Carbohydrate Metabolism Practice Questions

Q: Under anaerobic conditions, the glycolysis of one mole of glucose yields how many moles of ATP?

Two. **Explanation:** In anaerobic glycolysis (occurring in the absence of oxygen or in cells lacking mitochondria like RBCs), the metabolic pathway converts one mole of glucose into two moles of **lactate**. 1. **Why Option B is Correct:** The process involves two phases: * **Investment Phase:** 2 ATP molecules are consumed (at the Hexokinase and Phosphofructokinase-1 steps). * **Payoff Phase:** 4 ATP molecules are generated via substrate-level phosphorylation (2 ATP from 1,3-bisphosphoglycerate and 2 ATP from Phosphoenolpyruvate). * **Net Yield:** 4 (Produced) – 2 (Invested) = **2 ATP**. Crucially, the NADH produced during the glyceraldehyde-3-phosphate dehydrogenase step is re-oxidized to NAD+ by converting pyruvate to lactate (catalyzed by Lactate Dehydrogenase). This ensures the cycle continues but prevents the NADH from entering the electron transport chain, resulting in no additional ATP. 2. **Why Other Options are Incorrect:** * **Option A:** Incorrect; the payoff phase generates more than is invested. * **Option C:** 8 ATP is the net yield of **aerobic glycolysis** in certain tissues (using the Malate-Aspartate shuttle) where NADH is oxidized in the mitochondria. * **Option D:** 30 (or 32) ATP is the total yield from the **complete oxidation** of glucose through glycolysis, the TCA cycle, and oxidative phosphorylation. **High-Yield Clinical Pearls for NEET-PG:** * **Mature RBCs** rely exclusively on anaerobic glycolysis for energy because they lack mitochondria. * **Lactic Acidosis:** Occurs when there is excessive anaerobic glycolysis (e.g., during shock or severe hypoxia), leading to lactate accumulation. * **Rapoport-Luebering Cycle:** A shunt in RBC glycolysis that produces 2,3-BPG, which decreases hemoglobin's affinity for oxygen, shifting the dissociation curve to the right. This process yields **zero net ATP**.

Q: Which mucopolysaccharide does not contain uronic acid?

Keratan sulfate. **Explanation:** Mucopolysaccharides, also known as **Glycosaminoglycans (GAGs)**, are long, unbranched polysaccharides consisting of repeating disaccharide units. Typically, these units consist of an **amino sugar** (D-glucosamine or D-galactosamine) and a **uronic acid** (D-glucuronic acid or L-iduronic acid). **Keratan sulfate** is the unique exception to this rule. Instead of uronic acid, it contains **Galactose** linked to N-acetylglucosamine. Because it lacks uronic acid, it is the most distinct member of the GAG family in terms of chemical composition. **Analysis of Options:** * **Heparin:** Contains D-glucuronic acid or L-iduronic acid. It is the most highly anionic (acidic) GAG and acts as a natural anticoagulant. * **Chondroitin sulfate:** Contains D-glucuronic acid. It is the most abundant GAG in the body, found prominently in cartilage and bone. * **Dermatan sulfate:** Contains L-iduronic acid (formed by the epimerization of glucuronic acid). It is found mainly in the skin, blood vessels, and heart valves. **High-Yield Clinical Pearls for NEET-PG:** * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently bound to a protein (does not form proteoglycans). * **Heparin vs. Heparan Sulfate:** Heparin is intracellular (mast cells), while Heparan sulfate is extracellular (basement membranes). * **Hurler & Hunter Syndromes:** These are Mucopolysaccharidoses (MPS) caused by the inability to degrade GAGs (specifically Dermatan and Heparan sulfate), leading to skeletal deformities and mental retardation. * **Keratan Sulfate Location:** Primarily found in the **cornea** (maintains transparency) and cartilage.

Q: Cerebroside contains which sugar?

Galactose. **Explanation:** **Cerebrosides** are a type of **glycosphingolipid** (neutral glycolipids) that are essential components of nerve cell membranes, particularly the myelin sheath. 1. **Why Galactose is Correct:** A cerebroside consists of a **ceramide** unit (sphingosine + fatty acid) linked to a single sugar residue via a glycosidic bond. In the nervous system, the most common sugar found in cerebrosides is **Galactose**, forming **Galactosylceramide (Galactocerebroside)**. While Glucocerebrosides exist (found primarily in non-neural tissues), Galactose is the classic and most characteristic sugar associated with cerebrosides in medical biochemistry. 2. **Why Other Options are Incorrect:** * **Ribose:** This is a pentose sugar found in RNA and nucleotides (ATP/GTP), not in structural glycolipids. * **Fructose:** This is a ketohexose involved in the glycolytic pathway and seminal fluid; it does not form part of sphingolipids. * **Pentose:** This is a general category of 5-carbon sugars (like Ribose or Xylose). Cerebrosides specifically require hexose sugars (6-carbon). **NEET-PG High-Yield Clinical Pearls:** * **Krabbe’s Disease:** Caused by a deficiency of the enzyme **Galactocerebrosidase**, leading to the accumulation of galactocerebroside (presents with globoid cells and demyelination). * **Gaucher’s Disease:** The most common lysosomal storage disorder, caused by a deficiency of **Glucocerebrosidase**, leading to the accumulation of glucocerebroside (presents with "wrinkled paper" cytoplasm in macrophages). * **Gangliosides vs. Cerebrosides:** While cerebrosides contain a single sugar, gangliosides are complex sphingolipids containing oligosaccharides and at least one **Sialic acid (NANA)** residue.

Q: Which of the following is true for phosphofructokinase?

It catalyzes an irreversible reaction in glycolysis. **Phosphofructokinase-1 (PFK-1)** is the most important regulatory enzyme and the **rate-limiting step** of glycolysis. ### **Explanation of the Correct Answer** **Option C** is correct because PFK-1 catalyzes the phosphorylation of Fructose-6-Phosphate to Fructose-1,6-Bisphosphate. This reaction is highly exergonic ($\Delta G$ is strongly negative), making it **physiologically irreversible**. In the cell, this step serves as the "committed step"; once glucose is converted to Fructose-1,6-bisphosphate, it is destined to complete glycolysis. ### **Analysis of Incorrect Options** * **Option A:** PFK-1 uses **Fructose-6-Phosphate** and ATP as substrates. Fructose-1,6-bisphosphate is the *product* of the reaction. * **Option B:** PFK-1 **consumes ATP** rather than generating it. It transfers a phosphate group from ATP to the substrate. ATP generation in glycolysis occurs later via Phosphoglycerate Kinase and Pyruvate Kinase (substrate-level phosphorylation). * **Option D:** PFK-1 is **inhibited** by high levels of ATP and Citrate (signals of high energy status). It is **activated** by AMP and **Fructose-2,6-bisphosphate** (the most potent allosteric activator). ### **High-Yield Clinical Pearls for NEET-PG** * **Rate-Limiting Step:** PFK-1 is the "Pacemaker" of glycolysis. * **Hormonal Regulation:** Insulin increases PFK-1 activity (via Fructose-2,6-BP), while Glucagon decreases it. * **Tissues:** In the liver, citrate inhibition of PFK-1 helps prioritize glucose for glycogen synthesis when energy is abundant. * **Comparison:** Do not confuse PFK-1 with **PFK-2**, which synthesizes Fructose-2,6-bisphosphate, the regulator that turns on PFK-1.

Q: High-energy phosphate is not produced in which of the following metabolic pathways?

Hexose monophosphate pathway. The correct answer is **B. Hexose monophosphate (HMP) pathway**, also known as the Pentose Phosphate Pathway (PPP). ### **Why HMP Pathway is Correct** The HMP pathway is a unique metabolic shunt that does **not** involve the production or consumption of ATP (high-energy phosphate). Instead, its primary objectives are: 1. **Production of NADPH:** Used for reductive biosynthesis (fatty acids, steroids) and maintaining reduced glutathione to prevent oxidative stress. 2. **Production of Ribose-5-phosphate:** Essential for nucleotide and nucleic acid synthesis. Because it bypasses the ATP-generating steps of glycolysis, it is considered a non-energetic pathway. ### **Why Other Options are Incorrect** * **TCA Cycle:** Produces high-energy phosphate directly via **Substrate Level Phosphorylation** (Succinyl CoA to Succinate produces **GTP**, which is energetically equivalent to ATP). * **Glycolysis:** Produces ATP via Substrate Level Phosphorylation at two steps: Phosphoglycerate kinase and Pyruvate kinase. * **Beta Oxidation:** While the pathway itself produces FADH₂ and NADH, these enter the Electron Transport Chain to generate large amounts of ATP via oxidative phosphorylation. ### **NEET-PG High-Yield Pearls** * **Rate-limiting enzyme of HMP:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to produce NADPH, which is required to keep glutathione reduced in RBCs. * **Tissue Distribution:** HMP pathway is most active in tissues requiring NADPH (Adrenal cortex, Liver, Lactating mammary glands, and RBCs). * **Thiamine (B1) Connection:** Transketolase, an enzyme in the non-oxidative phase of HMP, requires Thiamine pyrophosphate as a cofactor. Measuring its activity is used to diagnose Thiamine deficiency.

Q: Glycogenin is a protein with self-glucosylation capacity. To which amino acid of glycogenin are glucose molecules attached?

Tyrosine. **Explanation:** **Glycogenin** is a specialized protein that acts as a primer for glycogen synthesis. Since **Glycogen Synthase** can only add glucose units to an existing chain of at least four glucose residues, glycogenin initiates the process through **autocatalysis**. 1. **Why Tyrosine is Correct:** Glycogenin possesses glucosyltransferase activity. It attaches the first glucose molecule from UDP-glucose to the **hydroxyl (-OH) group** of a specific **Tyrosine residue (Tyr-194)** within its own structure. This serves as the foundation upon which a short chain of about 8 glucose units is built, providing the necessary primer for Glycogen Synthase to take over. 2. **Why Other Options are Incorrect:** * **Serine & Threonine:** While these amino acids also have hydroxyl groups and are common sites for O-linked glycosylation in many glycoproteins (and phosphorylation sites for enzyme regulation), they are not the attachment sites for the glycogen primer. * **Hydroxylysine:** This is a modified amino acid primarily found in collagen, where it serves as a site for glycosylation (attachment of glucose and galactose), but it plays no role in glycogen metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Glycogenin** remains buried at the core of every mature glycogen granule. * **UDP-Glucose** is the active donor of glucose units for both glycogenin and glycogen synthase. * **Branching Enzyme (4:6 transferase)** is required to create $\alpha$1-6 linkages, while **Glycogen Synthase** only creates $\alpha$1-4 linkages. * In the absence of glycogenin, glycogen synthesis cannot be initiated, a concept relevant to understanding certain rare Glycogen Storage Diseases (GSD Type 0).

Question 1

What is the effect of oral administration of 50 gm of glucose on the body?

Accepted Answer

Decreased gluconeogenesis

Answer

Decreased ketone body production

Answer

Increased lactate production upon exercise

Answer

Increased gluconeogenesis

Question 2

Which of the following is a component of a polysaccharide?

Accepted Answer

Glucosamine

Answer

Synovium

Answer

Glucuronic acid

Answer

All of the above

Question 3

Why is fructose not used in intravenous infusions?

Accepted Answer

None of the above

Answer

It can cause irritability.

Answer

It can cause mental retardation.

Answer

It can increase erythrocyte protoporphyrin.

Question 4

Inulin, like other fructans, is used as a prebiotic because it is non-digestible. What causes its resistance to digestion in the upper gastrointestinal tract?

Accepted Answer

Beta configuration of the anomeric carbon (C2)

Answer

Absence of digestive enzymes in the upper gastrointestinal tract

Answer

Low pH of the stomach

Answer

Presence of alpha-glycosidic linkages

Question 5

Under anaerobic conditions, the glycolysis of one mole of glucose yields how many moles of ATP?

Accepted Answer

Two

Answer

One

Answer

Eight

Answer

Thirty

Question 6

Which mucopolysaccharide does not contain uronic acid?

Accepted Answer

Keratan sulfate

Answer

Heparin

Answer

Chondroitin sulfate

Answer

Dermatan sulfate

Question 7

Cerebroside contains which sugar?

Accepted Answer

Galactose

Answer

Ribose

Answer

Fructose

Answer

Pentose

Question 8

Which of the following is true for phosphofructokinase?

Accepted Answer

It catalyzes an irreversible reaction in glycolysis

Answer

It uses Fructose 1,6 bisphosphate as substrate.

Answer

It generates ATP

Answer

It is activated by increased levels of ATP and citrate.

Question 9

High-energy phosphate is not produced in which of the following metabolic pathways?

Accepted Answer

Hexose monophosphate pathway

Answer

TCA cycle

Answer

Glycolysis

Answer

Beta oxidation of fatty acid

Question 10

Glycogenin is a protein with self-glucosylation capacity. To which amino acid of glycogenin are glucose molecules attached?

Accepted Answer

Tyrosine

Answer

Serine

Answer

Threonine

Answer

Hydroxylysine

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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