Every Book Back multiple-choice question from Biomolecules (12th Standard Chemistry, Samacheer Kalvi) — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Which one of the following rotates the plane polarized light towards left?
- A. D(+) Glucose
- B. L(+) Glucose
- C. D(-) FructoseCorrect
- D. D(+) Galactose
Explanation. D(-) Fructose is levorotatory and rotates plane-polarized light to the left, as indicated by the negative sign in its name.
Q2
The correct corresponding order of names of four aldoses with configuration given below respectively is,
- A. L-Erythrose, L-Threose, L-Erythrose, D-Threose
- B. D-Threose, D-Erythrose, L-Threose, L-Erythrose
- C. L-Erythrose, L-Threose, D-Erythrose, D-Threose
- D. D-Erythrose, D-Threose, L-Erythrose, L-ThreoseCorrect
Explanation. The configuration of aldose stereoisomers is determined by comparing the orientation of hydroxyl groups at chiral centers with glyceraldehyde references.
Q3
Which one of the following is a non-reducing sugar?
- A. Glucose
- B. SucroseCorrect
- C. Maltose
- D. Lactose
Explanation. Sucrose is a non-reducing sugar because its reducing carbonyl groups from both glucose and fructose are involved in the glycosidic linkage.
Q4
Glucose reacts with hydrogen cyanide to form a product, which on hydrolysis followed by reduction with hydrogen iodide and heat gives compound A. Identify compound A:
- A. Heptanoic acidCorrect
- B. 2-Iodohexane
- C. Heptane
- D. Heptanol
Explanation. Glucose reacts with hydrogen cyanide to yield a cyanohydrin, which on hydrolysis yields a heptanoic acid derivative that reduces to heptanoic acid upon treatment with hydrogen iodide.
Q5
Assertion: A solution of sucrose in water is dextrorotatory. But on hydrolysis in the presence of little hydrochloric acid, it becomes levorotatory.
Reason: Sucrose hydrolysis gives equal amounts of glucose and fructose. As a result, this change in sign of rotation is observed.
- A. If both assertion and reason are true and reason is the correct explanation of assertionCorrect
- B. If both assertion and reason are true but reason is not the correct explanation of assertion
- C. If assertion is true but reason is false
- D. If both assertion and reason are false
Explanation. Sucrose is dextrorotatory, but its hydrolysis yields an equimolar mixture of dextrorotatory glucose and highly levorotatory fructose, making the overall solution levorotatory.
Q6
The central dogma of molecular genetics states that the genetic information flows from:
- A. Amino acids \(\rightarrow\) Protein \(\rightarrow\) DNA
- B. DNA \(\rightarrow\) Carbohydrates \(\rightarrow\) Proteins
- C. DNA \(\rightarrow\) RNA \(\rightarrow\) ProteinsCorrect
- D. DNA \(\rightarrow\) RNA \(\rightarrow\) Carbohydrates
Explanation. The central dogma states that genetic information flows sequentially from DNA through transcription to RNA, and then through translation to proteins.
Q7
In a protein, various amino acids are linked together by:
- A. Peptide bondCorrect
- B. Dative bond
- C. \(\alpha\)-Glycosidic bond
- D. \(\beta\)-Glycosidic bond
Explanation. Amino acids in proteins are joined covalently by peptide bonds formed between the carboxyl group of one amino acid and the amino group of the next.
Q8
Among the following, the achiral amino acid is:
- A. 2-ethylalanine
- B. 2-methylglycine
- C. 2-hydroxymethylserineCorrect
- D. Tryptophan
Explanation. 2-hydroxymethylserine contains two identical hydroxymethyl groups attached to the \(\alpha\)-carbon, which removes the asymmetry and makes the molecule achiral.
Q9
The correct statement regarding RNA and DNA respectively is:
- A. The sugar component in RNA is an arabinose and the sugar component in DNA is ribose
- B. The sugar component in RNA is 2'-deoxyribose and the sugar component in DNA is arabinose
- C. The sugar component in RNA is an arabinose and the sugar component in DNA is 2'-deoxyribose
- D. The sugar component in RNA is ribose and the sugar component in DNA is 2'-deoxyriboseCorrect
Explanation. RNA contains D-ribose sugar, while DNA contains 2'-deoxy-D-ribose sugar in their nucleotides.
Q10
In aqueous solution, amino acids mostly exist in the form of:
- A. NH2-CH(R)-COOH
- B. NH2-CH(R)-COO-
- C. H3N+-CH(R)-COOH
- D. H3N+-CH(R)-COO-Correct
Explanation. In aqueous solution, amino acids exist primarily as dipolar Zwitterions where the carboxyl group loses a proton and the amino group gains one.
Q11
Which one of the following is not produced by the human body?
- A. DNA
- B. Enzymes
- C. Hormones
- D. VitaminsCorrect
Explanation. Vitamins are essential organic compounds that cannot be synthesized by the human body in sufficient quantities and must be obtained through diet.
Q12
The number of sp2 and sp3 hybridised carbons in fructose are respectively:
- A. 1 and 4
- B. 4 and 2
- C. 5 and 1
- D. 1 and 5Correct
Explanation. In fructose, the carbonyl carbon is sp2 hybridized, while the other five carbons are single-bonded and therefore sp3 hybridized.
Q13
Vitamin B2 is also known as:
- A. RiboflavinCorrect
- B. Thiamine
- C. Nicotinamide
- D. Pyridoxine
Explanation. Vitamin B2 is chemically named Riboflavin, which is essential for cellular respiration and energy metabolism.
Q14
The pyrimidine bases present in DNA are
- A. Cytosine and Adenine
- B. Cytosine and Guanine
- C. Cytosine and ThymineCorrect
- D. Cytosine and Uracil
Explanation. DNA contains Cytosine and Thymine as its pyrimidine bases. Uracil is present only in RNA, whereas Adenine and Guanine are purine bases.
Q15
Among the following L-serine is
- A. (figure A)
- B. (figure B)
- C. (figure C)Correct
- D. (figure D)
Explanation. L-serine is determined by comparing its configuration to L-glyceraldehyde, where the amino group on the chiral carbon is situated on the left side.
Q16
The secondary structure of a protein refers to
- A. fixed configuration of the polypeptide backbone
- B. hydrophobic interaction
- C. sequence of \(\\alpha\)-amino acids
- D. \(\\alpha\)-helical backboneCorrect
Explanation. The secondary structure of proteins refers to regular, local folding patterns of the polypeptide backbone, predominantly stabilized by hydrogen bonds, such as the \(\\alpha\)-helix.
Q17
Which of the following vitamins is water soluble?
- A. Vitamin E
- B. Vitamin K
- C. Vitamin A
- D. Vitamin BCorrect
Explanation. Vitamins are classified into fat-soluble (A, D, E, K) and water-soluble groups. Vitamin B and Vitamin C are readily soluble in water and excreted in urine.
Q18
Complete hydrolysis of cellulose gives
- A. L-Glucose
- B. D-Fructose
- C. D-Ribose
- D. D-GlucoseCorrect
Explanation. Cellulose is a linear homopolysaccharide composed solely of D-glucose units linked by \(\\beta\)(1,4)-glycosidic bonds. Thus, its complete hydrolysis yields D-glucose.
Q19
Which of the following statement is not correct?
- A. Ovalbumin is a simple food reserve in egg-white
- B. Blood proteins thrombin and fibrinogen are involved in blood clotting
- C. Denaturation makes protein more activeCorrect
- D. Insulin maintains the sugar level of in the human body.
Explanation. Denaturation disrupts the secondary and tertiary structures of a protein, resulting in the complete loss of its biological function and making it inactive.
Q20
Glucose is an aldose. Which one of the following reactions is not expected with glucose?
- A. It does not form oxime
- B. It does not react with Grignard reagentCorrect
- C. It does not form osazones
- D. It does not reduce tollens reagent
Explanation. Glucose does not react with Grignard reagents to yield typical carbonyl addition products because its cyclic hemiacetal structure and multiple free hydroxyl groups deactivate the reagent.
Q21
If one strand of the DNA has the sequence 'ATGCTTGA', then the sequence of complementary strand would be
- A. TACGAACTCorrect
- B. TCCGAACT
- C. TACGTACT
- D. TACGRAGT
Explanation. In DNA base pairing, Adenine pairs with Thymine (A-T) and Cytosine pairs with Guanine (C-G). Hence, the complementary sequence for ATGCTTGA is TACGAACT.
Q22
Insulin, a hormone chemically is
- A. Fat
- B. Steroid
- C. ProteinCorrect
- D. Carbohydrates
Explanation. Insulin is a polypeptide hormone secreted by the pancreas that consists of 51 amino acid residues, making it chemically classified as a protein.
Q23
\(\\alpha\)-D (+) Glucose and \(\\beta\)-D (+) glucose are
- A. Epimers
- B. AnomersCorrect
- C. Enantiomers
- D. Conformational isomers
Explanation. \(\\alpha\)-D (+) glucose and \(\\beta\)-D (+) glucose are diastereomers that differ only in configuration at the hemiacetal carbon (C-1), which is the anomeric carbon.
Q24
Which of the following are epimers
- A. D(+)-Glucose and D(+)-Galactose
- B. D(+)-Glucose and D(+)-Mannose
- C. Neither (a) nor (b)
- D. Both (a) and (b)Correct
Explanation. D-mannose is the C-2 epimer of D-glucose, and D-galactose is the C-4 epimer of D-glucose. Since both pairs are epimers, option 4 is correct.
Q25
Which of the following amino acids are achiral?
- A. Alanine
- B. Leucine
- C. Proline
- D. GlycineCorrect
Explanation. Glycine is the simplest amino acid. Since its side chain is a hydrogen atom, its \(\\alpha\)-carbon is bonded to two identical hydrogen atoms, making it achiral.
