Biology · Biomolecules · NEET
The alpha-carbon has four valency positions. NCERT names them exactly: (1) a hydrogen atom (H), (2) a carboxyl group (-COOH, the acidic group), (3) an amino group (-NH2, the basic group), and (4) a variable group called the R group. All four are bonded to the same single carbon. NEET loves this list, so memorise all four.
Because the amino group (-NH2) and the acidic carboxyl group (-COOH) are both attached to the SAME carbon. That carbon is named the alpha-carbon. NCERT wording: amino acids are organic compounds containing an amino group and an acidic group as substituents on the same carbon, i.e., the alpha-carbon. That is why they are alpha-amino acids.
The R group is the one part that changes from one amino acid to another. NCERT says: based on the nature of the R group there are many amino acids, but only 20 types occur in proteins. If R is a hydrogen the amino acid is glycine, if R is a methyl group it is alanine, if R is hydroxy methyl it is serine. So the R group decides the identity of each amino acid.
Methane (CH4) is one carbon holding four hydrogens. In an amino acid, the central alpha-carbon also holds four groups, but three of the hydrogens are replaced (substituted) by -COOH, -NH2 and the R group, leaving one H. So the amino acid is a methane whose hydrogens are substituted. This is a direct NCERT line and appeared in NEET 2026.
No. Glycine is the simplest amino acid because its R group is just a hydrogen atom. It contains no sulphur. NEET 2016 asked exactly this as a wrong statement. The sulphur-containing amino acids are cysteine and methionine, not glycine.
NCERT states the chemical and physical properties of amino acids are essentially of the amino (-NH2), carboxyl (-COOH) and the R functional groups. Because -NH2 and -COOH can gain or lose a charge (ionizable), the structure of an amino acid changes in solutions of different pH, giving the zwitterion form.
Which one of the following statements is wrong?
Which of the following statements are correct regarding amino acids? A. They are substituted methanes. B. Serine is an aromatic amino acid. C. Valine is a neutral amino acid. D. Lysine is an acidic amino acid.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Four. The alpha-carbon has four valency positions filled by a hydrogen (H), a carboxyl group (-COOH), an amino group (-NH2), and a variable R group.
The R group. NCERT says based on the nature of the R group there are many amino acids, but only 20 occur in proteins. R = hydrogen gives glycine, R = methyl gives alanine, R = hydroxy methyl gives serine.
Methane is one carbon with four hydrogens. In an amino acid, three of those hydrogens are replaced by -COOH, -NH2 and R, so it is a substituted methane. This exact phrase was tested in NEET 2026.
Because both the amino group and the acidic carboxyl group are attached to the same carbon, and that carbon is the alpha-carbon. Hence the name alpha-amino acid.
The amino (-NH2), carboxyl (-COOH) and R groups. Because -NH2 and -COOH are ionizable, the structure changes at different pH, forming the zwitterion.
