Carbohydrate Metabolism Practice Questions

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

Dihydroxyacetone. ### 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.

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

Lactate. **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**.

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

Small particle size of food. **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.

Q: Which enzyme is common to both glycogenolysis and glycogenesis?

Phosphoglucomutase. **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.

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

Enolase. **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.

Q: Which of the following sugars is not synthesized or utilized in the human body?

L-fructose. **Explanation:** The human body primarily utilizes **D-isomers** of sugars (like D-glucose and D-fructose) for metabolism. While most sugars in our body belong to the D-series, there are specific exceptions where L-isomers are synthesized and utilized. **L-fructose**, however, is neither synthesized nor metabolized by human enzymes and has no physiological role in the body. **Analysis of Options:** * **L-fructose (Correct):** This is a synthetic sugar not found in human metabolic pathways. Human hexokinase and other glycolytic enzymes are stereospecific for D-fructose. * **L-fucose:** This is a crucial exception to the "D-sugar rule." It is an L-isomer sugar synthesized in the body and is a vital component of **glycoproteins** and **blood group substances** (H-substance). * **D-Glucose:** The primary metabolic fuel for the human body, especially for the brain and RBCs. * **D-Fructose:** A common dietary monosaccharide (found in fruits and honey) metabolized via the fructokinase pathway in the liver. **High-Yield Clinical Pearls for NEET-PG:** * **The "L-Sugar" Exceptions:** While most human sugars are D-isomers, remember these two L-isomers: **L-fucose** (found in glycoproteins) and **L-xylulose** (excreted in excess in Essential Pentosuria due to deficiency of L-xylulose reductase). * **Stereospecificity:** Enzymes are highly stereospecific; for example, glucose oxidase reacts only with D-glucose, not L-glucose. * **L-fucose Source:** It is derived from GDP-mannose. Deficiency in its transport leads to **Leukocyte Adhesion Deficiency Type II**.

Q: Which glucose transporter (GLUT) is primarily responsible for transporting glucose from the intestine to the liver?

GLUT2. **Explanation:** The correct answer is **GLUT2**. **Why GLUT2 is correct:** GLUT2 is a high-capacity, low-affinity (high $K_m$) bidirectional transporter. It is primarily located in the **basolateral membrane of intestinal mucosal cells**, the liver, pancreatic beta cells, and the renal tubular cells. After glucose is absorbed from the intestinal lumen via SGLT-1, it exits the enterocyte into the portal circulation via GLUT2. Because of its high $K_m$, GLUT2 acts as a "glucose sensor," ensuring that glucose uptake by the liver is proportional to blood glucose levels, especially in the postprandial (fed) state. **Why other options are incorrect:** * **GLUT1:** This is a high-affinity transporter found in almost all tissues, particularly **RBCs and the Blood-Brain Barrier**. It provides basal glucose uptake. * **GLUT3:** This is the primary transporter in **neurons**. It has a very low $K_m$, ensuring the brain receives glucose even during hypoglycemia. * **GLUT4:** This is the only **insulin-dependent** transporter. It is located in **skeletal muscle and adipose tissue**. In the absence of insulin, it remains sequestered in intracellular vesicles. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 vs. GLUT2:** Remember that SGLT-1 (Sodium-Glucose Co-transporter 1) handles the *active* uptake from the intestinal lumen, while GLUT2 handles the *facilitated* exit into the blood. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a congenital defect in **GLUT2**, leading to hepatomegaly and glucose intolerance. * **Bidirectionality:** GLUT2 is unique because it allows glucose to enter the liver for glycogenesis and exit the liver during gluconeogenesis/glycogenolysis.

Q: Which enzyme is responsible for the postprandial utilization of glucose?

Glucokinase. **Explanation:** The correct answer is **Glucokinase (Hexokinase IV)**. **Why Glucokinase is the correct answer:** Glucokinase is primarily located in the liver and pancreatic beta cells. Its unique kinetic properties make it ideal for the **postprandial (fed) state**: 1. **High $K_m$ (Low affinity):** It only becomes significantly active when blood glucose levels are high (e.g., after a meal). 2. **High $V_{max}$ (High capacity):** It can rapidly phosphorylate large amounts of glucose, allowing the liver to "clear" glucose from the portal blood and store it as glycogen, preventing postprandial hyperglycemia. 3. **Lack of Product Inhibition:** Unlike hexokinase, it is not inhibited by its product (Glucose-6-Phosphate), allowing continuous glucose uptake even when energy levels are high. **Why other options are incorrect:** * **Hexokinase (Types I-III):** These are found in extrahepatic tissues. They have a **low $K_m$ (high affinity)**, meaning they are already saturated at fasting glucose levels. Their role is to ensure tissues like the brain get glucose even during starvation, not to manage a postprandial surplus. * **Fructokinase:** This enzyme is specific to fructose metabolism (converting fructose to fructose-1-phosphate) and does not utilize glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Molecular Sensor:** Glucokinase acts as the "glucose sensor" in the pancreas for insulin release. * **MODY Type 2:** Mutations in the glucokinase gene lead to Maturity-Onset Diabetes of the Young (MODY) type 2. * **Localization:** Glucokinase is regulated by the **Glucokinase Regulatory Protein (GKRP)**, which sequesters it in the nucleus during fasting. * **Inducibility:** Glucokinase is induced by **Insulin**, further enhancing its role in the fed state.

Question 1

Which of the following carbohydrates has no asymmetric carbon atom?

Accepted Answer

Dihydroxyacetone

Answer

Glucose

Answer

Glyceraldehyde

Answer

Fructose

Question 2

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

Accepted Answer

Lactate

Answer

Alanine

Answer

Glutamate

Answer

None of the above

Question 3

Which of the following factors does NOT reduce postprandial glycemia?

Accepted Answer

Small particle size of food

Answer

Presence of enzyme inhibitors in food

Answer

Inadequate cooking of starch

Answer

Presence of protein and fat in association with carbohydrate

Question 4

Which enzyme is common to both glycogenolysis and glycogenesis?

Accepted Answer

Phosphoglucomutase

Answer

Glycogen phosphorylase

Answer

Glycogen synthase

Answer

Glucotransferase

Question 5

Which amino acid cannot be used for glycogen synthesis?

Accepted Answer

Brain

Answer

Muscle

Answer

RBCs

Answer

Kidney

Question 6

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

Accepted Answer

Enolase

Answer

ATPase

Answer

Pyruvate kinase

Answer

Fructose-6-phosphatase

Question 7

Which of the following statements about the Hexose Monophosphate (HMP) pathway is FALSE?

Accepted Answer

Ribose-5-phosphate is produced during the oxidative phase of the pathway.

Answer

The oxidative phase of the pathway produces NADPH.

Answer

The pathway does not produce ATP.

Answer

It occurs in the testes, ovaries, placenta, and adrenal cortex.

Question 8

Which of the following sugars is not synthesized or utilized in the human body?

Accepted Answer

L-fructose

Answer

L-fucose

Answer

D-Glucose

Answer

D-Fructose

Question 9

Which glucose transporter (GLUT) is primarily responsible for transporting glucose from the intestine to the liver?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

Question 10

Which enzyme is responsible for the postprandial utilization of glucose?

Accepted Answer

Glucokinase

Answer

Fructokinase

Answer

Hexokinase

Answer

All of the above

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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