Carbohydrate Metabolism Practice Questions

Q: Which of the following enzyme activities decreases during fasting?

Acetyl CoA Carboxylase. **Explanation:** The metabolic state of **fasting** is characterized by a high **Glucagon:Insulin ratio**. This hormonal shift triggers the phosphorylation of key regulatory enzymes, leading to the activation of catabolic pathways (to generate energy) and the inhibition of anabolic pathways (to conserve energy). **Why Acetyl CoA Carboxylase (ACC) is the correct answer:** ACC is the rate-limiting enzyme for **De novo Fatty Acid Synthesis** (lipogenesis). During fasting, elevated Glucagon and Epinephrine trigger cAMP-dependent protein kinase (PKA) and AMP-activated protein kinase (AMPK), which **phosphorylate and inactivate ACC**. This prevents the synthesis of Malonyl-CoA, thereby halting fatty acid production and allowing fatty acid oxidation to proceed. **Analysis of Incorrect Options:** * **A. Hormone Sensitive Lipase (HSL):** This enzyme catalyzes lipolysis in adipose tissue. During fasting, HSL is **activated** via phosphorylation by PKA to mobilize free fatty acids for fuel. * **B. Glycogen Phosphorylase:** This is the rate-limiting enzyme of glycogenolysis. It is **activated** (phosphorylated) during fasting to maintain blood glucose levels. * **C. CPS I (Carbamoyl Phosphate Synthetase I):** This urea cycle enzyme is typically **increased** or maintained during fasting/high-protein intake to handle the nitrogen load from increased amino acid catabolism (gluconeogenesis). **High-Yield NEET-PG Pearls:** * **"Fed State" Enzymes (Dephosphorylated = Active):** Glucokinase, PFK-1, Pyruvate Kinase, Acetyl CoA Carboxylase, HMG-CoA Reductase. * **"Fasting State" Enzymes (Phosphorylated = Active):** Glycogen Phosphorylase, Fructose-2,6-Bisphosphatase, Hormone Sensitive Lipase. * **Malonyl-CoA** (produced by ACC) is a potent inhibitor of **Carnitine Palmitoyltransferase-1 (CPT-1)**; thus, when ACC is inactive, CPT-1 is active, facilitating Beta-oxidation.

Q: Which of the following enzymes is required for glycolysis?

Pyruvate kinase. ### Explanation **Correct Option: A. Pyruvate kinase** Glycolysis is the metabolic pathway that converts glucose into pyruvate. **Pyruvate kinase** is the enzyme responsible for the final step of glycolysis, where it catalyzes the irreversible transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, yielding one molecule of pyruvate and one molecule of ATP. This is a key regulatory step and an example of substrate-level phosphorylation. **Why the other options are incorrect:** * **B. Pyruvate carboxylase:** This is a gluconeogenic enzyme that converts pyruvate to oxaloacetate. It is located in the mitochondria and requires biotin as a cofactor. * **C. Glucose-6-phosphatase:** This enzyme is involved in gluconeogenesis and glycogenolysis (found in the liver and kidneys). It converts glucose-6-phosphate back to free glucose, allowing it to enter the bloodstream. It is absent in muscle tissue. * **D. Glycerokinase:** This enzyme is involved in lipid metabolism, specifically the phosphorylation of glycerol to glycerol-3-phosphate. It is primarily found in the liver, which is why adipose tissue cannot reuse glycerol for TG synthesis. **High-Yield NEET-PG Pearls:** * **Rate-limiting step of glycolysis:** Phosphofructokinase-1 (PFK-1). * **Irreversible steps of glycolysis:** Glucokinase/Hexokinase, PFK-1, and Pyruvate Kinase (Steps 1, 3, and 10). * **Clinical Correlation:** **Pyruvate Kinase deficiency** is the second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). It leads to decreased ATP production, causing RBC membrane instability and "echinocytes" (burr cells) on peripheral smear. * **Regulation:** Pyruvate kinase is allosterically activated by Fructose-1,6-bisphosphate (feed-forward activation) and inhibited by ATP and Alanine.

Q: What is the net generation of ATP in glycolysis?

7. **Explanation:** In the context of NEET-PG and modern biochemistry (based on Harper’s Illustrated Biochemistry), the net ATP yield of glycolysis is calculated based on the **Malate-Aspartate Shuttle**, which is the predominant shuttle in the liver and heart. **Why 7 is the correct answer:** The net yield is calculated by subtracting the ATP consumed from the total ATP produced: 1. **ATP Consumed:** 2 ATP (at the Hexokinase and Phosphofructokinase-1 steps). 2. **ATP Produced (Substrate-level phosphorylation):** 4 ATP (at the Phosphoglycerate kinase and Pyruvate kinase steps). 3. **ATP from NADH (Oxidative phosphorylation):** 2 NADH are produced. Using the current oxidative phosphorylation ratios (1 NADH = 2.5 ATP), 2 NADH yield **5 ATP**. * **Calculation:** (4 + 5) - 2 = **7 ATP**. **Analysis of Incorrect Options:** * **Option A (5):** This would be the net yield if the **Glycerol-3-Phosphate Shuttle** (common in muscle/brain) were used, where 1 NADH yields only 1.5 ATP (Total: 4 + 3 - 2 = 5). However, unless specified, the higher yield is generally considered the standard "net" for the pathway. * **Option C (15) & D (20):** These values are incorrect for glycolysis alone. 15 ATP is the yield for one turn of the TCA cycle (including the PDH reaction), and 30-32 ATP is the total yield for the complete aerobic oxidation of one glucose molecule. **High-Yield Clinical Pearls for NEET-PG:** * **Anaerobic Glycolysis:** The net yield is always **2 ATP** because NADH is consumed to reduce pyruvate to lactate. * **Rate-limiting step:** Phosphofructokinase-1 (PFK-1). * **Rapoport-Luebering Cycle:** In RBCs, 2,3-BPG is produced, bypassing the first substrate-level phosphorylation, resulting in a net yield of **0 ATP**. * **Arsenic Poisoning:** Inhibits ATP production in glycolysis by competing with inorganic phosphate at the Glyceraldehyde-3-phosphate dehydrogenase step.

Q: Cori's cycle is a term used for which of the following pathways?

Lactic acid cycle. **Explanation:** **Cori’s Cycle (Lactic Acid Cycle)** is a metabolic pathway that describes the interplay between the skeletal muscle and the liver. 1. **Why Option A is correct:** During vigorous exercise, muscular demand for ATP exceeds the oxygen supply, leading to **anaerobic glycolysis**. In this process, pyruvate is converted to **lactate** by the enzyme Lactate Dehydrogenase (LDH). This lactate is released into the bloodstream and taken up by the **liver**, where it is converted back into glucose via **gluconeogenesis**. This glucose is then returned to the muscle to be used as energy. This recycling of lactate to glucose is why it is synonymous with the Lactic acid cycle. 2. **Why other options are incorrect:** * **Option B (Citric acid cycle):** Also known as the Krebs cycle or TCA cycle, this occurs in the mitochondria and is the final common pathway for the oxidation of carbohydrates, fats, and proteins. * **Option C (Pentose phosphate pathway):** Also known as the Hexose Monophosphate (HMP) Shunt, this pathway generates NADPH and ribose-5-phosphate; it does not involve lactate recycling. **High-Yield Clinical Pearls for NEET-PG:** * **Net Energy Cost:** The Cori cycle is energy-consuming. It costs **6 ATP** in the liver to synthesize glucose, while only **2 ATP** are produced during anaerobic glycolysis in the muscle (Net loss of 4 ATP). * **Purpose:** Its primary role is to prevent **lactic acidosis** in the muscle and provide a continuous glucose supply during fasting or intense exercise. * **Glucose-Alanine Cycle (Cahill Cycle):** Often confused with Cori’s cycle, the Cahill cycle involves the transport of amino groups from muscle to liver via **Alanine** instead of lactate.

Q: All of the following vitamins are required in the citric acid cycle, EXCEPT:

Ascorbic acid. **Explanation:** The Citric Acid Cycle (TCA cycle) is the central metabolic pathway for the oxidation of acetyl-CoA. It requires several B-complex vitamins acting as essential cofactors for its enzymatic reactions. **Why Ascorbic Acid (Vitamin C) is the correct answer:** Ascorbic acid is primarily involved in collagen synthesis (hydroxylation of proline and lysine), antioxidant defense, and iron absorption. It plays **no direct role** as a cofactor in the enzymes of the citric acid cycle. **Why the other options are incorrect:** The TCA cycle requires four specific B-vitamins to function: * **Thiamin (B1):** As Thiamin Pyrophosphate (TPP), it is a mandatory cofactor for the **α-Ketoglutarate dehydrogenase** complex. * **Riboflavin (B2):** As FAD, it acts as a prosthetic group for **Succinate dehydrogenase**. * **Niacin (B3):** As NAD+, it serves as an electron acceptor for **Isocitrate dehydrogenase**, **α-Ketoglutarate dehydrogenase**, and **Malate dehydrogenase**. * **Pantothenic acid (B5):** (Though not listed in options) It is a structural component of **Coenzyme A**, essential for the formation of Acetyl-CoA and Succinyl-CoA. **High-Yield Clinical Pearls for NEET-PG:** 1. **The "Big Four" Vitamins:** Remember that the α-Ketoglutarate dehydrogenase complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**evolving **N**ew **P**ants **L**oose). 2. **Arsenite Poisoning:** Arsenite inhibits the α-Ketoglutarate dehydrogenase complex by binding to the -SH groups of Lipoic acid, leading to a clinical presentation similar to pyruvate dehydrogenase deficiency. 3. **Succinate Dehydrogenase:** This is the only enzyme of the TCA cycle that is also part of the Electron Transport Chain (Complex II) and is located on the inner mitochondrial membrane.

Q: For the tricarboxylic acid cycle to be continuous, what molecule requires regeneration?

Oxaloacetic acid. The Tricarboxylic Acid (TCA) cycle, also known as the Krebs cycle, is a series of reactions occurring in the mitochondrial matrix. For the cycle to remain continuous, it must function as a true "closed loop." **Explanation of the Correct Answer:** **Oxaloacetic acid (OAA)** is the correct answer because it acts as the "starting material" and the "final product" of the cycle. The cycle begins when the 2-carbon Acetyl-CoA condenses with the 4-carbon **Oxaloacetate** to form Citrate (catalyzed by Citrate Synthase). Through a series of redox and decarboxylation reactions, OAA is eventually regenerated from Malate. If OAA is not regenerated, the cycle halts because there is no acceptor molecule for the incoming Acetyl-CoA. **Why the other options are incorrect:** * **Pyruvic acid:** This is a precursor to the TCA cycle (converted to Acetyl-CoA via the PDH complex) but is not a component *within* the cycle itself. * **Alpha-oxoglutaric acid (α-Ketoglutarate):** This is an intermediate produced during the cycle. While essential, it is consumed to form Succinyl-CoA and is not the specific molecule required to restart the condensation step. * **Malic acid:** This is the immediate precursor to OAA. While its oxidation is necessary, it is the regeneration of OAA specifically that allows the cycle to accept a new unit of Acetyl-CoA. **NEET-PG High-Yield Pearls:** * **Anaplerotic Reactions:** These are "filling up" reactions that replenish TCA intermediates. The most important anaplerotic reaction is the conversion of Pyruvate to OAA by **Pyruvate Carboxylase** (requires Biotin and ATP). * **Rate-Limiting Step:** Isocitrate Dehydrogenase is the key rate-limiting enzyme of the TCA cycle. * **Energy Yield:** One turn of the TCA cycle produces 3 NADH, 1 FADH₂, and 1 GTP (Total ~10 ATP equivalents).

Q: Glucose-6-phosphate dehydrogenase deficiency causes which of the following conditions?

Hemolytic anemia. **Explanation:** **Glucose-6-phosphate dehydrogenase (G6PD) deficiency** is an X-linked recessive disorder and the most common enzyme deficiency worldwide. **Why Hemolytic Anemia is Correct:** G6PD is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary role in red blood cells (RBCs) is to produce **NADPH**. NADPH is essential for maintaining a pool of **reduced glutathione**, which acts as an antioxidant to neutralize reactive oxygen species (like H2O2). In G6PD deficiency, the lack of NADPH leads to oxidative stress, causing hemoglobin to denature and precipitate as **Heinz bodies**. These damaged RBCs are destroyed in the spleen, resulting in **episodic hemolytic anemia**, typically triggered by infections, fava beans, or oxidant drugs (e.g., Primaquine, Sulfa drugs). **Why Other Options are Incorrect:** * **Megaloblastic Anemia:** Caused by Vitamin B12 or Folic acid deficiency, leading to impaired DNA synthesis and macrocytic RBCs. * **Sickle Cell Anemia:** A qualitative hemoglobinopathy caused by a point mutation (Glu → Val) in the beta-globin chain. * **Microcytic Anemia:** Most commonly caused by Iron deficiency or Thalassemia, characterized by small RBCs (low MCV). **High-Yield Clinical Pearls for NEET-PG:** * **Peripheral Smear:** Look for **Heinz bodies** (supravital stain) and **Bite cells** (degluticytes) formed by splenic macrophages. * **Inheritance:** X-linked recessive (primarily affects males). * **Protective Effect:** G6PD deficiency offers a survival advantage against *Plasmodium falciparum* malaria. * **Key Trigger:** Primaquine is a classic board-exam trigger for a hemolytic crisis in these patients.

Question 1

Which of the following enzyme activities decreases during fasting?

Accepted Answer

Acetyl CoA Carboxylase

Answer

Hormone sensitive lipase

Answer

Glycogen Phosphorylase

Answer

CPS I

Question 2

Which of the following enzymes is required for glycolysis?

Accepted Answer

Pyruvate kinase

Answer

Pyruvate carboxylase

Answer

Glucose-6-phosphatase

Answer

Glycerokinase

Question 3

What is the net generation of ATP in glycolysis?

Accepted Answer

7

Answer

5

Answer

15

Answer

20

Question 4

Cori's cycle is a term used for which of the following pathways?

Accepted Answer

Lactic acid cycle

Answer

Citric acid cycle

Answer

Pentose phosphate pathway

Answer

None of the above

Question 5

All of the following vitamins are required in the citric acid cycle, EXCEPT:

Accepted Answer

Ascorbic acid

Answer

Riboflavin

Answer

Niacin

Answer

Thiamin

Question 6

For the tricarboxylic acid cycle to be continuous, what molecule requires regeneration?

Accepted Answer

Oxaloacetic acid

Answer

Pyruvic acid

Answer

Alpha-oxoglutaric acid

Answer

Malic acid

Question 7

Glucose-6-phosphate dehydrogenase deficiency causes which of the following conditions?

Accepted Answer

Hemolytic anemia

Answer

Megaloblastic anemia

Answer

Sickle cell anemia

Answer

Microcytic anemia

Question 8

Which form of carbohydrate is present in glycoproteins?

Accepted Answer

Monosaccharide

Answer

Sugar alcohol

Answer

Homo polysaccharide

Answer

Hetero polysaccharide

Question 9

Within the red blood cell, hypoxia stimulates glycolysis via which of the following regulatory mechanisms?

Accepted Answer

Hypoxia stimulates the release of all glycolytic enzymes from band 3 or RBC membranes

Answer

Hypoxia stimulates pyruvate dehydrogenase by increased 2,3-DPG

Answer

Hypoxia inhibits hexokinase

Answer

Activation of regulatory enzymes by high pH

Question 10

Which compound can give rise to glucose by gluconeogenesis?

Accepted Answer

Lactate

Answer

Acetyl CoA

Answer

Palmitic acid

Answer

Fructose

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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