Carbohydrate Metabolism Practice Questions

Q: What is the end product of anaerobic glycolysis?

2 ATP + 2 NAD. ### Explanation **Concept:** In anaerobic glycolysis (occurring in RBCs or exercising muscle), the primary goal is to generate energy in the absence of oxygen. The process follows the standard glycolytic pathway until the formation of **Pyruvate**. However, to keep glycolysis running, the cell must regenerate **NAD+** from **NADH**. This is achieved by the enzyme **Lactate Dehydrogenase (LDH)**, which reduces Pyruvate to **Lactate** while simultaneously oxidizing NADH back to NAD+. **Why Option A is Correct:** 1. **Net ATP:** Glycolysis consumes 2 ATP and produces 4 ATP, resulting in a **net gain of 2 ATP**. 2. **NAD+ Regeneration:** The 2 NADH molecules produced during the glyceraldehyde-3-phosphate dehydrogenase step are consumed by LDH to produce **2 NAD+**. Therefore, the "end products" in terms of coenzymes are 2 NAD+. **Why Other Options are Incorrect:** * **Option B:** 2 NADH is the product of *aerobic* glycolysis (before the electron transport chain). In anaerobic conditions, NADH is used up to create lactate. * **Option C:** FADH2 is produced in the TCA cycle (Succinate Dehydrogenase step), not in glycolysis. * **Option D:** While 4 ATP are produced in the payoff phase, the *net* yield is 2 ATP because 2 were invested in the preparatory phase. **NEET-PG High-Yield Pearls:** * **RBCs:** Always undergo anaerobic glycolysis because they lack mitochondria. * **Key Enzyme:** **Lactate Dehydrogenase (LDH)** is the marker for anaerobic metabolism. * **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). * **Arsenite Poisoning:** Inhibits enzymes requiring lipoic acid, but **Arsenate** competes with inorganic phosphate in glycolysis, resulting in **zero net ATP** production.

Q: Which glucose transporter is primarily present in Red Blood Cells?

GLUT-1. **Explanation:** **GLUT-1** is the correct answer because it is the primary glucose transporter found in **Red Blood Cells (RBCs)** and the **Blood-Brain Barrier**. It is a high-affinity, insulin-independent transporter that ensures a constant basal glucose uptake, which is critical for RBCs as they rely exclusively on glycolysis for energy (due to the absence of mitochondria). **Analysis of Incorrect Options:** * **GLUT-2:** A low-affinity, high-capacity transporter found in the **Liver, Pancreas (beta cells), and Kidney**. It acts as a glucose sensor and is involved in bidirectional glucose transport. * **GLUT-3:** A high-affinity transporter primarily located in **Neurons**. It ensures glucose delivery to the brain even during low blood glucose concentrations. * **GLUT-4:** 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 found in **Skeletal Muscle and Adipose Tissue**. **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. * **GLUT-5:** Specifically transports **Fructose** and is located in the small intestine and spermatozoa. * **SGLT-1/SGLT-2:** These are Sodium-Glucose Co-transporters (Active Transport) found in the small intestine and renal tubules, unlike the GLUT family which uses **Facilitated Diffusion**. * **Mnemonic:** "BRICK L" for Insulin-Independent tissues: **B**rain, **R**BCs, **I**ntestine, **C**ornea, **K**idney, **L**iver.

Q: Deficiency of lysosomal maltase causes which condition?

Pompe disease. **Explanation:** **Correct Answer: D. Pompe disease** Pompe disease (Glycogen Storage Disease Type II) is unique among glycogen storage diseases because it is also a **lysosomal storage disorder**. It is caused by a deficiency of the enzyme **lysosomal alpha-1,4-glucosidase** (also known as **Acid Maltase**). While most glycogen breakdown occurs in the cytosol, about 1–3% of glycogen is degraded within lysosomes. In Pompe disease, glycogen accumulates within the lysosomes of all organs, most significantly affecting cardiac and skeletal muscle, leading to massive cardiomegaly and respiratory failure. **Analysis of Incorrect Options:** * **A. McArdle's disease (GSD Type V):** Caused by a deficiency of **muscle glycogen phosphorylase**. It presents with exercise intolerance, muscle cramps, and myoglobinuria, but lacks cardiac involvement. * **B. Andersen disease (GSD Type IV):** Caused by a deficiency of the **branching enzyme**. It leads to the accumulation of abnormal glycogen with long outer chains (amylopectin-like), resulting in early-onset liver cirrhosis. * **C. Cori disease (GSD Type III):** Caused by a deficiency of the **debranching enzyme**. It results in the accumulation of "limit dextrin" and presents with hepatomegaly and hypoglycemia, similar to Von Gierke disease but milder. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (heart)." * **Key Feature:** It is the only GSD that is a lysosomal storage disease. * **Clinical Presentation:** Infantile form presents with "floppy baby" syndrome (hypotonia) and **massive cardiomegaly**. * **Biochemical Marker:** Normal blood glucose levels (unlike Type I, III, and VI) because cytosolic glycogenolysis remains intact.

Q: Which of the following sites are known to have Hexose Monophosphate (HMP) shunt activity?

All of these. The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is an alternative pathway for glucose oxidation. Unlike glycolysis, its primary purpose is not ATP production, but the generation of **NADPH** and **Ribose-5-phosphate**. ### Why "All of these" is correct: The HMP shunt is most active in tissues that require high amounts of NADPH for reductive biosynthesis (fatty acids, steroids) or for maintaining antioxidant defenses. 1. **Liver (Option A):** The liver is the primary site for the synthesis of fatty acids and cholesterol, both of which require NADPH as a reducing agent. 2. **Lactating Mammary Gland (Option B):** During lactation, mammary glands undergo intense fatty acid synthesis for milk production, necessitating high HMP shunt activity. 3. **Testes (Option C):** Along with the adrenal cortex and ovaries, the testes require NADPH for the synthesis of steroid hormones (testosterone). ### High-Yield NEET-PG Clinical Pearls: * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Subcellular site:** Cytosol. * **Key Products:** * **NADPH:** Used for fatty acid/steroid synthesis and keeping glutathione reduced (protecting RBCs from oxidative stress). * **Ribose-5-Phosphate:** Essential for nucleotide (DNA/RNA) synthesis. * **Tissues with NO HMP Shunt:** Skeletal muscle (due to lack of G6PD). * **Clinical Correlation:** G6PD deficiency leads to hemolytic anemia because RBCs cannot generate NADPH to neutralize reactive oxygen species (ROS), leading to Heinz bodies and Bite cells.

Q: In pregnancy, what is the standard amount of glucose used in a Glucose Tolerance Test?

100g. **Explanation** The standard diagnostic test for Gestational Diabetes Mellitus (GDM) traditionally follows the **Carpenter-Coustan criteria**, which utilizes a **100g oral glucose load**. This is part of a "two-step" approach: first, a 50g screening test is performed; if positive, it is followed by the definitive 100g, 3-hour Oral Glucose Tolerance Test (OGTT). **Analysis of Options:** * **Option C (100g) - Correct:** This is the gold standard dose for the 3-hour OGTT in pregnancy. Blood glucose is measured at fasting, 1 hour, 2 hours, and 3 hours. Diagnosis is confirmed if at least two values are elevated. * **Option A (50g):** This is used for the **Glucose Challenge Test (GCT)**, a non-fasting screening test. It is not diagnostic on its own. * **Option B (75g):** This is the standard dose for non-pregnant adults (WHO criteria). While the IADPSG/DIPSI guidelines now advocate for a 75g "one-step" test in pregnancy, the 100g test remains the classic textbook answer for standard diagnostic protocols in many examination contexts. * **Option D (125g):** This dose is not used in any standardized clinical glucose tolerance protocol. **High-Yield Clinical Pearls for NEET-PG:** * **DIPSI Guidelines:** In India, the Diabetes in Pregnancy Study Group India (DIPSI) recommends a **75g glucose load** regardless of the last meal (single-step procedure). * **Renal Threshold:** In pregnancy, the renal threshold for glucose decreases (from 180 mg/dL to ~150 mg/dL), leading to physiological glucosuria. * **Hormonal Basis:** GDM is primarily driven by **Human Placental Lactogen (hPL)**, which acts as an anti-insulin hormone to ensure glucose availability for the fetus.

Q: Which of the following glycolytic enzymes is used in gluconeogenesis?

Aldolase. ### Explanation The key to understanding this question lies in distinguishing between **reversible** and **irreversible** steps of glycolysis. **1. Why Aldolase is Correct:** Gluconeogenesis is not a simple reversal of glycolysis; it shares the reversible enzymes but must bypass the three irreversible "bottleneck" steps. **Aldolase** (specifically Aldolase B in the liver) catalyzes a reversible reaction: it cleaves Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate during glycolysis, and **condenses** them back into Fructose-1,6-bisphosphate during gluconeogenesis. Because this reaction is at equilibrium, the same enzyme functions in both pathways. **2. Why the Other Options are Incorrect:** Options A, B, and D represent the three **irreversible, regulatory steps** of glycolysis. These enzymes cannot be used in gluconeogenesis and must be bypassed by specific gluconeogenic enzymes: * **Glucokinase (Step 1):** Bypassed by *Glucose-6-phosphatase*. * **Phosphofructokinase-1 (Step 3):** Bypassed by *Fructose-1,6-bisphosphatase* (the rate-limiting step of gluconeogenesis). * **Pyruvate Kinase (Step 10):** Bypassed by the two-step process involving *Pyruvate carboxylase* and *PEP carboxykinase (PEPCK)*. **3. NEET-PG High-Yield Pearls:** * **Reversible Enzymes:** Besides Aldolase, other shared enzymes include Phosphohexose isomerase, Enolase, and Phosphoglycerate kinase. * **Aldolase Deficiency:** Deficiency of **Aldolase B** leads to *Hereditary Fructose Intolerance*, characterized by severe hypoglycemia after fructose ingestion due to the trapping of intracellular phosphate. * **Location:** Gluconeogenesis occurs primarily in the **liver** (90%) and **kidney** (10%). It starts in the mitochondria but occurs mostly in the cytosol.

Q: Which of the following is NOT a positive signal for glycogen breakdown?

Blood glucose. **Explanation:** Glycogenolysis (glycogen breakdown) is regulated by the enzyme **Glycogen Phosphorylase**. This process is activated when the body requires energy or needs to maintain blood glucose levels. **Why Blood Glucose is the correct answer:** High levels of **blood glucose** act as a negative signal (inhibitor) for glycogenolysis. In the liver, glucose binds to glycogen phosphorylase $a$, inducing a conformational change that makes it susceptible to inactivation by protein phosphatase-1. This is a feedback mechanism: if blood glucose is already high, the body does not need to break down glycogen. **Analysis of Incorrect Options:** * **Cyclic AMP (cAMP):** This is a classic positive signal. Glucagon (in liver) or Epinephrine (in muscle) binds to G-protein coupled receptors, increasing cAMP. This activates Protein Kinase A, which eventually activates glycogen phosphorylase. * **Epinephrine:** Known as the "fight or flight" hormone, it stimulates glycogen breakdown in both liver and muscle to provide immediate fuel. * **Ca²⁺:** In contracting muscles, calcium is released from the sarcoplasmic reticulum. It binds to the calmodulin subunit of **Phosphorylase Kinase**, activating it even without phosphorylation. This ensures glycogen is broken down to support muscle contraction. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glycogen Phosphorylase. * **Allosteric Activators:** AMP (specifically in muscle during exercise) and Ca²⁺. * **Allosteric Inhibitors:** ATP, Glucose-6-Phosphate, and Free Glucose. * **Hormonal Control:** Glucagon and Epinephrine stimulate breakdown (via phosphorylation); Insulin inhibits breakdown (via dephosphorylation).

Q: All of the following biological reactions take place in mitochondria, EXCEPT:

EMP pathway. **Explanation:** The correct answer is **B. EMP pathway**. The **EMP (Embden-Meyerhof-Parnas) pathway**, commonly known as **Glycolysis**, is the sequence of reactions that converts glucose into pyruvate. This entire process occurs exclusively in the **cytosol** of the cell. It does not require oxygen or mitochondrial machinery, which is why it can function in cells lacking mitochondria, such as mature erythrocytes. **Analysis of Options:** * **Fatty acid oxidation (Beta-oxidation):** This process occurs within the **mitochondrial matrix**. Long-chain fatty acids are transported into the mitochondria via the carnitine shuttle to be broken down into Acetyl-CoA. * **Electron transport chain (ETC):** The components of the ETC and ATP synthase are located on the **inner mitochondrial membrane**. This is the site of oxidative phosphorylation. * **Citric acid cycle (Kreb’s Cycle):** All enzymes for this cycle (except succinate dehydrogenase, which is on the inner membrane) are located in the **mitochondrial matrix**. **High-Yield Clinical Pearls for NEET-PG:** * **Purely Cytosolic Pathways:** Glycolysis, HMP Shunt, Fatty acid synthesis, and Translation. * **Purely Mitochondrial Pathways:** TCA cycle, Beta-oxidation, ETC, and Ketogenesis. * **Dual Compartment Pathways (Both Cytosol & Mitochondria):** Remember the mnemonic **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. * **Erythrocyte Metabolism:** Since RBCs lack mitochondria, they rely solely on the EMP pathway for energy and cannot perform the TCA cycle or Beta-oxidation.

Question 1

What is the end product of anaerobic glycolysis?

Accepted Answer

2 ATP + 2 NAD

Answer

2 ATP + 2 NADH

Answer

2 ATP + 2 FADH2

Answer

4 ATP + 2 NAD

Question 2

Which glucose transporter is primarily present in Red Blood Cells?

Accepted Answer

GLUT-1

Answer

GLUT-2

Answer

GLUT-3

Answer

GLUT-4

Question 3

Deficiency of lysosomal maltase causes which condition?

Accepted Answer

Pompe disease

Answer

McArdle's disease

Answer

Andersen disease

Answer

Cori disease

Question 4

Which of the following sites are known to have Hexose Monophosphate (HMP) shunt activity?

Accepted Answer

All of these

Answer

Liver

Answer

Lactating mammary gland

Answer

Testes

Question 5

Hepatomegaly is an essential feature of which of the following conditions?

Accepted Answer

Hepatic porphyria

Answer

Von Gierke's disease

Answer

Niemann-Pick disease

Answer

Hurler syndrome

Question 6

In pregnancy, what is the standard amount of glucose used in a Glucose Tolerance Test?

Accepted Answer

100g

Answer

50g

Answer

75g

Answer

125g

Question 7

Which of the following glycolytic enzymes is used in gluconeogenesis?

Accepted Answer

Aldolase

Answer

Glucokinase

Answer

Pyruvate kinase

Answer

Phosphofructokinase

Question 8

Which of the following is NOT a positive signal for glycogen breakdown?

Accepted Answer

Blood glucose

Answer

Cyclic AMP

Answer

Epinephrine

Answer

Ca2+

Question 9

Which is the rate-limiting step in the Hexose Monophosphate (HMP) shunt?

Accepted Answer

Formation of 6-phosphogluconolactone from glucose-6-phosphate

Answer

Formation of 6-phosphogluconate from 6-phosphogluconolactone

Answer

Formation of ribulose-5-phosphate from 6-phosphogluconate

Answer

Formation of xylulose-5-phosphate from ribulose-5-phosphate

Question 10

All of the following biological reactions take place in mitochondria, EXCEPT:

Accepted Answer

EMP pathway

Answer

Fatty acid oxidation

Answer

Electron transport chain

Answer

Citric acid cycle

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?