Carbohydrate Metabolism Practice Questions

Q: Which of the following is the insulin-dependent glucose transporter?

GLUT-4. ### Explanation **Correct Option: D (GLUT-4)** GLUT-4 is the only **insulin-dependent** glucose transporter. It is primarily expressed in **skeletal muscle, cardiac muscle, and adipose tissue**. In the resting state, 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, increasing glucose uptake. This mechanism is crucial for lowering postprandial blood glucose levels. **Analysis of Incorrect Options:** * **GLUT-1 (Option A):** This is an insulin-independent transporter found in **RBCs**, the blood-brain barrier, and kidneys. It provides a basal level of glucose uptake. * **GLUT-5 (Option B):** This is a unique transporter primarily located in the small intestine and spermatozoa. It is specific for **fructose** transport, not glucose. * **GLUT-3 (Option C):** This is an insulin-independent transporter with a very high affinity for glucose. It is the primary transporter in **neurons**, ensuring the brain receives glucose even during hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-2:** A bidirectional transporter found in the **Liver, Pancreas (B-cells), and Kidney**. It acts as a "glucose sensor." * **Exercise & GLUT-4:** Muscle contraction can also trigger GLUT-4 translocation to the cell membrane independent of insulin, which is why exercise helps manage Type 2 Diabetes. * **SGLT-1/2:** Unlike GLUTs (facilitated diffusion), SGLTs are **Secondary Active Transporters** (Sodium-Glucose co-transporters) found in the intestine and renal tubules. * **Mnemonic:** "BRICK L" for insulin-independent tissues: **B**rain, **R**BCs, **I**ntestine, **C**ornea, **K**idney, **L**iver.

Q: Cellulose is not broken down due to beta anomerism at which carbon atom?

C1. **Explanation:** The correct answer is **A. C1**. **Why C1 is correct:** Cellulose is a linear homopolysaccharide composed of D-glucose units linked by **β(1→4) glycosidic bonds**. In glucose, the **C1 carbon** is the anomeric carbon. In cellulose, the hydroxyl group at C1 is in the **beta (β) configuration** (pointing upwards). Human digestive enzymes, specifically **α-amylase** (found in saliva and pancreatic juice), are stereospecific; they can only hydrolyze **α(1→4)** linkages (found in starch and glycogen). Humans lack the enzyme **cellulase** (β-glucosidase), which is required to break the β(1→4) bond at the C1 anomeric position. Consequently, cellulose remains undigested and serves as dietary fiber. **Why the other options are incorrect:** * **C2:** This carbon carries a hydroxyl group but is not involved in the glycosidic linkage or the determination of anomerism. * **C5:** This carbon is part of the pyranose ring structure and determines the D or L isomerism, but it does not form the glycosidic bond. * **C6:** This is the primary alcohol group ($CH_2OH$) located outside the ring. While it can be modified in other sugars, it is not the site of anomerism or the glycosidic bond in cellulose. **High-Yield Facts for NEET-PG:** * **Dietary Fiber:** Because cellulose cannot be digested, it adds bulk to the stool, promotes peristalsis, and prevents constipation. * **Ruminants:** Animals like cows can digest cellulose because they harbor symbiotic bacteria in their gut that secrete the enzyme cellulase. * **Starch vs. Cellulose:** Starch (amylose) has **α(1→4)** bonds and is digestible; Cellulose has **β(1→4)** bonds and is indigestible. * **Iodine Test:** Cellulose does not give a color with iodine, unlike starch (blue) or glycogen (reddish-brown).

Q: NADPH+ and H+ is generated in the reaction catalyzed by which enzyme?

Glucose-6-phosphate dehydrogenase (G-6-PD). **Explanation:** The correct answer is **Glucose-6-phosphate dehydrogenase (G-6-PD)**. This enzyme catalyzes the first and rate-limiting step of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). In this reaction, Glucose-6-phosphate is oxidized to 6-phosphogluconolactone, and **NADP+ is reduced to NADPH + H+**. This pathway is the primary source of NADPH in the body, which is essential for reductive biosynthesis (e.g., fatty acids, steroids) and maintaining reduced glutathione to protect cells against oxidative stress. **Analysis of Incorrect Options:** * **Lactate dehydrogenase (LDH):** Involved in anaerobic glycolysis; it interconverts pyruvate and lactate using **NADH/NAD+**, not NADPH. * **Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD):** A key enzyme in glycolysis that converts Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, producing **NADH**. * **Alcohol dehydrogenase:** Catalyzes the oxidation of ethanol to acetaldehyde in the cytosol, utilizing **NAD+** as a coenzyme to produce **NADH**. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. Since RBCs lack mitochondria, they rely solely on the HMP shunt for NADPH. Deficiency leads to inadequate reduced glutathione, resulting in **hemolysis** triggered by oxidative stress (e.g., Fava beans, Primaquine, or infections). * **Heinz Bodies & Bite Cells:** Classic peripheral smear findings in G6PD deficiency. * **Tissue Distribution:** The HMP shunt is highly active in the liver, lactating mammary glands, adrenal cortex, and RBCs.

Q: The pentose phosphate pathway produces which of the following?

NADPH. **Explanation:** The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is an alternative pathway for glucose oxidation. Unlike glycolysis, its primary purpose is not energy production but the generation of biosynthetic precursors. **Why NADPH is the correct answer:** The oxidative phase of the PPP is the body's primary source of **NADPH** (Reduced Nicotinamide Adenine Dinucleotide Phosphate). This occurs via the rate-limiting enzyme **Glucose-6-Phosphate Dehydrogenase (G6PD)**. NADPH is essential for: 1. **Reductive Biosynthesis:** Synthesis of fatty acids, cholesterol, and steroid hormones. 2. **Antioxidant Defense:** Maintaining reduced glutathione to protect cells (especially RBCs) against reactive oxygen species (ROS). **Why other options are incorrect:** * **ATP & ADP:** The PPP is unique because it **neither consumes nor produces ATP**. It is an energy-neutral pathway focused on carbon shuffling and redox potential. * **NADH:** While NADH is structurally similar to NADPH, it is primarily used in the **Electron Transport Chain (ETC)** for ATP generation. The PPP specifically produces NADPH, which has a phosphate group that directs it toward anabolic (building) reactions rather than catabolic (breaking down) energy production. **NEET-PG Clinical Pearls:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Non-oxidative phase:** Produces **Ribose-5-Phosphate**, essential for nucleotide (DNA/RNA) synthesis. * **Clinical Correlation:** G6PD deficiency leads to **hemolytic anemia** because RBCs cannot generate enough NADPH to combat oxidative stress (e.g., after eating fava beans or taking Primaquine), leading to the formation of **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** Highly active in the liver, adrenal cortex, lactating mammary glands, and RBCs.

Q: Which metabolic pathway occurs partly in the mitochondria and partly in the cytoplasm?

Gluconeogenesis. **Explanation:** Metabolic pathways are often compartmentalized to ensure efficient regulation. While many pathways occur entirely in one compartment, a few "dual-compartment" pathways are high-yield for NEET-PG. **1. Why Gluconeogenesis is Correct:** Gluconeogenesis begins in the **mitochondria** with the conversion of Pyruvate to Oxaloacetate (OAA) by *Pyruvate Carboxylase*. Since OAA cannot cross the mitochondrial membrane, it is converted to Malate (or Aspartate), shuttled into the **cytoplasm**, and converted back to OAA. The remaining steps, starting from the conversion of OAA to Phosphoenolpyruvate (PEP) by *PEPCK*, occur in the cytoplasm. **2. Analysis of Incorrect Options:** * **A. Electron Transport Chain:** Occurs exclusively in the **Inner Mitochondrial Membrane**. * **B. Link Reaction:** The conversion of Pyruvate to Acetyl-CoA by the PDH complex occurs entirely within the **Mitochondrial Matrix**. * **C. Fatty Acid Synthesis:** Occurs primarily in the **Cytoplasm** (the "Extramitochondrial" pathway). Note: Fatty acid *oxidation* occurs in the mitochondria. **3. Clinical Pearls & High-Yield Facts:** * **Mnemonic for Dual-Compartment Pathways:** Remember **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. These three pathways require enzymes from both the mitochondria and cytoplasm. * **Key Enzyme:** *Pyruvate Carboxylase* (the first step of gluconeogenesis) is a mitochondrial enzyme and requires **Biotin** as a cofactor. * **The "Shuttle":** The Malate-Aspartate shuttle is crucial for transporting carbon skeletons out of the mitochondria during gluconeogenesis.

Q: In glycogen metabolism, which enzyme acts on the 1-6 glycosidic bond of glycogen to produce glucose?

Glucose. **Explanation:** In glycogenolysis, the breakdown of glycogen involves two primary enzymes: **Glycogen Phosphorylase** and the **Debranching Enzyme**. The **Debranching Enzyme** is a bifunctional protein. Its second activity, **$\alpha$-1,6-glucosidase**, specifically hydrolyzes the $\alpha$-1,6-glycosidic bond at the branch point. Unlike the action of phosphorylase (which uses inorganic phosphate to release Glucose-1-Phosphate), the $\alpha$-1,6-glucosidase activity uses water (hydrolysis) to release a **free Glucose** molecule directly. For every branch point, one molecule of free glucose is produced, accounting for approximately 8-10% of the total glucose released from glycogen. **Analysis of Options:** * **Option A (Glucose-1-phosphate):** This is the product of Glycogen Phosphorylase acting on $\alpha$-1,4-linkages. It is the major product of glycogenolysis but is not released from the 1-6 bond. * **Option B (Glucose-6-phosphate):** This is formed in the liver by the isomerization of Glucose-1-phosphate via *Phosphoglucomutase*. It is not a direct product of the debranching enzyme. * **Option C (Maltose):** This is a disaccharide produced during the digestion of starch by amylase, not during intracellular glycogen metabolism. **NEET-PG High-Yield Pearls:** * **Debranching Enzyme Deficiency:** Leads to **Cori’s Disease (GSD Type III)**, characterized by the accumulation of "limit dextrins" (abnormal glycogen with short outer branches). * **Ratio:** Glycogenolysis yields Glucose-1-Phosphate and free Glucose in a ratio of approximately **10:1**. * **Key Difference:** Phosphorylase performs *phosphorolysis* (saves ATP), while debranching enzyme performs *hydrolysis*.

Q: Which of the following statements about GLUT is false?

GLUT-2 is needed in the brain. The correct answer is **A. GLUT-2 is needed in the brain**. This statement is false because the brain primarily relies on **GLUT-1** (for crossing the blood-brain barrier) and **GLUT-3** (for neuronal uptake). GLUT-2 is a high-capacity, low-affinity transporter found in the liver, pancreatic beta cells, small intestine, and renal tubules, where it acts as a "glucose sensor." ### Explanation of Options: * **Option A (False/Correct):** GLUT-2 is not the primary transporter for the brain. The brain requires a constant glucose supply regardless of blood sugar levels; therefore, it utilizes GLUT-3, which has a very low Km (high affinity). * **Option B (True):** GLUT-3 is indeed present in the placenta to ensure a steady transfer of glucose to the fetus. * **Option C (True):** GLUT-4 is the only **insulin-dependent** glucose transporter. It is sequestered in intracellular vesicles and translocates to the cell membrane only in the presence of insulin. It is primarily found in skeletal muscle, cardiac muscle, and **adipose tissue**. * **Option D (True):** GLUT-3 has a wide distribution, including the brain (neurons), placenta, liver, kidneys, and specifically the **testis and intestine**. ### High-Yield Clinical Pearls for NEET-PG: * **GLUT-1 Deficiency Syndrome:** Presents with infantile seizures and developmental delay due to impaired glucose transport across the blood-brain barrier. * **SGLT vs. GLUT:** Remember that SGLT (1 & 2) are active transporters (secondary active) used in the intestine and kidneys, whereas all GLUTs (1–5) facilitate **passive diffusion**. * **GLUT-5:** Unique because it is primarily a **fructose** transporter, located in the small intestine and spermatozoa. * **Bidirectional Transport:** GLUT-2 is unique for its ability to transport glucose in both directions (e.g., during gluconeogenesis in the liver).

Question 1

Which of the following is the insulin-dependent glucose transporter?

Accepted Answer

GLUT-4

Answer

GLUT-1

Answer

GLUT-5

Answer

GLUT-3

Question 2

A 3-year-old child presents with hepatomegaly and recurrent episodes of vomiting, jaundice, and hypoglycemia, beginning from the later part of infancy. These episodes seem to have no temporal relation with the intake of milk or milk products. The mother also gives a history that the child has an aversion to sweet food. Which of the following enzymes may be deficient in this child?

Accepted Answer

Aldolase B

Answer

Glucose-6-phosphatase

Answer

Branching enzyme

Answer

Galactose 1 phosphate uridyl transferase

Question 3

Cellulose is not broken down due to beta anomerism at which carbon atom?

Accepted Answer

C1

Answer

C2

Answer

C5

Answer

C6

Question 4

NADPH+ and H+ is generated in the reaction catalyzed by which enzyme?

Accepted Answer

Glucose-6-phosphate dehydrogenase (G-6-PD)

Answer

Lactate dehydrogenase (LDH)

Answer

Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD)

Answer

Alcohol dehydrogenase

Question 5

The pentose phosphate pathway produces which of the following?

Accepted Answer

NADPH

Answer

ATP

Answer

ADP

Answer

NADH

Question 6

Which metabolic pathway occurs partly in the mitochondria and partly in the cytoplasm?

Accepted Answer

Gluconeogenesis

Answer

Electron transport chain

Answer

Link reaction

Answer

Fatty acid synthesis

Question 7

A male patient presents with pain in his calf muscles upon exercise. Muscle biopsy reveals the presence of glycogen. Which enzyme deficiency is most likely responsible for this condition?

Accepted Answer

Phosphofructokinase I

Answer

Branching enzyme

Answer

Debranching enzyme

Answer

Glucose 6 phosphatase

Question 8

In glycogen metabolism, which enzyme acts on the 1-6 glycosidic bond of glycogen to produce glucose?

Accepted Answer

Glucose

Answer

Glucose-1-phosphate

Answer

Glucose-6-phosphate

Answer

Maltose

Question 9

Which of the following statements about GLUT is false?

Accepted Answer

GLUT-2 is needed in the brain

Answer

GLUT-3 is present in the placenta

Answer

GLUT-4 is present in adipose tissue

Answer

GLUT-3 is present in both intestine and testis

Question 10

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

Accepted Answer

Mucopolysaccharidosis

Answer

Diabetes mellitus

Answer

Urea cycle defect

Answer

Glycogen storage disease

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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