Which one of the following statements is wrong?
Cellulose is a polysaccharide
Uracil is a pyrimidine
Serine is a sulphur containing amino acid
Sucrose is a disaccharide
The wrong statement is: “Serine is a sulphur containing amino acid.” Serine does not contain sulphur; sulphur-containing amino acids are mainly cysteine and methionine.
Introduction
In competitive exams like NEET and CSIR NET Life Sciences, MCQs on biomolecules often test subtle conceptual differences between similar-looking statements. In the question that compares cellulose, uracil, serine and sucrose, only one statement is incorrect, and identifying it requires clear understanding of carbohydrates, nitrogenous bases and amino acid chemistry.
Option 1: “Cellulose is a polysaccharide”
This statement is correct. Cellulose is a polysaccharide composed of long, unbranched chains of β-D-glucose units linked by β-1,4-glycosidic bonds. It is the main structural component of plant cell walls and is one of the most abundant organic molecules in nature.
In cellulose, each glucose unit is repeated many times (often thousands), which is why it is classified as a complex carbohydrate or polysaccharide rather than a monosaccharide or disaccharide. Humans lack the enzyme cellulase, so cellulose acts as dietary fibre rather than a digestible energy source.
Option 2: “Uracil is a pyrimidine”
This statement is correct. Uracil is one of the pyrimidine nitrogenous bases found in RNA. In RNA, uracil pairs with adenine, taking the place that thymine occupies in DNA.
Pyrimidines are single-ring bases that include cytosine, thymine and uracil, whereas purines (adenine and guanine) have a double-ring structure. The presence of uracil instead of thymine is one of the key chemical differences between RNA and DNA.
Option 3: “Serine is a sulphur containing amino acid”
This statement is wrong and is the correct answer to the MCQ. Serine is a polar, uncharged amino acid whose side chain contains a hydroxymethyl group −CH2OH, not sulphur. Its general structure is HO-CH₂-CH(NH₂)-COOH, which includes oxygen in the side chain but no sulphur atom.
The typical sulphur-containing amino acids in proteins are cysteine (with a thiol −SH group) and methionine (with a thioether in its side chain). Confusing serine with these sulphur-containing amino acids is a common exam trap, so remembering that serine has an –OH group, not –SH, is crucial.
Option 4: “Sucrose is a disaccharide”
This statement is correct. Sucrose (table sugar) is a disaccharide composed of one molecule of glucose and one molecule of fructose linked by a glycosidic bond. Its molecular formula is C12H22O11, typical of many disaccharides.
Because both anomeric carbons of glucose and fructose are involved in the glycosidic linkage, sucrose behaves as a non-reducing sugar. On hydrolysis, sucrose yields equimolar amounts of D-glucose and D-fructose, a reaction important in digestion and in various industrial processes.
