Questions & explanations
1. Which of the following statements correctly describes hormones as biomolecules?
- Hormones are inorganic molecules that regulate metabolism.
- Hormones are biomolecules that act as chemical messengers to coordinate physiological processes.
- Hormones are exclusively proteins that are synthesized in endocrine glands.
- Hormones are enzymes that catalyze biochemical reactions.
Answer: Hormones are biomolecules that act as chemical messengers to coordinate physiological processes.
Hormones are organic biomolecules (such as peptides, steroids, or amino acid derivatives) that function as chemical messengers. They are secreted by endocrine glands and travel through the bloodstream to target organs, where they regulate various physiological functions. Option A is incorrect because hormones are organic, not inorganic. Option C is wrong because hormones include steroids and amines, not only proteins. Option D is false because hormones are signaling molecules, not catalysts like enzymes.
2. The key structural difference between starch and cellulose is that:
- Starch contains α-1,4-glycosidic linkages while cellulose contains β-1,4-glycosidic linkages
- Starch is a branched polymer but cellulose is not
- Cellulose is a homopolysaccharide of glucose but starch is not
- Starch has β-1,4 linkages and cellulose has α-1,4 linkages
Answer: Starch contains α-1,4-glycosidic linkages while cellulose contains β-1,4-glycosidic linkages
Starch (amylose and amylopectin) has α-1,4 linkages, whereas cellulose has β-1,4 linkages. This difference in anomeric configuration makes cellulose fibrous and indigestible by humans. Both are homopolysaccharides of glucose. Starch can be branched (amylopectin) but the primary difference is the type of glycosidic bond.
3. The change in specific rotation of α-D-glucose from +112° to an equilibrium value of +52.5° when dissolved in water is called:
- Optical activity
- Inversion
- Mutarotation
- Racemization
Answer: Mutarotation
Mutarotation is the interconversion between α and β anomers of a sugar, leading to a change in optical rotation until equilibrium is reached. Optical activity is a general property; inversion refers to a change in sign of rotation (e.g., sucrose hydrolysis); racemization involves loss of chirality at a stereocenter.
4. Which of the following is a non-reducing sugar?
- Glucose
- Maltose
- Sucrose
- Lactose
Answer: Sucrose
Sucrose is a non-reducing sugar because its anomeric carbons (C1 of glucose and C2 of fructose) are both involved in the glycosidic bond, leaving no free aldehyde or ketone group to reduce reagents like Fehling's. Glucose, maltose, and lactose all have a free anomeric carbon and are reducing sugars.
5. Deficiency of which vitamin causes scurvy?
- Vitamin A
- Vitamin D
- Vitamin C
- Vitamin K
Answer: Vitamin C
Scurvy is caused by a deficiency of vitamin C (ascorbic acid), leading to symptoms such as weak connective tissues, bleeding gums, and poor wound healing. Vitamin A deficiency causes night blindness, vitamin D deficiency causes rickets, and vitamin K deficiency affects blood clotting.
6. The α-helix structure in proteins is primarily stabilized by:
- Hydrogen bonds
- Disulfide bonds
- Ionic bonds
- Hydrophobic interactions
Answer: Hydrogen bonds
The α-helix is maintained by regular hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, typically four residues apart. Disulfide bonds stabilize tertiary structure, while ionic and hydrophobic interactions contribute to tertiary folding.
7. Glucose is classified as which of the following?
- Disaccharide
- Ketohexose
- Aldohexose
- Polysaccharide
Answer: Aldohexose
Glucose is a monosaccharide with an aldehyde group and six carbon atoms, making it an aldohexose. Disaccharides (e.g., sucrose) consist of two monosaccharides; ketohexoses (e.g., fructose) have a ketone group; polysaccharides (e.g., starch) are polymers of monosaccharides.
8. Enzymes are highly specific. This means that an enzyme:
- Can catalyze only one particular reaction or a specific class of reactions
- Requires a specific pH and temperature
- Is always a protein
- Increases the activation energy of a reaction
Answer: Can catalyze only one particular reaction or a specific class of reactions
Enzyme specificity refers to the ability of an enzyme to bind to a specific substrate and catalyze a particular reaction. While many enzymes require optimal pH/temperature and most are proteins, the defining feature of specificity is the substrate–reaction selectivity.
9. Which of the following tests is used to detect the reducing property of glucose?
- Iodoform test
- Biuret test
- Fehling's test
- Molisch's test
Answer: Fehling's test
Fehling's test (using copper sulfate and alkaline tartrate) detects reducing sugars like glucose by forming a red precipitate of cuprous oxide. The iodoform test is for methyl ketones; the biuret test is for proteins; Molisch's test is a general test for carbohydrates.
10. Which of the following correctly describes fructose?
- It is an aldohexose
- It exists only as a furanose ring
- It is a ketohexose also known as fruit sugar
- It does not undergo mutarotation
Answer: It is a ketohexose also known as fruit sugar
Fructose is a monosaccharide with a ketone group (ketohexose) and is commonly called fruit sugar. It is not an aldohexose (glucose is). It can form both pyranose and furanose rings, not only furanose. Fructose does undergo mutarotation due to anomeric interconversion.
11. Which one of the following is a nucleoside?
- ATP
- Adenosine
- AMP
- dAMP
Answer: Adenosine
A nucleoside consists of a nitrogenous base attached to a sugar (ribose or deoxyribose) without a phosphate group. Adenosine (adenine + ribose) is a nucleoside. ATP, AMP, and dAMP are nucleotides because they contain one or more phosphate groups.
12. Which of the following disaccharides contains a β-1,4-glycosidic linkage?
- Sucrose
- Maltose
- Lactose
- All of the above
Answer: Lactose
Lactose (milk sugar) consists of β-D-galactose and β-D-glucose linked by a β-1,4-glycosidic bond. Sucrose has an α-1,2-β linkage, and maltose has an α-1,4 linkage. 'All of the above' is incorrect because only lactose has a β-1,4 linkage.