Carbohydrate Metabolism Practice Questions

Q: Which of the following statements about GLUT is false?

GLUT-2 is needed in the brain. The correct answer is **A. GLUT-2 is needed in the brain**. This statement is false because the brain primarily relies on **GLUT-1** (for crossing the blood-brain barrier) and **GLUT-3** (for neuronal uptake). GLUT-2 is a high-capacity, low-affinity transporter found in the liver, pancreatic beta cells, small intestine, and renal tubules, where it acts as a "glucose sensor." ### Explanation of Options: * **Option A (False/Correct):** GLUT-2 is not the primary transporter for the brain. The brain requires a constant glucose supply regardless of blood sugar levels; therefore, it utilizes GLUT-3, which has a very low Km (high affinity). * **Option B (True):** GLUT-3 is indeed present in the placenta to ensure a steady transfer of glucose to the fetus. * **Option C (True):** GLUT-4 is the only **insulin-dependent** glucose transporter. It is sequestered in intracellular vesicles and translocates to the cell membrane only in the presence of insulin. It is primarily found in skeletal muscle, cardiac muscle, and **adipose tissue**. * **Option D (True):** GLUT-3 has a wide distribution, including the brain (neurons), placenta, liver, kidneys, and specifically the **testis and intestine**. ### High-Yield Clinical Pearls for NEET-PG: * **GLUT-1 Deficiency Syndrome:** Presents with infantile seizures and developmental delay due to impaired glucose transport across the blood-brain barrier. * **SGLT vs. GLUT:** Remember that SGLT (1 & 2) are active transporters (secondary active) used in the intestine and kidneys, whereas all GLUTs (1–5) facilitate **passive diffusion**. * **GLUT-5:** Unique because it is primarily a **fructose** transporter, located in the small intestine and spermatozoa. * **Bidirectional Transport:** GLUT-2 is unique for its ability to transport glucose in both directions (e.g., during gluconeogenesis in the liver).

Q: Which of the following enzymes are NOT involved in glycogen metabolism?

Glycogen synthase A. ### Explanation In glycogen metabolism, enzymes exist in two interconvertible forms: an active form and an inactive form, regulated primarily by covalent modification (phosphorylation/dephosphorylation). **Why Option D is the Correct Answer:** The term **"Glycogen synthase A"** is a misnomer in standard biochemical nomenclature. While "Glycogen Synthase **a**" (lowercase) refers to the active, dephosphorylated form of the enzyme, "Glycogen Synthase **A**" (uppercase) is not a recognized clinical or biochemical designation for the enzyme. In the context of this specific competitive exam question, it serves as the "distractor" that does not exist in the metabolic pathway. **Analysis of Other Options:** * **Glycogen phosphorylase b (Option A):** This is the **inactive**, dephosphorylated form of the enzyme responsible for glycogenolysis. It is converted to the active 'a' form by phosphorylase kinase. * **Glycogen synthase I (Option B):** The "I" stands for **Independent**. This is the active, dephosphorylated form of glycogen synthase that functions independently of the glucose-6-phosphate concentration. * **Glycogen synthase D (Option C):** The "D" stands for **Dependent**. This is the inactive, phosphorylated form of the enzyme which requires high concentrations of the allosteric activator, glucose-6-phosphate, to function. **High-Yield Clinical Pearls for NEET-PG:** * **Reciprocal Regulation:** Glucagon and Epinephrine trigger phosphorylation, which **activates** Glycogen Phosphorylase (promoting breakdown) but **inactivates** Glycogen Synthase (inhibiting synthesis). * **Insulin Effect:** Insulin triggers dephosphorylation via Protein Phosphatase-1, activating Glycogen Synthase (I-form). * **Rate-Limiting Enzymes:** Glycogen Synthase is the rate-limiting enzyme for glycogenesis; Glycogen Phosphorylase is the rate-limiting enzyme for glycogenolysis.

Q: Which of the following reactions generates ATP?

Phosphoenolpyruvate to Pyruvate. **Explanation:** The generation of ATP in glycolysis occurs through **Substrate-Level Phosphorylation**, where a high-energy phosphate group is directly transferred from a metabolic intermediate to ADP. **1. Why Option C is Correct:** The conversion of **Phosphoenolpyruvate (PEP) to Pyruvate** is the final step of glycolysis, catalyzed by the enzyme **Pyruvate Kinase**. PEP contains a high-energy enol-phosphate bond. The breakdown of this bond releases enough energy to drive the phosphorylation of ADP to ATP. This is one of the two substrate-level phosphorylation steps in glycolysis (the other being the conversion of 1,3-BPG to 3-phosphoglycerate). **2. Analysis of Incorrect Options:** * **Option A (Glyceraldehyde-3-phosphate to 1,3-BPG):** This reaction, catalyzed by G3P Dehydrogenase, generates **NADH**, not ATP. It incorporates inorganic phosphate but does not produce energy in the form of ATP at this specific step. * **Option B (Glyceraldehyde-3-phosphate to DHAP):** This is a reversible isomerization reaction catalyzed by **Triose Phosphate Isomerase**. It involves the rearrangement of atoms with no net gain or loss of energy/ATP. * **Option C (Pyruvate to Lactate):** This occurs under anaerobic conditions catalyzed by **Lactate Dehydrogenase**. This reaction actually **consumes NADH** (oxidizing it to NAD+) to allow glycolysis to continue; it does not generate ATP. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Step:** Pyruvate Kinase is one of the three irreversible enzymes of glycolysis. * **Clinical Correlation:** **Pyruvate Kinase deficiency** is the second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). Since RBCs lack mitochondria, they rely solely on glycolysis for ATP; a deficiency leads to ATP depletion, causing rigid membranes and hemolysis. * **Net Yield:** The net gain of glycolysis is **2 ATP** and **2 NADH** per molecule of glucose.

Q: In what form does the product of glycolysis enter the TCA cycle?

Acetyl-CoA. **Explanation:** The end product of aerobic glycolysis is **Pyruvate**. However, the TCA cycle (Krebs cycle) occurs in the mitochondrial matrix and cannot directly utilize pyruvate. **1. Why Acetyl-CoA is correct:** Before entering the TCA cycle, pyruvate must undergo **oxidative decarboxylation** to form **Acetyl-CoA**. This reaction is catalyzed by the **Pyruvate Dehydrogenase (PDH) Complex**, a multi-enzyme cluster located in the inner mitochondrial membrane. Acetyl-CoA then condenses with Oxaloacetate (OAA) to form Citrate, marking the official start of the TCA cycle. Therefore, Acetyl-CoA is the "connecting link" or the actual substrate that enters the cycle. **2. Why other options are incorrect:** * **Pyruvate:** While it is the product of glycolysis, it is a 3-carbon molecule that must be converted to the 2-carbon Acetyl-CoA before it can participate in the TCA cycle. * **NADH:** This is a coenzyme produced during glycolysis and the TCA cycle. It enters the Electron Transport Chain (ETC) to generate ATP but does not "enter" the TCA cycle as a substrate. * **Glucose:** This is the starting substrate for glycolysis, not the product. **3. High-Yield Clinical Pearls for NEET-PG:** * **PDH Complex Requirements:** It requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**T**ender **R**eeds **N**ever **P**lay **L**oose). * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to lipoic acid, leading to lactic acidosis and neurological symptoms. * **Regulation:** PDH is inhibited by its products (Acetyl-CoA and NADH) and activated by ADP and $Ca^{2+}$.

Q: Phosphofructokinase is the key enzyme of which metabolic pathway?

Glycolysis. **Explanation:** **Phosphofructokinase-1 (PFK-1)** is the most important regulatory and rate-limiting enzyme of **Glycolysis** (the Embden-Meyerhof pathway). It catalyzes the irreversible conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate using one molecule of ATP. This step is known as the "committed step" because once phosphorylated by PFK-1, the glucose metabolite is destined to complete the glycolytic pathway. **Analysis of Incorrect Options:** * **Glycogenolysis:** The rate-limiting enzyme is **Glycogen Phosphorylase**, which breaks down glycogen into glucose-1-phosphate. * **Glycogenesis:** The rate-limiting enzyme is **Glycogen Synthase**, responsible for forming $\alpha$-1,4-glycosidic bonds to synthesize glycogen. * **TCA Cycle:** The key regulatory enzyme is **Isocitrate Dehydrogenase**, which catalyzes the oxidative decarboxylation of isocitrate to $\alpha$-ketoglutarate. **High-Yield Clinical Pearls for NEET-PG:** * **Allosteric Regulation:** PFK-1 is allosterically **inhibited by ATP and Citrate** (signaling high energy status) and **activated by AMP and Fructose-2,6-bisphosphate**. * **Potent Activator:** Fructose-2,6-bisphosphate is the most potent physiological activator of PFK-1, produced by the bifunctional enzyme PFK-2. * **Clinical Correlation:** Deficiency of the M-isoform of PFK-1 in muscles leads to **Tarui Disease (Glycogen Storage Disease Type VII)**, characterized by exercise intolerance and muscle cramping. * **Insulin vs. Glucagon:** Insulin increases PFK-1 activity (promoting glycolysis), while glucagon decreases it.

Q: Which test is used to differentiate monosaccharides from disaccharides?

Barfoed's test. **Explanation:** The correct answer is **Barfoed’s test**. This test is specifically designed to distinguish between reducing monosaccharides and reducing disaccharides based on the speed of the reaction. **1. Why Barfoed’s test is correct:** Barfoed’s reagent consists of cupric acetate in dilute acetic acid (an acidic medium). Both monosaccharides and disaccharides can reduce cupric ions to cuprous oxide, forming a red precipitate. However, because the medium is acidic, monosaccharides (being stronger reducing agents) react much faster, typically within **2–3 minutes** of boiling. Reducing disaccharides (like lactose or maltose) require prolonged boiling (7–10 minutes) to show a reaction. **2. Analysis of Incorrect Options:** * **Benedict’s test:** Used to detect **reducing sugars** in general (e.g., glucose, fructose, lactose). It cannot differentiate between a monosaccharide and a disaccharide. * **Seliwanoff’s test:** Used to differentiate **keto-hexoses** (like fructose) from aldo-hexoses. It produces a cherry-red complex with ketoses. * **Rapid Furfural test:** Used to distinguish between **fructose (a keto-hexose)** and sucrose. It is a faster version of the Molisch test specifically for ketoses. **High-Yield Clinical Pearls for NEET-PG:** * **Barfoed’s Reagent:** Cupric acetate + Glacial acetic acid. * **Key distinction:** Monosaccharides = Scanty red precipitate in 7 mins. * **Osazone Test:** Another high-yield test where glucose and fructose form needle-shaped crystals, while lactose forms "powder-puff" or "hedgehog" shaped crystals. * **Bial’s Test:** Used specifically to detect **Pentoses** (e.g., ribose).

Q: What is the primary location of GLUT 5?

Small intestine. **Explanation:** Glucose transporters (GLUT) are a family of transmembrane proteins that facilitate the transport of glucose and other hexoses across cell membranes. **Why the Small Intestine is Correct:** **GLUT 5** is unique among the GLUT family because it is primarily a **fructose transporter** rather than a glucose transporter. Its primary location is the apical membrane of the enterocytes in the **small intestine**, where it facilitates the absorption of dietary fructose from the intestinal lumen. It is also found in high concentrations in **spermatozoa** (testis), as fructose is their main energy source. **Analysis of Incorrect Options:** * **A. RBC:** The primary transporter in Red Blood Cells is **GLUT 1**, which provides a basal glucose uptake independent of insulin. * **B. Liver:** The liver primarily utilizes **GLUT 2**. This is a high-capacity, high-Km (low affinity) transporter that allows the liver to "sense" and respond to high postprandial blood glucose levels. * **D. Placenta:** The placenta predominantly expresses **GLUT 1** and **GLUT 3** to ensure a continuous supply of glucose to the fetus, even during maternal hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT 4** is the only **insulin-dependent** transporter, located in skeletal muscle and adipose tissue. * **GLUT 2** is bidirectional and found in the "Liver, Kidney, and Pancreatic Beta cells." * **SGLT-1 vs. GLUT 5:** Remember that glucose and galactose are absorbed via SGLT-1 (active transport), while fructose is absorbed via GLUT 5 (facilitated diffusion). * **Mnemonic:** "GLUT **5** is for **F**ructose" (Both start with the 'F' sound).

Question 1

Which of the following statements about GLUT is false?

Accepted Answer

GLUT-2 is needed in the brain

Answer

GLUT-3 is present in the placenta

Answer

GLUT-4 is present in adipose tissue

Answer

GLUT-3 is present in both intestine and testis

Question 2

Regarding the HMP shunt, all of the following statements are true EXCEPT:

Accepted Answer

The oxidative phase generates NADPH, while the non-oxidative phase generates pyruvate.

Answer

It occurs in the cytosol.

Answer

No ATP is produced during the cycle.

Answer

It is active in adipose tissue, the liver, and gonads.

Question 3

Which of the following enzymes are NOT involved in glycogen metabolism?

Accepted Answer

Glycogen synthase A

Answer

Glycogen phosphorylase b

Answer

Glycogen synthase I

Answer

Glycogen synthase C

Question 4

Which of the following reactions generates ATP?

Accepted Answer

Phosphoenolpyruvate to Pyruvate

Answer

Glucose-3-phosphate to 1,3 BPG

Answer

Glucose-3-phosphate to Dihydroxyacetone phosphate

Answer

Pyruvate to Lactate

Question 5

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

Accepted Answer

Decrease ketone body production

Answer

Increased lactate production upon exercise

Answer

Decreased gluconeogenesis

Answer

Increased gluconeogenesis

Question 6

In what form does the product of glycolysis enter the TCA cycle?

Accepted Answer

Acetyl-CoA

Answer

Pyruvate

Answer

NADH

Answer

Glucose

Question 7

Phosphofructokinase is the key enzyme of which metabolic pathway?

Accepted Answer

Glycolysis

Answer

Glycogenolysis

Answer

Glycogenesis

Answer

TCA cycle

Question 8

Which test is used to differentiate monosaccharides from disaccharides?

Accepted Answer

Barfoed's test

Answer

Benedict's test

Answer

Seliwanoff's test

Answer

Rapid furfural test

Question 9

Glyconeogenic capability of a cell is determined by the presence of which enzyme?

Accepted Answer

Fructose 1,6-bisphosphatase

Answer

Pyruvate dehydrogenase

Answer

Glucose 6 phosphatase

Answer

Pyruvate carboxylase

Question 10

What is the primary location of GLUT 5?

Accepted Answer

Small intestine

Answer

RBC

Answer

Liver

Answer

Placenta

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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