Carbohydrate Metabolism Practice Questions

Q: Beta-glucosidase is defective in which disease?

Gaucher's disease. **Explanation:** **1. Why Gaucher's Disease is Correct:** Gaucher’s disease is the most common **Lysosomal Storage Disorder (LSD)**. It is caused by a deficiency of the enzyme **Acid Beta-glucosidase** (also known as **Glucocerebrosidase**). Under normal conditions, this enzyme breaks down glucocerebroside into glucose and ceramide. When defective, glucocerebroside accumulates within the lysosomes of macrophages, transforming them into characteristic **"Gaucher cells"** (described as having a "wrinkled tissue paper" appearance). **2. Why the Other Options are Incorrect:** * **Tay-Sachs Disease:** This is caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 gangliosides. It is clinically characterized by a cherry-red spot on the macula and progressive neurodegeneration. * **Galactosaemia:** This is a disorder of carbohydrate metabolism, most commonly due to a deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. It is not a lysosomal storage disease. * **Diabetes Mellitus:** This is a metabolic disorder characterized by hyperglycemia due to insulin deficiency or resistance, unrelated to lysosomal enzyme defects. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gaucher Cells:** Pathognomonic macrophages with fibrillary cytoplasm (wrinkled tissue paper appearance) found in the bone marrow, liver, and spleen. * **Clinical Triad:** Hepatosplenomegaly, bone involvement (Erlenmeyer flask deformity of the femur, bone crises), and pancytopenia. * **Biochemical Marker:** Elevated levels of **Serum Acid Phosphatase** (Tartrate-resistant) are often seen. * **Treatment:** Enzyme Replacement Therapy (ERT) with recombinant glucocerebrosidase (Imiglucerase) is the gold standard.

Q: Which of the following dietary components determines the glycemic index of food?

Fibre. ### Explanation **1. Why Fibre is the Correct Answer:** The **Glycemic Index (GI)** is a ranking of carbohydrates (0–100) based on how quickly they raise blood glucose levels after consumption. Dietary **fibre**, particularly soluble fibre (e.g., pectin, gums), is the primary determinant that lowers the GI of a food. It functions by increasing the viscosity of the intestinal contents, which slows down gastric emptying and delays the enzymatic digestion of starch. This results in a slower, more gradual absorption of glucose into the bloodstream, preventing rapid postprandial "spikes." **2. Why Other Options are Incorrect:** * **Protein (A) and Fat (B):** While protein and fat can influence the overall glycemic *load* of a meal by slowing digestion, they are not the primary dietary components used to define or determine the GI of a specific carbohydrate-rich food. GI is specifically a property of the carbohydrates present in the food. * **None of the above (D):** Incorrect, as fibre is a well-established physiological modulator of the glycemic response. **3. NEET-PG High-Yield Pearls:** * **Glycemic Load (GL):** Unlike GI, GL accounts for the **quantity** of carbohydrates in a typical serving (GL = GI × grams of carbohydrate / 100). * **Clinical Significance:** Low-GI diets (rich in fibre) are crucial in managing **Diabetes Mellitus**, metabolic syndrome, and PCOS, as they improve insulin sensitivity. * **Processing Factor:** Highly processed or overcooked foods generally have a higher GI because the physical barriers to digestion are removed, allowing for rapid glucose release.

Q: Which of the following is not an aldose sugar?

Fructose. ### Explanation **Core Concept: Aldose vs. Ketose Sugars** Monosaccharides are classified based on the type of carbonyl group they contain. * **Aldoses:** Contain an **aldehyde group (-CHO)**, typically located at the C1 position. * **Ketoses:** Contain a **keto group (>C=O)**, typically located at the C2 position. **Why Fructose is the Correct Answer:** Fructose is a **ketohexose**. It contains six carbon atoms but features a functional keto group at the second carbon (C2). Therefore, it is not an aldose. **Analysis of Incorrect Options:** * **A. Erythrose:** This is a four-carbon sugar (**aldotetrose**) used in the Pentose Phosphate Pathway. It contains an aldehyde group. * **B. Glucose:** The most common **aldohexose**. It serves as the primary metabolic fuel and contains an aldehyde group at C1. * **D. Galactose:** An **aldohexose** and a C4-epimer of glucose. It is a constituent of lactose (milk sugar). --- ### NEET-PG High-Yield Clinical Pearls 1. **Functional Isomers:** Glucose and Fructose are functional isomers because they have the same molecular formula ($C_6H_{12}O_6$) but different functional groups (aldehyde vs. ketone). 2. **Reducing Sugars:** Both aldoses and ketoses are reducing sugars. Fructose (a ketose) can reduce Benedict’s reagent because it undergoes **tautomerization** (enediol formation) in alkaline conditions to convert into glucose and mannose. 3. **Seliwanoff’s Test:** This biochemical test is specifically used to distinguish ketoses from aldoses. Ketoses (like Fructose) give a **cherry-red color** rapidly. 4. **Sorbitol Pathway:** In the polyol pathway, Glucose (aldose) is reduced to Sorbitol, which is then oxidized to Fructose (ketose) by sorbitol dehydrogenase. This is the primary source of energy for sperm in seminal fluid.

Q: Which of the following are sources of NADPH?

All of the above. **Explanation:** The correct answer is **D (All of the above)** because NADPH is primarily generated through specific enzymatic reactions in the cytoplasm to support reductive biosynthesis (like fatty acid and steroid synthesis) and to maintain antioxidant defenses. 1. **Pentose Phosphate Pathway (PPP/HMP Shunt):** This is the **major source** of NADPH in most tissues. The rate-limiting enzyme, **Glucose-6-Phosphate Dehydrogenase (G6PD)**, along with 6-phosphogluconate dehydrogenase, reduces $NADP^+$ to $NADPH$ during the oxidative phase. 2. **Malic Enzyme:** This enzyme converts Malate to Pyruvate in the cytoplasm. It plays a crucial role in the "Citrate-Malate Shuttle," providing the NADPH necessary for fatty acid synthesis in the liver and adipose tissue. 3. **Cytoplasmic Isocitrate Dehydrogenase (IDH1):** While the mitochondrial version (IDH3) produces NADH for the TCA cycle, the cytosolic isoform (NADP-dependent IDH) produces NADPH. It is particularly important in the brain and for maintaining the cytosolic pool of reduced glutathione. **Why other options are not "the only" answer:** Options A, B, and C are all individual, valid sources of NADPH. Therefore, selecting any single one would be incomplete, making "All of the above" the most accurate choice. **NEET-PG High-Yield Pearls:** * **G6PD Deficiency:** The most common enzymopathy worldwide; it leads to hemolytic anemia because RBCs lack mitochondria and rely *exclusively* on the PPP for NADPH to keep glutathione reduced. * **NADPH vs. NADH:** Remember: **NADH** is for **ATP** production (catabolic), while **NADPH** is for **B**iosynthesis (anabolic) and **B**reaking down free radicals. * **Tissues rich in PPP:** Adrenal cortex, liver, mammary glands, and testes (sites of active steroid or fatty acid synthesis).

Q: What are the enzymes required for the formation of Phosphoenolpyruvate from Pyruvate?

Pyruvate carboxylase and Phosphoenolpyruvate Carboxykinase. ### Explanation The conversion of Pyruvate to Phosphoenolpyruvate (PEP) is the first major hurdle in **Gluconeogenesis**. In Glycolysis, the conversion of PEP to Pyruvate by *Pyruvate Kinase* is irreversible. To bypass this, the body employs a two-step "bypass" mechanism: 1. **Pyruvate Carboxylase (PC):** Located in the mitochondria, this enzyme converts Pyruvate into Oxaloacetate (OAA). This reaction requires **Biotin** as a cofactor and ATP. 2. **Phosphoenolpyruvate Carboxykinase (PEPCK):** This enzyme converts OAA into PEP. This step occurs in the cytosol (or mitochondria) and requires **GTP**. **Analysis of Incorrect Options:** * **Pyruvate Dehydrogenase (PDH):** This enzyme converts Pyruvate to Acetyl-CoA. This is an irreversible step that leads into the TCA cycle, not gluconeogenesis. "Acetyl-CoA cannot be converted back to glucose." * **Lactate Dehydrogenase (LDH):** This catalyzes the reversible conversion of Pyruvate to Lactate (anaerobic glycolysis). It does not participate in the bypass of Pyruvate Kinase. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Pyruvate Carboxylase is a **mitochondrial** enzyme, while PEPCK is found in both the mitochondria and cytosol. * **Obligatory Activator:** Pyruvate Carboxylase is allosterically activated by **Acetyl-CoA**. High levels of Acetyl-CoA signal that the cell has enough energy, shunting pyruvate toward gluconeogenesis. * **Cofactor:** Remember the mnemonic **ABC** for carboxylases: **A**TP, **B**iotin, and **C**O₂ are required. * **Rate-limiting step:** PEPCK is often considered a key regulatory point in gluconeogenesis.

Q: Which of the following compounds is characterized by a large amount of carbohydrate and a small amount of protein?

Proteoglycan. **Explanation:** The distinction between these compounds lies in the **ratio of carbohydrate to protein** and the nature of the carbohydrate chains. **1. Why Proteoglycan is Correct:** Proteoglycans consist of a core protein to which long, unbranched polysaccharide chains called **Glycosaminoglycans (GAGs)** are covalently attached. In a proteoglycan molecule, the carbohydrate content is dominant, typically accounting for **90-95% of the weight**, while the protein core makes up only 5-10%. This makes them "carbohydrate-heavy" molecules. **2. Why other options are incorrect:** * **Glycoprotein (A):** These are "protein-heavy" molecules. They consist primarily of protein with short, often branched oligosaccharide chains. The carbohydrate content is usually small (1-20%). * **Glycosaminoglycan (B):** These are the carbohydrate components themselves (e.g., Heparin, Hyaluronic acid). They are pure polysaccharides and do not contain a protein core unless they are part of a proteoglycan. * **Glycocalyx (D):** This is a descriptive term for the "sugar coat" on the outer surface of the plasma membrane, composed of both glycoproteins and glycolipids. It is a structural feature, not a specific biochemical compound defined by a fixed carbohydrate-protein ratio. **Clinical Pearls for NEET-PG:** * **Aggrecan** is the major proteoglycan found in cartilage. * **Hyaluronic Acid** is the only GAG that is **not sulfated** and **not covalently attached** to a protein core. * **Mucopolysaccharidoses (MPS):** These are lysosomal storage disorders caused by the deficiency of enzymes required to degrade GAGs (e.g., Hurler Syndrome, Hunter Syndrome). * **Heparin** is the GAG with the highest negative charge density in the body.

Q: Dehydrogenases of the HMP shunt are specific for which coenzyme?

Nicotinamide adenine dinucleotide phosphate (NADP+). **Explanation:** The Hexose Monophosphate (HMP) Shunt, also known as the Pentose Phosphate Pathway, occurs in the cytosol and is primarily an anabolic pathway. The rate-limiting step and the subsequent oxidative step are catalyzed by **Glucose-6-Phosphate Dehydrogenase (G6PD)** and **6-Phosphogluconate Dehydrogenase**, respectively. Both of these enzymes are strictly specific for **NADP+** as their coenzyme, reducing it to **NADPH**. Unlike NADH, which is used in the electron transport chain for ATP production, NADPH is essential for: 1. **Reductive Biosynthesis:** Synthesis of fatty acids and steroids. 2. **Antioxidant Defense:** Maintaining glutathione in its reduced state to protect cells against reactive oxygen species (ROS), particularly in RBCs. **Analysis of Incorrect Options:** * **A. Thiamine pyrophosphate (TPP):** This is a derivative of Vitamin B1. While it is a coenzyme in the HMP shunt, it is used by **Transketolase** (non-oxidative phase), not by dehydrogenases. * **C & D. FMN and FAD:** These are flavin-derived coenzymes (Vitamin B2) typically involved in redox reactions within the mitochondria (e.g., TCA cycle or Complex I/II of ETC). They are not utilized by the oxidative enzymes of the HMP shunt. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It leads to hemolytic anemia because RBCs cannot generate NADPH, leaving them vulnerable to oxidative stress (e.g., fava beans, primaquine, infections) and resulting in **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** The HMP shunt is most active in tissues requiring high NADPH, such as the adrenal cortex (steroidogenesis), liver (fatty acid synthesis), and lactating mammary glands. * **Transketolase Activity:** Measuring erythrocyte transketolase activity is the gold standard for diagnosing **Thiamine deficiency**.

Q: Glucagon receptors are NOT found in which organ?

Cornea. **Explanation:** Glucagon is a peptide hormone primarily known for its role in glucose homeostasis. It acts via **G-protein coupled receptors (GPCRs)** that activate the Adenylate Cyclase-cAMP pathway. **Why Cornea is the Correct Answer:** Glucagon receptors are primarily localized in tissues involved in fuel mobilization and metabolic regulation. The **cornea** is an avascular, specialized ocular tissue that relies on atmospheric oxygen and glucose from the aqueous humor. It does not possess glucagon receptors because it does not participate in systemic glucose regulation or glycogenolysis. **Analysis of Other Options:** * **Kidney:** Glucagon receptors are present in the renal cortex and distal tubules. Glucagon plays a role in stimulating gluconeogenesis in the kidney and influencing electrolyte excretion (e.g., increasing glomerular filtration rate). * **Stomach:** Receptors are found in the gastric mucosa. Glucagon inhibits gastric acid secretion and reduces gastrointestinal motility. * **Adrenal Gland:** Glucagon receptors are present in the adrenal medulla, where glucagon can stimulate the release of catecholamines (epinephrine and norepinephrine). **High-Yield NEET-PG Pearls:** 1. **Primary Site:** The **Liver** has the highest density of glucagon receptors (stimulating glycogenolysis and gluconeogenesis). 2. **Muscle Paradox:** Glucagon receptors are **NOT** found in **Skeletal Muscle**. This is a frequent exam trap; muscle glycogen is used only for local contraction and cannot be released into the blood because muscle lacks Glucose-6-Phosphatase. 3. **Adipose Tissue:** Glucagon receptors are present here to stimulate lipolysis via Hormone-Sensitive Lipase (HSL). 4. **Clinical Correlation:** Glucagon is used as an antidote for **Beta-blocker overdose** because it bypasses the beta-receptor to increase cAMP in cardiac myocytes, improving heart rate and contractility.

Question 1

Beta-glucosidase is defective in which disease?

Accepted Answer

Gaucher's disease

Answer

Tay-Sachs disease

Answer

Galactosaemia

Answer

Diabetes Mellitus

Question 2

Which of the following dietary components determines the glycemic index of food?

Accepted Answer

Fibre

Answer

Protein

Answer

Fat

Answer

None of the above

Question 3

Which of the following is not an aldose sugar?

Accepted Answer

Fructose

Answer

Erythrose

Answer

Glucose

Answer

Galactose

Question 4

Mutarotation refers to a change in which of the following properties?

Accepted Answer

Optical rotation

Answer

pH

Answer

Conductance

Answer

Chemical properties

Question 5

Which of the following are sources of NADPH?

Accepted Answer

All of the above

Answer

Malic enzyme

Answer

Cytoplasmic isocitrate dehydrogenase

Answer

Pentose phosphate pathway

Question 6

What are the enzymes required for the formation of Phosphoenolpyruvate from Pyruvate?

Accepted Answer

Pyruvate carboxylase and Phosphoenolpyruvate Carboxykinase

Answer

Pyruvate dehydrogenase and Pyruvate carboxylase

Answer

Lactate Dehydrogenase and Pyruvate Dehydrogenase

Answer

Phosphoenolpyruvate carboxykinase and Pyruvate Dehydrogenase

Question 7

All of the following are true about changes in brain metabolism after traumatic brain injury, except:

Accepted Answer

Increased CSF lactate is associated with good prognosis

Answer

Shut down of pyruvate dehydrogenase activity

Answer

Accumulation of lactate in brain

Answer

Increased lactate uptake from circulation

Question 8

Which of the following compounds is characterized by a large amount of carbohydrate and a small amount of protein?

Accepted Answer

Proteoglycan

Answer

Glycoprotein

Answer

Glycosaminoglycan

Answer

Glycocalyx

Question 9

Dehydrogenases of the HMP shunt are specific for which coenzyme?

Accepted Answer

Nicotinamide adenine dinucleotide phosphate (NADP+)

Answer

Thiamine pyrophosphate (TPP)

Answer

Flavin mononucleotide (FMN)

Answer

Flavin adenine dinucleotide (FAD)

Question 10

Glucagon receptors are NOT found in which organ?

Accepted Answer

Cornea

Answer

Kidney

Answer

Stomach

Answer

Adrenal gland

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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