Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 501: Insulin-dependent glucose transport occurs in all of the following tissues except:

A. Heart
B. Adipose tissues
C. Liver (Correct Answer)
D. Skeletal muscles

Explanation: **Explanation:** The core concept behind this question is the tissue-specific distribution of **Glucose Transporters (GLUT)**. Glucose uptake into cells occurs via facilitated diffusion, mediated by different GLUT isoforms. **Why Liver is the correct answer:** The liver expresses **GLUT-2**, which is an **insulin-independent** transporter. GLUT-2 has a high $K_m$ (low affinity) for glucose, allowing the liver to sense and uptake glucose proportionally to blood glucose levels (e.g., after a meal) without requiring insulin signaling to move the transporter to the cell membrane. This ensures the liver can perform essential functions like gluconeogenesis or glycogenolysis even when insulin levels are low. **Why the other options are incorrect:** * **Skeletal Muscle & Heart (Options A & D):** These tissues primarily utilize **GLUT-4**, which is the only **insulin-dependent** glucose transporter. In the resting state, GLUT-4 is sequestered in intracellular vesicles. Insulin binding to its receptor triggers the translocation of these vesicles to the plasma membrane to allow glucose entry. * **Adipose Tissue (Option B):** Like muscle, adipocytes also rely on **GLUT-4** for glucose uptake to provide the glycerol backbone (glycerol-3-phosphate) required for triglyceride synthesis. **NEET-PG High-Yield Pearls:** * **GLUT-1:** Found in RBCs and the Blood-Brain Barrier (basal uptake). * **GLUT-2:** Found in Liver, Pancreatic beta cells, Kidney, and Small Intestine (Bidirectional). * **GLUT-3:** Found in Neurons (highest affinity/lowest $K_m$). * **GLUT-4:** Found in Skeletal muscle, Heart, and Adipose tissue (**Insulin-dependent**). * **GLUT-5:** Specifically for **Fructose** transport (found in small intestine and spermatozoa). * **SGLT-1/2:** Active transport (Sodium-dependent) found in the intestine and renal tubules.

Question 502: The parent alcohol in carbohydrates is:

A. Glycerol (Correct Answer)
B. Ethanol
C. Methanol
D. Cholesterol

Explanation: **Explanation:** **Why Glycerol is the Correct Answer:** Carbohydrates are chemically defined as **polyhydroxy aldehydes or ketones**, or substances that yield these on hydrolysis. The simplest carbohydrates (monosaccharides) are derived from **Glycerol** ($C_3H_8O_3$), a trihydric alcohol. By oxidizing the hydroxyl groups of glycerol, we obtain the simplest sugars (trioses): 1. Oxidation of the primary alcohol group yields **Glyceraldehyde** (an aldose). 2. Oxidation of the secondary alcohol group yields **Dihydroxyacetone** (a ketose). Because all higher sugars are structurally built upon these three-carbon foundations, glycerol is considered the "parent alcohol" of the carbohydrate family. **Analysis of Incorrect Options:** * **B. Ethanol & C. Methanol:** These are monohydric alcohols (containing only one -OH group). They do not possess the polyhydroxy structure required to form the backbone of a carbohydrate. * **D. Cholesterol:** This is a complex steroid alcohol (sterol). While it is a lipid component of cell membranes, it has no structural relationship to the basic framework of carbohydrates. **NEET-PG High-Yield Pearls:** * **Simplest Sugars:** Glyceraldehyde and Dihydroxyacetone are the smallest possible monosaccharides (Trioses). * **Glycerol-Lipid Link:** Glycerol also serves as the backbone for **Triacylglycerols (TAGs)** and phospholipids, linking carbohydrate metabolism to lipid metabolism via Glycerol-3-Phosphate. * **Gluconeogenesis:** Glycerol released from adipose tissue during lipolysis is a significant non-carbohydrate precursor for glucose synthesis in the liver.

Question 503: In the fasting state, which one of the following is utilized by the liver for gluconeogenesis?

A. Glycerol (Correct Answer)
B. Even-chain fatty acids
C. Liver glycogen
D. Ketone bodies

Explanation: **Explanation:** **1. Why Glycerol is Correct:** Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. During fasting, adipose tissue undergoes lipolysis, breaking down triacylglycerols (TAGs) into free fatty acids and **glycerol**. While fatty acids cannot be converted to glucose, glycerol is transported to the liver. Here, it is phosphorylated by **glycerol kinase** to glycerol-3-phosphate and then oxidized to **dihydroxyacetone phosphate (DHAP)**, a direct intermediate of the gluconeogenic pathway. **2. Why the Other Options are Incorrect:** * **Even-chain fatty acids:** These are oxidized to Acetyl-CoA. In humans, there is no metabolic pathway to convert Acetyl-CoA into pyruvate or oxaloacetate (the Pyruvate Dehydrogenase reaction is irreversible). Thus, even-chain fatty acids cannot serve as a substrate for gluconeogenesis. * **Liver glycogen:** While liver glycogen is broken down to maintain blood glucose during fasting, this process is called **glycogenolysis**, not gluconeogenesis. Gluconeogenesis specifically refers to the de novo synthesis of glucose from non-carbohydrate sources. * **Ketone bodies:** These are alternative fuels produced from Acetyl-CoA during prolonged fasting, but they cannot be reversed back into glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Key Substrates:** The major gluconeogenic precursors are **Lactate** (Cori Cycle), **Glucogenic Amino Acids** (mainly Alanine), and **Glycerol**. * **Odd-chain fatty acids:** Unlike even-chain, these *can* be gluconeogenic because their terminal metabolism yields **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA. * **Enzyme Localization:** Glycerol kinase is primarily present in the **liver and kidneys**, which is why adipose tissue itself cannot reuse glycerol.

Question 504: A child presents with hypoglycaemia, hepatomegaly, and the accumulation of highly branched glycogen, known as limit dextrins. What is the most likely diagnosis?

A. Cori's disease (Correct Answer)
B. Anderson's disease
C. McArdle's disease
D. Von Gierke's disease

Explanation: **Explanation:** The clinical presentation of hypoglycemia, hepatomegaly, and the presence of abnormally structured glycogen with short outer branches (**limit dextrins**) is pathognomonic for **Cori’s disease (GSD Type III)**. 1. **Why Cori’s Disease is Correct:** Cori’s disease is caused by a deficiency of the **Debranching enzyme** (α-1,6-glucosidase and 4-α-glucanotransferase). During glycogenolysis, phosphorylase can only break down glycogen until four glucose residues remain before a branch point. Without the debranching enzyme, the process halts, leading to the accumulation of "limit dextrins." This results in fasting hypoglycemia (as glycogen cannot be fully mobilized) and hepatomegaly. 2. **Why Other Options are Incorrect:** * **Anderson’s Disease (GSD Type IV):** Caused by **Branching enzyme** deficiency. It results in long, unbranched glucose chains (amylopectin-like) which trigger an immune response, leading to infantile cirrhosis and death. * **McArdle’s Disease (GSD Type V):** Caused by **Muscle Phosphorylase** deficiency. It presents with exercise-induced cramps and myoglobinuria, not hypoglycemia or hepatomegaly. * **Von Gierke’s Disease (GSD Type I):** Caused by **Glucose-6-Phosphatase** deficiency. While it features severe hypoglycemia and hepatomegaly, the glycogen structure is **normal**. It is also associated with hyperuricemia and hyperlipidemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **A**nderson’s = **A**bnormal branches (too few); **C**ori’s = **C**omplex branches (too many/limit dextrins). * Cori’s disease (Type III) often presents with milder symptoms than Type I because **gluconeogenesis remains intact**, allowing for some glucose production from amino acids. * Type IIIa involves both liver and muscle; Type IIIb involves only the liver.

Question 505: Which of the following pairs of enzymes is required for the process of gluconeogenesis?

A. Fructose-1,6-bisphosphatase and pyruvate carboxylase (Correct Answer)
B. Glucose-6-phosphatase and phosphofructokinase-1
C. Glucose-6-phosphatase and pyruvate dehydrogenase
D. Phosphoenolpyruvate carboxykinase and glucokinase

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that generates glucose from non-carbohydrate precursors (like lactate, glycerol, and glucogenic amino acids). It is essentially the reverse of glycolysis but must bypass **three irreversible steps** of glycolysis using four specific "bypass enzymes." **1. Why Option A is Correct:** The correct answer includes two of the four key regulatory enzymes of gluconeogenesis: * **Pyruvate Carboxylase:** Converts pyruvate to oxaloacetate (requires Biotin and ATP). This bypasses the Pyruvate Kinase step. * **Fructose-1,6-bisphosphatase (FBPase-1):** Converts Fructose-1,6-bisphosphate to Fructose-6-phosphate. This is the **rate-limiting step** of gluconeogenesis and bypasses Phosphofructokinase-1 (PFK-1). **2. Why Other Options are Incorrect:** * **Option B:** While Glucose-6-phosphatase is a gluconeogenic enzyme, **PFK-1** is a glycolytic enzyme. They act in opposite directions. * **Option C:** **Pyruvate Dehydrogenase (PDH)** converts pyruvate to Acetyl-CoA for the TCA cycle; it is inhibited during gluconeogenesis to prevent the loss of carbon atoms. * **Option D:** **Glucokinase** is a glycolytic enzyme found in the liver that phosphorylates glucose; gluconeogenesis requires its counterpart, Glucose-6-phosphatase, to release free glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Gluconeogenesis occurs mainly in the **Liver** (90%) and Kidney (10%). * **Subcellular Sites:** It is a "mixed" pathway; Pyruvate carboxylase is **mitochondrial**, while the rest are **cytosolic** (except Glucose-6-phosphatase, which is in the **ER**). * **Obligatory Activator:** Acetyl-CoA is an obligatory activator of Pyruvate Carboxylase. * **Von Gierke’s Disease:** Caused by a deficiency of Glucose-6-phosphatase, leading to severe fasting hypoglycemia.

Question 506: Which of the following is a glycogen storage disorder due to deficiency of the myophosphorylase enzyme?

A. McArdle's disease (Correct Answer)
B. Alport syndrome
C. Marfan syndrome
D. Ehlers-Danlos syndrome

Explanation: ### Explanation **Correct Answer: A. McArdle's disease** **1. Why McArdle’s Disease is Correct:** McArdle’s disease, also known as **Glycogen Storage Disease (GSD) Type V**, is caused by a deficiency of **myophosphorylase** (muscle glycogen phosphorylase). This enzyme is responsible for the rate-limiting step of glycogenolysis in skeletal muscle: breaking down glycogen into glucose-1-phosphate. Without it, muscles cannot mobilize glucose during exercise, leading to symptoms like exercise intolerance, muscle cramps, and myoglobinuria. **2. Why the Other Options are Incorrect:** * **B. Alport Syndrome:** A genetic disorder affecting **Type IV collagen**, primarily involving the basement membranes of the kidney (nephritis), eyes, and inner ear (sensorineural hearing loss). * **C. Marfan Syndrome:** A connective tissue disorder caused by a mutation in the **FBN1 gene** (fibrillin-1), characterized by skeletal abnormalities, ectopia lentis, and aortic root dilation. * **D. Ehlers-Danlos Syndrome:** A group of disorders characterized by hyperextensible skin and joint hypermobility, primarily due to defects in **collagen synthesis** (e.g., Type III or V). **3. High-Yield Clinical Pearls for NEET-PG:** * **"Second Wind" Phenomenon:** A classic clinical sign of McArdle’s where patients experience relief from fatigue after a few minutes of exercise as the body switches to using fatty acids and blood glucose. * **Ischemic Forearm Test:** Classically shows a **failure of blood lactate to rise** after exercise (since glycogen cannot be converted to lactate). * **Histology:** Muscle biopsy reveals **subsarcolemmal glycogen blebs** (PAS-positive). * **Contrast with Von Gierke (Type I):** Von Gierke involves *liver* phosphorylase/glucose-6-phosphatase deficiency, leading to hypoglycemia; McArdle’s involves *muscle* and does **not** cause hypoglycemia.

Question 507: What is the mechanism by which pyruvate is transported from the cytosol to the mitochondria?

A. Chloride antiporter
B. Proton symporter (Correct Answer)
C. ATP-dependent antiporter
D. Facilitated uniporter

Explanation: ### Explanation **Mechanism of Pyruvate Transport** Pyruvate is generated in the cytosol via glycolysis. However, its oxidative decarboxylation into Acetyl-CoA (by the Pyruvate Dehydrogenase Complex) occurs within the mitochondrial matrix. To cross the inner mitochondrial membrane (IMM), which is impermeable to polar molecules, pyruvate utilizes a specific transport protein called the **Mitochondrial Pyruvate Carrier (MPC)**. The transport is a **Proton (H⁺) Symporter** mechanism. Pyruvate is co-transported with a proton into the matrix, driven by the electrochemical gradient (proton motive force) generated by the electron transport chain. This ensures that pyruvate moves from the cytosol into the mitochondria effectively to fuel the TCA cycle. **Analysis of Incorrect Options:** * **A. Chloride antiporter:** Chloride antiporters (like the Band 3 protein/Bicarbonate-Chloride exchanger) are primarily involved in gas exchange in RBCs (Chloride shift), not organic acid transport in mitochondria. * **C. ATP-dependent antiporter:** While some mitochondrial transporters (like the Adenine Nucleotide Translocase) exchange molecules, pyruvate transport is driven by the proton gradient, not direct ATP hydrolysis. * **D. Facilitated uniporter:** Pyruvate does not move down its own concentration gradient alone; it requires the symport of a proton to overcome the electrochemical barrier of the IMM. **High-Yield Clinical Pearls for NEET-PG:** * **MPC Inhibition:** Thiazolidinediones (TZDs), used in Type 2 Diabetes, have been shown to modulate the Mitochondrial Pyruvate Carrier. * **Lactic Acidosis:** If pyruvate transport or the PDH complex is defective, pyruvate is converted to lactate in the cytosol, leading to metabolic acidosis. * **Outer vs. Inner Membrane:** Pyruvate crosses the *outer* mitochondrial membrane freely through voltage-dependent anion channels (VDACs/porins), but requires the *MPC symporter* for the *inner* membrane.

Question 508: Which of the following is associated with the malate shuttle?

A. Glycolysis (Correct Answer)
B. Gluconeogenesis
C. Glycogenolysis
D. Ketone body synthesis

Explanation: **Explanation:** The **Malate-Aspartate Shuttle** is a crucial biochemical mechanism used to transport reducing equivalents from the cytosol into the mitochondrial matrix. **1. Why Glycolysis is Correct:** During aerobic **glycolysis**, the enzyme Glyceraldehyde-3-phosphate dehydrogenase produces **NADH** in the cytosol. However, the inner mitochondrial membrane is impermeable to NADH. To enter the Electron Transport Chain (ETC) and generate ATP, the electrons from cytosolic NADH are transferred to oxaloacetate to form **malate**. Malate then crosses into the mitochondria, where it is converted back to oxaloacetate, regenerating NADH for oxidative phosphorylation. This shuttle is primarily active in the heart, liver, and kidneys. **2. Why Other Options are Incorrect:** * **Gluconeogenesis:** While malate is involved here (transporting oxaloacetate out of the mitochondria), the "Malate Shuttle" specifically refers to the cyclic process of moving reducing equivalents for ATP production, which is a hallmark of the terminal phase of the glycolytic pathway. * **Glycogenolysis:** This is the breakdown of glycogen into glucose-1-phosphate; it occurs in the cytosol and does not directly utilize the malate shuttle. * **Ketone body synthesis:** This occurs primarily within the mitochondrial matrix of liver cells and does not require the transport of cytosolic NADH via this shuttle. **Clinical Pearls & High-Yield Facts:** * **ATP Yield:** The Malate-Aspartate shuttle is more efficient than the Glycerol-3-phosphate shuttle (found in muscle/brain), yielding **2.5 ATP** per NADH instead of 1.5 ATP. * **Key Enzymes:** Malate Dehydrogenase (MDH) and Aspartate Aminotransferase (AST). * **Transamination:** The shuttle requires glutamate and alpha-ketoglutarate to facilitate the conversion of oxaloacetate to aspartate for the return trip to the cytosol.

Question 509: Which of the following statements is not true regarding glycoproteins?

A. They are proteins to which oligosaccharides are covalently attached.
B. The carbohydrate units of glycoproteins often contain repeating sequences.
C. Carbohydrate content is the same, but the proteins are different. (Correct Answer)
D. All of the above.

Explanation: ### Explanation **1. Why Option C is the Correct Answer (The "Not True" Statement)** In glycoproteins, the carbohydrate content is **highly variable**, ranging from less than 1% to over 85% of the total weight. There is no fixed ratio; both the protein structure and the carbohydrate composition (length, branching, and sugar types) differ significantly between different glycoproteins (e.g., Collagen vs. Erythropoietin). Therefore, the statement that "carbohydrate content is the same" is biologically incorrect. **2. Analysis of Other Options** * **Option A (True):** Glycoproteins are defined by the **covalent attachment** of oligosaccharide chains to a protein backbone. These are typically linked via N-glycosidic bonds (to Asparagine) or O-glycosidic bonds (to Serine/Threonine). * **Option B (True):** Unlike Proteoglycans, which contain long, unbranched GAG chains with strictly repeating disaccharides, the carbohydrate units in glycoproteins are often shorter and branched. However, they can contain specific **repeating sequences** or motifs (like the "core" pentasaccharide in N-linked glycans) that are essential for molecular recognition. **3. High-Yield NEET-PG Clinical Pearls** * **Glycoproteins vs. Proteoglycans:** Glycoproteins are primarily protein by weight and lack the long, uronic acid-containing GAG chains found in proteoglycans. * **Functions:** They serve as cell surface receptors, blood group antigens (ABO system), hormones (HCG, TSH, FSH), and plasma proteins (except Albumin). * **I-Cell Disease:** A high-yield clinical correlate where a defect in the phosphorylation of mannose residues on glycoproteins leads to the failure of lysosomal enzyme targeting. * **Key Linkages:** * **N-linked:** Attached to **Asparagine** (occurs in ER). * **O-linked:** Attached to **Serine/Threonine** (occurs in Golgi).

Question 510: What are the primary products generated by colonic bacteria during the digestion of dietary fibers?

A. Free radicals
B. Glycerol
C. Butyrate (Correct Answer)
D. Butyrate

Explanation: **Explanation:** Dietary fibers (non-starch polysaccharides like cellulose and pectin) cannot be digested by human enzymes in the small intestine. When they reach the large intestine, they undergo **anaerobic fermentation** by commensal colonic bacteria. **1. Why Butyrate is Correct:** The primary products of this fermentation are **Short-Chain Fatty Acids (SCFAs)**, specifically **Acetate (2C), Propionate (3C), and Butyrate (4C)**. Among these, **Butyrate** is of significant clinical importance as it serves as the primary energy source for colonocytes (epithelial cells of the colon). It promotes mucosal integrity and has anti-inflammatory and anti-cancer properties. **2. Why Other Options are Incorrect:** * **Free Radicals:** These are unstable molecules produced during oxidative stress. Bacterial fermentation is a reductive process aimed at energy production, not the generation of damaging free radicals. * **Glycerol:** This is a backbone of triglycerides and is released during lipolysis (breakdown of fats), not the fermentation of dietary fibers. * **Note on Options C & D:** Both options list Butyrate; in a standard exam format, this confirms the focus on SCFAs as the key metabolic byproduct. **Clinical Pearls for NEET-PG:** * **Acetate** is the most abundant SCFA and is used by peripheral tissues (muscle/brain) for energy. * **Propionate** is primarily taken up by the liver for **gluconeogenesis**. * **High-Yield Fact:** SCFAs lower the colonic pH, which inhibits the growth of pathogenic bacteria and enhances the absorption of minerals like calcium and magnesium. * **Prebiotics** are the non-digestible fibers themselves, while **Probiotics** are the live beneficial bacteria that perform this fermentation.

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

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free