Carbohydrate Metabolism — MCQs

A patient with type 1 diabetes self-injected insulin prior to their evening meal, but then was distracted and forgot to eat. A few hours later, the individual fainted, and after the paramedics arrived, they did a STAT blood glucose level and found it to be 45 mg/dL. The blood glucose level was so low because which one of the following tissues assimilated most of it under these conditions?

Q339Medium

Which of the following statements regarding enzymes involved in liver dephosphorylation and phosphorylation is TRUE?

Q340Easy

The pentose phosphate pathway is essential for the production of which molecule?

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 331: GLUT 4 Receptors are primarily found in which tissues?

A. Brain, red blood cells
B. Neurons and placenta
C. Liver, kidney, ileum
D. Skeletal muscle, fat and cardiac tissue (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** GLUT 4 is the only **insulin-dependent** glucose transporter. Under basal conditions, GLUT 4 is sequestered in intracellular vesicles. When insulin binds to its receptor, it triggers a signaling cascade that causes these vesicles to fuse with the plasma membrane, allowing glucose uptake. This mechanism is vital for post-prandial glucose disposal in **skeletal muscle** (the primary site for glucose utilization), **adipose tissue** (fat), and **cardiac muscle**. **2. Analysis of Incorrect Options:** * **Option A (Brain, RBCs):** Brain primarily uses **GLUT 3** (high affinity for constant supply), while RBCs use **GLUT 1**. Both are insulin-independent to ensure the brain and blood cells receive glucose even during fasting. * **Option B (Neurons, Placenta):** These tissues predominantly express **GLUT 3**. The placenta also utilizes GLUT 1 for maternal-fetal glucose transfer. * **Option C (Liver, Kidney, Ileum):** The liver and kidneys utilize **GLUT 2**, a high-capacity, low-affinity bidirectional transporter. The ileum uses **SGLT-1** (active transport) for glucose absorption and GLUT 2 for basolateral exit. **3. Clinical Pearls & High-Yield Facts:** * **Exercise & GLUT 4:** Muscle contraction can trigger GLUT 4 translocation to the membrane *independent* of insulin. This is why exercise helps manage blood glucose in Type 2 Diabetes. * **GLUT 2:** Acts as a "glucose sensor" in Pancreatic Beta cells. * **SGLT-2:** Located in the proximal convoluted tubule of the kidney; inhibited by "Gliflozin" drugs to treat diabetes. * **Mnemonic:** "GLUT **4** is for the **4**-chambered heart and muscles that move the **4** limbs."

Question 332: Gluconeogenesis occurs in which organ?

A. Muscle
B. Neurons
C. Spleen
D. Liver (Correct Answer)

Explanation: **Explanation:** **Gluconeogenesis** is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors (such as lactate, glycerol, and glucogenic amino acids). 1. **Why Liver is Correct:** The **Liver** is the primary site of gluconeogenesis (accounting for ~90% of production during overnight fasting). This is because the liver possesses the complete set of four key regulatory enzymes: Pyruvate carboxylase, PEP carboxykinase, Fructose-1,6-bisphosphatase, and **Glucose-6-phosphatase**. The Kidney cortex is the secondary site (~10%), becoming more significant during prolonged starvation. 2. **Why Other Options are Incorrect:** * **Muscle:** While muscles have high glycogen stores, they **lack Glucose-6-phosphatase**. Therefore, they cannot release free glucose into the blood; they can only use glucose for their own energy needs. * **Neurons:** The brain is a major *consumer* of glucose, not a producer. It lacks the enzymatic machinery for gluconeogenesis. * **Spleen:** The spleen is involved in lymphoid function and RBC sequestration; it does not play a role in glucose synthesis. **High-Yield NEET-PG Pearls:** * **Subcellular Localization:** Gluconeogenesis occurs in both the **Mitochondria** (initial steps) and the **Cytosol**. * **Key Enzyme:** **Glucose-6-phosphatase** is the "marker enzyme" for gluconeogenesis, also found in the kidney and small intestine. * **Cori Cycle:** Lactate produced by muscles travels to the liver to be converted back to glucose via gluconeogenesis. * **Energy Requirement:** Gluconeogenesis is an endergonic process, requiring **6 ATP/GTP** molecules to produce one molecule of glucose from two molecules of pyruvate.

Question 333: Which is the most abundant glucose transporter in Red Blood Cells (RBCs)?

A. GLUT-1 (Correct Answer)
B. GLUT-3
C. GLUT-4
D. GLUT-5

Explanation: **Explanation:** **1. Why GLUT-1 is the Correct Answer:** GLUT-1 is the primary glucose transporter found in **Red Blood Cells (RBCs)** and the **Blood-Brain Barrier**. RBCs lack mitochondria and are entirely dependent on anaerobic glycolysis for energy; therefore, they require a constant, insulin-independent supply of glucose. GLUT-1 has a high affinity (low $K_m$) for glucose, ensuring a steady basal uptake even during fasting states. In RBCs, GLUT-1 constitutes about 5% of the total membrane protein. **2. Why Other Options are Incorrect:** * **GLUT-3:** While it also has a high affinity for glucose, it is the primary transporter for **neurons** (Brain). It ensures glucose delivery to the brain even at low blood sugar levels. * **GLUT-4:** This is the only **insulin-dependent** glucose transporter. It is primarily located in **skeletal muscle and adipose tissue**. It is sequestered in intracellular vesicles and moves to the cell membrane only in the presence of insulin. * **GLUT-5:** This is a specialized transporter primarily responsible for the absorption of **fructose**, located mainly in the small intestine and spermatozoa. **3. High-Yield Clinical Pearls for NEET-PG:** * **GLUT-2:** A high-capacity, low-affinity (high $K_m$) transporter found in the **Liver, Pancreatic beta-cells, and Kidney**. It acts as a "glucose sensor." * **SGLT-1/SGLT-2:** These are sodium-dependent active transporters (Secondary active transport) found in the small intestine and renal tubules, unlike the GLUT family which facilitates **passive diffusion**. * **GLUT-1 Deficiency Syndrome:** Can lead to infantile seizures and developmental delay because glucose cannot cross the blood-brain barrier efficiently.

Question 334: What are the main products of the Hexose Monophosphate (HMP) shunt, excluding one?

A. NADPH
B. Fructose-6-phosphate
C. Glyceraldehyde-3-phosphate
D. CO2 (Correct Answer)

Explanation: ### **Explanation** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is an alternative pathway for glucose oxidation that occurs in the cytosol. Unlike glycolysis, its primary purpose is not the generation of ATP, but the production of specialized intermediates for biosynthesis. **1. Why CO2 is the correct answer (in the context of "excluding one"):** The question asks for the main products of the pathway. While CO2 is indeed produced during the oxidative phase (specifically during the conversion of 6-phosphogluconate to Ribulose-5-phosphate by the enzyme *6-phosphogluconate dehydrogenase*), it is a **by-product** (waste product) rather than a "main functional product" of the pathway. The primary metabolic goals of the HMP shunt are the generation of **NADPH** and **Pentose sugars** (like Ribose-5-phosphate). In many NEET-PG contexts, if a question asks for the "main products" and includes metabolic intermediates used for energy or synthesis, CO2 is often the outlier. **2. Analysis of Incorrect Options:** * **A. NADPH:** This is the most important product of the **Oxidative Phase**. It is essential for reductive biosynthesis (fatty acids/steroids) and maintaining reduced glutathione to prevent oxidative stress. * **B. Fructose-6-phosphate:** A key product of the **Non-oxidative Phase**. It allows the pathway to link back to glycolysis. * **C. Glyceraldehyde-3-phosphate:** Another product of the **Non-oxidative Phase** (shunted back to glycolysis). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to regenerate reduced glutathione, causing oxidative damage (Heinz bodies and Bite cells). * **Thiamine (B1) Connection:** The enzyme **Transketolase** requires Thiamine pyrophosphate as a cofactor. Measuring erythrocyte transketolase activity is the gold standard for diagnosing Thiamine deficiency (Wernicke-Korsakoff syndrome). * **Tissues involved:** Highly active in the adrenal cortex, liver, mammary glands, and RBCs.

Question 335: Gluconeogenesis occurs in all except:

A. Glycerol
B. Amino acid
C. Lactic acid
D. Palmitate (Correct Answer)

Explanation: **Explanation:** The core concept tested here is the **irreversibility of the Pyruvate Dehydrogenase (PDH) complex**. Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. To form glucose, a substrate must be capable of being converted into Pyruvate or an intermediate of the TCA cycle (like Oxaloacetate). **Why Palmitate is the correct answer:** Palmitate is a 16-carbon saturated fatty acid. Through beta-oxidation, even-chain fatty acids are broken down into **Acetyl-CoA**. In humans, Acetyl-CoA cannot be converted back into Pyruvate because the PDH reaction is irreversible. Furthermore, for every two carbons of Acetyl-CoA that enter the TCA cycle, two carbons are lost as $CO_2$. Therefore, there is no net synthesis of glucose from even-chain fatty acids like palmitate. **Why the other options are incorrect:** * **Glycerol:** Derived from triacylglycerol breakdown, it is phosphorylated to glycerol-3-phosphate and converted to **Dihydroxyacetone phosphate (DHAP)**, a direct intermediate of glycolysis/gluconeogenesis. * **Amino acids:** Glucogenic amino acids (e.g., Alanine) are deaminated to form Pyruvate or TCA cycle intermediates (alpha-ketoglutarate, succinyl-CoA), which can then enter the gluconeogenic pathway. * **Lactic acid:** Via the **Cori Cycle**, lactate produced by anaerobic glycolysis in muscles/RBCs is transported to the liver and converted back to Pyruvate by Lactate Dehydrogenase (LDH) to enter gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** 1. **Odd-chain fatty acids** ARE gluconeogenic because their final breakdown product is **Propionyl-CoA**, which converts to Succinyl-CoA. 2. **Leucine and Lysine** are the only purely ketogenic amino acids (cannot form glucose). 3. The major site of gluconeogenesis is the **Liver**, followed by the **Kidney** (especially during prolonged starvation). 4. The rate-limiting enzyme of gluconeogenesis is **Fructose-1,6-bisphosphatase**.

Question 336: Which of the following carbohydrate metabolism tests is used for liver function assessment?

A. Galactose intolerance test (Correct Answer)
B. Sucrose intolerance test
C. Glucose intolerance test
D. Lactose intolerance test

Explanation: **Explanation:** The **Galactose Tolerance Test (GTT)** is a specific biochemical assessment used to evaluate the metabolic capacity of the liver. Unlike glucose, which is utilized by almost all tissues in the body, galactose is almost exclusively metabolized in the **liver** (converted to glucose via the Leloir pathway). 1. **Why Galactose?** In a healthy individual, the liver rapidly clears galactose from the blood. If there is significant hepatocellular damage (e.g., cirrhosis or hepatitis), the liver's ability to convert galactose is impaired, leading to prolonged elevation of blood galactose levels or increased excretion in the urine. This makes it a sensitive marker for **functional hepatic reserve**. 2. **Analysis of Incorrect Options:** * **Sucrose & Lactose Intolerance Tests:** These are primarily tests of **intestinal brush border enzyme activity** (sucrase and lactase) and intestinal absorption. They diagnose malabsorption syndromes, not liver dysfunction. * **Glucose Intolerance Test:** While the liver plays a role in glycogenesis, glucose levels are heavily influenced by extrahepatic factors, most notably **insulin and glucagon** secretion from the pancreas. Therefore, it is used to diagnose Diabetes Mellitus rather than primary liver disease. **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of galactose to glucose-1-phosphate requires the enzyme **Galactose-1-phosphate uridyltransferase (GALT)**. Deficiency leads to Classic Galactosemia. * **Intravenous vs. Oral:** The IV Galactose Tolerance Test is preferred for liver assessment to bypass intestinal absorption variables. * **High-Yield Fact:** Galactose is often used to measure **Liver Blood Flow** because of its high hepatic extraction ratio.

Question 337: Cataract in diabetic patients is due to the accumulation of sorbitol in the lens. Which enzyme is responsible for this conversion?

A. Hexokinase
B. NADPH+ dependent aldose reductase (Correct Answer)
C. Glucokinase
D. Phosphofructokinase

Explanation: ### Explanation **1. Why Option B is Correct:** The formation of cataracts in diabetic patients is explained by the **Polyol Pathway** (Sorbitol Pathway). In states of hyperglycemia, the enzyme **Aldose Reductase** reduces excess glucose into **Sorbitol** (a sugar alcohol or polyol). This reaction requires **NADPH** as a cofactor. Sorbitol is osmotically active; it draws water into the lens, leading to swelling, crystalline precipitation, and eventual opacity (cataract). While some tissues can convert sorbitol to fructose via sorbitol dehydrogenase, the lens has very low levels of this enzyme, leading to toxic sorbitol accumulation. **2. Why Other Options are Incorrect:** * **Option A (Hexokinase):** This enzyme phosphorylates glucose to Glucose-6-Phosphate in most tissues. It has a low Km (high affinity) for glucose and is involved in glycolysis, not the polyol pathway. * **Option C (Glucokinase):** This is the liver and pancreatic B-cell specific isoenzyme of hexokinase. It has a high Km and functions primarily when glucose levels are high to initiate glycogen synthesis or insulin release. * **Option D (Phosphofructokinase-1):** This is the rate-limiting enzyme of **Glycolysis**. It converts Fructose-6-Phosphate to Fructose-1,6-Bisphosphate and does not interact with free glucose or sorbitol. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tissues involved:** "LUKE" – **L**ens, **U**rethra (Kidney), **K**idney, and **E**pinerium (Peripheral nerves) are susceptible to sorbitol damage because they lack **Sorbitol Dehydrogenase**. * **Cofactor Switch:** Aldose reductase uses **NADPH** (consuming it and causing oxidative stress), while Sorbitol Dehydrogenase uses **NAD+**. * **Galactosemia Connection:** Aldose reductase also converts Galactose to **Galactitol** (Dulcitol), which causes "Oil-drop cataracts" in infants with galactosemia.

Question 338: A patient with type 1 diabetes self-injected insulin prior to their evening meal, but then was distracted and forgot to eat. A few hours later, the individual fainted, and after the paramedics arrived, they did a STAT blood glucose level and found it to be 45 mg/dL. The blood glucose level was so low because which one of the following tissues assimilated most of it under these conditions?

A. Brain
B. Liver
C. Red blood cells
D. Adipose tissue (Correct Answer)

Explanation: ### Explanation The core concept in this question is the **insulin-dependent regulation of glucose transporters (GLUT)**. **1. Why Adipose Tissue is Correct:** Glucose uptake in tissues occurs via facilitated diffusion using GLUT transporters. While most GLUT transporters are insulin-independent, **GLUT-4** is strictly **insulin-dependent**. GLUT-4 is primarily located in **skeletal muscle** and **adipose tissue**. In this clinical scenario, the patient injected exogenous insulin. This high level of circulating insulin causes the rapid translocation of GLUT-4 from intracellular vesicles to the plasma membrane of adipocytes and muscle cells. Consequently, these tissues "sequester" or assimilate the majority of the available blood glucose, leading to profound hypoglycemia (45 mg/dL) when not countered by dietary intake. **2. Why Other Options are Incorrect:** * **A. Brain:** The brain utilizes **GLUT-1 and GLUT-3**, which are insulin-independent. While the brain is a major consumer of glucose, its uptake rate does not increase in response to insulin; rather, it suffers during hypoglycemia because it cannot increase uptake to compensate for low serum levels. * **B. Liver:** The liver uses **GLUT-2**. Although insulin affects hepatic metabolism (promoting glycogenesis), the actual *uptake* of glucose via GLUT-2 is insulin-independent and depends primarily on the blood glucose concentration gradient. * **C. Red Blood Cells (RBCs):** RBCs rely on **GLUT-1** for glucose uptake. This process is insulin-independent, ensuring the cells can survive even during fasting states. **3. High-Yield NEET-PG Pearls:** * **GLUT-4** is the only insulin-responsive transporter (found in Heart, Skeletal Muscle, and Adipose tissue). * **GLUT-2** has a high $K_m$ (low affinity) and acts as a glucose sensor in Pancreatic $\beta$-cells and the Liver. * **GLUT-1** is responsible for basal glucose uptake and is the primary transporter in the Blood-Brain Barrier and RBCs. * **GLUT-5** is unique as it primarily transports **fructose**, not glucose.

Question 339: Which of the following statements regarding enzymes involved in liver dephosphorylation and phosphorylation is TRUE?

A. Phosphorylase is always activated by cAMP-dependent Protein Kinase. (Correct Answer)
B. The enzyme is always activated.
C. Catecholamines do not play a role in this process.
D. Phosphorylation of glycogen synthase increases its activity.

Explanation: ### Explanation The regulation of glycogen metabolism in the liver is a high-yield topic for NEET-PG, centered on the reciprocal control of **Glycogen Phosphorylase** (breakdown) and **Glycogen Synthase** (synthesis) via covalent modification (phosphorylation). **1. Why Option A is Correct:** In the liver, glucagon or epinephrine binds to G-protein coupled receptors, increasing **cAMP** levels. This activates **Protein Kinase A (PKA)**. PKA then phosphorylates **Phosphorylase Kinase**, which in turn phosphorylates **Glycogen Phosphorylase**, converting it from the inactive 'b' form to the active 'a' form. Thus, phosphorylation (triggered by the cAMP cascade) always activates phosphorylase to mobilize glucose. **2. Why the Other Options are Incorrect:** * **Option B:** Enzymes are not "always activated." They are strictly regulated by hormonal signals (insulin vs. glucagon) to maintain blood glucose homeostasis. * **Option C:** Catecholamines (Epinephrine/Norepinephrine) play a crucial role. In the liver, they act via $\beta$-receptors (cAMP pathway) and $\alpha_1$-receptors ($Ca^{2+}$-calmodulin pathway) to stimulate glycogenolysis during stress or hypoglycemia. * **Option D:** Phosphorylation **decreases** the activity of **Glycogen Synthase**. It is active in the dephosphorylated state (Synthase 'a') and inactive when phosphorylated (Synthase 'b'). This ensures that glycogen synthesis and breakdown do not occur simultaneously (reciprocal regulation). **Clinical Pearls for NEET-PG:** * **Mnemonic:** **P**hosphorylation **P**romotes **P**hosphorylase (The "3 Ps"). * **Insulin** acts via **Protein Phosphatase-1**, which dephosphorylates both enzymes, thereby activating Glycogen Synthase and inactivating Glycogen Phosphorylase. * **Von Gierke’s Disease (GSD Type I):** Deficiency of Glucose-6-Phosphatase; the final step of glycogenolysis is blocked, leading to severe fasting hypoglycemia and hepatomegaly.

Question 340: The pentose phosphate pathway is essential for the production of which molecule?

A. NAD
B. FAD
C. NADPH (Correct Answer)
D. NADH

Explanation: ### Explanation The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is a unique pathway of glucose metabolism that occurs in the cytosol. Unlike glycolysis, its primary purpose is not the production of ATP, but rather the generation of two key products: **NADPH** and **Ribose-5-phosphate**. **Why NADPH is the correct answer:** NADPH is generated during the oxidative phase of the PPP by the enzyme **Glucose-6-Phosphate Dehydrogenase (G6PD)**. NADPH is crucial for: 1. **Reductive Biosynthesis:** Providing reducing equivalents for fatty acid and steroid synthesis (active in liver, mammary glands, and adrenal cortex). 2. **Antioxidant Defense:** Maintaining **reduced glutathione**, which protects cells (especially RBCs) from oxidative damage by reactive oxygen species (ROS). **Why other options are incorrect:** * **NAD and FAD:** These are oxidized coenzymes. The body typically synthesizes them from B-vitamins (Niacin and Riboflavin, respectively), not via the PPP. * **NADH:** This is primarily produced during glycolysis and the TCA cycle. While chemically similar to NADPH, NADH is used by the electron transport chain for **ATP production**, whereas NADPH is used for **biosynthesis and detoxification**. **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It leads to **hemolytic anemia** because RBCs lack mitochondria and depend solely on the PPP for NADPH to neutralize H₂O₂. * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (inhibited by high levels of NADPH). * **Non-oxidative phase:** Uses **Transketolase** (requires Thiamine/B1 as a cofactor), which links PPP intermediates back to glycolysis. * **Tissues involved:** "Livers, Glands, and RBCs"—Liver, lactating mammary glands, adrenal cortex, and erythrocytes.

Practice by Chapter

Carbohydrate Chemistry and Classification

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Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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