Nitrogen is NOT an essential constituents of
- (a)Chlorophyll
- (b)RNA
- (c)DNA
- (d)Carbohydrate
Correct — D, Carbohydrate. Carbohydrates (sugars, starch, cellulose) have the general composition Cx(H2O)y — they are built only of carbon, hydrogen and oxygen and contain no nitrogen. The other three all contain nitrogen: chlorophyll has a magnesium–porphyrin ring in which four nitrogen atoms coordinate the central Mg; RNA and DNA carry nitrogen in their nitrogenous bases (adenine, guanine, cytosine, thymine/uracil). So the one biomolecule that does NOT need nitrogen is the carbohydrate.
- (a)Chlorophyll — Wrong — chlorophyll contains nitrogen: its porphyrin (tetrapyrrole) head has four nitrogen atoms holding the central magnesium ion, so nitrogen is essential to it.
- (b)RNA — Wrong — RNA's nitrogenous bases (adenine, guanine, cytosine, uracil) are rich in nitrogen; nitrogen is a defining constituent of every nucleic acid.
- (c)DNA — Wrong — DNA likewise contains nitrogen in its bases (adenine, guanine, cytosine, thymine); it cannot exist without nitrogen.
Nitrogen is a key element in many biomolecules: proteins (amino/–NH2 groups), nucleic acids (DNA & RNA, via nitrogenous bases), and pigments such as chlorophyll (a nitrogen-containing porphyrin ring around magnesium). Carbohydrates are the exception — as Cx(H2O)y they are composed only of carbon, hydrogen and oxygen, with no nitrogen in the basic sugar/starch/cellulose structure.
Read the question as 'which of these does NOT contain nitrogen'. Chlorophyll, DNA and RNA are all classic nitrogen-bearing molecules, so they can be struck out. What remains, carbohydrate, is the only class defined by C, H and O alone — making it the correct 'odd one out'.
- Carbohydrates have the formula Cx(H2O)y — only carbon, hydrogen and oxygen, no nitrogen.
- Chlorophyll contains four nitrogen atoms in its Mg–porphyrin ring.
- DNA and RNA carry nitrogen in their nitrogenous bases (purines and pyrimidines).
- Proteins also contain nitrogen (in the amino group of amino acids).
Only carbohydrates lack nitrogen → option (d).
- Assuming every organic biomolecule contains nitrogen (carbohydrates and simple lipids do not)
- Missing the 'NOT' and picking a nitrogen-containing molecule like chlorophyll
UPPSC & UPSC test elemental composition of biomolecules — remember carbohydrates = C, H, O only; proteins/nucleic acids/chlorophyll all carry nitrogen.
Which of the following elements are present in all proteins? I. Carbon II. Hydrogen III. Oxygen IV. Nitrogen Select the correct answer from the codes given below:
- (a) I and II
- (b) I, II and IV
- (c) I, III and IV
- (d) I, II, III and IV
Answer(d) I, II, III and IV
Same concept — elemental composition of biomolecules. It confirms nitrogen is a constituent of proteins (alongside DNA/RNA/chlorophyll here), the flip side of noting that carbohydrates carry no nitrogen.
- practice — not a real PYQ
Which of the following biomolecules does NOT contain phosphorus?
- (a)DNA
- (b)RNA
- (c)ATP
- (d)Glucose
Answer(d) Glucose — a simple carbohydrate of C, H and O; DNA, RNA and ATP all contain phosphate groups.
- practice — not a real PYQ
The central metal atom held by the four nitrogen atoms in the chlorophyll molecule is
- (a)Iron
- (b)Magnesium
- (c)Manganese
- (d)Zinc
Answer(b) Magnesium — chlorophyll's porphyrin ring coordinates a central Mg ion via four nitrogen atoms.