Carbohydrate Metabolism Practice Questions

Q: Which of the following enzymes are active in the dephosphorylated state? 1. Glycogen synthase 2. Glycogen phosphorylase 3. Fructose-2,6-bisphosphatase 4. Phosphofructokinase-2 (PFK-2)

1 and 4. ***1 and 4*** - **Glycogen synthase** is activated by dephosphorylation (via **Protein Phosphatase-1**) in response to **insulin**, promoting glycogenesis. - **Phosphofructokinase-2 (PFK-2)** is active in its dephosphorylated state, where its **kinase domain** predominates to produce **Fructose-2,6-bisphosphate**, stimulating glycolysis. *1 and 2* - **Glycogen synthase** is indeed active when dephosphorylated, but **Glycogen phosphorylase** is the opposite; it requires phosphorylation (by **phosphorylase kinase**) to become active for glycogenolysis. - This pair incorrectly groups an enzyme activated by dephosphorylation with one activated by phosphorylation. *2 and 3* - **Glycogen phosphorylase** and **Fructose-2,6-bisphosphatase** are both enzymes that are active in their **phosphorylated** states. - They function during fasting states (mediated by **glucagon/epinephrine** and **protein kinase A**) to promote glycogen breakdown and gluconeogenesis, respectively. *3 and 4* - **Fructose-2,6-bisphosphatase** is active only when phosphorylated, which activates its phosphatase domain to lower levels of **Fructose-2,6-bisphosphate**. - Conversely, **PFK-2** is inactive when phosphorylated, making this option incorrect as it pairs an enzyme inactive in the dephosphorylated state with one active in it.

Q: Which of the following gives a negative Benedict's test?

Sucrose. ***Sucrose*** - Sucrose is a **non-reducing sugar** because the anomeric carbons of both glucose and fructose are involved in the **glycosidic bond**, leaving no free aldehyde or ketone group to reduce copper sulfate in Benedict's reagent. - Consequently, it fails to cause the characteristic **color change** from blue to brick-red, resulting in a **negative Benedict's test**. *Glucose* - Glucose is a **reducing sugar** possessing a free aldehyde group at the C1 position. - This functional group allows it to reduce cupric ions (Cu2+) to cuprous oxide (Cu+), yielding a **positive result**. *Fructose* - Despite being a ketose, fructose is a **reducing sugar** because it undergoes **tautomerization** in the alkaline conditions of the reagent to form glucose and mannose, which possess free aldehyde groups. - This chemical conversion allows it to reduce the reagent, giving a **positive result**. *Ascorbic acid* - Ascorbic acid (Vitamin C) acts as a **strong reducing agent** due to its enediol group. - Because of its potent **reducing properties**, it causes a false **positive Benedict's test** even in the absence of reducing sugars.

Q: Insulin is essential for the entry of glucose into which of the following tissues?

Skeletal muscles. **Explanation:** The entry of glucose into cells is mediated by a family of glucose transporters known as **GLUT**. The correct answer is **Skeletal muscles** because they primarily express **GLUT-4**, which is the only insulin-dependent glucose transporter. 1. **Why Skeletal Muscle is Correct:** In the resting state, GLUT-4 transporters are sequestered in intracellular vesicles. When insulin binds to its receptor, it triggers a signaling cascade that translocates these vesicles to the plasma membrane, allowing glucose uptake. This mechanism is also found in **Adipose tissue** and the **Heart**. 2. **Why Other Options are Incorrect:** * **Neurons (Cerebral Cortex):** Use **GLUT-3** (and GLUT-1), which has a high affinity for glucose and is insulin-independent, ensuring the brain receives glucose even during fasting. * **Renal Tubular Cells & Intestinal Mucosa:** These tissues utilize **SGLT-1 and SGLT-2** (Sodium-Glucose Linked Transporters) for active transport against a concentration gradient, and **GLUT-2** for facilitated diffusion. Both are insulin-independent. **NEET-PG High-Yield Pearls:** * **GLUT-1:** Found in RBCs and the Blood-Brain Barrier (Basal uptake). * **GLUT-2:** Bidirectional transporter found in the **Liver, Pancreas (B-cells), and Kidney**. It acts as a glucose sensor. * **GLUT-4:** The only **insulin-responsive** transporter (Muscle, Fat). * **GLUT-5:** Specifically a **Fructose** transporter found in the small intestine and spermatozoa. * **Exercise** can also trigger GLUT-4 translocation in skeletal muscle independent of insulin, which is why exercise helps manage Blood Glucose in Type 2 Diabetes.

Q: Which of the following statements is NOT true with respect to glycogen?

Insulin stimulates glycogenolysis.. ### Explanation The correct answer is **D**. This statement is false because **insulin inhibits glycogenolysis** and stimulates glycogenesis. #### 1. Why Option D is the Correct Choice (The False Statement) Insulin is an anabolic hormone secreted by the pancreatic $\beta$-cells in the fed state. Its primary goal is to lower blood glucose levels. It achieves this by: * **Stimulating Glycogenesis:** Activating glycogen synthase. * **Inhibiting Glycogenolysis:** Deactivating glycogen phosphorylase through dephosphorylation (via protein phosphatase-1). In contrast, **Glucagon** and **Epinephrine** are the hormones that stimulate glycogenolysis to increase blood glucose during fasting or stress. #### 2. Analysis of Other Options * **Option A:** Glycogen is indeed the primary storage polysaccharide in humans (analogous to starch in plants), providing a rapidly mobilizable source of glucose. * **Option B:** While most cells contain some glycogen, the **liver** (maintains blood glucose) and **skeletal muscle** (provides energy for contraction) are the major reservoirs. * **Option C:** Glycogenesis is the biochemical pathway that synthesizes glycogen from glucose-6-phosphate. #### 3. NEET-PG High-Yield Pearls * **Rate-Limiting Enzymes:** Glycogen Synthase (Glycogenesis) and Glycogen Phosphorylase (Glycogenolysis). * **Linkage Types:** $\alpha(1\to4)$ glycosidic bonds form the linear chain; $\alpha(1\to6)$ bonds create branches (via Branching Enzyme). * **Tissue Specificity:** Muscle glycogen cannot maintain blood glucose because muscles lack the enzyme **Glucose-6-Phosphatase**. * **Clinical Correlation:** Von Gierke’s Disease (GSD Type I) is caused by a deficiency in Glucose-6-Phosphatase, leading to severe hypoglycemia and hepatomegaly.

Q: What is the threshold value for a high glycemic index?

70. ### Explanation The **Glycemic Index (GI)** is a ranking system (0–100) that measures how quickly a carbohydrate-containing food increases blood glucose levels compared to a reference food (usually pure glucose or white bread). **1. Why Option C (70) is Correct:** According to the World Health Organization (WHO) and international standards, carbohydrates are categorized into three tiers based on their GI value: * **Low GI:** ≤ 55 (e.g., whole grains, legumes, most fruits) * **Medium GI:** 56–69 (e.g., brown rice, sweet potato) * **High GI:** **≥ 70** (e.g., white bread, white rice, glucose) Foods with a GI of 70 or above cause a rapid spike in blood glucose and insulin levels, making them less ideal for patients with diabetes or metabolic syndrome. **2. Why Other Options are Incorrect:** * **Option A (55):** This is the upper limit for **Low GI** foods. * **Option B (60):** This falls within the **Medium GI** range (56–69). * **Option D (100):** This is the **reference value** assigned to pure glucose. While it is a high GI value, it is not the *threshold* (starting point) for the high category. **3. Clinical Pearls for NEET-PG:** * **Glycemic Load (GL):** A more accurate clinical predictor than GI because it accounts for the **portion size** (GL = GI × grams of carbohydrate / 100). * **Factors affecting GI:** Particle size (smaller = higher GI), fiber content (higher = lower GI), and acidity (lowers GI by slowing gastric emptying). * **Clinical Utility:** Low GI diets are recommended in **Diabetes Mellitus** and **PCOS** to improve insulin sensitivity and reduce postprandial hyperglycemia.

Q: In G6PD deficiency, which red blood cells are more prone to hemolysis?

Older red cells. **Explanation:** In G6PD deficiency, the correct answer is **Older red cells** because of the unique way red blood cells (RBCs) manage enzyme production. **Why Older Red Cells are Susceptible:** RBCs lack a nucleus and ribosomes; therefore, they cannot synthesize new proteins or enzymes once they enter circulation. The G6PD enzyme has a specific half-life. In individuals with G6PD deficiency (especially the common **G6PD A- variant**), the mutant enzyme is unstable and degrades more rapidly than normal. * **Younger cells** and reticulocytes have recently been released from the bone marrow and still possess sufficient enzymatic activity to maintain adequate levels of **reduced glutathione (GSH)**. * **Older cells**, having circulated for weeks, have exhausted their supply of functional G6PD. Without G6PD, they cannot generate NADPH, leaving them unable to neutralize reactive oxygen species (ROS). This leads to hemoglobin oxidation, Heinz body formation, and subsequent hemolysis during oxidative stress. **Analysis of Incorrect Options:** * **B & C (Young cells/Reticulocytes):** These cells have the highest residual G6PD activity. During an acute hemolytic crisis, the reticulocyte count rises, which can lead to a **false-normal** G6PD assay result because these young cells still have enough enzyme to pass the test. * **D (Equally susceptible):** This is incorrect because the "age-dependent" decline in enzyme activity is the hallmark of the most common variants of this disorder. **NEET-PG High-Yield Pearls:** * **Inheritance:** X-linked recessive. * **Diagnosis:** Avoid testing during an acute episode (due to high reticulocyte count); wait 6–8 weeks. * **Morphology:** Look for **Heinz bodies** (denatured Hb) and **Bite cells** (degmacytes) created by splenic macrophages. * **Triggers:** Fava beans, infections, and drugs (Primaquine, Sulfa drugs, Nitrofurantoin).

Q: If only the terminal aldehyde group of glucose is oxidized, what is the product?

Gluconic acid. ### Explanation The oxidation of glucose can yield different sugar acids depending on which carbon atom is oxidized. **1. Why Gluconic Acid is Correct:** Glucose is an aldose sugar with an aldehyde group at **Carbon-1 (C1)** and a primary alcohol group at **Carbon-6 (C6)**. When **only the terminal aldehyde group (C1)** is oxidized to a carboxyl group (-COOH), the resulting compound is **Gluconic acid**. This reaction is catalyzed by the enzyme glucose oxidase and is the fundamental principle behind many glucose estimation tests. **2. Analysis of Incorrect Options:** * **Glucuronic acid:** This is formed when **only the terminal primary alcohol group (C6)** is oxidized, leaving the aldehyde group intact. It is crucial for detoxification (conjugation) in the liver. * **Glucosaccharic acid (Glucaric acid):** This is a dicarboxylic acid formed when **both the C1 (aldehyde) and C6 (primary alcohol)** groups are oxidized simultaneously (usually by strong oxidizing agents like concentrated nitric acid). * **Gluconolactone:** This is an intermediate cyclic ester formed during the oxidation of glucose (specifically in the Pentose Phosphate Pathway) before it is hydrolyzed to gluconic acid. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Uronic Acid Pathway:** Glucuronic acid is the precursor for **bilirubin conjugation** and the synthesis of **Glycosaminoglycans (GAGs)**. * **Essential Pentosuria:** A deficiency of L-xylulose reductase in the uronic acid pathway leads to the excretion of L-xylulose in urine (a benign condition). * **Sorbitol Pathway:** While oxidation produces acids, the **reduction** of the glucose aldehyde group produces **Sorbitol** (polyol), which is implicated in diabetic complications like cataracts and neuropathy. * **Benedict’s Test:** This test relies on the oxidation of the aldehyde group to a carboxyl group, reducing cupric ions to cuprous oxide.

Q: Which mucopolysaccharide possesses anticoagulant activity?

Heparin. **Explanation:** **Heparin** is the correct answer because it is a classic example of a **Glycosaminoglycan (GAG)**, also known as a **mucopolysaccharide**. It is a highly sulfated, linear polysaccharide consisting of repeating disaccharide units (D-glucosamine and uronic acid). * **Mechanism:** Heparin acts as an indirect anticoagulant by binding to **Antithrombin III**. This binding induces a conformational change in Antithrombin III, accelerating its ability to inactivate Thrombin (Factor IIa) and Factor Xa by 1,000-fold. * **Location:** It is naturally produced by mast cells and basophils. **Analysis of Incorrect Options:** * **Phenindione (B):** This is a synthetic vitamin K antagonist (indanedione derivative). While it is an anticoagulant, it is a small molecule drug, not a mucopolysaccharide. * **Rivaroxaban (C):** This is a Direct Factor Xa inhibitor (NOAC). It is a synthetic small molecule, not a carbohydrate-based molecule. * **Dabigatran (D):** This is a Direct Thrombin Inhibitor (DTI). Like the others, it is a synthetic drug and does not belong to the class of mucopolysaccharides. **High-Yield Clinical Pearls for NEET-PG:** * **Most Acidic GAG:** Heparin is the most acidic (most negatively charged) substance in the human body due to its high sulfate content. * **Antidote:** The negative charge of Heparin is neutralized by the positively charged **Protamine Sulfate**. * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently bound to a protein core. * **Chondroitin Sulfate:** The most abundant GAG in the body (found in cartilage and bone).

Question 1

Which of the following enzymes are active in the dephosphorylated state?
1. Glycogen synthase
2. Glycogen phosphorylase
3. Fructose-2,6-bisphosphatase
4. Phosphofructokinase-2 (PFK-2)

Accepted Answer

1 and 4

Answer

1 and 2

Answer

2 and 3

Answer

3 and 4

Question 2

Assertion: The structural diversity of glycans exceeds that of proteins.
Reason: Glycans are not assembled using a genetic template and can form branched chains with
variable linkages.

Accepted Answer

Both A and R are true, and R is the correct explanation of A

Answer

A is true, but R is false

Answer

A is false, but R is true

Answer

Both A and R are true, but R is not the correct explanation of A

Question 3

Which of the following gives a negative Benedict's test?

Accepted Answer

Sucrose

Answer

Glucose

Answer

Fructose

Answer

Ascorbic acid

Question 4

Insulin is essential for the entry of glucose into which of the following tissues?

Accepted Answer

Skeletal muscles

Answer

Most neurons in the cerebral cortex

Answer

Renal tubular cells

Answer

Mucosa of the small intestine

Question 5

Which of the following statements is NOT true with respect to glycogen?

Accepted Answer

Insulin stimulates glycogenolysis.

Answer

The principle storage form of carbohydrate in the human body is glycogen.

Answer

The liver and muscles are the main sites of glycogen storage.

Answer

Glycogen is produced by glycogenesis.

Question 6

What is the threshold value for a high glycemic index?

Accepted Answer

70

Answer

55

Answer

60

Answer

100

Question 7

In G6PD deficiency, which red blood cells are more prone to hemolysis?

Accepted Answer

Older red cells

Answer

Young red cells

Answer

Reticulocytes

Answer

All are equally susceptible

Question 8

If only the terminal aldehyde group of glucose is oxidized, what is the product?

Accepted Answer

Gluconic acid

Answer

Glucuronic acid

Answer

Glucosaccharic acid

Answer

Gluconalactone

Question 9

What is the correct sequence of enzymatic events in glycogenolysis?

Accepted Answer

Glycogen phosphorylase, glucan transferase, debranching enzyme (alpha-1,6-glucosidase), glycogen phosphorylase

Answer

Debranching enzyme, glycogen phosphorylase, transferase, phosphorylase

Answer

Transferase, glycogen phosphorylase, debranching enzyme, glycogen phosphorylase

Answer

Any of the above

Question 10

Which mucopolysaccharide possesses anticoagulant activity?

Accepted Answer

Heparin

Answer

Phenindione

Answer

Rivaroxaban

Answer

Dabigatran

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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