Carbohydrate Metabolism Practice Questions

Q: What is the purpose of Lactate dehydrogenase in anaerobic glycolysis?

Replenishment of NAD+. **Explanation:** In anaerobic glycolysis, the primary objective is to maintain a continuous flow of energy (ATP) when oxygen is scarce. The enzyme **Lactate Dehydrogenase (LDH)** plays a pivotal role in this metabolic adaptation. **1. Why "Replenishment of NAD+" is correct:** During the payoff phase of glycolysis, the enzyme *Glyceraldehyde-3-phosphate dehydrogenase* (G3PDH) requires **NAD+** as a cofactor to convert Glyceraldehyde-3-phosphate into 1,3-Bisphosphoglycerate. In aerobic conditions, NADH is re-oxidized to NAD+ via the electron transport chain. However, under anaerobic conditions, the mitochondria cannot process NADH. LDH solves this by reducing Pyruvate to Lactate, simultaneously oxidizing **NADH back to NAD+**. This ensures a steady supply of NAD+ for G3PDH, allowing glycolysis to continue producing ATP. **2. Why other options are incorrect:** * **Production of Lactate:** While LDH does produce lactate, this is a metabolic "dead-end" byproduct. The *purpose* of the reaction is not to make lactate, but to recycle the coenzyme. * **Production of ATP:** LDH does not directly generate ATP. ATP is produced by *Phosphoglycerate kinase* and *Pyruvate kinase*. * **Replenishment of NADH:** LDH consumes NADH; it does not replenish it. NADH is replenished by G3PDH. **Clinical Pearls for NEET-PG:** * **Cori Cycle:** The lactate produced by LDH in muscles travels to the liver, where it is converted back to glucose (gluconeogenesis). * **Diagnostic Marker:** LDH is a non-specific marker of tissue injury (e.g., hemolysis, MI, or malignancy). * **Isoenzymes:** LDH has 5 isoenzymes; LDH-1 is predominant in the heart, while LDH-5 is found in skeletal muscle and the liver.

Q: Which of the following is a homopolysaccharide?

Chitin. **Explanation:** Polysaccharides are classified into two types based on their composition: **Homopolysaccharides**, which consist of a single type of monosaccharide unit, and **Heteropolysaccharides** (Glycosaminoglycans/GAGs), which contain different types of sugar units and often amino sugars or uronic acids. **Why Chitin is the Correct Answer:** Chitin is a **homopolysaccharide** composed of repeating units of **N-acetyl-D-glucosamine** linked by **β(1→4) glycosidic bonds**. It provides structural support in the exoskeleton of arthropods and the cell walls of fungi. Like cellulose, it is a linear polymer that provides high tensile strength. **Why Other Options are Incorrect:** * **Heparin, Chondroitin sulphate, and Hyaluronic acid** are all examples of **Heteropolysaccharides** (specifically Glycosaminoglycans). * They are composed of repeating **disaccharide units** (typically an amino sugar and a uronic acid). * **Hyaluronic acid** is unique among GAGs because it is non-sulfated and not covalently attached to a protein core. * **Heparin** is the most highly sulfated (negatively charged) molecule in the body. **High-Yield Clinical Pearls for NEET-PG:** * **Storage Homopolysaccharides:** Glycogen (animals), Starch (plants). * **Structural Homopolysaccharides:** Cellulose (plants), Chitin (insects/fungi). * **Inulin:** A homopolysaccharide of fructose (fructan) used to measure **Glomerular Filtration Rate (GFR)** because it is freely filtered but neither secreted nor reabsorbed. * **Dextran:** A homopolysaccharide of glucose used as a **plasma volume expander**. * **Agar:** A heteropolysaccharide derived from seaweed, used in laboratories.

Q: Which molecule is common to both glycolysis and the pentose phosphate pathway?

Glucose 6 phosphate. **Explanation:** The correct answer is **Glucose 6-phosphate (G6P)**. This molecule serves as a critical metabolic branch point in carbohydrate metabolism. **1. Why Glucose 6-phosphate is correct:** Upon entering a cell, glucose is immediately phosphorylated by Hexokinase (or Glucokinase) to form Glucose 6-phosphate. This molecule is the **initial substrate** for both: * **Glycolysis:** Where it is isomerized to Fructose 6-phosphate by phosphohexose isomerase. * **Pentose Phosphate Pathway (PPP):** Where it is oxidized by G6P Dehydrogenase (G6PD), the rate-limiting enzyme of the pathway, to generate NADPH and pentose sugars. **2. Why the other options are incorrect:** * **NAD:** Glycolysis utilizes **NAD+** (Nicotinamide adenine dinucleotide) as a coenzyme. In contrast, the PPP exclusively uses **NADP+** (Nicotinamide adenine dinucleotide phosphate). These two are not interchangeable in metabolic pathways. * **ATP:** While glycolysis requires an initial investment of ATP and subsequently produces it, the **PPP does not consume or produce any ATP**. It is an anabolic pathway focused on reducing power (NADPH) and ribose synthesis. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. Since the PPP is the only source of NADPH in RBCs (needed to keep glutathione reduced), a deficiency leads to hemolysis under oxidative stress (e.g., Fava beans, Primaquine). * **Rate-limiting enzymes:** Remember **PFK-1** for Glycolysis and **G6PD** for the PPP. * **Localization:** Both pathways occur entirely in the **cytosol**. * **Tissues:** PPP is highly active in tissues involved in fatty acid or steroid synthesis (Adrenal cortex, Liver, Mammary glands) to provide the necessary NADPH.

Q: Which enzyme catalyzes substrate-level phosphorylation in glycolysis?

Pyruvate kinase. **Explanation:** Substrate-level phosphorylation (SLP) is the direct synthesis of ATP (or GTP) from ADP (or GDP) by the transfer of a high-energy phosphate group from a phosphorylated intermediate, independent of the electron transport chain. **1. Why Pyruvate Kinase is Correct:** In the final step of glycolysis, **Pyruvate Kinase** catalyzes the conversion of Phosphoenolpyruvate (PEP) to Pyruvate. PEP contains a high-energy phosphate bond; its hydrolysis releases enough energy to drive the phosphorylation of ADP to **ATP**. This is one of the two SLP steps in glycolysis (the other being Phosphoglycerate kinase). **2. Analysis of Incorrect Options:** * **Succinate thiokinase (Succinyl-CoA Synthetase):** While this enzyme *does* catalyze substrate-level phosphorylation (converting Succinyl-CoA to Succinate and producing GTP), it occurs in the **TCA Cycle**, not glycolysis. * **Enolase:** This enzyme catalyzes the dehydration of 2-phosphoglycerate to PEP. It creates a high-energy bond but does not synthesize ATP. * **Pyruvate dehydrogenase (PDH):** This is a multi-enzyme complex that converts pyruvate to Acetyl-CoA (oxidative decarboxylation). It produces NADH but does not perform SLP. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP in Glycolysis:** 4 ATP are generated per glucose molecule (2 from Phosphoglycerate kinase, 2 from Pyruvate kinase). The *net* gain is 2 ATP. * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH step, bypasses the first SLP (Phosphoglycerate kinase), and results in **zero net ATP** production in glycolysis. * **Pyruvate Kinase Deficiency:** The second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). Since RBCs lack mitochondria, they rely solely on glycolysis; PK deficiency leads to ATP depletion and cell membrane damage.

Q: During gluconeogenesis, which mechanism transports reducing equivalents from the mitochondria to the cytosol?

Malate-aspartate shuttle. ### Explanation **1. Why Malate-Aspartate Shuttle is Correct:** Gluconeogenesis requires **NADH** in the cytosol for the conversion of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (catalyzed by GAPDH). While NADH is generated in the mitochondria, the inner mitochondrial membrane is impermeable to it. To overcome this, mitochondrial **Oxaloacetate (OAA)** is reduced to **Malate** by mitochondrial Malate Dehydrogenase, consuming NADH. Malate then crosses into the cytosol via a specific transporter, where it is re-oxidized to OAA by cytosolic Malate Dehydrogenase, regenerating **NADH** in the process. This "shuttle" effectively moves reducing equivalents to the cytosol to drive glucose synthesis. **2. Analysis of Incorrect Options:** * **Pyruvate-Malate Shuttle:** This primarily functions in **Lipogenesis** to transport Acetyl-CoA units (as Citrate) from the mitochondria to the cytosol, generating NADPH via Malic Enzyme, rather than providing NADH for gluconeogenesis. * **Glycerophosphate Shuttle:** This shuttle moves reducing equivalents from the **cytosol to the mitochondria** for the Electron Transport Chain (ETC). It is irreversible and cannot transport NADH out to the cytosol. * **Fatty Acid Oxidation:** While it provides the **ATP and NADH** required to power gluconeogenesis, it is a metabolic pathway, not a transport mechanism for reducing equivalents. **3. NEET-PG High-Yield Pearls:** * **The "OAA Dilemma":** OAA cannot cross the mitochondrial membrane directly. It must be converted to **Malate** (if NADH is needed in the cytosol) or **Aspartate** (if only the carbon skeleton is needed). * **Key Enzyme:** Pyruvate Carboxylase (the first step of gluconeogenesis) is located exclusively in the **mitochondria** and requires **Biotin** and **Acetyl-CoA** as an allosteric activator. * **Energy Yield:** The Malate-Aspartate shuttle is more efficient than the Glycerophosphate shuttle, yielding **2.5 ATP** per NADH.

Q: Which of the following hormones causes inhibition of glycogenolysis and gluconeogenesis?

Insulin. **Explanation:** The correct answer is **Insulin**. Insulin is the primary anabolic hormone of the body, secreted by the β-cells of the pancreas in the fed state. Its primary role is to lower blood glucose levels by promoting glucose uptake and storage while inhibiting pathways that produce glucose. **Why Insulin is correct:** Insulin inhibits **glycogenolysis** (breakdown of glycogen) by promoting the dephosphorylation (inactivation) of *Glycogen Phosphorylase*. Simultaneously, it inhibits **gluconeogenesis** (synthesis of glucose from non-carbohydrate sources) by repressing the expression of key rate-limiting enzymes, specifically *Phosphoenolpyruvate carboxykinase (PEPCK)* and *Glucose-6-phosphatase*. **Why the other options are incorrect:** * **Glucagon:** Secreted by α-cells during fasting, it is the primary counter-regulatory hormone that **stimulates** both glycogenolysis (via cAMP/PKA pathway) and gluconeogenesis to raise blood glucose. * **Glucocorticoids (e.g., Cortisol):** These are "diabetogenic" hormones. They **stimulate** gluconeogenesis by increasing the induction of PEPCK and promoting muscle proteolysis to provide amino acid precursors. * **Epinephrine:** Released during stress (fight or flight), it rapidly **stimulates** glycogenolysis in both the liver and muscle to provide immediate energy. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Gluconeogenesis:** Fructose-1,6-bisphosphatase. * **Rate-limiting enzyme of Glycogenolysis:** Glycogen phosphorylase. * **The "Bifunctional Enzyme":** Insulin decreases cAMP levels, leading to the dephosphorylation of the PFK-2/FBPase-2 complex, which increases Fructose-2,6-bisphosphate levels, thereby stimulating glycolysis and inhibiting gluconeogenesis. * **Key Concept:** Insulin is the only hormone that lowers blood glucose; Glucagon, Epinephrine, Cortisol, and Growth Hormone all act to increase it.

Q: A medical student fasts for 12 hours and then consumes a large quantity of pretzels. What metabolic effect will this meal have on the student?

Liver glycogen stores will be replenished.. **Explanation:** This question tests the transition from the fasting state to the **well-fed (absorptive) state**. **1. Why Option A is Correct:** Pretzels are high in carbohydrates (starch). Upon ingestion, they are broken down into glucose, leading to an increase in blood glucose levels. This triggers the release of **insulin** from the pancreas. Insulin activates **Glycogen Synthase** (via dephosphorylation) and inhibits Glycogen Phosphorylase. In the liver, this promotes **glycogenesis**, effectively replenishing the glycogen stores that were depleted during the 12-hour fast to maintain blood glucose. **2. Why the Incorrect Options are Wrong:** * **Option B:** Gluconeogenesis is a fasting-state pathway stimulated by glucagon. In the fed state, high insulin levels suppress key gluconeogenic enzymes (like PEPCK and Fructose-1,6-bisphosphatase), decreasing the rate of glucose synthesis. * **Option C:** Insulin promotes **lipogenesis**. It activates Acetyl-CoA Carboxylase and provides glycerol-3-phosphate (via glycolysis). This *increases* the rate at which fatty acids are esterified into triacylglycerols for storage in adipose tissue. * **Option D:** Hyperglycemia inhibits the alpha cells of the pancreas, leading to a **decrease** in glucagon levels, not an increase. **Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Glycogenesis:** Glycogen Synthase (Active in dephosphorylated state). * **Insulin/Glucagon Ratio:** The metabolic direction is determined by this ratio. A high ratio (fed state) favors storage (Glycogenesis, Lipogenesis, Protein synthesis). * **Liver vs. Muscle Glycogen:** Liver glycogen maintains blood glucose levels, whereas muscle glycogen is used exclusively for muscle contraction (as muscles lack Glucose-6-Phosphatase).

Question 1

Regarding proteoglycans, which of the following statements is false?

Accepted Answer

They hold a large amount of water.

Answer

Chondroitin sulfate is a proteoglycan.

Answer

They are made up of sugar and amino acids.

Answer

They carry a negative charge.

Question 2

What is the purpose of Lactate dehydrogenase in anaerobic glycolysis?

Accepted Answer

Replenishment of NAD+

Answer

Production of Lactate

Answer

Production of ATP

Answer

Replenishment of NADH

Question 3

Which of the following is a homopolysaccharide?

Accepted Answer

Chitin

Answer

Heparin

Answer

Chondroitin sulphate

Answer

Hyaluronic acid

Question 4

Although glycolysis and the pentose phosphate pathway have several common metabolites, they are markedly different otherwise. All of the following statements are true, EXCEPT:

Accepted Answer

Thiamine is used as a coenzyme in both glycolysis and the pentose phosphate pathway.

Answer

CO2 is produced in the pentose phosphate pathway, but not in glycolysis.

Answer

ATP is generated in glycolysis, but not in the pentose phosphate pathway.

Answer

Mg2+ ions are required for both glycolysis and the pentose phosphate pathway.

Question 5

Which molecule is common to both glycolysis and the pentose phosphate pathway?

Accepted Answer

Glucose 6 phosphate

Answer

NAD

Answer

ATP

Answer

All of the above

Question 6

Which enzyme catalyzes substrate-level phosphorylation in glycolysis?

Accepted Answer

Pyruvate kinase

Answer

Succinate thiokinase

Answer

Enolase

Answer

Pyruvate dehydrogenase (PDH)

Question 7

Mucopolysaccharides are?

Accepted Answer

Homopolysaccharides

Answer

Heteropolysaccharides

Answer

Proteins

Answer

Amino acids

Question 8

During gluconeogenesis, which mechanism transports reducing equivalents from the mitochondria to the cytosol?

Accepted Answer

Malate-aspartate shuttle

Answer

Pyruvate-malate shuttle

Answer

Glycerophosphate shuttle

Answer

Fatty acid oxidation

Question 9

Which of the following hormones causes inhibition of glycogenolysis and gluconeogenesis?

Accepted Answer

Insulin

Answer

Glucagon

Answer

Glucocorticoid

Answer

Epinephrine

Question 10

A medical student fasts for 12 hours and then consumes a large quantity of pretzels. What metabolic effect will this meal have on the student?

Accepted Answer

Liver glycogen stores will be replenished.

Answer

The rate of gluconeogenesis will be increased.

Answer

The rate at which fatty acids are converted to adipose triacylglycerols will be reduced.

Answer

Blood glucagon levels will increase.

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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