting step. ATP acts as an allosteric inhibitor by binding to a specific regulatory site (distinct from the catalytic site), decreasing the enzyme's affinity for Fructose-6-Phosphate. * **Pyruvate Kinase:** This is the final step of glycolysis. It is also allosterically inhibited by ATP. This prevents the unnecessary conversion of Phosphoenolpyruvate (PEP) to Pyruvate when energy stores are sufficient. #### 2. Why Other Options are Wrong * **Hexokinase (Option A):** Hexokinase is inhibited by its product, **Glucose-6-Phosphate**, not by ATP. (Note: Glucokinase in the liver is not inhibited by G6P). * **PFK-2 (Option B):** PFK-2 is primarily regulated by **cAMP-dependent phosphorylation** (via Glucagon/Insulin ratio) rather than direct allosteric inhibition by ATP. * **G3P Dehydrogenase (Option D):** This is a reversible step in glycolysis and is not a major site of allosteric regulation by ATP. #### 3. NEET-PG High-Yield Clinical Pearls * **PFK-1 Activators:** The most potent allosteric activator of PFK-1 is **Fructose-2,6-bisphosphate**. * **Feed-forward Activation:** Pyruvate kinase is allosterically *activated* by **Fructose-1,6-bisphosphate** (the product of the PFK-1 reaction). * **Inhibitors of PFK-1:** ATP and **Citrate** (linking Glycolysis to the TCA cycle). * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the G3P Dehydrogenase reaction, resulting in zero net ATP production during glycolysis.

Question 476: Lactate is formed in all except?

A. RBC
B. Lens
C. Brain (Correct Answer)
D. Testis

Explanation: **Explanation:** The production of lactate is the hallmark of **anaerobic glycolysis**, occurring in tissues that either lack mitochondria or have a relatively low oxygen supply. **Why Brain is the Correct Answer:** Under normal physiological conditions, the brain is an **obligate aerobic organ**. It utilizes glucose as its primary fuel source and completely oxidizes it via the TCA cycle and Oxidative Phosphorylation to produce CO₂ and H₂O. Because the brain has a high density of mitochondria and a constant, rich oxygen supply, it does not typically produce lactate as a metabolic end-product. **Analysis of Other Options:** * **RBCs (A):** Mature erythrocytes lack mitochondria. Therefore, they are incapable of aerobic metabolism and must rely entirely on anaerobic glycolysis, converting all glucose to **lactate**. * **Lens (B) & Cornea:** These structures are largely avascular to maintain optical clarity. They have limited mitochondrial activity and rely on anaerobic glycolysis to meet energy needs, producing lactate. * **Testis (D):** The interior of the testis is relatively hypoxic. Spermatocytes and the germinal epithelium utilize anaerobic pathways significantly, making the testis a recognized site of lactate production. **High-Yield NEET-PG Pearls:** * **Major sites of lactate production:** RBCs, Lens, Cornea, Kidney Medulla, Testis, and actively exercising Skeletal Muscle. * **Cori Cycle:** The lactate produced by these tissues is transported to the liver, where it is converted back to glucose via gluconeogenesis. * **Key Enzyme:** Lactate Dehydrogenase (LDH) catalyzes the reversible conversion of Pyruvate to Lactate, regenerating **NAD+** required for glycolysis to continue.

Question 477: Which of the following hormones stimulates glucose utilization?

A. Insulin (Correct Answer)
B. Growth hormone
C. Corticosteroid
D. Glucagon

Explanation: **Explanation:** The regulation of blood glucose is a balance between **anabolic (hypoglycemic)** and **catabolic (hyperglycemic)** hormones. **Why Insulin is Correct:** Insulin is the only major anabolic hormone that lowers blood glucose by stimulating **glucose utilization**. It achieves this through three primary mechanisms: 1. **Increased Uptake:** It promotes the translocation of **GLUT-4** transporters to the cell membranes of skeletal muscle and adipose tissue. 2. **Glycolysis:** It activates key enzymes like Phosphofructokinase-1 (PFK-1) and Glucokinase, favoring glucose breakdown. 3. **Glycogenesis:** It stimulates Glycogen Synthase to store glucose as glycogen in the liver and muscles. **Why Other Options are Incorrect:** Options B, C, and D are all **counter-regulatory (diabetogenic) hormones** that increase blood glucose levels: * **Glucagon:** Stimulates hepatic glycogenolysis and gluconeogenesis during fasting states. * **Corticosteroids (Cortisol):** Increase gluconeogenesis and decrease peripheral glucose uptake (insulin resistance) to ensure glucose availability during stress. * **Growth Hormone:** Inhibits glucose uptake in peripheral tissues (anti-insulin effect) and stimulates lipolysis. **High-Yield NEET-PG Pearls:** * **GLUT-4** is the only insulin-dependent glucose transporter (found in heart, skeletal muscle, and adipose tissue). * **Brain and Liver** do not require insulin for glucose uptake (GLUT-1 and GLUT-2 respectively). * **Rate-limiting enzyme of Glycolysis:** Phosphofructokinase-1 (PFK-1), which is induced by insulin via Fructose 2,6-bisphosphate. * **C-peptide** levels can be used to differentiate endogenous insulin production from exogenous insulin injection.

Question 478: Which one of the following human tissues contains the greatest amount of body glycogen?

A. Liver
B. Kidney
C. Skeletal muscle (Correct Answer)
D. Cardiac muscle

Explanation: **Explanation:** The correct answer is **Skeletal muscle**. This question hinges on the distinction between **concentration** (percentage per gram of tissue) and **total content** (total mass in the body). 1. **Why Skeletal Muscle is Correct:** While the liver has a higher *concentration* of glycogen (approx. 5–8% of its weight), the total mass of skeletal muscle in the human body is significantly larger (approx. 40% of body weight). Consequently, skeletal muscle stores about **three-quarters (approx. 400g)** of the body's total glycogen, whereas the liver stores only about **one-quarter (approx. 100g)**. 2. **Why Other Options are Incorrect:** * **Liver:** It has the highest *density/concentration* of glycogen, but its smaller total organ mass means it holds less total glycogen than the muscular system. * **Kidney & Cardiac Muscle:** These tissues store only negligible amounts of glycogen for local, emergency metabolic needs. They do not serve as systemic reservoirs. **High-Yield NEET-PG Pearls:** * **Function:** Liver glycogen maintains **blood glucose levels** during fasting (via Glucose-6-Phosphatase). Muscle glycogen is used **locally** for ATP production during contraction because muscles lack Glucose-6-Phosphatase and cannot release glucose into the blood. * **Regulation:** Muscle glycogen is stimulated by **epinephrine** and calcium ions, while liver glycogen is primarily regulated by **glucagon** and insulin. * **Glycogen Storage Diseases (GSD):** Type I (Von Gierke’s) affects the liver; Type II (Pompe) and Type V (McArdle) primarily affect the muscles.

Question 479: Muscle cannot participate in gluconeogenesis due to the absence of which enzyme?

A. Enolase
B. Glucose 6 phosphatase (Correct Answer)
C. Pyruvate kinase
D. All of the above

Explanation: **Explanation:** The primary goal of gluconeogenesis is to maintain blood glucose levels during fasting. While the liver (and to a lesser extent, the kidney) can perform this task, skeletal muscle cannot. **Why Glucose-6-Phosphatase is the Correct Answer:** Gluconeogenesis involves the synthesis of glucose from non-carbohydrate precursors. The final step of this pathway is the conversion of **Glucose-6-Phosphate to free Glucose**. This reaction is catalyzed by the enzyme **Glucose-6-Phosphatase**. Skeletal muscle lacks this enzyme; therefore, even though muscle can break down glycogen to Glucose-6-Phosphate, it cannot dephosphorylate it to release free glucose into the bloodstream. Instead, the Glucose-6-Phosphate enters the glycolytic pathway to provide energy (ATP) for muscle contraction. **Why the Other Options are Incorrect:** * **Enolase:** This is a reversible enzyme used in both glycolysis and gluconeogenesis. It is present in almost all tissues, including muscle. * **Pyruvate Kinase:** This is a key regulatory enzyme of glycolysis. While it is bypassed during gluconeogenesis (by Pyruvate Carboxylase and PEP Carboxykinase), its presence or absence does not define a tissue's ability to perform gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **The Cori Cycle:** Since muscle cannot release glucose, it exports **Lactate** to the liver. The liver then converts lactate back to glucose via gluconeogenesis and sends it back to the muscle. * **Von Gierke’s Disease (GSD Type I):** This condition is caused by a deficiency of Glucose-6-Phosphatase. It leads to severe fasting hypoglycemia because neither glycogenolysis nor gluconeogenesis can release glucose from the liver. * **Key Gluconeogenic Organs:** Liver (90%) and Kidney (10%). During prolonged starvation, the kidney's contribution increases significantly.

Question 480: Amino sugars are formed from which of the following precursors?

A. Glucose-1-phosphate
B. Glucose-6-phosphate
C. Fructose-1-phosphate
D. Fructose-6-phosphate (Correct Answer)

Explanation: ### Explanation The synthesis of amino sugars (hexosamines) occurs via the **Hexosamine Biosynthetic Pathway (HBP)**. **1. Why Fructose-6-phosphate is correct:** The key rate-limiting step in the formation of amino sugars is the conversion of **Fructose-6-phosphate (F6P)** into **Glucosamine-6-phosphate**. This reaction is catalyzed by the enzyme **Glutamine-fructose-6-phosphate amidotransferase (GFAT)**. In this process, an amino group is transferred from the donor amino acid, **Glutamine**, to Fructose-6-phosphate. Glucosamine-6-phosphate then serves as the precursor for N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GalNAc), and sialic acids, which are essential components of glycoproteins, glycolipids, and glycosaminoglycans (GAGs). **2. Why the other options are incorrect:** * **Glucose-6-phosphate (B):** While G6P is the starting point of glycolysis and the HMP shunt, it must first be isomerized to Fructose-6-phosphate by phosphohexose isomerase before it can enter the hexosamine pathway. * **Glucose-1-phosphate (A):** This is primarily involved in glycogenesis (glycogen synthesis) and uronic acid pathways, not direct amino sugar formation. * **Fructose-1-phosphate (C):** This is an intermediate of fructose metabolism in the liver (via fructokinase) and does not serve as a substrate for GFAT. ### High-Yield Clinical Pearls for NEET-PG: * **Nitrogen Donor:** Glutamine is the obligatory nitrogen donor for amino sugar synthesis. * **Feedback Inhibition:** The end product, UDP-GlcNAc, inhibits the rate-limiting enzyme GFAT. * **Clinical Significance:** The hexosamine pathway is linked to **insulin resistance**. High intracellular glucose levels increase flux through this pathway, leading to the modification of signaling proteins (O-GlcNAcylation), which can impair insulin sensitivity. * **Essential Amino Sugars:** Glucosamine, Galactosamine, and N-acetylneuraminic acid (NANA/Sialic acid).

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

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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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