Carbohydrate Metabolism Practice Questions

Q: Which of the following is NOT a substrate for gluconeogenesis?

Leucine. **Explanation:** Gluconeogenesis is the metabolic pathway that generates glucose from non-carbohydrate precursors. To be a substrate for gluconeogenesis, a molecule must be capable of being converted into **Pyruvate** or an intermediate of the **TCA cycle** (like Oxaloacetate). **Why Leucine is the Correct Answer:** Amino acids are classified as glucogenic, ketogenic, or both. **Leucine and Lysine** are the only two **purely ketogenic** amino acids. They are metabolized directly into Acetyl-CoA or Acetoacetate. Because the Pyruvate Dehydrogenase reaction (Pyruvate → Acetyl-CoA) is irreversible in humans, Acetyl-CoA cannot be converted back into glucose. Therefore, Leucine cannot serve as a substrate for gluconeogenesis. *Note: While Option D (Lysine) is also purely ketogenic and technically correct, in standard medical examinations, Leucine is the classic "textbook" answer for this question.* **Analysis of Other Options:** * **Lactate:** Produced via anaerobic glycolysis, it enters the liver and is converted to pyruvate by Lactate Dehydrogenase (Cori Cycle). * **Alanine:** The primary glucogenic amino acid. It undergoes transamination to form Pyruvate (Glucose-Alanine Cycle). * **Lysine:** Like Leucine, it is purely ketogenic. (In many MCQ formats, if both are present, Leucine is the preferred answer, though both are non-glucogenic). **High-Yield Clinical Pearls for NEET-PG:** * **Purely Ketogenic:** Leucine and Lysine (The "L"s). * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (Mnemonic: **PITTT**). * **Major Site:** 90% occurs in the Liver; 10% in the Kidney (increases during prolonged starvation). * **Key Regulatory Enzyme:** Fructose-1,6-bisphosphatase (inhibited by Fructose-2,6-bisphosphate).

Q: GLUT-5 is a transporter for which of the following?

Fructose. **Explanation:** The correct answer is **Fructose**. Glucose transporters (GLUTs) are a family of transmembrane proteins that facilitate the passive diffusion of monosaccharides across cell membranes. **GLUT-5** is unique among the GLUT family because it has a high affinity specifically for **fructose** and lacks the ability to transport glucose or galactose under physiological conditions. It is primarily expressed in the apical membrane of enterocytes in the small intestine, where it facilitates the absorption of dietary fructose. **Analysis of Options:** * **A. Glucose:** Transported primarily by GLUT-1 to GLUT-4. GLUT-1 and GLUT-3 are for basal uptake, GLUT-2 is high-capacity (liver/pancreas), and GLUT-4 is insulin-dependent (muscle/adipose). * **C. Mannose:** While mannose can be transported by some hexose transporters, it is not the primary substrate for GLUT-5. * **D. Galactose:** Along with glucose, galactose is transported into the enterocyte via **SGLT-1** (active transport) and exits into the blood via **GLUT-2**. GLUT-5 does not recognize galactose. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 vs. GLUT-5:** Remember that SGLT-1 is a secondary active transporter (Sodium-dependent) for Glucose and Galactose, whereas GLUT-5 is a facilitated diffuser (Sodium-independent) for Fructose. * **GLUT-2:** This is the "bidirectional" transporter found in the liver, pancreas, and the basolateral membrane of the intestine; it transports glucose, galactose, and fructose. * **Spermatozoa:** GLUT-5 is also found in mature spermatozoa, as fructose is their primary energy source. * **Insulin Independence:** GLUT-5, like most GLUTs (except GLUT-4), is **not** regulated by insulin.

Q: Which of the following diseases occurs due to the deficiency of glucocerebroside?

Gaucher disease. **Explanation:** The question addresses **Sphingolipidoses**, a subgroup of Lysosomal Storage Diseases. These occur due to the deficiency of specific lysosomal hydrolases, leading to the accumulation of complex lipids. **Correct Option: A. Gaucher disease** Gaucher disease is the most common lysosomal storage disorder. It is caused by a deficiency of the enzyme **$\beta$-Glucocerebrosidase** (also known as acid $\beta$-glucosidase). This deficiency leads to the accumulation of **Glucocerebroside** (glucosylceramide) in the macrophages of the reticuloendothelial system. These lipid-laden macrophages are called **Gaucher cells**, classically described as having a "wrinkled tissue paper" appearance. **Incorrect Options:** * **B. Pompe disease:** This is a Glycogen Storage Disease (Type II) caused by a deficiency of **$\alpha$-1,4-glucosidase** (Acid Maltase). It primarily affects the heart and skeletal muscles. * **C. Fabry disease:** This is an X-linked disorder caused by a deficiency of **$\alpha$-galactosidase A**, leading to the accumulation of Ceramide trihexoside. Clinical features include angiokeratomas and renal failure. * **D. Krabbe disease:** This is caused by a deficiency of **$\beta$-galactocerebrosidase**, leading to the accumulation of Galactocerebroside and psychosine, which destroys myelin-producing oligodendrocytes. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Disease:** Look for hepatosplenomegaly, bone pain (Erlenmeyer flask deformity of the femur), and Gaucher cells in bone marrow. * **Enzyme Replacement Therapy (ERT):** Recombinant glucocerebrosidase (Imiglucerase) is the treatment of choice for Gaucher Type 1. * **Niemann-Pick Disease:** Often confused with Gaucher; it is due to **Sphingomyelinase** deficiency and presents with a "Cherry-red spot" on the macula and foam cells.

Q: What is the chemical nature of streptomycin?

Glycoside. **Explanation:** **Streptomycin** is a classic example of an **Aminoglycoside** antibiotic. In biochemistry, a **glycoside** is defined as a molecule where a sugar is bound to another functional group (either another sugar or a non-sugar moiety) via a glycosidic bond. Streptomycin consists of three components linked together: 1. **Streptidine** (an aminocyclitol/aglycone part) 2. **Streptose** (a 5-carbon sugar) 3. **N-methyl-L-glucosamine** (an amino sugar) Because these components are joined by glycosidic linkages, its chemical nature is fundamentally that of a glycoside. **Analysis of Incorrect Options:** * **A. Peptide:** Peptides are chains of amino acids linked by peptide bonds. While some antibiotics (like Vancomycin or Bacitracin) are glycopeptides or cyclic peptides, Streptomycin does not contain amino acid chains. * **C. Phospholipid:** These are lipids containing a phosphate group (e.g., Lecithin). Streptomycin is highly polar and water-soluble, lacking the long fatty acid chains characteristic of phospholipids. * **D. Glycolipid:** These are lipids with a carbohydrate attached (e.g., Cerebrosides). Streptomycin does not have a lipid/fatty acid component. **NEET-PG Clinical Pearls:** * **Mechanism of Action:** Streptomycin binds to the **30S ribosomal subunit**, causing misreading of mRNA and inhibition of protein synthesis. * **Clinical Use:** It is a first-line drug for **Tuberculosis** (part of the RIPE regimen) and is also used for Plague and Tularemia. * **Adverse Effects:** Highly high-yield for exams—it is **Ototoxic** (specifically vestibulotoxic, affecting cranial nerve VIII) and **Nephrotoxic**. It is contraindicated in pregnancy as it can cause fetal deafness.

Q: What is the carbohydrate reserve of a human adult?

500 g. **Explanation:** The carbohydrate reserve of a healthy human adult (weighing approximately 70 kg) is stored primarily as **glycogen** in the liver and skeletal muscles. The total body glycogen content is approximately **500 g**, which provides roughly 2,000 kcal of energy. 1. **Muscle Glycogen (~400 g):** This constitutes the largest portion of the reserve. It is used locally by muscles during exercise to generate ATP via glycolysis. It cannot contribute to blood glucose levels because muscle tissue lacks the enzyme *glucose-6-phosphatase*. 2. **Liver Glycogen (~100 g):** This serves as the primary source for maintaining blood glucose levels during fasting (post-absorptive state) through glycogenolysis. 3. **Blood Glucose (~2-5 g):** A very small amount of glucose is present in the extracellular fluid. **Analysis of Options:** * **A (100 g):** This represents only the liver glycogen store, not the total body reserve. * **B (200 g):** This is an underestimate of the combined muscle and liver stores. * **D (1100 g):** This value is too high for a standard adult; such levels are rarely reached even with intensive "carb-loading." **High-Yield Clinical Pearls for NEET-PG:** * **Glycogen Storage:** Liver glycogen concentration is higher (up to 10% of tissue weight), but muscle contains a larger total amount due to its greater mass. * **Depletion:** Liver glycogen is typically depleted after **12–18 hours of fasting**, after which gluconeogenesis becomes the sole source of blood glucose. * **Key Enzyme:** *Glycogen synthase* is the rate-limiting enzyme for glycogenesis; *Glycogen phosphorylase* is the rate-limiting enzyme for glycogenolysis.

Q: Which compound is produced in the hexose monophosphate (pentose phosphate) pathway?

NADPH. **Explanation:** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is an alternative pathway for glucose oxidation. Unlike glycolysis, its primary purpose is not energy production (ATP) but the generation of specialized products for biosynthesis and antioxidant defense. **Why NADPH is the correct answer:** The oxidative phase of the HMP shunt, catalyzed by the rate-limiting enzyme **Glucose-6-Phosphate Dehydrogenase (G6PD)**, reduces NADP⁺ to **NADPH**. This molecule is crucial for: 1. **Reductive Biosynthesis:** Synthesis of fatty acids, cholesterol, and steroid hormones. 2. **Antioxidant Defense:** Maintaining **reduced glutathione** to protect cells (especially RBCs) from reactive oxygen species (ROS). **Analysis of Incorrect Options:** * **A. ATP:** The HMP shunt is unique because it neither consumes nor produces ATP. * **B. NADH:** NADH is primarily generated in glycolysis and the TCA cycle for the Electron Transport Chain. The HMP shunt specifically uses the NADP⁺/NADPH pool. * **D. Fructose 1,6-bisphosphate:** This is an intermediate of glycolysis and gluconeogenesis, not the HMP shunt. The HMP shunt produces Pentose phosphates (like Ribose-5-P) and glycolytic intermediates like Glyceraldehyde-3-P and Fructose-6-P. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It leads to hemolytic anemia under oxidative stress (e.g., Fava beans, Primaquine, Infection) because RBCs cannot generate NADPH to neutralize free radicals. * **Tissue Distribution:** The pathway is most active in the liver, lactating mammary glands, adrenal cortex, and RBCs. * **Thiamine (B1) Connection:** The non-oxidative phase uses **Transketolase**, which requires Thiamine pyrophosphate as a cofactor. Measuring transketolase activity is used to diagnose B1 deficiency.

Q: Which one of the following is NOT a fuel for gluconeogenesis?

Acetyl CoA. **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates. **Why Acetyl CoA is the correct answer:** Acetyl CoA cannot be used as a substrate for gluconeogenesis in humans. This is because the **Pyruvate Dehydrogenase (PDH) complex** reaction—which converts pyruvate to Acetyl CoA—is **irreversible**. Furthermore, in the TCA cycle, the two carbons that enter as Acetyl CoA are lost as two molecules of $\text{CO}_2$ before reaching oxaloacetate. Therefore, there is no net gain of carbon to enter the gluconeogenic pathway. Instead, Acetyl CoA acts as an **allosteric activator** of Pyruvate Carboxylase, signaling that enough energy is available to start gluconeogenesis. **Analysis of other options:** * **Glycerol:** Derived from the hydrolysis of triacylglycerols in adipose tissue. It is phosphorylated to glycerol-3-phosphate and then oxidized to dihydroxyacetone phosphate (DHAP), a direct intermediate of gluconeogenesis. * **Lactate:** Produced by anaerobic glycolysis in skeletal muscle and RBCs. It is converted back to pyruvate by **Lactate Dehydrogenase** in the liver (Cori Cycle) to enter gluconeogenesis. * **Glucogenic Amino Acids:** (Note: Option D appears to be a distractor/typo in the prompt, but typically refers to amino acids like Alanine). Alanine is converted to pyruvate via transamination to serve as a major substrate. **High-Yield NEET-PG Pearls:** * **Key Regulatory Enzyme:** Fructose-1,6-bisphosphatase (inhibited by Fructose-2,6-bisphosphate). * **Location:** Occurs primarily in the **Liver** (90%) and Kidney (10%). * **Odd-chain Fatty Acids:** Unlike even-chain fatty acids, these *can* be gluconeogenic because they yield **Propionyl CoA**, which enters the TCA cycle as Succinyl CoA. * **Leucine and Lysine:** These are the only two purely ketogenic amino acids and cannot provide glucose.

Q: Which of the following statements about glycosaminoglycans is FALSE?

Glycosaminoglycans are highly positively charged.. **Explanation:** **1. Why Option C is the Correct (False) Statement:** Glycosaminoglycans (GAGs) are **highly negatively charged** molecules, not positively charged. This intense negative charge is due to the presence of **sulfate groups** and **uronic acid (carboxyl) groups**. This property is physiologically vital: the negative charges repel each other, causing the GAG chains to remain extended in solution. Furthermore, they attract water molecules (hydration), creating a "cushioning" effect or lubrication, which is essential for joint health and structural integrity. **2. Analysis of Other Options:** * **Option A (True):** Most GAGs (except hyaluronic acid) are covalently linked to a **core protein**, forming a complex known as a **proteoglycan**. * **Option B (True):** GAGs are major components of the **extracellular matrix (ECM)** and connective tissues (e.g., cartilage, bone, skin, and vitreous humor), providing structural support. * **Option D (True):** As explained above, GAGs are polyanionic due to sulfate and carboxyl groups; thus, they are indeed negatively charged. **3. High-Yield Clinical Pearls for NEET-PG:** * **Hyaluronic Acid:** The only GAG that is **non-sulfated**, not covalently attached to a protein, and not limited to animal tissues (also found in bacteria). * **Heparin:** The GAG with the **highest negative charge density**; it acts as a natural anticoagulant by activating Antithrombin III. * **Chondroitin Sulfate:** The most abundant GAG in the body (found in cartilage and bone). * **Mucopolysaccharidoses (MPS):** Genetic disorders (e.g., Hurler and Hunter syndromes) caused by the deficiency of lysosomal enzymes required to degrade GAGs, leading to their accumulation in tissues.

Question 1

Which of the following is NOT a substrate for gluconeogenesis?

Accepted Answer

Leucine

Answer

Lactate

Answer

Alanine

Answer

Lysine

Question 2

GLUT-5 is a transporter for which of the following?

Accepted Answer

Fructose

Answer

Glucose

Answer

Mannose

Answer

Galactose

Question 3

Which of the following diseases occurs due to the deficiency of glucocerebroside?

Accepted Answer

Gaucher disease

Answer

Pompe disease

Answer

Fabry disease

Answer

Krabbe disease

Question 4

What is the chemical nature of streptomycin?

Accepted Answer

Glycoside

Answer

Peptide

Answer

Phospholipid

Answer

Glycolipid

Question 5

What is the carbohydrate reserve of a human adult?

Accepted Answer

500 g

Answer

100 g

Answer

200 g

Answer

1100 g

Question 6

Which compound is produced in the hexose monophosphate (pentose phosphate) pathway?

Accepted Answer

NADPH

Answer

ATP

Answer

NADH

Answer

Fructose 1,6-bisphosphate

Question 7

Which one of the following is NOT a fuel for gluconeogenesis?

Accepted Answer

Acetyl CoA

Answer

Glycerol

Answer

Lactate

Answer

Shoening of the cell cycle

Question 8

Which enzyme is common to both gluconeogenesis and glycolysis pathways?

Accepted Answer

Phosphofructokinase

Answer

Fructose 2,6-bisphosphatase

Answer

Hexokinase

Answer

Glucose 6-phosphatase

Question 9

Which of the following statements about glycosaminoglycans is FALSE?

Accepted Answer

Glycosaminoglycans are highly positively charged.

Answer

Glycosaminoglycans are associated with core proteins.

Answer

Glycosaminoglycans may be associated with connective tissues.

Answer

Glycosaminoglycans are negatively charged.

Question 10

A 28-year-old man complains of abdominal bloating and diarrhea following a meal containing paneer, a dairy product. These symptoms are absent with other foods. Which of the following enzymes is likely deficient in this patient?

Accepted Answer

Sucrase

Answer

a-amylase

Answer

b-galactosidase

Answer

a-glucosidase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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