Carbohydrate Metabolism Practice Questions

Q: Which of the following is an action of insulin?

Increased glucose uptake in muscle. **Explanation:** Insulin is the body’s primary **anabolic hormone**, secreted by the beta cells of the pancreas in response to high blood glucose levels. Its primary goal is to lower blood glucose by promoting storage and utilization. **1. Why Option B is Correct:** Insulin facilitates glucose uptake in **skeletal muscle** and **adipose tissue** by stimulating the translocation of **GLUT-4** (an insulin-dependent glucose transporter) from intracellular vesicles to the plasma membrane. Without insulin, GLUT-4 remains sequestered inside the cell, making these tissues the primary sites for insulin-mediated glucose disposal. **2. Why Other Options are Incorrect:** * **A & C (Gluconeogenesis and Glycogenolysis):** These are **catabolic** processes that increase blood glucose levels. They are stimulated by counter-regulatory hormones like **glucagon** and epinephrine during fasting. Insulin actively inhibits these pathways to prevent hyperglycemia. * **D (Glucose uptake in endothelium):** Glucose uptake in the endothelium (as well as the brain, liver, and RBCs) is **insulin-independent**. These tissues utilize transporters like **GLUT-1, GLUT-2, or GLUT-3**, which are always present on the cell membrane regardless of insulin levels. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-4** is the only insulin-dependent glucose transporter. * **Exercise** can also trigger GLUT-4 translocation in muscles via an insulin-independent pathway (AMPK-mediated), which is why exercise helps manage Type 2 Diabetes. * **Brain and RBCs** rely on GLUT-1/3; they do not require insulin for glucose entry, ensuring a constant energy supply even during starvation. * **Liver (GLUT-2)**: Insulin does not increase glucose *uptake* via transporters here (GLUT-2 is bidirectional), but it promotes glucose *utilization* by inducing Glucokinase.

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 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: All the following are features of Von Gierke disease except?

Muscle hypotonia. **Von Gierke Disease (GSD Type I)** is caused by a deficiency of **Glucose-6-Phosphatase**, the enzyme responsible for converting Glucose-6-Phosphate into free glucose. This enzyme is primarily expressed in the **liver and kidneys**, but notably **absent in skeletal muscle**. ### Why "Muscle Hypotonia" is the Correct Answer: In Von Gierke disease, skeletal muscle is unaffected because muscle tissue lacks Glucose-6-Phosphatase even under normal physiological conditions (it lacks the ability to release glucose into the blood). Therefore, there is no primary muscle pathology, weakness, or hypotonia. Muscle involvement (hypotonia/cardiomyopathy) is instead a hallmark of **Pompe Disease (GSD Type II)**. ### Explanation of Incorrect Options: * **Hypoglycemia:** Since the liver cannot perform the final step of glycogenolysis or gluconeogenesis, severe fasting hypoglycemia occurs, often leading to seizures. * **Lactic Acidosis:** Excess Glucose-6-Phosphate is shunted into the glycolytic pathway, leading to increased pyruvate and subsequently elevated lactate levels. * **Hyperlipidemia:** Low insulin and high glucagon levels trigger lipolysis. Additionally, the liver increases VLDL synthesis, leading to "doll-like" facies due to fat deposition. ### High-Yield Clinical Pearls for NEET-PG: * **Hyperuricemia:** Increased G-6-P enters the HMP shunt, increasing ribose-5-phosphate, which accelerates purine synthesis and degradation to uric acid (Gout). * **Clinical Presentation:** Massive hepatomegaly (due to glycogen storage), stunted growth, and "doll-like" face. * **Treatment:** Frequent oral glucose/cornstarch and avoidance of fructose and galactose (which exacerbate metabolite buildup).

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.

Question 1

A 5-month-old child presents with hepatomegaly, ketosis, hyperuricemia, and malaise. What is the most likely diagnosis?

Accepted Answer

Glycogen storage disease

Answer

Diabetes mellitus

Answer

Urea cycle defect

Answer

Mucopolysaccharidosis

Question 2

Cyclic AMP increases the rate of glycogenolysis by which of the following mechanisms?

Accepted Answer

Promoting the activation of phosphorylase kinase, which then activates glycogen phosphorylase.

Answer

Acting as a cofactor for glycogen phosphorylase.

Answer

Furnishing phosphate for the phosphorylysis of glycogen.

Answer

Acting as a precursor of 5' AMP, which is a cofactor for glycogen phosphorylase.

Question 3

Which of the following is an action of insulin?

Accepted Answer

Increased glucose uptake in muscle

Answer

Gluconeogenesis

Answer

Glycogenolysis

Answer

Increased glucose uptake in endothelium

Question 4

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 5

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 6

Which of the following is a homopolysaccharide?

Accepted Answer

Chitin

Answer

Heparin

Answer

Chondroitin sulphate

Answer

Hyaluronic acid

Question 7

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 8

Which enzyme catalyzes substrate-level phosphorylation in glycolysis?

Accepted Answer

Pyruvate kinase

Answer

Succinate thiokinase

Answer

Enolase

Answer

Pyruvate dehydrogenase (PDH)

Question 9

All the following are features of Von Gierke disease except?

Accepted Answer

Muscle hypotonia

Answer

Hypoglycemia

Answer

Lactic acidosis

Answer

Hyperlipidemia

Question 10

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

Accepted Answer

Insulin

Answer

Glucagon

Answer

Glucocorticoid

Answer

Epinephrine

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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