15 extra multiple-choice questions for Biomolecules (12th Standard Chemistry, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Which carbon atom's configuration in D-glucose is compared with that of D-glyceraldehyde to assign the D notation?
- A. C1
- B. C2
- C. C5Correct
- D. C6
Explanation. D-glucose is assigned the D configuration because the stereocentre at C5 has the same relative configuration as the chiral carbon C2 in D-glyceraldehyde.
Q2
When alpha-D-glucose and beta-D-glucose undergo mutarotation in water, they establish an equilibrium mixture with a constant specific rotation of:
- A. +112 degrees
- B. +19 degrees
- C. +53 degreesCorrect
- D. +100 degrees
Explanation. Mutarotation is the slow interconversion of alpha-D-glucose and beta-D-glucose in water until an equilibrium is established with a constant specific rotation of +53 degrees.
Q3
At the branching points of amylopectin in starch, new chains are linked by which type of glycosidic bond?
- A. alpha(1,4) glycosidic bond
- B. beta(1,4) glycosidic bond
- C. alpha(1,6) glycosidic bondCorrect
- D. beta(1,6) glycosidic bond
Explanation. While the linear chains of amylopectin contain alpha(1,4) linkages, the branch points are linked by alpha(1,6) glycosidic bonds.
Q4
Why are humans unable to use cellulose as a food source?
- A. Cellulose contains alpha(1,4) glycosidic bonds which cannot be cleaved by human amylase.
- B. Humans lack the digestive enzymes like glycosidases or cellulases to hydrolyse beta(1,4) glycosidic bonds.Correct
- C. Cellulose is highly branched and cannot be packed into human tissue layers.
- D. Cellulose undergoes spontaneous mutarotation in the body to form toxic ketoses.
Explanation. Humans lack the cellulase enzymes required to hydrolyse the beta(1,4) glycosidic linkages present in cellulose.
Q5
Which of the following is a non-essential amino acid synthesised by the human body?
- A. Valine
- B. Phenylalanine
- C. AlanineCorrect
- D. Leucine
Explanation. Alanine can be synthesised by the human body and is classified as a non-essential amino acid, whereas valine, phenylalanine, and leucine are essential.
Q6
At a pH value below its isoelectric point, an amino acid exists predominantly as:
- A. A negatively charged anion
- B. A positively charged cationCorrect
- C. A neutral Zwitterion
- D. A non-polar molecule
Explanation. Below its isoelectric point, the amino group accepts a proton to become positively charged, making the amino acid exist as a cation.
Q7
In an alpha-helix sub-structure of proteins, the right-handed helical conformation is stabilised by hydrogen bonding between the carbonyl oxygen of the nth residue and the amino hydrogen of which residue?
- A. n+2th residue
- B. n+3th residue
- C. n+4th residueCorrect
- D. n+5th residue
Explanation. The alpha-helix is stabilised by hydrogen bonding between the C=O group of the nth amino acid residue and the N-H group of the n+4th residue.
Q8
Which amino acid has a rigid cyclic structure that produces a kink in the polypeptide chain and is known as a "helix breaker"?
- A. Glycine
- B. ProlineCorrect
- C. Histidine
- D. Serine
Explanation. Proline has a rigid cyclic structure that disrupts right-handed helical structures, introducing a kink and acting as a helix breaker.
Q9
During the denaturation of a protein, which level of protein structure remains completely intact?
- A. Quaternary structure
- B. Tertiary structure
- C. Secondary structure
- D. Primary structureCorrect
Explanation. Denaturation involves the loss of higher-order structures due to disruption of weak interactions, leaving the covalent primary peptide backbone intact.
Q10
Simple lipids classified as waxes are chemically defined as esters of long-chain fatty acids with:
- A. Glycerol
- B. Long-chain monohydric alcoholsCorrect
- C. Amino alcohols
- D. Phosphoric acid
Explanation. Waxes are esters of long-chain fatty acids with long-chain monohydric alcohols, whereas fats are esters with glycerol.
Q11
Which class of vitamins cannot be stored in the body and must be regularly supplied through diet because any excess is excreted in urine?
- A. Vitamins A and D
- B. Vitamins E and K
- C. Vitamins B and CCorrect
- D. Vitamins A and K
Explanation. Water-soluble vitamins like Vitamin B and C cannot be stored in the body, as any excess is readily excreted through urine.
Q12
According to the text, which vitamin acts as a coenzyme in amino acid metabolism and is essential for the formation of heme in hemoglobin?
- A. Vitamin B2 (Riboflavin)
- B. Vitamin B5 (Pantothenic acid)
- C. Vitamin B6 (Pyridoxine)Correct
- D. Vitamin B7 (Biotin)
Explanation. Vitamin B6 serves as a coenzyme in amino acid metabolism and is required for the biosynthesis of heme in hemoglobin.
Q13
Which pyrimidine base is uniquely present in RNA molecules and replaces the thymine found in DNA?
- A. Adenine
- B. Cytosine
- C. UracilCorrect
- D. Guanine
Explanation. Both DNA and RNA contain cytosine, but the second pyrimidine base is thymine in DNA and uracil in RNA.
Q14
The biological process by which a single-stranded messenger RNA (mRNA) molecule is synthesised from a DNA template strand is known as:
- A. Translation
- B. TranscriptionCorrect
- C. Replication
- D. Denaturation
Explanation. The synthesis of a complementary messenger RNA strand from a template DNA strand is called transcription.
Q15
Chemically, hormones are organic substances secreted by endocrine glands and are classified into which two major groups?
- A. Carbohydrates and lipids
- B. Proteins and steroidsCorrect
- C. Amino acids and nucleotides
- D. Nucleic acids and vitamins
Explanation. Chemically, hormones may be classified as either protein-based, such as insulin and epinephrine, or steroid-based, such as estrogen and androgen.
Frequently asked questions
How many MCQs are there in Biomolecules?
This chapter has 15 book-back multiple-choice questions, each with the correct answer and a step-by-step explanation.
Are these 12th Standard Chemistry MCQs free to practise online?
Yes. Every question, answer and explanation here is free, and you can also take them as a timed practice test.
Where can I find the Biomolecules book-back answers?
The correct option for each question is highlighted on this page with a worked explanation, plus a quick answer-key summary at the top.
