Carbohydrate Metabolism Practice Questions

Q: What is the primary product of the pentose phosphate pathway besides pentoses?

NADPH. ### Explanation The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is an alternative pathway for glucose oxidation. Unlike glycolysis, its primary purpose is not energy production (ATP) but the generation of two specific products: **Pentose phosphates** (for nucleotide synthesis) and **NADPH**. **Why NADPH is the correct answer:** NADPH (Nicotinamide Adenine Dinucleotide Phosphate) is generated during the oxidative phase of the pathway, primarily by the rate-limiting enzyme **Glucose-6-Phosphate Dehydrogenase (G6PD)**. NADPH is essential for: 1. **Reductive Biosynthesis:** Providing reducing equivalents for fatty acid and steroid synthesis (active in liver, lactating mammary glands, and adrenal cortex). 2. **Antioxidant Defense:** Maintaining a pool of reduced **glutathione** to protect cells (especially RBCs) against reactive oxygen species (ROS). **Analysis of Incorrect Options:** * **A & C (ATP/ADP):** The HMP shunt is unique because it **neither consumes nor produces ATP**. It is an energy-neutral pathway focused on biosynthetic precursors. * **D (Acetyl CoA):** This is the end product of the Pyruvate Dehydrogenase complex following glycolysis. While NADPH is used to synthesize fatty acids from Acetyl CoA, the HMP shunt itself does not produce Acetyl CoA. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. A lack of NADPH leads to the inability to regenerate reduced glutathione, resulting in oxidative hemolysis and the presence of **Heinz bodies** and **Bite cells** on peripheral smears. * **Thiamine (B1) Requirement:** The non-oxidative phase uses the enzyme **Transketolase**, which requires Thiamine Pyrophosphate (TPP) as a cofactor. Measuring erythrocyte transketolase activity is a diagnostic test for Thiamine deficiency. * **Localization:** Occurs entirely in the **cytosol**.

Q: In anaerobic glycolysis, what is the net gain of ATP?

2 ATP. ### Explanation **1. Why Option B is Correct:** In anaerobic glycolysis (which occurs in the cytosol), the net energy yield is **2 ATP per molecule of glucose**. * **Investment Phase:** 2 ATP are consumed (catalyzed by Hexokinase and Phosphofructokinase-1). * **Payoff Phase:** 4 ATP are produced via substrate-level phosphorylation (catalyzed by Phosphoglycerate kinase and Pyruvate kinase). * **The Redox Balance:** Under anaerobic conditions (e.g., exercising muscle or RBCs), the 2 NADH produced during the oxidation of Glyceraldehyde-3-phosphate must be re-oxidized to **NAD+** to allow glycolysis to continue. This is achieved by reducing Pyruvate to **Lactate** (catalyzed by Lactate Dehydrogenase). Consequently, there is **no net gain of NADH**. **2. Why Other Options are Incorrect:** * **Option A:** While 2 ATP are gained, NAD+ is a coenzyme that is recycled, not a "net gain" product of the pathway. * **Option C:** This describes **aerobic glycolysis**. In the presence of oxygen, NADH is shuttled into the mitochondria for the Electron Transport Chain (ETC) rather than being used to form lactate. * **Option D:** 4 ATP is the *gross* yield, not the net yield. FADH2 is produced in the TCA cycle, not glycolysis. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mature RBCs:** They lack mitochondria and rely **exclusively** on anaerobic glycolysis for energy. * **Rapoport-Luebering Cycle:** A shunt in RBC glycolysis that produces **2,3-BPG**, which shifts the oxygen dissociation curve to the right (facilitating O2 unloading). This process yields **0 net ATP**. * **Key Enzyme:** Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme of glycolysis. * **Lactic Acidosis:** Occurs when the rate of glycolysis exceeds the capacity of mitochondria to oxidize pyruvate (e.g., shock, severe hypoxia).

Q: In starvation, the brain utilizes glucose preferentially. Why does the liver not do the same?

Low Km value of hexokinase. This question tests your understanding of the kinetic differences between **Hexokinase** and **Glucokinase**, which is a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer** In starvation, blood glucose levels are low. The brain must prioritize glucose uptake to maintain cellular function. This is achieved through **Hexokinase**, which has a **low $K_m$** (high affinity) for glucose. A low $K_m$ means the enzyme can function at maximum velocity ($V_{max}$) even when glucose concentrations are minimal. Therefore, the brain can efficiently "trap" glucose even during hypoglycemia. ### **Why the Other Options are Incorrect** * **B & C (Glucokinase kinetics):** Glucokinase (found in the liver and pancreatic beta cells) has a **high $K_m$** (low affinity). It only becomes active when blood glucose levels are high (post-prandial). In starvation, the liver's glucokinase is inactive, preventing the liver from competing with the brain for scarce glucose. This ensures the liver performs gluconeogenesis to *supply* glucose rather than *consuming* it. * **D (High blood flow):** While the brain receives significant cardiac output, blood flow alone does not determine the biochemical trapping of glucose; enzymatic affinity ($K_m$) is the regulatory bottleneck. ### **High-Yield Clinical Pearls for NEET-PG** * **Hexokinase:** Found in most extrahepatic tissues; inhibited by its product, **Glucose-6-Phosphate** (feedback inhibition). * **Glucokinase (Hexokinase IV):** Not inhibited by G6P; induced by **Insulin**. It acts as a "glucose sensor." * **MODY Type 2:** Caused by a mutation in the Glucokinase gene, leading to a higher threshold for insulin release and mild chronic hyperglycemia. * **GLUT Transporters:** Remember that the brain primarily uses **GLUT-1 and GLUT-3** (insulin-independent, low $K_m$), while the liver uses **GLUT-2** (high $K_m$, high capacity).

Q: Which of the following intermediates of the TCA cycle is depleted in Type-I Diabetes mellitus to suppress the TCA cycle?

Oxaloacetate. ### Explanation **Correct Option: D. Oxaloacetate** In Type-I Diabetes Mellitus (TIDM), there is an absolute deficiency of insulin and an excess of glucagon. This hormonal imbalance triggers a state of "starvation in the midst of plenty," leading to the following metabolic consequences: 1. **Increased Gluconeogenesis:** To compensate for perceived hypoglycemia, the liver aggressively synthesizes glucose. **Oxaloacetate (OAA)** is the primary substrate for gluconeogenesis (via PEP carboxykinase). Consequently, OAA is diverted away from the TCA cycle to produce glucose. 2. **Impaired TCA Cycle:** OAA is the "limiting factor" of the TCA cycle; it must condense with Acetyl-CoA to form Citrate. When OAA is depleted, the TCA cycle cannot proceed efficiently. 3. **Ketogenesis:** Simultaneously, increased lipolysis floods the liver with Acetyl-CoA. Since the TCA cycle is suppressed due to low OAA, the excess Acetyl-CoA is diverted into the **ketogenic pathway**, leading to Diabetic Ketoacidosis (DKA). --- ### Why Other Options are Incorrect: * **A, B, and C (Succinate, Malate, α-Ketoglutarate):** While these are all intermediates of the TCA cycle, they are not the primary "bottleneck" diverted for gluconeogenesis in the same capacity as OAA. OAA is the direct precursor to Phosphoenolpyruvate (PEP), making its withdrawal the critical event that halts the cycle. --- ### NEET-PG High-Yield Pearls: * **The "Metabolic Crossroad":** Oxaloacetate is required for both the TCA cycle and Gluconeogenesis. Its depletion is the biochemical link between hyperglycemia and ketosis. * **Acetyl-CoA Source:** In TIDM, Acetyl-CoA levels are high (from β-oxidation of fatty acids), but it cannot enter the TCA cycle without OAA. * **Key Enzyme:** **Pyruvate Carboxylase** (requires Biotin) converts Pyruvate to OAA to replenish the cycle (anaplerosis), but in TIDM, the flux is heavily toward glucose production.

Q: What is the carbohydrate component of blood group substances?

Fucose. ### Explanation **Why Fucose is Correct:** Blood group antigens (ABO system) are complex glycosphingolipids or glycoproteins found on the surface of red blood cells. The specificity of these antigens is determined by the terminal sugar residues attached to a precursor substance known as the **H-substance**. **L-Fucose** (6-deoxy-L-galactose) is the essential carbohydrate component that must be attached to the precursor chain by the enzyme *fucosyltransferase* to form the H-antigen. * **H-antigen:** Precursor + L-Fucose. * **A-antigen:** H-antigen + N-acetylgalactosamine. * **B-antigen:** H-antigen + D-galactose. Since the H-antigen is the foundation for both A and B antigens, Fucose is the fundamental carbohydrate component of blood group substances. **Why Incorrect Options are Wrong:** * **A. Sucrose:** A disaccharide (Glucose + Fructose) primarily involved in dietary energy; it is not a structural component of human cell surface antigens. * **C. Arabinose:** A five-carbon aldopentose found mainly in plant polysaccharides (like gum arabic) and certain bacterial cell walls, but not in human blood group substances. * **D. Maltose:** A disaccharide (Glucose + Glucose) produced during the digestion of starch; it does not play a role in membrane glycoconjugates. **High-Yield Clinical Pearls for NEET-PG:** 1. **Bombay Phenotype:** Individuals lacking the *H gene* cannot attach L-Fucose to the precursor. They lack H, A, and B antigens, appearing as Type O, but they produce anti-H antibodies, making cross-matching difficult. 2. **Secretors:** In 80% of the population (secretors), these blood group substances are also found in saliva, sweat, and other body fluids. 3. **Deoxy-sugar:** Fucose is a "deoxy-sugar," specifically 6-deoxy-L-galactose, a common feature in high-yield biochemistry questions.

Q: Pick the odd one out?

Cellulose. ### Explanation The correct answer is **Cellulose**. The primary classification criterion used here is the complexity of the carbohydrate structure (Oligosaccharides vs. Polysaccharides). **1. Why Cellulose is the odd one out:** Cellulose is a **Polysaccharide**. It is a high-molecular-weight polymer consisting of thousands of glucose units linked by **$\beta(1\to4)$ glycosidic bonds**. Unlike the other options, it is a structural carbohydrate found in plant cell walls. In humans, it remains undigested due to the lack of the enzyme cellulase, contributing to dietary fiber. **2. Why the other options are incorrect:** * **Lactose:** It is a **Disaccharide** (reducing sugar) composed of Glucose and Galactose linked by a $\beta(1\to4)$ bond. It is the primary sugar in milk. * **Maltose:** It is a **Disaccharide** (reducing sugar) composed of two Glucose units linked by an $\alpha(1\to4)$ bond. It is an intermediate product of starch digestion. * **Sucrose:** It is a **Disaccharide** (non-reducing sugar) composed of Glucose and Fructose linked by an $\alpha1\to\beta2$ bond. It is common table sugar. **3. NEET-PG High-Yield Clinical Pearls:** * **Reducing vs. Non-reducing:** All common monosaccharides and disaccharides (Maltose, Lactose) are reducing sugars **except Sucrose**, because its functional groups are involved in the glycosidic bond. * **Invert Sugar:** Sucrose is called "invert sugar" because its optical rotation changes from dextrorotatory to levorotatory upon hydrolysis into glucose and fructose. * **Lactose Intolerance:** Caused by a deficiency of the enzyme lactase (brush border disaccharidase), leading to osmotic diarrhea and abdominal bloating. * **Cellulose in Diet:** While indigestible, it is essential for increasing fecal bulk and preventing constipation and diverticulosis.

Q: Which of the following is the insulin-dependent glucose transporter?

GLUT-4. ### Explanation **Correct Option: D (GLUT-4)** GLUT-4 is the only **insulin-dependent** glucose transporter. It is primarily expressed in **skeletal muscle, cardiac muscle, and adipose tissue**. In the resting state, GLUT-4 is sequestered in intracellular vesicles. When insulin binds to its receptor, it triggers a signaling cascade that causes these vesicles to fuse with the plasma membrane, increasing glucose uptake. This mechanism is crucial for lowering postprandial blood glucose levels. **Analysis of Incorrect Options:** * **GLUT-1 (Option A):** This is an insulin-independent transporter found in **RBCs**, the blood-brain barrier, and kidneys. It provides a basal level of glucose uptake. * **GLUT-5 (Option B):** This is a unique transporter primarily located in the small intestine and spermatozoa. It is specific for **fructose** transport, not glucose. * **GLUT-3 (Option C):** This is an insulin-independent transporter with a very high affinity for glucose. It is the primary transporter in **neurons**, ensuring the brain receives glucose even during hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-2:** A bidirectional transporter found in the **Liver, Pancreas (B-cells), and Kidney**. It acts as a "glucose sensor." * **Exercise & GLUT-4:** Muscle contraction can also trigger GLUT-4 translocation to the cell membrane independent of insulin, which is why exercise helps manage Type 2 Diabetes. * **SGLT-1/2:** Unlike GLUTs (facilitated diffusion), SGLTs are **Secondary Active Transporters** (Sodium-Glucose co-transporters) found in the intestine and renal tubules. * **Mnemonic:** "BRICK L" for insulin-independent tissues: **B**rain, **R**BCs, **I**ntestine, **C**ornea, **K**idney, **L**iver.

Question 1

What is the primary product of the pentose phosphate pathway besides pentoses?

Accepted Answer

NADPH

Answer

ATP

Answer

ADP

Answer

Acetyl CoA

Question 2

In anaerobic glycolysis, what is the net gain of ATP?

Accepted Answer

2 ATP

Answer

2 ATP and 2 NAD

Answer

2 ATP and 2 NADH

Answer

4 ATP and 2 FADH2

Question 3

In starvation, the brain utilizes glucose preferentially. Why does the liver not do the same?

Accepted Answer

Low Km value of hexokinase

Answer

Low Km of glucokinase

Answer

High Km of glucokinase

Answer

High blood flow

Question 4

What is the characteristic function of Glucokinase?

Accepted Answer

It has a high Km for glucose, making it important for glucose phosphorylation primarily after ingestion of a carbohydrate-rich meal.

Answer

It is widely distributed and occurs in most mammalian tissues.

Answer

It is widely distributed in prokaryotes.

Answer

None of the above.

Question 5

Which of the following intermediates of the TCA cycle is depleted in Type-I Diabetes mellitus to suppress the TCA cycle?

Accepted Answer

Oxaloacetate

Answer

Succinate

Answer

Malate

Answer

a-Ketoglutarate

Question 6

What is the carbohydrate component of blood group substances?

Accepted Answer

Fucose

Answer

Sucrose

Answer

Arabinose

Answer

Maltose

Question 7

Pick the odd one out?

Accepted Answer

Cellulose

Answer

Lactose

Answer

Maltose

Answer

Sucrose

Question 8

In one cycle of glycolysis under aerobic conditions, how many molecules of ATP and NADPH are formed?

Accepted Answer

2 ATP, 2 NADPH

Answer

4 ATP, 2 NADPH

Answer

4 ATP, 4 NADPH

Answer

2 ATP, 4 NADPH

Question 9

Which of the following is the insulin-dependent glucose transporter?

Accepted Answer

GLUT-4

Answer

GLUT-1

Answer

GLUT-5

Answer

GLUT-3

Question 10

A 3-year-old child presents with hepatomegaly and recurrent episodes of vomiting, jaundice, and hypoglycemia, beginning from the later part of infancy. These episodes seem to have no temporal relation with the intake of milk or milk products. The mother also gives a history that the child has an aversion to sweet food. Which of the following enzymes may be deficient in this child?

Accepted Answer

Aldolase B

Answer

Glucose-6-phosphatase

Answer

Branching enzyme

Answer

Galactose 1 phosphate uridyl transferase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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