Carbohydrate Chemistry and Classification — MCQs
What is the optimal time for alginate impression material gel formation at 20°C?
History of dislike for sweet food items is typically present in:
Which carbon of glucose is oxidized to form glucuronic acid?
Which of the following GAG is not sulfated?
Which of the following statements about isozymes is true?
Iodine gives red color with:
Which of the following is false about starvation ketoacidosis?
What type of carbohydrate is inulin classified as?
How many stereoisomers are possible for an aldohexose?
What is the respiratory quotient of carbohydrates?
Carbohydrate Chemistry and Classification Indian Medical PG Practice Questions and MCQs
Question 1: What is the optimal time for alginate impression material gel formation at 20°C?
- A. 6-8 mins.
- B. 15-16 mins.
- C. 3-4 mins.
- D. 1-2 min. (Correct Answer)
Explanation: ***1-2 min.*** - The **optimal gelation time** for common alginate impression materials at 20°C (room temperature) is typically between 1 to 2 minutes, allowing sufficient working time before setting. - This rapid setting is crucial for patient comfort and minimizing distortion from patient movement during the impression-taking process. *3-4 mins.* - A gelation time of 3-4 minutes is generally considered on the **longer side** for conventional alginate and might be indicative of factors like cooler water temperature or the use of **slow-set alginate**. - While still acceptable in some clinical situations, it increases the risk of patient discomfort and potential impression inaccuracies if the patient moves. *6-8 mins.* - A setting time of 6-8 minutes is **too long** for most clinical applications of alginate, leading to **patient fatigue** and a high probability of impression distortion due to movement. - This duration is typically seen when inhibitors are added or if the water used is significantly colder than recommended. *15-16 mins.* - A gelation time of 15-16 minutes is **impractical and unacceptable** for alginate impressions in a clinical setting. - Such an extended setting time would be highly uncomfortable for the patient and would almost certainly result in a **compromised, unusable impression**.
Question 2: History of dislike for sweet food items is typically present in:
- A. Glycogen storage disease
- B. Diabetes mellitus
- C. Galactosemia
- D. Hereditary fructose intolerance (Correct Answer)
Explanation: ***Hereditary fructose intolerance*** - Patients with hereditary fructose intolerance develop severe symptoms like **nausea, vomiting, abdominal pain, and hypoglycemia** after ingesting fructose, leading to an aversive response and **dislike for sweet food items**. - This aversion is a protective mechanism, as avoiding fructose-containing foods (including many sweets) prevents the accumulation of toxic metabolites due to a deficiency in **hepatic aldolase B**. *Glycogen storage disease* - While glycogen storage diseases can cause hypoglycemia, they typically do not lead to a specific **aversion to sweet foods**. - The primary defect is in **glycogen synthesis or breakdown**, leading to symptoms like hepatomegaly, muscle weakness, and exercise intolerance. *Diabetes mellitus* - Patients with diabetes mellitus often have a **craving for sweet foods** due to uncontrolled blood glucose levels and insulin resistance, rather than a dislike. - The condition is characterized by **hyperglycemia** and may involve polydipsia, polyuria, and polyphagia. *Galactosemia* - Galactosemia involves an inability to metabolize galactose, leading to symptoms such as **vomiting, lethargy, and jaundice** upon milk ingestion [1]. - While patients will avoid milk, their aversion is not generally to all sweet foods, as sweet foods do not always contain galactose [1].
Question 3: Which carbon of glucose is oxidized to form glucuronic acid?
- A. Oxidation of the aldehyde group only
- B. No oxidation occurs
- C. Oxidation of the terminal alcohol group only (Correct Answer)
- D. Oxidation of both the aldehyde and terminal alcohol groups
Explanation: ***Oxidation of the terminal alcohol group only*** - Glucuronic acid is formed by the **oxidation of the C6 carbon (terminal alcohol group)** of glucose, while the aldehyde group (C1) remains intact. - This specific oxidation converts glucose into a **uronic acid**, essential for detoxification and connective tissue synthesis. *Oxidation of the aldehyde group only* - The oxidation of the **aldehyde group (C1)** of glucose would yield **gluconic acid**, not glucuronic acid. - This reaction typically occurs during the conversion of glucose to gluconolactone, a step in the pentose phosphate pathway for example. *No oxidation occurs* - The formation of glucuronic acid is explicitly an **oxidative process**, as a hydroxyl group is converted to a carboxyl group. - If no oxidation occurred, glucose would remain glucose, or undergo other non-oxidative transformations. *Oxidation of both the aldehyde and terminal alcohol groups* - Oxidation of **both the aldehyde (C1) and terminal alcohol (C6)** groups of glucose would lead to the formation of **glucaric acid (saccharic acid)**. - Glucaric acid has carboxyl groups at both ends, making it different from glucuronic acid, which only has a carboxyl group at C6.
Question 4: Which of the following GAG is not sulfated?
- A. Keratan sulfate
- B. Dermatan sulfate
- C. Chondroitin sulfate
- D. Hyaluronic acid (Correct Answer)
Explanation: ***Hyaluronic acid*** - **Hyaluronic acid** is unique among glycosaminoglycans (GAGs) because it is the only one that is **not sulfated**. - It also distinguishes itself by being the only GAG that does **not form proteoglycans** and is not synthesized in the Golgi apparatus. *Chondroitin sulfate* - **Chondroitin sulfate** is a sulfated glycosaminoglycan that is a major component of the **extracellular matrix**, particularly in cartilage. - Its sulfate groups contribute to its **negative charge**, allowing it to attract water and provide resistance to compression. *Dermatan sulfate* - **Dermatan sulfate** is another sulfated GAG, found predominantly in the skin, blood vessels, and heart valves. - It contains **sulfate groups**, which are crucial for its interactions with various proteins and its role in tissue structure. *Keratan sulfate* - **Keratan sulfate** is a sulfated GAG found in the cornea, cartilage, and bone. - It is distinct from other GAGs due to its **lack of uronic acid** and the presence of sulfate groups.
Question 5: Which of the following statements about isozymes is true?
- A. They catalyze the same reaction but may differ in structure. (Correct Answer)
- B. They have the same quaternary structure.
- C. They have the same enzyme classification but differ in number and name.
- D. They are distributed uniformly across different organs.
Explanation: ***They catalyze the same reaction but may differ in structure.*** - Isozymes are **different forms of an enzyme** that catalyze the **same biochemical reaction** but have distinct amino acid sequences. - Due to their different amino acid sequences, isozymes can exhibit variations in their **molecular structure**, kinetic properties, and regulatory mechanisms. *They have the same quaternary structure.* - While some isozymes might have a similar quaternary structure (e.g., both being tetramers), it is not a defining characteristic; they often have **different subunit compositions** or arrangements. - Their structural differences, including quaternary structure, contribute to their distinct properties and often reflect their expression in **different tissues or developmental stages**. *They have the same enzyme classification but differ in number and name.* - Isozymes belong to the **same enzyme classification** (e.g., EC number) because they catalyze the identical reaction, but they are **not necessarily numbered differently** as distinct enzymes. - Their differing names typically reflect the tissue they are found in or their specific subunits (e.g., lactate dehydrogenase isozymes **LDH-1 to LDH-5**). *They are distributed uniformly across different organs.* - Isozymes typically exhibit a **tissue-specific distribution**, meaning their presence and relative abundance vary significantly between different organs and tissues. - This differential distribution allows for **fine-tuning metabolic pathways** to meet the specific physiological demands of each tissue.
Question 6: Iodine gives red color with:
- A. Glycogen
- B. Starch
- C. Dextrin (Correct Answer)
- D. Inulin
Explanation: ***Dextrin*** - **Dextrin** gives a characteristic **bright red to red-violet color** with iodine solution, which is the most distinctive "red" color among polysaccharides. - Dextrin is an intermediate product formed during **starch hydrolysis**, with a molecular structure between starch and simple sugars. - This color reaction is used as a **qualitative test** to identify dextrin and monitor starch breakdown during digestion. *Glycogen* - **Glycogen** produces a **red-brown to mahogany brown color** with iodine, which is more brown than red. - This brownish-red color is due to its **highly branched structure** with shorter chain lengths compared to starch. - While it has a reddish tinge, the predominant color is **brown**, making it distinct from the bright red of dextrin. *Starch* - **Starch** (particularly amylose) gives a distinctive **blue-black color** with iodine solution. - This occurs due to formation of a **starch-iodine complex** where iodine molecules fit into the helical structure of amylose. - Amylopectin (branched component) produces a **red-purple color**, but whole starch appears blue-black due to amylose dominance. *Inulin* - **Inulin**, a fructose polymer, shows **no color reaction** with iodine solution. - This absence of reaction is because inulin lacks the helical structure needed for iodine complex formation.
Question 7: Which of the following is false about starvation ketoacidosis?
- A. Smell of acetone in breath
- B. Metabolic acidosis
- C. Benedict's test +ve (Correct Answer)
- D. Rothera's test +ve
Explanation: ***Benedict's test +ve (FALSE)*** - **Benedict's test** detects the presence of **reducing sugars** (glucose) in the urine. - In starvation ketoacidosis, there is **no significant glucose in the urine** because blood glucose levels are low to normal. - The body is in a state of **prolonged fasting** with depleted glycogen stores, utilizing **fats for energy** instead of carbohydrates. - Unlike diabetic ketoacidosis where glucosuria occurs due to hyperglycemia, starvation ketoacidosis typically presents with **hypoglycemia or normoglycemia**. - Therefore, Benedict's test would be **negative**, making this statement FALSE. *Smell of acetone in breath (TRUE)* - During starvation, the body breaks down fats into **ketone bodies** (beta-hydroxybutyrate, acetoacetate, and acetone). - **Acetone** is volatile and exhaled through the lungs, producing a characteristic **fruity or sweet smell** on the breath. - This is a classic clinical feature of ketoacidosis. *Metabolic acidosis (TRUE)* - The accumulation of **beta-hydroxybutyrate** and **acetoacetate** (both acidic ketone bodies) in the blood leads to decreased pH. - This results in **high anion gap metabolic acidosis** as the excess acids consume the body's **bicarbonate buffer system**. - The anion gap increases due to unmeasured anions (ketone bodies). *Rothera's test +ve (TRUE)* - **Rothera's test** specifically detects **ketone bodies**, particularly **acetoacetate**, in urine. - In starvation ketoacidosis, there is significant production and excretion of ketone bodies. - This causes a **positive Rothera's test**, confirming ketonuria.
Question 8: What type of carbohydrate is inulin classified as?
- A. Glucosan
- B. Fructosan (Correct Answer)
- C. Galactosan
- D. Mannosan
Explanation: ***Fructosan*** - **Inulin** is a naturally occurring **polysaccharide** composed primarily of **fructose** units. - As such, it is classified as a **fructosan**, a type of **fructan**, meaning its main monosaccharide component is fructose. *Glucosan* - A **glucosan** is a polysaccharide primarily made up of **glucose** units, such as **starch** or **glycogen**. - Inulin's monomeric units are predominantly fructose, not glucose. *Galactosan* - A **galactosan** is a polysaccharide primarily composed of **galactose** units. - Inulin does not primarily consist of galactose units. *Mannosan* - A **mannosan** is a polysaccharide primarily composed of **mannose** units. - Inulin's structure is based on fructose, not mannose.
Question 9: How many stereoisomers are possible for an aldohexose?
- A. 32
- B. 64
- C. 16 (Correct Answer)
- D. 8
Explanation: ***16*** - An aldohexose (like glucose) has **four chiral centers** (C2, C3, C4, and C5 in the open-chain form). - The number of possible stereoisomers for a molecule with 'n' chiral centers is given by the formula **2^n**. Therefore, 2^4 = **16 stereoisomers**. - These 16 stereoisomers include D-glucose, D-mannose, D-galactose, D-allose, and their corresponding L-forms. *32* - This number would be true if an aldohexose had **five chiral centers** (2^5 = 32), which it does not. - Aldohexoses are six-carbon sugars, but C1 (aldehyde carbon) and C6 (primary alcohol) are not chiral centers. *64* - This number would imply **six chiral centers** (2^6 = 64), which is incorrect for aldohexoses. - This would require all six carbons to be chiral centers, which is structurally impossible in an aldohexose. *8* - This number suggests **three chiral centers** (2^3 = 8), which is an underestimation. - Aldohexoses have **four chiral centers**, not three, resulting in 16 possible stereoisomers.
Question 10: What is the respiratory quotient of carbohydrates?
- A. 0.5
- B. 0.8
- C. 0.75
- D. 1 (Correct Answer)
Explanation: ***Option: 1 (Correct Answer)*** - The **respiratory quotient (RQ)** is the ratio of **carbon dioxide produced to oxygen consumed** during metabolism. - For carbohydrates, complete oxidation yields equal moles of CO2 and O2, resulting in an **RQ of 1.0**. - Example: C6H12O6 + 6O2 → 6CO2 + 6H2O, giving RQ = 6CO2/6O2 = 1.0 - This value reflects that carbohydrates are highly oxygenated molecules, requiring less external oxygen for their oxidation relative to the CO2 produced. *Option: 0.5* - An RQ of 0.5 is not observed for any major macronutrient during complete oxidation. - This value would imply significantly lower CO2 production relative to O2 consumption, which doesn't match any physiological substrate metabolism. *Option: 0.8* - An RQ of approximately 0.8 is characteristic of a **mixed diet** or the average value sometimes cited for **protein metabolism**. - Protein RQ typically ranges from 0.8-0.85, as proteins require more oxygen for their oxidation compared to the CO2 produced. - The exact RQ can vary depending on the specific amino acids being metabolized. *Option: 0.75* - An RQ around 0.7-0.75 may represent **fat-predominant metabolism** or a mixed diet with fats and carbohydrates. - Pure **fat metabolism** has an RQ of approximately **0.7**, as fats require substantial oxygen for oxidation due to their lower oxygen content relative to carbon and hydrogen. - Fats contain many C-H bonds and few C-O bonds, necessitating more oxygen for complete combustion.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free