Carbohydrate Metabolism Practice Questions

Q: What is the Pasteur effect?

Inhibition of glycolysis by oxygen. **Explanation:** The **Pasteur Effect** refers to the observation that the rate of glycolysis is significantly decreased in the presence of oxygen compared to anaerobic conditions. **Why Option A is Correct:** In the presence of oxygen (aerobic conditions), cells can utilize the **Electron Transport Chain (ETC)** and oxidative phosphorylation. This process is far more efficient, yielding **30–32 ATP** per glucose molecule, compared to only **2 ATP** produced during anaerobic glycolysis. Because the cell’s energy requirements are met more efficiently, high levels of ATP and citrate act as allosteric inhibitors of **Phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis. Thus, oxygen "inhibits" the rapid consumption of glucose. **Why Other Options are Incorrect:** * **Option B:** The absence of oxygen actually *stimulates* glycolysis (the opposite of the Pasteur effect) to compensate for low ATP yields. * **Option C:** Glycolysis contains three irreversible steps (Hexokinase, PFK-1, and Pyruvate Kinase); the Pasteur effect is a regulatory phenomenon, not a description of reversibility. * **Option D:** Oxygen decelerates glycolysis; the *acceleration* of glycolysis in the presence of oxygen is known as the **Warburg Effect**, typically seen in cancer cells. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** The Pasteur effect primarily targets **PFK-1**. * **Warburg Effect:** This is the "clinical opposite" where cancer cells prefer glycolysis even when oxygen is plentiful (aerobic glycolysis), allowing them to use glycolytic intermediates for cell growth. * **Crabtree Effect:** The inhibition of oxygen consumption by high concentrations of glucose (seen in yeast and some tumor cells).

Q: Muscle glycogen cannot contribute to blood glucose due to the absence of which enzyme?

Glucose-6-phosphatase. **Explanation:** The correct answer is **Glucose-6-phosphatase**. **1. Why Glucose-6-phosphatase is the correct answer:** Glycogenolysis in both the liver and muscle produces Glucose-1-phosphate, which is converted to Glucose-6-phosphate (G6P). However, G6P is a charged molecule that cannot cross the cell membrane to enter the bloodstream. To become free glucose, it must be dephosphorylated by the enzyme **Glucose-6-phosphatase**. This enzyme is present in the liver and kidneys but is **absent in skeletal muscle**. Consequently, muscle glycogen is used exclusively as an internal energy source for glycolysis to generate ATP during contraction, rather than for maintaining blood glucose levels. **2. Why the other options are incorrect:** * **Phosphoglucomutase (Option A):** This enzyme is present in muscle; it catalyzes the reversible conversion of Glucose-1-phosphate to Glucose-6-phosphate. * **Branching enzyme (Option B):** Also known as glucosyl 4:6 transferase, it is required for glycogen *synthesis* (glycogenesis), not breakdown. * **Debranching enzyme (Option C):** This enzyme is present in muscle and is essential for the complete breakdown of glycogen branches. Its deficiency leads to Cori’s disease (GSD Type III). **3. NEET-PG High-Yield Clinical Pearls:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperuricemia because the liver cannot release glucose. * **Muscle vs. Liver:** Muscle glycogen stores are larger in total mass, but liver glycogen is the primary source for blood glucose during the first 12–18 hours of fasting. * **GLUT-4:** Remember that glucose uptake in muscles is mediated by insulin-dependent GLUT-4 transporters, but glucose *release* is impossible due to the lack of the phosphatase enzyme.

Q: Alpha-amylase acts on which type of glycosidic bond?

Alpha 1-4 glycosidic bond. **Explanation:** **Alpha-amylase** (both salivary and pancreatic) is an **endo-glycosidase** that specifically hydrolyzes the internal **alpha 1-4 glycosidic bonds** of polysaccharides like starch and glycogen. By breaking these linear bonds, it converts complex carbohydrates into smaller units like maltose, maltotriose, and alpha-limit dextrins. **Analysis of Options:** * **Option A (Correct):** Alpha-amylase acts exclusively on alpha 1-4 bonds. It requires a pH of 6.7–7.0 and chloride ions ($Cl^-$) for optimal activity. * **Option B (Incorrect):** Alpha 1-6 glycosidic bonds are the "branch points" in amylopectin and glycogen. Alpha-amylase **cannot** hydrolyze these bonds. These are broken by "debranching enzymes" (like isomaltase in the gut). * **Option C (Incorrect):** Beta 1-4 bonds are found in **cellulose**. Humans lack the enzyme (cellulase) to break these bonds, which is why cellulose serves as dietary fiber. * **Option D (Incorrect):** Beta 1-6 bonds are not typically found in major dietary starches or glycogen. **High-Yield Clinical Pearls for NEET-PG:** * **Chloride Activation:** Alpha-amylase is a metalloenzyme that requires **Calcium ($Ca^{2+}$)** for stability and **Chloride ($Cl^-$)** for activation. * **Limit Dextrins:** Because amylase cannot bypass or break alpha 1-6 branch points, the remaining branched fragments are called **alpha-limit dextrins**. * **Diagnostic Marker:** Serum amylase rises in **Acute Pancreatitis** (within 2–12 hours), though lipase is considered more specific. * **Isoenzymes:** Salivary amylase (Ptyalin) is inactivated by gastric HCl, while Pancreatic amylase continues digestion in the duodenum.

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: What is the enzyme deficiency in McArdle syndrome?

Muscle phosphorylase. **Explanation:** **McArdle Syndrome (GSD Type V)** is a glycogen storage disease characterized by the deficiency of **Muscle Phosphorylase** (also known as myophosphorylase). 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 ATP depletion. **Why the correct answer is right:** * **Muscle Phosphorylase (Option A):** In GSD Type V, the muscle-specific isoform of glycogen phosphorylase is absent. This results in an inability to perform anaerobic glycolysis, leading to exercise intolerance, muscle cramps, and myoglobinuria. **Why the incorrect options are wrong:** * **Liver Phosphorylase (Option B):** Deficiency of this enzyme causes **Hers Disease (GSD Type VI)**, which presents with hepatomegaly and mild hypoglycemia, but no muscle symptoms. * **Liver Debranching Enzyme (Option C):** Deficiency causes **Cori Disease (GSD Type III)**. It affects both liver and muscle but is characterized by the accumulation of "limit dextrins" (abnormal glycogen structure). * **Glycogen Synthase (Option D):** Deficiency leads to **GSD Type 0**, characterized by fasting hypoglycemia and ketosis because glycogen cannot be stored at all. **High-Yield Clinical Pearls for NEET-PG:** 1. **"Second Wind" Phenomenon:** A classic sign where patients experience a decrease in heart rate and improved exercise tolerance after a few minutes of activity (due to a switch to fatty acid oxidation and increased blood flow). 2. **Ischemic Forearm Exercise Test:** Shows a **failure of blood lactate to rise** (flat lactate curve) with a significant rise in ammonia. 3. **Burgundy-colored urine:** Post-exercise myoglobinuria can lead to acute renal failure. 4. **Biopsy:** Shows subsarcolemmal deposits of normal-structured glycogen.

Question 1

What is the Pasteur effect?

Accepted Answer

Inhibition of glycolysis by oxygen

Answer

Inhibition of glycolysis due to absence of oxygen

Answer

Reversible reaction of glycolysis

Answer

Acceleration of glycolysis by oxygen

Question 2

Muscle glycogen cannot contribute to blood glucose due to the absence of which enzyme?

Accepted Answer

Glucose-6-phosphatase

Answer

Phosphoglutamase

Answer

Branching enzyme

Answer

Debranching enzyme

Question 3

Alpha-amylase acts on which type of glycosidic bond?

Accepted Answer

Alpha 1-4 glycosidic bond

Answer

Alpha 1-6 glycosidic bond

Answer

Beta 1-4 glycosidic bond

Answer

Beta 1-6 glycosidic bond

Question 4

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 5

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 6

Skeletal muscle is deficient in which of the following enzymes?

Accepted Answer

Glucose-6-phosphatase

Answer

Hexokinase

Answer

Isomerase

Answer

Phosphofructokinase

Question 7

The Rapaport-Luebering cycle causes a shift of the O2 dissociation curve to the right because of which of the following?

Accepted Answer

2,3-Bisphosphoglycerate (2,3-BPG)

Answer

1,3-Bisphosphoglycerate (1,3-BPG)

Answer

3-Phosphoglycerate

Answer

Fructose 1,6-bisphosphate

Question 8

A 2-week-old neonate presents with complete hypotonia, convulsions, failure to thrive, and metabolic acidosis. The urine has a 'burnt sugar' odor, and a positive DNPH test. What is the enzyme deficiency in this metabolic disorder?

Accepted Answer

Branched chain keto acid dehydrogenase

Answer

Isovaleryl CoA dehydrogenase

Answer

Dihydrolipoamide dehydrogenase

Answer

Transacylase

Question 9

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 10

What is the enzyme deficiency in McArdle syndrome?

Accepted Answer

Muscle phosphorylase

Answer

Liver phosphorylase

Answer

Liver debranching enzyme

Answer

Glycogen synthase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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