Carbohydrate Metabolism Practice Questions

Q: Which of the following enzymes does not catalyze a reversible step in glycolysis?

Phosphofructokinase-1 (PFK). ### Explanation In glycolysis, most reactions are reversible and occur near equilibrium. However, there are **three irreversible steps** that serve as the primary regulatory points of the pathway. These reactions have a large negative Gibbs free energy ($\Delta G$), making them "one-way" valves in the cell. **1. Why Phosphofructokinase-1 (PFK-1) is the correct answer:** PFK-1 catalyzes the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate. This is the **second irreversible step** and the **rate-limiting step** of glycolysis. Because it is highly exergonic, it cannot be reversed by the same enzyme; instead, gluconeogenesis must use a different enzyme (Fructose-1,6-bisphosphatase) to bypass this step. **2. Analysis of Incorrect Options:** * **Enolase:** Catalyzes the reversible dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). It is inhibited by **fluoride** (used in gray-top blood collection tubes to prevent glycolysis). * **Phosphoglyceromutase:** Catalyzes the reversible shift of the phosphate group from the 3rd to the 2nd carbon of glycerate. * **Glyceraldehyde-3-phosphate dehydrogenase (GAPDH):** Catalyzes the reversible oxidative phosphorylation of GAP to 1,3-bisphosphoglycerate. This step is notable for producing NADH and being inhibited by **arsenite**. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Three Irreversible Enzymes:** Remember the mnemonic **"H-P-K"**: **H**exokinase (Step 1), **P**hosphofructokinase-1 (Step 3), and **P**yruvate **K**inase (Step 10). * **PFK-1 Regulation:** It is allosterically activated by **AMP** and **Fructose-2,6-bisphosphate**, and inhibited by **ATP** and **Citrate**. * **Arsenite Poisoning:** Arsenite competes with inorganic phosphate in the GAPDH reaction, leading to the bypass of ATP synthesis at the substrate level.

Q: Oxidative decarboxylation of pyruvate requires which of the following?

COASH. **Explanation:** The oxidative decarboxylation of pyruvate to Acetyl-CoA is a critical link between glycolysis and the TCA cycle. This reaction is catalyzed by the **Pyruvate Dehydrogenase (PDH) Complex**, a multi-enzyme system located in the mitochondrial matrix. **Why COASH is correct:** The PDH complex requires five specific cofactors to function: **T**hiamine pyrophosphate (TPP/B1), **L**ipoic acid, **C**oenzyme A (CoA-SH/B5), **F**AD (B2), and **N**AD+ (B3). **CoA-SH** acts as the final acceptor of the acetyl group, forming **Acetyl-CoA**, which then enters the Krebs cycle. Without CoA-SH, the two-carbon unit cannot be activated for further metabolism. **Why other options are incorrect:** * **NADP+:** This is primarily used in reductive biosynthesis (e.g., fatty acid synthesis) and the HMP shunt. The PDH complex specifically uses **NAD+** as an electron acceptor. * **Cytochromes:** These are components of the Electron Transport Chain (ETC) involved in redox reactions for ATP production, not in the decarboxylation of keto-acids. * **Pyridoxal phosphate (PLP):** This is the active form of Vitamin B6, essential for **transamination** and decarboxylation of amino acids, but it plays no role in the PDH complex. **Clinical Pearls for NEET-PG:** * **Mnemonic:** "The Lovely Co-enzymes For Nerds" (TPP, Lipoate, CoA, FAD, NAD). * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Deficiency:** Thiamine (B1) deficiency (Wernicke-Korsakoff) impairs PDH activity, significantly affecting ATP-dependent organs like the brain.

Q: In glycolysis, insulin affects all of the following enzymes except?

Hexokinase. ### Explanation The correct answer is **Hexokinase**. In carbohydrate metabolism, insulin acts as an anabolic hormone that promotes glycolysis to lower blood glucose levels. It achieves this by inducing the synthesis of three key regulatory enzymes. However, **Hexokinase (Type I, II, and III)** is an exception because it is a constitutive enzyme. **1. Why Hexokinase is the correct answer:** Hexokinase is found in most extrahepatic tissues. It is **not induced by insulin**. Instead, it is primarily regulated by **allosteric inhibition** by its product, glucose-6-phosphate. This ensures that cells do not over-consume glucose if they already have sufficient energy, regardless of insulin levels. **2. Why the other options are incorrect:** Insulin regulates the three "irreversible" steps of glycolysis via gene induction (increasing enzyme synthesis): * **Glucokinase (Hexokinase IV):** Unlike other hexokinases, the liver-specific Glucokinase is **induced by insulin**. This allows the liver to trap glucose effectively after a meal. * **Phosphofructokinase-1 (PFK-1):** Insulin increases the synthesis of PFK-1 and also stimulates the production of Fructose-2,6-bisphosphate, its most potent allosteric activator. * **Pyruvate Kinase:** Insulin induces the synthesis of Pyruvate Kinase and keeps it in its active (dephosphorylated) state. **High-Yield Clinical Pearls for NEET-PG:** * **Glucokinase vs. Hexokinase:** Glucokinase has a **high Km** (low affinity) and **high Vmax**, making it ideal for glucose sensing in the liver and pancreas. Hexokinase has a **low Km** (high affinity), allowing tissues to take up glucose even at low blood concentrations. * **Mnemonic:** Insulin **"Induces"** the **"Key"** enzymes: **G**lucokinase, **P**FK, and **P**yruvate kinase (**G**o **P**lay **P**iano). * **MODY Type 2:** Mutations in the *GCK* gene (Glucokinase) lead to Maturity-Onset Diabetes of the Young.

Q: In glycolysis, the conversion of 1 mol of fructose 1,6-bisphosphate to 2 mol of pyruvate results in the formation of which of the following molecules?

2 mol NADH and 4 mol of ATP. **Explanation:** In the glycolytic pathway, the conversion of **Fructose 1,6-bisphosphate (F1,6BP)** to Pyruvate represents the "Pay-off Phase." 1. **Cleavage:** F1,6BP (6 carbons) is cleaved by Aldolase A into two 3-carbon units: Glyceraldehyde 3-phosphate (G3P) and Dihydroxyacetone phosphate (DHAP). DHAP is isomerized to G3P, resulting in **2 moles of G3P**. 2. **Oxidation (NADH generation):** Each G3P is oxidized by *G3P Dehydrogenase*. This step reduces NAD+ to NADH. Since there are 2 moles of G3P, **2 mol of NADH** are produced. 3. **ATP Generation (Substrate-Level Phosphorylation):** * Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate (by Phosphoglycerate kinase) yields 1 ATP per G3P. * Conversion of Phosphoenolpyruvate (PEP) to Pyruvate (by Pyruvate kinase) yields 1 ATP per G3P. * Total yield: 2 ATP × 2 moles of G3P = **4 mol of ATP**. **Why incorrect options are wrong:** * **A & B:** These suggest the formation of NAD+. In glycolysis, NAD+ is *consumed* (reduced), not formed. NAD+ is only regenerated during anaerobic glycolysis (lactate formation). * **C:** This option accounts for only 2 ATP. While the *net* gain of glycolysis starting from Glucose is 2 ATP, the question starts from **Fructose 1,6-bisphosphate**, bypassing the preparatory phase where 2 ATP are normally consumed. **Clinical Pearls for NEET-PG:** * **Net ATP Yield:** From Glucose to Pyruvate is 2 ATP; from F1,6BP to Pyruvate is 4 ATP. * **Arsenate Poisoning:** Arsenate competes with inorganic phosphate in the G3P Dehydrogenase reaction, resulting in 0 net ATP production despite glycolysis continuing. * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to 2,3-BPG, bypassing the first ATP-generating step, which shifts the O2 dissociation curve to the right.

Q: Polysaccharides are classified as which of the following types of organic molecules?

Polymers. **Explanation:** **Why Polymers is the Correct Answer:** Polysaccharides are complex carbohydrates composed of long chains of monosaccharide units (monomers) linked together by **glycosidic bonds**. In biochemistry, a polymer is defined as a large molecule made of repeating structural subunits. Polysaccharides like **glycogen** (the storage form of glucose in humans), **starch**, and **cellulose** fit this definition perfectly. They can be linear or branched and may consist of hundreds to thousands of sugar units. **Why the Other Options are Incorrect:** * **Acids:** While some polysaccharides can be acidic (e.g., Glycosaminoglycans like Hyaluronic acid due to glucuronic acid residues), the category "Polysaccharide" itself defines a structural carbohydrate, not a functional acid. * **Proteins:** These are polymers of **amino acids** linked by peptide bonds. While proteins can link with carbohydrates to form glycoproteins, they are chemically distinct. * **Oils:** These belong to the **Lipid** family, specifically triacylglycerols that are liquid at room temperature. They are composed of glycerol and fatty acids, not sugar units. **NEET-PG High-Yield Clinical Pearls:** * **Glycogen:** A highly branched homopolymer of glucose. It contains $\alpha(1\to4)$ linkages in the linear chain and $\alpha(1\to6)$ linkages at branch points. * **Inulin:** A polymer of fructose (fructosan) used to determine **Glomerular Filtration Rate (GFR)** because it is freely filtered but neither reabsorbed nor secreted by renal tubules. * **Glycosaminoglycans (GAGs):** These are heteropolysaccharides (e.g., Heparin, Chondroitin sulfate) that form the ground substance of the extracellular matrix. * **Cellulose:** A glucose polymer with **$\beta(1\to4)$ linkages**. Humans cannot digest it because we lack the enzyme cellulase, making it a key component of dietary fiber.

Q: What is glyconeogenesis?

Synthesis of glycogen from non-carbohydrate sources. **Explanation:** The term **Glyconeogenesis** is often confused with *Gluconeogenesis*, but they represent distinct metabolic pathways. 1. **Why Option D is Correct:** **Glyconeogenesis** refers specifically to the synthesis of **glycogen** from **non-carbohydrate precursors** (such as lactate, amino acids, or glycerol). This process occurs when these precursors are first converted into glucose-6-phosphate (via the gluconeogenesis pathway) and then immediately channeled into glycogen synthesis (glycogenesis) without the glucose ever entering the systemic circulation. This is a vital process in the liver during recovery from exercise or during fasting. 2. **Analysis of Incorrect Options:** * **Option A (Gluconeogenesis):** This is the synthesis of *glucose* from non-carbohydrate sources. While related, the end product is free glucose, not glycogen. * **Option B (Glycogenesis):** This is the synthesis of glycogen specifically from *glucose* molecules. * **Option C:** This is a specific subset of gluconeogenesis, not the definition of glyconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Site:** Primarily occurs in the **liver**. * **Key Difference:** * *Gluconeogenesis:* Non-carbohydrate $\rightarrow$ Glucose. * *Glycogenesis:* Glucose $\rightarrow$ Glycogen. * *Glyconeogenesis:* Non-carbohydrate $\rightarrow$ Glycogen. * **Cori Cycle Connection:** A classic example of glyconeogenesis is the conversion of muscle-derived **lactate** back into liver glycogen during the recovery phase after strenuous muscular activity. * **Enzymes:** It requires the coordinated action of both gluconeogenic enzymes (like PEPCK and Fructose-1,6-bisphosphatase) and glycogenic enzymes (like Glycogen Synthase).

Q: What is the source of maltose?

Germinating cereals. **Explanation:** **Maltose**, also known as "malt sugar," is a disaccharide composed of two glucose units linked by an **α(1→4) glycosidic bond**. **Why Germinating Cereals is Correct:** The primary source of maltose is the enzymatic breakdown of starch. During the germination of cereals (like barley), the enzyme **α-amylase** is activated. This enzyme hydrolyzes the starch stored in the grain into maltose. This process is fundamental in the brewing industry (malting), where germinated grains provide the fermentable sugars necessary for alcohol production. In the human body, maltose is similarly produced as an intermediate during the digestion of dietary starch by salivary and pancreatic amylase. **Analysis of Incorrect Options:** * **A. Beet sugar:** This is a primary source of **Sucrose** (cane sugar), which is a disaccharide of glucose and fructose. * **B. Milk:** This is the source of **Lactose** (milk sugar), a disaccharide composed of glucose and galactose. * **D. Yeast:** While yeast ferments maltose into ethanol and CO₂, it is not a "source" of the sugar itself. Yeast contains the enzyme maltase to break down maltose for energy. **NEET-PG High-Yield Pearls:** * **Reducing Sugar:** Maltose is a reducing sugar because it retains a free anomeric carbon (unlike sucrose). * **Isomaltose:** A structural isomer of maltose where glucose units are linked by an **α(1→6)** bond; it is a limit dextrin produced during starch digestion. * **Maltotriose:** A trisaccharide consisting of three glucose units, also produced during starch hydrolysis. * **Enzymatic Defect:** Deficiency of the **sucrase-isomaltase complex** leads to osmotic diarrhea and abdominal distention after ingesting starch or sucrose.

Question 1

Which of the following enzymes does not catalyze a reversible step in glycolysis?

Accepted Answer

Phosphofructokinase-1 (PFK)

Answer

Enolase

Answer

Phosphoglyceromutase

Answer

Glyceraldehyde-3-phosphate dehydrogenase

Question 2

Oxidative decarboxylation of pyruvate requires which of the following?

Accepted Answer

COASH

Answer

NADP+

Answer

Cytochromes

Answer

Pyridoxal phosphate

Question 3

Which of the following is a component of dietary fibre?

Accepted Answer

Pectin

Answer

Collagen

Answer

Proteoglycans

Answer

Starch

Question 4

Which of the following statements about glycolysis is true?

Accepted Answer

Glycolysis involves the conversion of glucose to 3-carbon units.

Answer

Glycolysis occurs in the mitochondria.

Answer

Glycolysis is the complete breakdown of glucose.

Answer

3 ATP molecules are used in the anaerobic pathway of glycolysis.

Question 5

In glycolysis, insulin affects all of the following enzymes except?

Accepted Answer

Hexokinase

Answer

Phosphofructokinase

Answer

Pyruvate kinase

Answer

Glucokinase

Question 6

In glycolysis, the conversion of 1 mol of fructose 1,6-bisphosphate to 2 mol of pyruvate results in the formation of which of the following molecules?

Accepted Answer

2 mol NADH and 4 mol of ATP

Answer

1 mol NAD+ and 2 mol of ATP

Answer

2 mol NAD+ and 4 mol of ATP

Answer

2 mol NADH and 2 mol of ATP

Question 7

The uptake of glucose by the liver increases following a carbohydrate meal because:

Accepted Answer

There is an increase in the phosphorylation of glucose by glucokinase.

Answer

GLUT-2 is stimulated by insulin.

Answer

Glucokinase has a low Km for glucose.

Answer

Hexokinase in the liver has a high affinity for glucose.

Question 8

Polysaccharides are classified as which of the following types of organic molecules?

Accepted Answer

Polymers

Answer

Acids

Answer

Proteins

Answer

Oils

Question 9

What is glyconeogenesis?

Accepted Answer

Synthesis of glycogen from non-carbohydrate sources

Answer

Synthesis of glucose from non-carbohydrate sources

Answer

Synthesis of glycogen from glucose

Answer

Synthesis of glucose from glycerol

Question 10

What is the source of maltose?

Accepted Answer

Germinating cereals

Answer

Beet sugar

Answer

Milk

Answer

Yeast

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?