Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 121: Synthesis of 1 molecule of glucose from 2 molecules of lactate requires how many ATP molecules?

A. 2
B. 4
C. 6 (Correct Answer)
D. 8

Explanation: **Explanation:** The synthesis of glucose from non-carbohydrate precursors (like lactate) is known as **Gluconeogenesis**. This process is not a simple reversal of glycolysis; it bypasses three irreversible steps of glycolysis using four specific enzymes, requiring a significant input of energy. **Why 6 ATP is the correct answer:** To convert 2 molecules of lactate into 1 molecule of glucose, the following energy-consuming steps occur (per glucose molecule): 1. **Pyruvate to Oxaloacetate:** 2 ATP are consumed (catalyzed by Pyruvate Carboxylase). 2. **Oxaloacetate to Phosphoenolpyruvate (PEP):** 2 GTP (equivalent to 2 ATP) are consumed (catalyzed by PEP Carboxykinase). 3. **3-Phosphoglycerate to 1,3-Bisphosphoglycerate:** 2 ATP are consumed (catalyzed by Phosphoglycerate Kinase). **Total Energy Cost: 4 ATP + 2 GTP = 6 High-energy phosphate bonds.** **Analysis of Incorrect Options:** * **Option A (2 ATP):** This is the net *gain* of ATP during anaerobic glycolysis. Gluconeogenesis is an anabolic process and requires much more energy. * **Option B (4 ATP):** This accounts only for the bypass steps converting pyruvate to PEP (2 ATP + 2 GTP) but misses the energy required for the later stages of the pathway. * **Option D (8 ATP):** This is an overestimation. While 2 NADH are also required, they are used for reduction, not as direct phosphate donors in the stoichiometry of the pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Cori Cycle:** This describes the metabolic pathway where lactate produced by anaerobic glycolysis in muscles moves to the liver and is converted back to glucose via gluconeogenesis. * **Key Regulatory Enzyme:** Fructose-1,6-bisphosphatase is the rate-limiting step of gluconeogenesis. * **Biotin Dependency:** Pyruvate carboxylase requires Biotin (Vitamin B7) as a cofactor. * **Energy Deficit:** Gluconeogenesis is "expensive." The body spends 6 ATP to recover a glucose molecule that only yields 2 ATP during anaerobic metabolism.

Question 122: GLP-1 is broken down by which enzyme?

A. α-glucosidase
B. Rasburicase
C. Dipeptidyl peptidase (Correct Answer)
D. Glucose 1 phosphatase

Explanation: **Explanation:** **Correct Answer: C. Dipeptidyl peptidase (DPP-4)** GLP-1 (Glucagon-like peptide-1) is an **incretin hormone** secreted by the L-cells of the intestine in response to food intake. It stimulates insulin secretion, inhibits glucagon release, and slows gastric emptying. However, endogenous GLP-1 has a very short half-life (1–2 minutes) because it is rapidly degraded by the enzyme **Dipeptidyl peptidase-4 (DPP-4)**. DPP-4 cleaves the two N-terminal amino acids from the GLP-1 peptide, rendering it inactive. This physiological process is the basis for **DPP-4 inhibitors** (e.g., Sitagliptin, Vildagliptin) used in Type 2 Diabetes to prolong GLP-1 action. **Analysis of Incorrect Options:** * **A. α-glucosidase:** This enzyme is located in the intestinal brush border and breaks down complex carbohydrates into glucose. Inhibitors like Acarbose act here, not on GLP-1. * **B. Rasburicase:** This is a recombinant urate oxidase enzyme used to treat hyperuricemia (Tumor Lysis Syndrome) by converting uric acid to allantoin. * **D. Glucose 1 phosphatase:** This enzyme is involved in glycogen metabolism (converting Glucose-1-P to Glucose), not in the degradation of peptide hormones. **High-Yield Clinical Pearls for NEET-PG:** * **Incretin Effect:** Oral glucose causes a much higher insulin response than intravenous glucose due to GLP-1 and GIP release. * **GLP-1 Agonists (Exenatide, Liraglutide):** These are "DPP-4 resistant" synthetic analogs used for weight loss and diabetes. * **DPP-4 Inhibitors (Gliptins):** These are weight-neutral drugs that increase endogenous GLP-1 levels. * **Key GLP-1 Action:** It acts via G-protein coupled receptors (GPCR) to increase cAMP in pancreatic beta cells.

Question 123: Glucose on reduction with sodium amalgam forms which of the following?

A. Dulcitol
B. Sorbitol (Correct Answer)
C. Mannitol
D. Mannitol and sorbitol

Explanation: **Explanation:** **Underlying Concept:** Monosaccharides undergo reduction of their aldehyde or ketone groups to form polyhydroxy alcohols known as **sugar alcohols (polyols)**. Glucose is an aldohexose with an aldehyde group at the C1 position. When treated with reducing agents like **sodium amalgam (Na-Hg)** or sodium borohydride, the aldehyde group (-CHO) of glucose is reduced to a primary alcohol group (-CH2OH), resulting in the formation of **Sorbitol** (also known as D-glucitol). **Analysis of Options:** * **Sorbitol (Correct):** This is the specific polyol produced by the reduction of glucose. * **Dulcitol (Incorrect):** Also known as Galactitol, this is the sugar alcohol formed by the reduction of **Galactose**. * **Mannitol (Incorrect):** This is the sugar alcohol formed by the reduction of **Mannose**. * **Mannitol and Sorbitol (Incorrect):** This mixture is formed by the reduction of **Fructose**. Since Fructose is a ketose (C2=O), reduction creates a new chiral center at C2, yielding two epimeric alcohols: Sorbitol and Mannitol. **Clinical Pearls for NEET-PG:** 1. **Sorbitol & Cataracts:** In diabetes mellitus, excess glucose enters the **Polyol Pathway**. Aldose reductase converts glucose to sorbitol. Because sorbitol is osmotically active and cannot easily cross cell membranes, it accumulates in the lens, leading to osmotic swelling and **diabetic cataracts**. 2. **Mannitol:** Used clinically as an **osmotic diuretic** to reduce intracranial pressure. 3. **Essential Fructosuria:** A deficiency in fructokinase leads to fructose appearing in urine; it is a benign condition compared to Hereditary Fructose Intolerance.

Question 124: In muscle, phosphorylase b is inactivated by:

A. cAMP
B. Ca ions
C. Glucose
D. ATP (Correct Answer)

Explanation: **Explanation:** Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis. In muscle, it exists in two forms: **Phosphorylase *a*** (active, phosphorylated) and **Phosphorylase *b*** (less active, dephosphorylated). **Why ATP is the correct answer:** Muscle glycogenolysis is primarily regulated by the energy status of the cell. Phosphorylase *b* is subject to **allosteric regulation**. When the cell has high energy levels, **ATP** and **Glucose-6-Phosphate** act as allosteric inhibitors, binding to the enzyme and stabilizing its inactive state. This prevents unnecessary breakdown of glycogen when energy is abundant. **Analysis of Incorrect Options:** * **A. cAMP:** This acts as a second messenger that activates Protein Kinase A (PKA), which eventually leads to the *activation* (phosphorylation) of phosphorylase *b* into phosphorylase *a*. * **B. Ca ions:** During muscle contraction, Ca²⁺ levels rise. Calcium binds to the calmodulin subunit of Phosphorylase Kinase, *activating* it, which in turn activates phosphorylase. * **C. Glucose:** While glucose is a potent allosteric inhibitor of glycogen phosphorylase in the **liver**, it is not a significant regulator in the **muscle**. Muscle cells lack Glucose-6-Phosphatase and do not regulate blood glucose levels. **High-Yield Facts for NEET-PG:** 1. **AMP** is the potent allosteric **activator** of muscle phosphorylase *b* (signaling low energy). 2. **Covalent modification:** Phosphorylation (via Phosphorylase Kinase) converts the *b* form to the *a* form. 3. **McArdle Disease (GSD Type V):** Caused by a deficiency of muscle glycogen phosphorylase, leading to exercise intolerance and myoglobinuria.

Question 125: What is invert sugar?

A. An equimolar mixture of glucose and fructose (Correct Answer)
B. An equimolar mixture of sucrose and glucose
C. An equimolar mixture of glucose and lactose
D. An equimolar mixture of glucose and galactose

Explanation: **Explanation:** **Invert sugar** is an equimolar mixture of **D-glucose and D-fructose**. It is produced by the hydrolysis of sucrose (table sugar) by the enzyme **sucrase (invertase)** or by dilute acids. **Why the correct answer is right:** The term "invert" refers to the change in the **optical rotation** of the solution. * **Sucrose** is dextrorotatory ($+66.5^\circ$). * Upon hydrolysis, it yields equal parts glucose ($+52.7^\circ$) and fructose ($-92.4^\circ$). * Because the levorotatory (left-turning) power of fructose is much stronger than the dextrorotatory (right-turning) power of glucose, the overall net rotation of the mixture becomes **levorotatory**. This "inversion" from $(+)$ to $(-)$ gives the mixture its name. **Why the incorrect options are wrong:** * **Option B:** Sucrose is the parent disaccharide; it is not a component of the hydrolyzed mixture. * **Option C & D:** Lactose is a disaccharide of glucose and galactose. Hydrolysis of lactose yields these two, but this mixture is not called invert sugar and does not exhibit the specific optical inversion characteristic of sucrose hydrolysis. **High-Yield Facts for NEET-PG:** 1. **Sweetness:** Invert sugar is significantly sweeter than sucrose because free fructose is the sweetest naturally occurring sugar. 2. **Honey:** Honey is a natural source of invert sugar, containing high concentrations of free glucose and fructose. 3. **Specific Rotation:** Remember the values for the exam: Sucrose ($+66.5^\circ$), Glucose ($+52.7^\circ$), and Fructose ($-92.4^\circ$). 4. **Seliwanoff’s Test:** Invert sugar will give a positive result (cherry red color) due to the presence of fructose (a ketose).

Question 126: What is the glycemic index for glucose?

A. 0.5
B. 1 (Correct Answer)
C. 1.5
D. 2

Explanation: **Explanation:** The **Glycemic Index (GI)** is a numerical scale (0–100) used to measure how quickly a carbohydrate-containing food raises blood glucose levels compared to a reference food. In the standard GI scale, **pure glucose** is used as the universal reference standard and is assigned a value of **100** (or **1.0** when expressed as a ratio). 1. **Why Option B is Correct:** By definition, glucose is the benchmark against which all other foods are measured because it requires no digestion and is absorbed directly into the bloodstream, causing the most rapid rise in blood sugar. Therefore, its relative value is **1 (or 100%)**. 2. **Why Options A, C, and D are Incorrect:** * **0.5 (50):** This represents a "Low GI" food (e.g., legumes, whole grains). * **1.5 and 2:** These values are mathematically impossible on the standard GI scale, as no food is typically expected to raise blood glucose faster than pure intravenous-equivalent oral glucose. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Reference Standards:** While glucose (GI = 100) is the most common reference, some scales use **white bread** as the standard. * **Classification:** * **Low GI:** ≤ 55 (e.g., pulses, milk, apples). * **Medium GI:** 56–69 (e.g., sucrose, basmati rice). * **High GI:** ≥ 70 (e.g., white bread, honey, potatoes). * **Glycemic Load (GL):** A more accurate clinical predictor than GI, as it accounts for the **quantity** of carbohydrates in a typical serving (GL = GI × Net Carbs / 100). * **Factors lowering GI:** High fiber content, presence of fats/proteins, and less processing (e.g., whole grains vs. flour).

Question 127: Which shuttle system transports substrates from the cytoplasm to the mitochondria?

A. Glycerophosphate shuttle
B. Malate shuttle (Correct Answer)
C. Phosphoenol pyruvate
D. Oxaloacetate

Explanation: **Explanation:** The inner mitochondrial membrane is impermeable to NADH produced during glycolysis in the cytoplasm. To utilize these reducing equivalents for ATP production in the Electron Transport Chain (ETC), shuttle systems are required. **1. Why the Malate-Aspartate Shuttle is Correct:** The **Malate Shuttle** is the most efficient system, primarily active in the heart, liver, and kidneys. In the cytosol, Oxaloacetate is reduced to **Malate** by cytosolic Malate Dehydrogenase, consuming NADH. Malate then crosses the mitochondrial membrane via a specific transporter. Inside the mitochondria, Malate is oxidized back to Oxaloacetate, regenerating **NADH**. This NADH enters Complex I of the ETC, yielding approximately **2.5 ATP** per molecule. **2. Analysis of Incorrect Options:** * **Glycerophosphate Shuttle:** While this also transports reducing equivalents, it transfers electrons to FAD (forming FADH₂), which enters the ETC at Complex II, yielding only **1.5 ATP**. While it is a shuttle, the Malate shuttle is the classic answer for substrate transport involving the malate-aspartate cycle. * **Phosphoenolpyruvate (PEP):** This is an intermediate in glycolysis and gluconeogenesis, not a shuttle system for reducing equivalents. * **Oxaloacetate:** Although involved in the Malate shuttle, Oxaloacetate **cannot** directly cross the inner mitochondrial membrane; it must be converted to Malate or Aspartate to move between compartments. **Clinical Pearls for NEET-PG:** * **ATP Yield:** Malate shuttle = 32 ATP per glucose; Glycerophosphate shuttle = 30 ATP per glucose. * **Tissue Specificity:** Glycerophosphate shuttle is predominant in the **brain and skeletal muscle**. * **Key Enzymes:** Malate shuttle requires Malate Dehydrogenase and Aspartate Aminotransferase (AST).

Question 128: Which of the following is NOT a heteropolysaccharide?

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

Explanation: **Explanation:** The distinction between polysaccharides is based on their monomeric composition. **Inulin** is a **homopolysaccharide**, specifically a polymer of **fructose** (fructosan) linked by β(2→1) glycosidic bonds. Because it consists of only one type of monosaccharide unit, it is not a heteropolysaccharide. **Why the other options are incorrect:** Options A, B, and C are all examples of **Glycosaminoglycans (GAGs)**, which are classic **heteropolysaccharides**. They are composed of repeating disaccharide units consisting of an amino sugar (e.g., glucosamine) and a uronic acid (e.g., glucuronic acid). * **Hyaluronic acid:** Composed of D-glucuronic acid and N-acetyl-D-glucosamine. It is unique among GAGs as it is non-sulfated and not protein-bound. * **Chondroitin sulfate:** The most abundant GAG in the body, found in cartilage and bone. * **Heparan sulfate:** Found on cell surfaces and in the extracellular matrix; it contains higher levels of acetylated glucosamine compared to heparin. **High-Yield Clinical Pearls for NEET-PG:** * **Inulin Clearance:** Inulin is the "Gold Standard" for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered by the glomeruli but neither reabsorbed nor secreted by the renal tubules. * **Storage forms:** Starch (plants) and Glycogen (animals) are homopolysaccharides of glucose. * **Dextran vs. Dextrin:** Dextran is a branched glucose polymer used as a plasma volume expander; Dextrin is an intermediate product of starch hydrolysis. * **Agar:** A heteropolysaccharide derived from seaweed, used in laboratories as a culture medium.

Question 129: Glucose diffusion in RBC occurs via which transporter?

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

Explanation: **Explanation:** Glucose enters the Red Blood Cell (RBC) via **facilitated diffusion**, a process mediated by specific carrier proteins known as Glucose Transporters (GLUT). **1. Why GLUT 1 is correct:** GLUT 1 is the primary glucose transporter found in **RBCs** and the **Blood-Brain Barrier**. It is a high-affinity transporter (low $K_m$), ensuring a constant basal uptake of glucose regardless of blood sugar levels. Since RBCs lack mitochondria and rely exclusively on anaerobic glycolysis for energy, a steady supply of glucose via GLUT 1 is vital for their survival. **2. Why the other options are incorrect:** * **GLUT 2:** Found in the **Liver, Pancreas (beta cells), and Kidneys**. It has a high $K_m$ (low affinity), acting as a "glucose sensor" that functions primarily when blood glucose levels are high. * **GLUT 3:** Primarily located in **Neurons**. Like GLUT 1, it has a very high affinity to ensure the brain receives glucose even during hypoglycemia. * **GLUT 4:** Found in **Skeletal Muscle and Adipose Tissue**. It is the only **insulin-dependent** transporter. In the absence of insulin, these transporters are sequestered inside the cell. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT 5:** Unique because it is a **Fructose** transporter (found in the small intestine and spermatozoa). * **SGLT 1/2:** These are Sodium-Glucose Co-transporters (Active Transport) found in the small intestine and renal tubules, unlike the GLUT family which uses facilitated diffusion. * **De Vivo Disease:** Caused by a deficiency in GLUT 1, leading to infantile seizures and developmental delay due to low glucose levels in the CSF.

Question 130: What is the net number of ATP molecules produced during anaerobic glycolysis?

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

Explanation: **Explanation:** The net ATP yield of glycolysis depends on the starting substrate. In this specific question, the correct answer is **3 ATP**, which refers to anaerobic glycolysis starting from **Glycogen** (via glycogenolysis). 1. **Why Option C is Correct:** When glycolysis begins with **Glycogen**, the enzyme *Glycogen Phosphorylase* releases Glucose-1-Phosphate, which is converted to Glucose-6-Phosphate without consuming ATP. Therefore, only **1 ATP** is invested (at the Phosphofructokinase-1 step). Since the payoff phase generates **4 ATP**, the net yield is **4 - 1 = 3 ATP**. In anaerobic conditions, the NADH produced is used to reduce pyruvate to lactate, resulting in no additional mitochondrial ATP. 2. **Why Other Options are Incorrect:** * **Option B (2 ATP):** This is the net yield if the starting substrate is **free Glucose**. Here, 2 ATPs are invested (Hexokinase and PFK-1 steps), and 4 are produced, resulting in **4 - 2 = 2 ATP**. * **Option A & D:** These do not represent the net stoichiometry of any standard glycolytic pathway. 4 ATP is the *gross* yield, not the net yield. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme of glycolysis. * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH reaction, resulting in **zero net ATP** production even if glycolysis continues. * **Mature RBCs:** Since they lack mitochondria, RBCs always derive a net of **2 ATP** per glucose molecule via anaerobic glycolysis. * **Rapoport-Luebering Cycle:** In RBCs, 2,3-BPG is produced, bypassing the phosphoglycerate kinase step, which further reduces the net ATP yield to zero for that specific shunt.

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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