Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 401: α-D-glucose and β-D-glucose are related as:

A. Stereoisomers
B. Epimers
C. Anomers (Correct Answer)
D. Keto-aldo pairs

Explanation: **Explanation:** **Why Anomers is the correct answer:** Anomers are a specific type of stereoisomer found in cyclic sugars. When glucose forms a ring structure (pyranose), the carbonyl carbon (C1) becomes a new chiral center, known as the **anomeric carbon**. The difference between α-D-glucose and β-D-glucose lies solely in the orientation of the hydroxyl (-OH) group at this C1 position. In the α-form, the -OH is below the plane of the ring (trans to the CH₂OH group), while in the β-form, it is above the plane (cis to the CH₂OH group). **Analysis of Incorrect Options:** * **Stereoisomers:** While anomers are technically a subset of stereoisomers, "Anomers" is the most specific and accurate term. In NEET-PG, always choose the most specific classification. * **Epimers:** These are isomers that differ at only one chiral center *other than* the anomeric carbon. For example, Glucose and Galactose are C4 epimers; Glucose and Mannose are C2 epimers. * **Keto-aldo pairs:** These are functional isomers. Glucose is an aldose (aldehyde group), while Fructose is a ketose (ketone group). **High-Yield Clinical Pearls for NEET-PG:** * **Mutarotation:** This is the change in specific optical rotation when α and β anomers are interconverted in an aqueous solution until an equilibrium is reached (approx. 36% α and 64% β). * **Reducing Sugars:** All monosaccharides (including both anomers of glucose) are reducing sugars because the cyclic form can open back into a free aldehyde chain to reduce Benedict’s or Fehling’s reagent. * **Starch vs. Cellulose:** Humans can digest α-1,4-glycosidic bonds (starch/glycogen) but lack the enzyme (cellulase) to break β-1,4-glycosidic bonds (cellulose).

Question 402: Where does gluconeogenesis primarily take place?

A. Liver (Correct Answer)
B. Red Blood Cells (RBCs)
C. Adipocytes
D. Myocytes

Explanation: **Explanation:** **1. Why Liver is the Correct Answer:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors (like lactate, glycerol, and glucogenic amino acids). The **liver** is the primary site, accounting for approximately **90%** of glucose production during overnight fasting. This is because the liver possesses a complete complement of the four key regulatory enzymes: Pyruvate carboxylase, PEP carboxykinase, Fructose-1,6-bisphosphatase, and **Glucose-6-phosphatase**. The latter is essential for releasing free glucose into the bloodstream to maintain glycemic levels. **2. Why Other Options are Incorrect:** * **Red Blood Cells (RBCs):** RBCs lack mitochondria. Since the initial steps of gluconeogenesis (pyruvate to oxaloacetate) occur in the mitochondria, RBCs cannot perform this process. Instead, they are major *producers* of lactate, a substrate for gluconeogenesis. * **Adipocytes:** These cells focus on lipogenesis and lipolysis. While they provide glycerol (a substrate), they lack the enzymatic machinery to convert it back to glucose. * **Myocytes (Muscle):** Muscles lack **Glucose-6-phosphatase**. Therefore, even though they can synthesize glycogen, they cannot release free glucose into the blood; they use glucose-6-phosphate internally for energy. **3. NEET-PG High-Yield Pearls:** * **Secondary Site:** The **Kidney cortex** is the secondary site of gluconeogenesis (contributing ~10%, increasing up to 40% during prolonged starvation). * **Small Intestine:** Recent evidence suggests the small intestine also possesses gluconeogenic activity during fasting. * **Key Substrates:** Lactate (Cori Cycle), Alanine (Cahill Cycle), and Glycerol. * **Hormonal Control:** Stimulated by Glucagon and Glucocorticoids; inhibited by Insulin.

Question 403: Which of the following occurs during glycolysis?

A. Glucose is oxidized to pyruvate in the cytosol of all cells.
B. The rate-limiting step is the formation of glucose-6-phosphate.
C. Glucose is phosphorylated by hexokinase.
D. Pyruvate, NADH, and ATP are produced. (Correct Answer)

Explanation: **Explanation:** **Why Option D is Correct:** Glycolysis (the Embden-Meyerhof pathway) is the primary metabolic pathway that breaks down one molecule of glucose into two molecules of **pyruvate**. This process occurs in the cytosol and results in a net gain of **2 ATP** (via substrate-level phosphorylation) and **2 NADH** (via the oxidation of glyceraldehyde-3-phosphate). These products are essential for cellular energy production and downstream aerobic respiration. **Analysis of Incorrect Options:** * **Option A:** While glycolysis occurs in the cytosol, it does not always end in pyruvate. In cells lacking mitochondria (e.g., **mature RBCs**) or under anaerobic conditions (e.g., exercising muscle), pyruvate is reduced to **lactate** to regenerate NAD+. * **Option B:** The rate-limiting and committed step of glycolysis is the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate, catalyzed by **Phosphofructokinase-1 (PFK-1)**, not the formation of glucose-6-phosphate. * **Option C:** While glucose is indeed phosphorylated by hexokinase (or glucokinase), this is a single step within the pathway, not a summary of what "occurs during glycolysis" as a whole. Option D provides a more comprehensive description of the pathway's output. **High-Yield Clinical Pearls for NEET-PG:** * **Key Regulatory Enzymes:** Hexokinase/Glucokinase, PFK-1 (Rate-limiting), and Pyruvate Kinase. * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to **2,3-BPG**, which shifts the oxygen dissociation curve to the right (facilitating O2 unloading). * **Arsenic Poisoning:** Inhibits the net ATP gain in glycolysis by competing with inorganic phosphate at the glyceraldehyde-3-phosphate dehydrogenase step. * **Fluoride:** Inhibits the enzyme **Enolase**; this is why fluoride is added to blood collection tubes (grey top) to prevent glycolysis during glucose estimation.

Question 404: What is the major contributor to gluconeogenesis?

A. Lactate (Correct Answer)
B. Glutamate
C. Ketones
D. Alanine

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that generates glucose from non-carbohydrate precursors during periods of fasting or intense exercise. **Why Lactate is the correct answer:** Lactate is considered the **major quantitative contributor** to gluconeogenesis. It is primarily produced by anaerobic glycolysis in skeletal muscles and red blood cells. Through the **Cori Cycle**, lactate is transported to the liver, where it is converted back into pyruvate by lactate dehydrogenase (LDH) and subsequently into glucose. This cycle is essential for maintaining blood glucose levels and recycling lactate to prevent lactic acidosis. **Analysis of Incorrect Options:** * **Glutamate:** While glutamate is a glucogenic amino acid (entering the TCA cycle as $\alpha$-ketoglutarate), it is not the primary precursor compared to the sheer volume of lactate produced daily. * **Ketones:** Ketone bodies (acetoacetate and $\beta$-hydroxybutyrate) **cannot** be converted into glucose in humans because the reaction catalyzed by pyruvate dehydrogenase is irreversible; acetyl-CoA cannot be converted back to pyruvate. * **Alanine:** Alanine is the most important **amino acid** precursor for gluconeogenesis (via the Glucose-Alanine cycle), but in terms of total daily flux, lactate contributes a higher percentage of the glucose pool. **NEET-PG High-Yield Pearls:** * **Major Precursors:** Lactate > Glycerol > Alanine. * **Rate-limiting enzyme:** Fructose-1,6-bisphosphatase. * **Location:** Primarily the liver (90%), followed by the kidney cortex (10%). * **Key distinction:** While Alanine is the primary precursor during **prolonged fasting**, Lactate remains the overall major contributor under normal physiological conditions.

Question 405: Which of the following is not a product of the glycolytic pathway?

A. Fructose 2,6 bisphosphate
B. Fructose 1,6 bisphosphate
C. Glyceraldehyde-3-phosphate
D. None of the above (Correct Answer)

Explanation: ### Explanation The correct answer is **None of the above** because all the molecules listed in options A, B, and C are either direct intermediates or crucial regulatory bypass products of the glycolytic pathway. **1. Why "None of the above" is correct:** In the context of biochemistry, a "product" of a pathway includes its intermediates. * **Fructose 1,6-bisphosphate (F1,6BP):** Produced by the action of Phosphofructokinase-1 (PFK-1) on Fructose 6-phosphate. This is the "committed step" of glycolysis. * **Glyceraldehyde-3-phosphate (G3P):** Formed when Aldolase A cleaves F1,6BP into two trioses (G3P and DHAP). * **Fructose 2,6-bisphosphate (F2,6BP):** While not a direct intermediate in the main flow toward pyruvate, it is synthesized from Fructose 6-phosphate by the bifunctional enzyme PFK-2. It is considered a product of the glycolytic machinery and serves as the **most potent allosteric activator** of PFK-1. **2. Analysis of Incorrect Options:** * **Option B & C:** These are standard intermediates found in the Embden-Meyerhof pathway. Excluding them would be factually incorrect. * **Option A:** Students often mistake F2,6BP as purely a regulatory molecule; however, it is synthesized within the cell specifically to drive glycolysis forward by overriding the inhibitory effects of ATP. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** PFK-1 is the key regulatory enzyme of glycolysis. * **PFK-2/FBPase-2:** This bifunctional enzyme is regulated by covalent modification. In the **fed state** (insulin), it is dephosphorylated, activating the PFK-2 kinase domain to produce F2,6BP, which stimulates glycolysis. * **Rapoport-Luebering Shunt:** In RBCs, the glycolytic intermediate 1,3-BPG can be converted to **2,3-BPG**, which decreases hemoglobin's affinity for oxygen (shifting the dissociation curve to the right).

Question 406: Muscle glycolysis is increased by which of the following substances?

A. Epinephrine (Correct Answer)
B. Acetylcholine
C. Histamine
D. Serotonin

Explanation: **Explanation:** **1. Why Epinephrine is Correct:** In skeletal muscle, epinephrine (adrenaline) acts as a potent stimulator of glycolysis during the "fight or flight" response. It binds to **$\beta_2$-adrenergic receptors**, leading to an increase in intracellular cAMP. This triggers a signaling cascade that activates **Glycogen Phosphorylase**, accelerating glycogenolysis to provide Glucose-1-Phosphate. Crucially, in muscle, the increased cAMP and calcium levels (from contraction) activate **Phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis. Unlike the liver, where epinephrine inhibits glycolysis to save glucose for the brain, in muscle, it promotes glycolysis to generate immediate ATP for contraction. **2. Why the Other Options are Incorrect:** * **B. Acetylcholine:** This is the primary neurotransmitter at the neuromuscular junction responsible for muscle contraction. While contraction indirectly increases glycolysis (via $Ca^{2+}$ ions), acetylcholine itself does not directly regulate the glycolytic enzymatic pathway. * **C. Histamine:** Primarily involved in inflammatory responses, vasodilation, and gastric acid secretion; it has no direct regulatory role in skeletal muscle carbohydrate metabolism. * **D. Serotonin:** Acts mainly as a neurotransmitter in the CNS and a local hormone in the GI tract; it does not modulate muscle glycolytic flux. **3. High-Yield NEET-PG Pearls:** * **Organ-Specific Regulation:** Remember the "Reciprocal Regulation" difference: Epinephrine **stimulates** glycolysis in **muscle** but **inhibits** it in the **liver** (via PFK-2 inhibition). * **PFK-1 Activators:** The most potent allosteric activator of PFK-1 is **Fructose-2,6-bisphosphate**. * **AMP vs. ATP:** High AMP levels (signaling low energy) activate glycolysis, while high ATP and Citrate levels inhibit it. * **Key Enzyme:** PFK-1 is the "Pacemaker" of glycolysis.

Question 407: The level of glucose transporters is reduced after an overnight fast in which of the following cells?

A. Liver cells
B. Brain cells
C. Red blood cells
D. Cardiac muscle cells (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Cardiac muscle cells.** **1. Why Cardiac Muscle Cells is Correct:** The regulation of glucose transporters (GLUT) depends on whether they are insulin-dependent or insulin-independent. **GLUT-4** is the primary insulin-responsive glucose transporter found in **cardiac muscle, skeletal muscle, and adipose tissue.** During an overnight fast, insulin levels drop while glucagon levels rise. In the absence of insulin, GLUT-4 transporters are sequestered from the cell membrane and moved into intracellular vesicles. This physiological mechanism ensures that glucose is conserved for glucose-dependent organs (like the brain) while muscles shift to fatty acid oxidation for energy. **2. Why the Other Options are Incorrect:** * **A. Liver cells:** These contain **GLUT-2**, which is insulin-independent. GLUT-2 is always present on the membrane to allow for bidirectional glucose flux (glycogenolysis and gluconeogenesis during fasting). * **B. Brain cells:** These primarily use **GLUT-1 and GLUT-3**, which are insulin-independent and have a high affinity for glucose. This ensures the brain receives a constant supply of glucose even during low blood sugar states. * **C. Red blood cells:** These contain **GLUT-1**, which is insulin-independent. Since RBCs lack mitochondria and rely solely on glycolysis, they require constant glucose uptake regardless of insulin levels. **3. High-Yield Clinical Pearls for NEET-PG:** * **GLUT-4** is the only GLUT regulated by insulin. * **Exercise** can also trigger the translocation of GLUT-4 to the cell membrane in skeletal muscle, independent of insulin (important for managing Diabetes Mellitus). * **GLUT-2** has a high $K_m$ (low affinity) and acts as a "glucose sensor" in Pancreatic $\beta$-cells. * **SGLT-1/SGLT-2** are sodium-dependent active transporters found in the small intestine and kidneys, unlike the GLUT family which facilitates passive diffusion.

Question 408: In which of the following polysaccharides are monomers linked by a beta-glycosidic bond?

A. Amylose
B. Amylopectin
C. Glycogen
D. Cellulose (Correct Answer)

Explanation: ### Explanation The correct answer is **Cellulose**. **1. Why Cellulose is Correct:** Cellulose is a structural homopolysaccharide found in the cell walls of plants. It consists of long, unbranched chains of D-glucose units linked by **$\beta(1 \to 4)$ glycosidic bonds**. This $\beta$-linkage results in a linear, rigid structure that allows for the formation of hydrogen bonds between adjacent chains, providing high tensile strength. Humans lack the enzyme **cellulase**, which is specific for hydrolyzing $\beta$-glycosidic bonds; therefore, cellulose cannot be digested and serves as dietary fiber. **2. Why the Other Options are Incorrect:** * **Amylose (A):** A component of starch consisting of unbranched glucose chains linked by **$\alpha(1 \to 4)$ glycosidic bonds**. * **Amylopectin (B):** The branched component of starch. It contains **$\alpha(1 \to 4)$ bonds** in the linear chain and **$\alpha(1 \to 6)$ bonds** at the branching points. * **Glycogen (C):** The primary storage polysaccharide in animals (liver and muscle). Like amylopectin, it uses **$\alpha(1 \to 4)$ bonds** for the backbone and **$\alpha(1 \to 6)$ bonds** for branches, though it is more highly branched than amylopectin. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Digestibility:** Human salivary and pancreatic $\alpha$-amylase can only hydrolyze $\alpha(1 \to 4)$ bonds. * **Dietary Fiber:** Cellulose increases stool bulk and decreases intestinal transit time, protective against colon cancer and diverticulosis. * **Inulin:** Another high-yield polysaccharide (fructose polymer) with $\beta(2 \to 1)$ bonds, used to measure **GFR** because it is freely filtered but neither reabsorbed nor secreted. * **Lactose:** A disaccharide (Glucose + Galactose) that also contains a **$\beta(1 \to 4)$ bond**, requiring the enzyme lactase for digestion.

Question 409: Which of the following cannot be degraded by colonic microorganisms and gastrointestinal enzymes?

A. Pectin
B. Lignin (Correct Answer)
C. Cellulose
D. Glucose

Explanation: ### Explanation The question tests the understanding of **Dietary Fiber** and its digestibility within the human gastrointestinal tract. **1. Why Lignin is the Correct Answer:** Dietary fiber is broadly classified into soluble and insoluble types. **Lignin** is a complex polymer of aromatic alcohols (phenylpropane units) found in the woody parts of plants. Unlike other dietary fibers, it is **not a carbohydrate**. It is uniquely resistant to both human digestive enzymes (in the small intestine) and **bacterial fermentation** (by colonic microflora). Therefore, it passes through the entire GI tract completely unchanged, providing bulk to the stool. **2. Analysis of Incorrect Options:** * **Pectin (Option A):** A soluble fiber found in fruits. While indigestible by human enzymes, it is **almost completely fermented** by colonic bacteria into short-chain fatty acids (SCFAs). * **Cellulose (Option B):** An insoluble fiber (β-1,4-linked glucose polymer). Humans lack cellulase, but colonic microorganisms can **partially degrade** it through fermentation. * **Glucose (Option D):** A simple monosaccharide that is rapidly and completely absorbed in the small intestine via SGLT-1 and GLUT-2 transporters. It never reaches the colon under normal physiological conditions. **3. Clinical Pearls for NEET-PG:** * **Short-Chain Fatty Acids (SCFAs):** Products of bacterial fermentation (e.g., Butyrate, Propionate, Acetate). **Butyrate** is the primary energy source for colonocytes and has anti-inflammatory properties. * **Classification:** * *Non-polysaccharide Fiber:* Lignin (only one). * *Polysaccharide Fibers:* Cellulose, Hemicellulose, Pectins, Gums, and Mucilages. * **Health Benefits:** Dietary fiber increases stool bulk, decreases transit time, and lowers glycemic index and cholesterol absorption. High-fiber diets are protective against diverticulosis and colorectal cancer.

Question 410: Which of the following enzymes is active in its dephosphorylated form?

A. Glucokinase
B. PFK-2
C. Pyruvate kinase
D. All of the above (Correct Answer)

Explanation: **Explanation:** The metabolic state of the body is primarily regulated by the **Insulin:Glucagon ratio**. A fundamental rule in biochemistry is that **insulin-dependent (well-fed state) enzymes are active in their dephosphorylated form**, while glucagon-dependent (fasting state) enzymes are active when phosphorylated. 1. **Pyruvate Kinase:** This is a key regulatory enzyme of glycolysis. Insulin triggers its dephosphorylation (via Protein Phosphatase-1), activating it to convert PEP to pyruvate. Conversely, Glucagon/Epinephrine trigger phosphorylation via Protein Kinase A, inactivating it to inhibit glycolysis during fasting. 2. **PFK-2 (Phosphofructokinase-2):** This bifunctional enzyme controls the levels of Fructose-2,6-bisphosphate. In its **dephosphorylated state**, the kinase domain is active (forming F-2,6-BP), which potently stimulates glycolysis. When phosphorylated, the phosphatase domain takes over, inhibiting glycolysis. 3. **Glucokinase:** While primarily regulated by the Glucokinase Regulatory Protein (GKRP) in the nucleus, its overall activity and expression are induced by insulin. In the context of covalent modification patterns in the liver, it aligns with the dephosphorylated (active) glycolytic pathway. **High-Yield NEET-PG Pearls:** * **Mnemonic:** "P" for **P**hosphorylated is "P" for **P**umped up (active) in the **Fasting** state (e.g., Glycogen phosphorylase). * **Exceptions:** Almost all rate-limiting enzymes of **Glycolysis, Fatty Acid Synthesis, and Cholesterol Synthesis** are active when **dephosphorylated**. * **Key Enzyme:** **Glycogen Synthase** is also active in the dephosphorylated form, whereas **Glycogen Phosphorylase** is active when phosphorylated.

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