Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 461: Which of the following is NOT a product of the Pentose Phosphate Pathway?

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

Explanation: The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, occurs in the cytosol and consists of an oxidative (irreversible) phase and a non-oxidative (reversible) phase. ### **Why CO₂ is the Correct Answer** While CO₂ is produced during the **oxidative phase** of the PPP (specifically during the conversion of 6-phosphogluconate to Ribulose-5-phosphate by the enzyme 6-phosphogluconate dehydrogenase), it is technically considered a **byproduct** or waste gas. However, in the context of standard medical examinations like NEET-PG, the question focuses on the **carbon skeletons and reducing equivalents** generated for biosynthetic purposes. Among the options provided, CO₂ is often the "distractor" because the primary functional products are NADPH and pentose sugars. *Note: In some advanced biochemistry texts, CO₂ is listed as a product. However, in competitive exams, if the question implies "functional metabolic intermediates," CO₂ is the odd one out compared to the structural sugar phosphates.* ### **Analysis of Incorrect Options** * **NADPH (Option B):** 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. * **Sedoheptulose 7-phosphate (Option C):** A 7-carbon sugar produced during the **non-oxidative phase** via the action of the enzyme **Transaldolase**. * **Glyceraldehyde 3-phosphate (Option A):** A 3-carbon intermediate produced in the non-oxidative phase that can re-enter the glycolytic pathway. ### **High-Yield NEET-PG Clinical Pearls** 1. **Rate-Limiting Enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). 2. **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to regenerate reduced glutathione, causing **Heinz bodies** and **Bite cells**. 3. **Thiamine (B1) Connection:** The enzyme **Transketolase** requires Thiamine pyrophosphate (TPP) as a cofactor. Measuring erythrocyte transketolase activity is the gold standard for diagnosing Thiamine deficiency. 4. **Tissues involved:** Highly active in the adrenal cortex, liver, mammary glands (for fatty acid/steroid synthesis), and RBCs.

Question 462: In the conversion of lactic acid to glucose (gluconeogenesis), three reactions of the glycolytic pathway are circumvented. Which of the following enzymes do NOT participate in this process?

A. Pyruvate carboxylase
B. Phosphoenol pyruvate carboxy kinase
C. Pyruvate kinase (Correct Answer)
D. Glucose-6-phosphatase

Explanation: **Explanation:** Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (like lactate). While it shares many enzymes with glycolysis, it must bypass three **irreversible** steps of glycolysis to proceed energetically. These "bottleneck" steps are catalyzed by Hexokinase, Phosphofructokinase-1, and Pyruvate kinase. **Why Pyruvate Kinase is the Correct Answer:** Pyruvate kinase is a **glycolytic enzyme** that converts Phosphoenolpyruvate (PEP) to Pyruvate. In gluconeogenesis, this reaction cannot be reversed directly. Instead, the body bypasses it using a two-step process involving **Pyruvate carboxylase** and **PEP carboxykinase (PEPCK)**. Therefore, Pyruvate kinase does not participate in gluconeogenesis; it is actually inhibited during this process to prevent a futile cycle. **Analysis of Incorrect Options:** * **A. Pyruvate carboxylase:** A key gluconeogenic enzyme that converts pyruvate to oxaloacetate in the mitochondria. It requires **Biotin** as a cofactor. * **B. Phosphoenolpyruvate carboxykinase (PEPCK):** Converts oxaloacetate to PEP, completing the bypass of the pyruvate kinase step. * **D. Glucose-6-phosphatase:** The final bypass enzyme found in the ER of the liver and kidney. It converts Glucose-6-phosphate to free glucose, allowing it to enter the bloodstream. **High-Yield Clinical Pearls for NEET-PG:** * **The Four Key Gluconeogenic Enzymes:** Pyruvate carboxylase, PEPCK, Fructose-1,6-bisphosphatase (the rate-limiting step), and Glucose-6-phosphatase. * **Acetyl-CoA Connection:** Pyruvate carboxylase is **obligatorily activated** by Acetyl-CoA. * **Location:** Gluconeogenesis occurs primarily in the **Liver** (90%) and Kidney (10%). It does not occur in the muscle due to the absence of Glucose-6-phosphatase. * **Cori Cycle:** Lactate produced in muscles travels to the liver to be converted back to glucose via these bypass enzymes.

Question 463: Cancer cells derive their nutrition from which process?

A. Glycolysis (Correct Answer)
B. Oxidative phosphorylation
C. Increase in mitochondria
D. TCA cycle

Explanation: ### Explanation The correct answer is **Glycolysis**. This phenomenon is known as the **Warburg Effect**. **1. Why Glycolysis is Correct:** Even in the presence of abundant oxygen, cancer cells preferentially shift their metabolism from oxidative phosphorylation to **aerobic glycolysis**. While glycolysis is less efficient in terms of ATP yield per glucose molecule (2 ATP vs. 36 ATP), it occurs at a much faster rate. This rapid glucose uptake provides the metabolic intermediates (like Ribose-5-phosphate and Acetyl-CoA) necessary for the synthesis of nucleic acids, proteins, and lipids required for rapid cellular proliferation. **2. Why the Other Options are Incorrect:** * **B & D (Oxidative Phosphorylation & TCA Cycle):** In normal differentiated cells, these are the primary energy sources. However, cancer cells often downregulate these pathways or suffer from mitochondrial dysfunction to avoid the production of Reactive Oxygen Species (ROS), which could trigger apoptosis. * **C (Increase in mitochondria):** Cancer cells typically do not show an increase in functional mitochondria; instead, they often exhibit a relative decrease in mitochondrial activity or structural abnormalities to favor glycolytic pathways. **3. High-Yield Clinical Pearls for NEET-PG:** * **Warburg Effect:** The observation that cancer cells consume glucose at high rates and produce lactate even in aerobic conditions. * **PET Scan (Positron Emission Tomography):** This imaging modality utilizes the Warburg effect by using **18F-fluorodeoxyglucose (FDG)**, a glucose analog, to detect metabolically active tumor cells. * **HIF-1α (Hypoxia-Inducible Factor):** This transcription factor is often upregulated in tumors, leading to the overexpression of glucose transporters (GLUT1, GLUT3) and glycolytic enzymes. * **Lactate Production:** The end product of this process is lactate, which creates an acidic microenvironment that facilitates tumor invasion and immune evasion.

Question 464: Which of the following carbohydrates has no asymmetric carbon atom?

A. Glucose
B. Glyceraldehyde
C. Dihydroxyacetone (Correct Answer)
D. Fructose

Explanation: ### Explanation **Core Concept: Chirality in Carbohydrates** An asymmetric carbon atom (chiral center) is a carbon atom attached to four different groups. In carbohydrate chemistry, the presence of these centers determines optical activity and the existence of stereoisomers. **Why Dihydroxyacetone is the Correct Answer:** Dihydroxyacetone is a **ketotriose** (the simplest ketose). Its chemical structure is $CH_2OH-CO-CH_2OH$. * Carbon 1 and Carbon 3 are attached to two hydrogen atoms each (not four different groups). * Carbon 2 is part of a carbonyl group ($C=O$) and is $sp^2$ hybridized, meaning it cannot be asymmetric. Consequently, dihydroxyacetone is the **only monosaccharide that is optically inactive** because it lacks a chiral center. **Analysis of Incorrect Options:** * **Glyceraldehyde (Option B):** The simplest aldose. Its central carbon (C2) is attached to $-H$, $-OH$, $-CHO$, and $-CH_2OH$. It has **one** asymmetric carbon. * **Glucose (Option A):** An aldohexose. In its open-chain form, it contains **four** asymmetric carbon atoms (C2, C3, C4, and C5). * **Fructose (Option D):** A ketohexose. In its open-chain form, it contains **three** asymmetric carbon atoms (C3, C4, and C5). **NEET-PG High-Yield Pearls:** 1. **Formula for Isomers:** The number of possible stereoisomers for a sugar is $2^n$, where $n$ is the number of asymmetric carbon atoms. Since $n=0$ for dihydroxyacetone, it has only $1$ form ($2^0=1$). 2. **Reference Sugar:** Glyceraldehyde is the reference sugar used to designate the **D and L notations** of all other sugars. 3. **Epimers:** Glucose and Galactose are C4 epimers; Glucose and Mannose are C2 epimers. 4. **Clinical Link:** Dihydroxyacetone phosphate (DHAP) is a crucial intermediate in **Glycolysis** and **Gluconeogenesis**, acting as a bridge to lipid metabolism via glycerol-3-phosphate.

Question 465: Cori's cycle is concerned with the transport of which substance?

A. Alanine
B. Glutamate
C. Lactate (Correct Answer)
D. None of the above

Explanation: **Explanation:** The **Cori Cycle** (also known as the Lactic Acid Cycle) is a metabolic pathway that describes the metabolic cooperation between skeletal muscle and the liver. **Why Lactate is Correct:** During vigorous exercise, muscular demand for ATP exceeds the oxygen supply, leading to **anaerobic glycolysis**. In this process, pyruvate is converted into **lactate** by the enzyme *Lactate Dehydrogenase (LDH)* to regenerate NAD+. Since the muscle cannot convert lactate back to glucose, the lactate is released into the bloodstream and transported to the **liver**. In the liver, lactate is converted back to pyruvate and then to glucose via **gluconeogenesis**. This glucose is then released back into the blood to be used by the muscle again, completing the cycle. **Why Other Options are Incorrect:** * **Option A (Alanine):** Alanine is the primary transport molecule in the **Cahill Cycle** (Glucose-Alanine Cycle). This cycle is used to transport amino groups from muscle to the liver for urea synthesis while providing glucose back to the muscle. * **Option B (Glutamate):** While glutamate is a key intermediate in amino acid metabolism and nitrogen transport, it is not the specific substance cycled between muscle and liver in the Cori cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Net Energy Cost:** The Cori cycle is energy-expensive; it consumes **6 ATP** in the liver (gluconeogenesis) while producing only **2 ATP** in the muscle (glycolysis). * **Enzyme:** *Lactate Dehydrogenase* is the key enzyme; LDH-5 (M4) isoenzyme favors the conversion of pyruvate to lactate in the muscle. * **Clinical Relevance:** Failure of the Cori cycle (e.g., in severe liver disease or hypoxia) leads to **Lactic Acidosis**.

Question 466: Which one of the following is a monosaccharide?

A. Maltose
B. Sucrose
C. Fructose (Correct Answer)
D. Starch

Explanation: **Explanation:** Carbohydrates are classified based on the number of sugar units they contain. A **monosaccharide** is the simplest form of carbohydrate that cannot be hydrolyzed further into smaller units. **Why Fructose is Correct:** **Fructose** is a 6-carbon keto-sugar (hexose) and is a classic example of a monosaccharide. It is the sweetest of all natural sugars and is absorbed directly into the bloodstream during digestion via the GLUT-5 transporter. **Analysis of Incorrect Options:** * **Maltose (Disaccharide):** Composed of two glucose units linked by an $\alpha(1\to4)$ glycosidic bond. It is a product of starch digestion. * **Sucrose (Disaccharide):** Known as table sugar, it consists of one glucose and one fructose unit linked by an $\alpha1\to\beta2$ bond. It is a non-reducing sugar. * **Starch (Polysaccharide):** A complex polymer of glucose units (amylose and amylopectin) used for energy storage in plants. **High-Yield Clinical Pearls for NEET-PG:** * **Reducing Sugars:** All monosaccharides (including Fructose) are reducing sugars. Among disaccharides, Maltose and Lactose are reducing, while **Sucrose is NOT**. * **Seliwanoff’s Test:** This biochemical test is used to distinguish ketoses (like Fructose) from aldoses (like Glucose). Fructose gives a cherry-red color. * **Essential Fructosuria:** A benign deficiency of *fructokinase*, leading to fructose appearing in the urine (detected as a reducing sugar but negative on glucose dipstick). * **Hereditary Fructose Intolerance (HFI):** A severe deficiency of *Aldolase B*, leading to intracellular trapping of Fructose-1-Phosphate, causing hypoglycemia and jaundice.

Question 467: Which of the following factors does NOT reduce postprandial glycemia?

A. Small particle size of food (Correct Answer)
B. Presence of enzyme inhibitors in food
C. Inadequate cooking of starch
D. Presence of protein and fat in association with carbohydrate

Explanation: **Explanation:** The core concept behind postprandial glycemia is the **Glycemic Index (GI)**, which measures how quickly a carbohydrate-containing food raises blood glucose levels. Factors that slow down digestion or absorption reduce postprandial glycemia. **Why Option A is correct:** **Small particle size** (e.g., finely ground flour vs. whole grains) increases the surface area available for digestive enzymes like salivary and pancreatic amylase. This leads to **rapid digestion and faster glucose absorption**, thereby **increasing** postprandial glycemia rather than reducing it. **Why the other options are incorrect:** * **B. Enzyme inhibitors:** Naturally occurring inhibitors (like amylase inhibitors in some legumes) or pharmacological agents (like Acarbose) slow the breakdown of complex carbohydrates, reducing the rate of glucose entry into the blood. * **C. Inadequate cooking:** Raw or undercooked starch exists in a semi-crystalline form (resistant starch) that is difficult for enzymes to penetrate. Cooking causes "gelatinization," making starch digestible; hence, inadequate cooking keeps the glycemic response low. * **D. Presence of protein and fat:** These macronutrients delay **gastric emptying** and stimulate the release of incretins (like GIP and GLP-1), which slows the rise of blood glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Soluble Fiber:** (e.g., Pectin, Gums) increases the viscosity of intestinal contents, slowing glucose absorption and lowering postprandial peaks. * **Resistant Starch:** Acts similarly to dietary fiber and is fermented in the colon rather than absorbed in the small intestine. * **Acarbose/Miglitol:** These are competitive inhibitors of **$\alpha$-glucosidase**, used clinically to specifically target and reduce postprandial hyperglycemia in Type 2 Diabetes.

Question 468: Which enzyme is common to both glycogenolysis and glycogenesis?

A. Glycogen phosphorylase
B. Glycogen synthase
C. Glucotransferase
D. Phosphoglucomutase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Phosphoglucomutase**. This enzyme plays a pivotal role in carbohydrate metabolism by catalyzing the reversible interconversion of **Glucose-1-Phosphate (G-1-P)** and **Glucose-6-Phosphate (G-6-P)**. 1. **Why Phosphoglucomutase is correct:** * **In Glycogenesis:** After glucose is phosphorylated to G-6-P by hexokinase/glucokinase, Phosphoglucomutase converts G-6-P into G-1-P. G-1-P is then activated to UDP-glucose to be added to the glycogen chain. * **In Glycogenolysis:** Glycogen phosphorylase releases G-1-P from glycogen. Phosphoglucomutase then converts this G-1-P back into G-6-P, which can enter glycolysis (in muscle) or be dephosphorylated to free glucose (in the liver). 2. **Why other options are incorrect:** * **Glycogen phosphorylase:** The rate-limiting enzyme for **glycogenolysis** only. It breaks $\alpha$-1,4-glycosidic bonds. * **Glycogen synthase:** The rate-limiting enzyme for **glycogenesis** only. It creates $\alpha$-1,4-glycosidic bonds. * **Glucotransferase (Debranching enzyme):** Involved specifically in **glycogenolysis** to handle branch points ($\alpha$-1,6 bonds). **NEET-PG High-Yield Pearls:** * **Rate-limiting steps:** Glycogen Synthase (Synthesis) vs. Glycogen Phosphorylase (Breakdown). * **Cofactor:** Glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)**. * **Von Gierke’s Disease:** Deficiency of Glucose-6-Phosphatase; it affects the final step of glucose release in the liver but is not part of the glycogen polymer synthesis/breakdown itself. * **Key Intermediate:** UDP-Glucose is the "active" form of glucose used in glycogenesis.

Question 469: Which amino acid cannot be used for glycogen synthesis?

A. Muscle
B. RBCs
C. Brain (Correct Answer)
D. Kidney

Explanation: **Explanation:** The question asks which organ/tissue cannot synthesize glycogen. The correct answer is the **Brain**. **1. Why the Brain is the correct answer:** Glycogen synthesis (Glycogenesis) requires the presence of specific enzymes and serves as a storage form of glucose. The brain is highly dependent on a continuous supply of glucose from the blood because it has **negligible glycogen stores**. While astrocytes contain minute amounts of glycogen for emergency use during hypoglycemia, for the purposes of medical examinations, the brain is considered to have no significant glycogen storage capacity. It lacks the metabolic machinery to store glucose in bulk, relying instead on aerobic metabolism of blood-borne glucose. **2. Analysis of Incorrect Options:** * **Muscle:** Skeletal muscle is one of the two primary sites of glycogen storage (alongside the liver). It stores glycogen to provide a quick source of energy (ATP) during contraction via glycogenolysis. * **RBCs:** While RBCs rely solely on anaerobic glycolysis for energy, they do contain very small amounts of glycogen. However, compared to the brain, they are not the standard answer for "lack of synthesis" in this context. * **Kidney:** The kidney is capable of both glycogenesis and gluconeogenesis. It stores small amounts of glycogen, particularly in the renal cortex and medulla. **3. High-Yield Clinical Pearls for NEET-PG:** * **Liver vs. Muscle Glycogen:** Liver glycogen maintains blood glucose levels (contains Glucose-6-Phosphatase), whereas muscle glycogen is used only for local energy (lacks Glucose-6-Phosphatase). * **Rate-limiting enzyme:** Glycogen Synthase is the key regulatory enzyme for glycogenesis. * **Primer:** Glycogen synthesis cannot start *de novo*; it requires a protein primer called **Glycogenin**. * **Energy Requirement:** Synthesis of glycogen from glucose is an energy-consuming process, requiring **2 ATP** (one for phosphorylation to G6P and one for the conversion of UTP to UDP-glucose).

Question 470: What is the main site of fluoride inhibition in the Embden-Meyerhof pathway?

A. ATPase
B. Enolase (Correct Answer)
C. Pyruvate kinase
D. Fructose-6-phosphatase

Explanation: **Explanation:** **Correct Option: B. Enolase** The Embden-Meyerhof pathway (Glycolysis) is inhibited by fluoride at the step where **2-phosphoglycerate** is converted to **phosphoenolpyruvate (PEP)**. This reaction is catalyzed by the enzyme **Enolase**. Fluoride acts as a competitive inhibitor by forming a complex with magnesium ($Mg^{2+}$) and inorganic phosphate ($P_i$), which then binds to the active site of the enzyme. Since Enolase requires $Mg^{2+}$ as a cofactor, the formation of the **magnesium-fluorophosphate complex** effectively removes the available magnesium and blocks the enzyme's activity. **Analysis of Incorrect Options:** * **A. ATPase:** This enzyme is involved in ATP hydrolysis and energy transport, not the glycolytic pathway. While some ATPases are inhibited by substances like Ouabain, they are not the target of fluoride in glycolysis. * **C. Pyruvate kinase:** This is the final enzyme of glycolysis. While it is a regulated step, it is not inhibited by fluoride. * **D. Fructose-6-phosphatase:** This is a gluconeogenic enzyme (not part of the Embden-Meyerhof pathway) that converts Fructose-1,6-bisphosphate back to Fructose-6-phosphate. **Clinical Pearls & High-Yield Facts:** * **Blood Glucose Estimation:** In clinical practice, blood samples for glucose estimation are collected in **grey-topped vials** containing **Sodium Fluoride (NaF)**. This inhibits glycolysis in RBCs, preventing the artificial lowering of glucose levels before the sample reaches the lab. * **Anticoagulant Pair:** NaF is usually paired with **Potassium Oxalate**, which acts as the anticoagulant by chelating calcium. * **Reversibility:** The inhibition of Enolase by fluoride is reversible if the concentration of magnesium is significantly increased.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

Practice Questions

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