Carbohydrate Metabolism Practice Questions

Q: In which of the following tissues is glycogen incapable of contributing directly to blood glucose?

Muscle. **Explanation:** The correct answer is **Muscle** because it lacks the enzyme **Glucose-6-Phosphatase**. **1. Why Muscle is the Correct Answer:** While both the liver and skeletal muscle store glycogen, their physiological roles differ. In the liver, glycogenolysis produces Glucose-6-Phosphate (G6P), which is then converted into free glucose by the enzyme **Glucose-6-Phosphatase**. This free glucose can exit the cell via GLUT-2 transporters to maintain blood glucose levels. In contrast, skeletal muscle lacks Glucose-6-Phosphatase. Therefore, the G6P generated from muscle glycogenolysis is trapped within the muscle cell and must enter the **glycolytic pathway** to provide ATP for local muscle contraction. Consequently, muscle glycogen cannot contribute directly to blood glucose. **2. Why Other Options are Incorrect:** * **A. Liver:** The liver is the primary organ responsible for maintaining blood glucose during fasting. It contains high concentrations of Glucose-6-Phosphatase, allowing it to release glucose into the bloodstream. * **C & D:** These are incorrect based on the specific enzymatic deficiency in muscle tissue described above. **High-Yield Clinical Pearls for NEET-PG:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-Phosphatase. It results in severe fasting hypoglycemia because neither glycogenolysis nor gluconeogenesis can release glucose from the liver. * **Cori Cycle:** Muscle glycogen can *indirectly* contribute to blood glucose via the Cori Cycle, where muscle-derived lactate travels to the liver to be converted back into glucose. * **Key Enzyme:** Glycogen phosphorylase is the rate-limiting enzyme for glycogenolysis, but Glucose-6-Phosphatase is the "gatekeeper" for glucose release into the blood.

Q: Which enzyme is responsible for the complete oxidation of glucose to CO2 and water?

Mitochondria. **Explanation:** The complete oxidation of glucose involves three major stages: Glycolysis, the TCA (Krebs) cycle, and the Electron Transport Chain (ETC). While glycolysis occurs in the cytosol, it only partially oxidizes glucose to pyruvate. The **Mitochondria** is the definitive site for complete oxidation because it houses the Pyruvate Dehydrogenase (PDH) complex, the TCA cycle enzymes, and the machinery for Oxidative Phosphorylation. Within the mitochondrial matrix, acetyl-CoA is oxidized to **CO2**, and the resulting reducing equivalents (NADH/FADH2) are used by the ETC on the inner mitochondrial membrane to produce **water** and ATP. **Analysis of Incorrect Options:** * **A. Cytosol:** This is the site for glycolysis (partial oxidation) and the HMP shunt. It lacks the oxygen-dependent machinery required to break down pyruvate into CO2 and water. * **C. Lysosomes:** These are "suicide bags" involved in the degradation of macromolecules (proteolysis, lipolysis) via acid hydrolases, not energy metabolism. * **D. Endoplasmic Reticulum:** The ER is primarily involved in protein synthesis (RER), lipid synthesis, and detoxification (SER). It plays a role in gluconeogenesis (Glucose-6-phosphatase) but not glucose oxidation. **High-Yield NEET-PG Pearls:** * **Mitochondria** are often called the "Powerhouse of the cell" because they generate >90% of cellular ATP. * **RBCs** lack mitochondria; therefore, they can never oxidize glucose completely and rely solely on anaerobic glycolysis (lactate production). * **Key Mitochondrial Enzymes:** Pyruvate Dehydrogenase, Citrate Synthase, and Cytochrome Oxidase. * **Site Marker:** Succinate dehydrogenase is a marker enzyme for the inner mitochondrial membrane.

Q: Skeletal muscle is deficient in which of the following enzymes?

Glucose-6-phosphatase. ### Explanation **1. Why Glucose-6-phosphatase is the correct answer:** Glucose-6-phosphatase is the enzyme responsible for converting Glucose-6-phosphate into free glucose. This enzyme is primarily located in the **liver** and **kidneys**. Skeletal muscle lacks this enzyme; therefore, it cannot release free glucose into the bloodstream. Instead, the glucose-6-phosphate derived from muscle glycogenolysis enters the glycolytic pathway to provide ATP locally for muscle contraction. This ensures that muscle glycogen is a "selfish" fuel source, reserved exclusively for the muscle's own energy needs. **2. Analysis of Incorrect Options:** * **Hexokinase (Option B):** This enzyme is present in skeletal muscle. It catalyzes the first step of glycolysis (Glucose → Glucose-6-phosphate). It has a low Km (high affinity) for glucose, allowing muscles to utilize glucose even at low blood concentrations. * **Isomerase (Option C):** Specifically Phosphohexose Isomerase, this enzyme is essential for the second step of glycolysis (Glucose-6-P ↔ Fructose-6-P) and is present in all tissues, including muscle. * **Phosphofructokinase (Option D):** PFK-1 is the rate-limiting enzyme of glycolysis and is highly active in skeletal muscle to support rapid energy production during exercise. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia because neither glycogenolysis nor gluconeogenesis can export glucose from the liver. * **Cori Cycle:** Since muscles cannot release glucose, they release **lactate** during anaerobic exercise. This lactate travels to the liver, where it is converted back to glucose via gluconeogenesis. * **Glucokinase vs. Hexokinase:** Remember that the liver has Glucokinase (high Km), while muscles have Hexokinase (low Km).

Q: Which of the following reactions in the TCA cycle is an example of substrate-level phosphorylation?

Succinyl CoA to succinate. ### Explanation **Correct Answer: C. Succinyl CoA to succinate** In the TCA cycle, the conversion of **Succinyl CoA to Succinate** is the only step that generates a high-energy phosphate bond directly without the involvement of the electron transport chain. This process is called **Substrate-Level Phosphorylation (SLP)**. The enzyme **Succinate thiokinase** (also known as Succinyl CoA synthetase) cleaves the high-energy thioester bond of Succinyl CoA. The energy released is used to phosphorylate GDP to **GTP** (in the liver and kidneys) or ADP to **ATP** (in heart and muscle). This is a high-yield fact because it represents the only "direct" energy gain within the cycle itself. #### Analysis of Incorrect Options: * **Option A (Isocitrate to oxalosuccinate):** This is part of the reaction catalyzed by *Isocitrate dehydrogenase*. It involves the reduction of NAD+ to **NADH**, not the direct formation of ATP/GTP. * **Option B (Alpha-ketoglutarate to succinyl CoA):** Catalyzed by the *α-ketoglutarate dehydrogenase complex*, this oxidative decarboxylation produces **NADH** and CO₂. * **Option D (Succinate to fumarate):** Catalyzed by *Succinate dehydrogenase* (Complex II of ETC), this reaction involves the reduction of FAD to **FADH₂**. #### NEET-PG High-Yield Pearls: 1. **Location:** The TCA cycle occurs in the **mitochondrial matrix**, but Succinate dehydrogenase is the only enzyme attached to the **inner mitochondrial membrane**. 2. **Arsenite Poisoning:** The α-ketoglutarate dehydrogenase complex is inhibited by Arsenite. 3. **ATP Yield:** One turn of the TCA cycle yields **10 ATP** (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1). 4. **Regulatory Step:** The conversion of Isocitrate to α-ketoglutarate is the **rate-limiting step** of the cycle.

Q: The polyol pathway is responsible for the formation of which of the following?

Fructose from glucose. The **Polyol Pathway** (also known as the Sorbitol pathway) is a two-step metabolic process that bypasses glycolysis to convert Glucose into Fructose. ### 1. Why Option A is Correct The pathway occurs in two distinct enzymatic steps: * **Step 1:** Glucose is reduced to **Sorbitol** (a polyol/sugar alcohol) by the enzyme **Aldose Reductase**, using NADPH as a cofactor. * **Step 2:** Sorbitol is oxidized to **Fructose** by the enzyme **Sorbitol Dehydrogenase**, using NAD+ as a cofactor. This pathway is primarily active in the seminal vesicles (fructose is the main energy source for sperm) and the liver. ### 2. Why Other Options are Incorrect * **Options B & C:** Galactose metabolism involves the **Leloir pathway** (Galactokinase and GALT enzymes). While Aldose Reductase can convert Galactose to Galactitol (dulcitol), it does not produce Fructose from Galactose. * **Option D:** The conversion of Fructose to Glucose occurs via **Gluconeogenesis** (after fructose enters glycolysis as DHAP/Glyceraldehyde-3-P), not the polyol pathway. ### 3. NEET-PG High-Yield Clinical Pearls * **Osmotic Damage:** In **Diabetes Mellitus**, hyperglycemia leads to excess glucose entering the polyol pathway. Sorbitol is polar and cannot easily cross cell membranes. * **Tissue Vulnerability:** Tissues like the **Lens, Retina, and Schwann cells** have high Aldose Reductase but **lack Sorbitol Dehydrogenase**. Consequently, sorbitol accumulates, causing osmotic swelling. * **Complications:** This accumulation is a major mechanism behind **Diabetic Cataracts** and **Diabetic Neuropathy**. * **Cofactor Depletion:** Excessive use of NADPH by Aldose Reductase depletes the pool available for Glutathione Reductase, increasing **oxidative stress** in the cell.

Q: Diabetes mellitus is associated with which type of lactic acidosis?

Type B lactic acidosis. ### Explanation Lactic acidosis is classified into two primary types based on the presence or absence of tissue hypoxia. **Why Type B is Correct:** **Type B lactic acidosis** occurs in the **absence of clinical evidence of tissue hypoxia** or hypoperfusion. In Diabetes Mellitus (DM), lactic acidosis occurs primarily due to metabolic derangements rather than oxygen debt. 1. **Metabolic Dysfunction:** In DM, there is an impairment of the enzyme **pyruvate dehydrogenase (PDH)**, leading to the diversion of pyruvate to lactate. 2. **Drug-Induced:** A classic high-yield association is the use of **Metformin** (a Biguanide), which inhibits mitochondrial respiration and gluconeogenesis, leading to lactate accumulation. 3. **Ketoacidosis:** Severe DKA can impair lactate clearance in the liver. **Analysis of Incorrect Options:** * **Type A Lactic Acidosis:** This is caused by **hypoxia or hypoperfusion** (e.g., shock, septicemia, severe hemorrhage, or heart failure). While a diabetic patient *can* develop shock, the primary association of the disease process itself is Type B. * **Type D Lactic Acidosis:** This is caused by **D-lactate** produced by abnormal gut bacteria, typically seen in **Short Bowel Syndrome**. Standard laboratory tests for lactate only measure the L-isomer, making this a distinct clinical entity unrelated to DM. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Lactate Levels:** 5 mmol/L with a pH <7.35. * **Metformin Warning:** Metformin is contraindicated if the eGFR is <30 mL/min/1.73 m² due to the high risk of Type B lactic acidosis. * **Enzyme Link:** Deficiency of **Thiamine (B1)** can also cause Type B lactic acidosis because it is a cofactor for PDH.

Q: The muscle shown in the illustration predominantly uses which glucose transporter?

GLUT 4. ***GLUT 4*** - **GLUT 4** is the predominant glucose transporter in **skeletal muscle** and **adipose tissue**, requiring **insulin** for translocation to the cell membrane. - It has **high affinity** for glucose (low Km) and is crucial for **postprandial glucose uptake**, making it clinically relevant in **insulin resistance** and **type 2 diabetes**. *GLUT 1* - **GLUT 1** is found in **brain**, **red blood cells**, and **placenta**, providing **basal glucose uptake** independent of insulin. - It has **high affinity** for glucose but is **constitutively expressed** at the cell membrane, not translocated like GLUT 4. *GLUT 2* - **GLUT 2** is predominantly found in **liver**, **pancreatic beta cells**, **kidney**, and **small intestine** with **low affinity** (high Km) for glucose. - It functions as a **glucose sensor** in pancreatic beta cells and allows **bidirectional glucose transport** in hepatocytes, unlike muscle tissue. *GLUT 3* - **GLUT 3** is primarily expressed in **brain tissue** and **neurons**, providing **high-affinity glucose transport** for neural metabolism. - It is **insulin-independent** and constitutively active, unlike the **insulin-dependent GLUT 4** required for muscle glucose uptake.

Q: Which molecule, known to initiate cataract formation in the eye lens, also has a 1-phosphate derivative responsible for liver failure?

Galactitol. **Explanation:** The correct answer is **Galactitol** (also known as dulcitol). This question tests the integration of the polyol pathway and galactose metabolism. **1. Why Galactitol is correct:** In patients with **Galactosemia** (due to Galactose-1-phosphate uridyltransferase or Galactokinase deficiency), excess galactose is diverted into the polyol pathway. The enzyme **Aldose Reductase** reduces galactose into **Galactitol**. Unlike sorbitol, galactitol cannot be further metabolized and accumulates in the lens. It is osmotically active, drawing water into the lens, causing swelling and **cataract formation**. The second part of the question refers to **Classic Galactosemia** (GALT deficiency), where **Galactose-1-phosphate** accumulates in the liver. This metabolite is toxic, leading to ATP depletion and phosphate sequestration, resulting in **liver failure**, jaundice, and hepatomegaly. **2. Why other options are incorrect:** * **Sorbitol:** While sorbitol causes cataracts in diabetic patients (via glucose reduction), its 1-phosphate derivative is not associated with liver failure. Fructose-1-phosphate (from fructose metabolism) causes liver damage in Hereditary Fructose Intolerance. * **Mannitol:** It is a sugar alcohol used clinically as an osmotic diuretic to reduce intracranial pressure; it is not a primary cause of metabolic cataracts or liver failure. * **Inositol:** A precursor for signaling molecules (IP3/DAG); it does not accumulate to cause cataracts or liver toxicity in this metabolic context. **High-Yield NEET-PG Pearls:** * **Enzyme Deficiency:** Classic Galactosemia is due to **GALT** deficiency (more severe); Galactokinase deficiency causes cataracts but *not* liver failure. * **Cataract Type:** Galactosemia typically presents with "Oil-drop cataracts." * **Dietary Management:** Immediate removal of lactose and galactose from the diet is life-saving.

Q: Hers disease is due to deficiency of which enzyme?

Liver phosphorylase. **Explanation:** **Hers Disease (GSD Type VI)** is a glycogen storage disease caused by a deficiency of **Liver Phosphorylase**. This enzyme is responsible for the rate-limiting step of glycogenolysis in the liver, breaking down glycogen into glucose-1-phosphate. When deficient, glycogen accumulates in the liver, leading to hepatomegaly and mild fasting hypoglycemia (milder than Von Gierke’s because gluconeogenesis remains intact). **Analysis of Options:** * **Option A (Correct):** Liver phosphorylase deficiency leads to Hers disease. It presents with growth retardation and hepatomegaly in early childhood. * **Option B (Incorrect):** Deficiency of **Muscle Phosphorylase** causes **McArdle Disease (GSD Type V)**, characterized by exercise-induced muscle cramps and myoglobinuria. * **Option C (Incorrect):** Deficiency of the **Branching Enzyme** (α-1,4 → α-1,6 transglucosidase) causes **Andersen Disease (GSD Type IV)**, which typically leads to liver cirrhosis and early infantile death. * **Option D (Incorrect):** Deficiency of **Phosphofructokinase (PFK-1)** in muscles and RBCs causes **Tarui Disease (GSD Type VII)**, presenting similarly to McArdle disease but with associated hemolytic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember **"H"** for **H**ers and **H**epatic phosphorylase; **"M"** for **M**cArdle and **M**uscle phosphorylase. * Unlike Type I (Von Gierke), Hers disease typically has **normal** levels of lactate and uric acid. * Ketosis is a common finding in Hers disease during fasting. * **GSD Type VI and IX** (Phosphorylase kinase deficiency) are clinically similar and often grouped together as they both affect the activation of liver phosphorylase.

Question 1

In which of the following tissues is glycogen incapable of contributing directly to blood glucose?

Accepted Answer

Muscle

Answer

Liver

Answer

Both

Answer

None

Question 2

Which enzyme is responsible for the complete oxidation of glucose to CO2 and water?

Accepted Answer

Mitochondria

Answer

Cytosol

Answer

Lysosomes

Answer

Endoplasmic Reticulum

Question 3

Skeletal muscle is deficient in which of the following enzymes?

Accepted Answer

Glucose-6-phosphatase

Answer

Hexokinase

Answer

Isomerase

Answer

Phosphofructokinase

Question 4

Which of the following reactions in the TCA cycle is an example of substrate-level phosphorylation?

Accepted Answer

Succinyl CoA to succinate

Answer

Isocitrate to oxalosuccinate

Answer

Alpha-ketoglutarate to succinyl CoA

Answer

Succinate to fumarate

Question 5

A pregnant woman with lactase deficiency cannot tolerate milk in her diet and is concerned about producing milk of insufficient caloric value to nourish her baby. What is the best advice to give her?

Accepted Answer

The production of lactose by the mammary gland does not require the ingestion of milk or milk products.

Answer

She must consume pure galactose to produce the galactose moiety of lactose.

Answer

She will be unable to breastfeed her baby because she cannot produce lactose.

Answer

She can produce lactose directly by degrading a-lactalbumin.

Question 6

The polyol pathway is responsible for the formation of which of the following?

Accepted Answer

Fructose from glucose

Answer

Galactose from fructose

Answer

Galactose from glucose

Answer

Glucose from fructose

Question 7

Diabetes mellitus is associated with which type of lactic acidosis?

Accepted Answer

Type B lactic acidosis

Answer

Type A lactic acidosis

Answer

Type D lactic acidosis

Answer

All of the above

Question 8

The muscle shown in the illustration predominantly uses which glucose transporter?

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Question 9

Which molecule, known to initiate cataract formation in the eye lens, also has a 1-phosphate derivative responsible for liver failure?

Accepted Answer

Galactitol

Answer

Sorbitol

Answer

Mannitol

Answer

Inositol

Question 10

Hers disease is due to deficiency of which enzyme?

Accepted Answer

Liver phosphorylase

Answer

Muscle phosphorylase

Answer

Branching enzyme

Answer

RBC phosphofructokinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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