General Structure of an Amino Acid
An amino acid is a molecule that carries two functional groups: an amino group ($\ce{-NH2}$) and a carboxyl group ($\ce{-COOH}$). The combination of an acidic carboxyl and a basic amino group within one small molecule is what gives amino acids their entire characteristic chemistry. NCERT defines them precisely as compounds containing amino and carboxyl functional groups, and notes that depending on the relative position of the amino group with respect to the carboxyl group, the acids can be classified as $\alpha$, $\beta$, $\gamma$, $\delta$ and so on.
The biologically and examination-relevant form is the $\alpha$-amino acid. Here the $\ce{-NH2}$ group is bonded to the carbon atom immediately adjacent to the carboxyl carbon — the carbon called the $\alpha$-carbon. That same $\alpha$-carbon also carries a hydrogen atom and a fourth group, the side chain R, which is what distinguishes one amino acid from another. The general skeleton can be written compactly as $\ce{R-CH(NH2)-COOH}$.
Schematic of the general $\alpha$-amino acid: –NH2 and –COOH on the same (alpha) carbon, with side chain R.
The side chain R is the variable that drives all classification. In the simplest amino acid, glycine, R is just a hydrogen atom. In alanine R is $\ce{-CH3}$; in serine it is $\ce{-CH2OH}$; in cysteine it is $\ce{-CH2SH}$. NCERT lists twenty such naturally occurring acids in Table 10.2, and their trivial names usually reflect a property or source — glycine for its sweet taste (Greek glykos, sweet) and tyrosine for cheese (Greek tyros).
Why Only Alpha-Amino Acids
Although the amino group can in principle sit at the $\beta$, $\gamma$ or $\delta$ position, NCERT is explicit that only $\alpha$-amino acids are obtained on hydrolysis of proteins. This is the structural anchor of the whole protein chapter: when a protein is broken down into its building blocks, every fragment is an $\alpha$-amino acid in which the $\ce{-NH2}$ is on the carbon next to $\ce{-COOH}$. NIOS records the same fact, stating that all amino acids found in proteins carry an amino group on the carbon atom adjacent to the carbonyl group, and that all twenty have the L-configuration.
Classification: Acidic, Basic, Neutral
The first classification scheme rests entirely on counting groups. NCERT classifies amino acids as acidic, basic or neutral depending on the relative number of amino and carboxyl groups in the molecule. The rule is mechanical and easy marks in an examination.
| Class | Group balance | NCERT examples |
|---|---|---|
| Neutral | Equal number of $\ce{-NH2}$ and $\ce{-COOH}$ | Glycine, Alanine, Valine, Serine |
| Acidic | More $\ce{-COOH}$ than $\ce{-NH2}$ | Glutamic acid, Aspartic acid |
| Basic | More $\ce{-NH2}$ than $\ce{-COOH}$ | Lysine, Arginine, Histidine |
Aspartic acid, $\ce{HOOC-CH2-CH(NH2)-COOH}$, carries two carboxyl groups against one amino group, so it is acidic. Lysine, $\ce{H2N-(CH2)4-CH(NH2)-COOH}$, carries two amino groups against one carboxyl, so it is basic — the exact reasoning NEET 2020 demanded.
Count the side-chain groups, not just the backbone
Every $\alpha$-amino acid backbone already has one $\ce{-NH2}$ and one $\ce{-COOH}$. Whether the acid is acidic, basic or neutral is decided by what the side chain R adds. A spare $\ce{-COOH}$ in R (glutamic, aspartic) tips it acidic; a spare $\ce{-NH2}$ in R (lysine, arginine) tips it basic.
Tyrosine and serine have polar side chains but no extra ionisable acid/base group counted here, so they are treated as neutral.
Essential vs Non-Essential
The second classification is biological rather than structural. NCERT divides amino acids by whether the body can make them. The acids that can be synthesised in the body are non-essential; those that cannot be synthesised and must be obtained through diet are essential. The chapter summary fixes the count: of the roughly twenty amino acids, ten are essential.
| Type | Source | NCERT examples (Table 10.2) |
|---|---|---|
| Essential | Must come from diet | Valine, Leucine, Isoleucine, Lysine, Arginine, Threonine, Methionine, Phenylalanine, Tryptophan, Histidine |
| Non-essential | Synthesised in the body | Glycine, Alanine, Glutamic acid, Aspartic acid, Serine, Tyrosine, Cysteine |
In NCERT Table 10.2 the essential amino acids are marked with an asterisk. Note that a single acid can fall in two schemes at once: lysine is both basic (group count) and essential (dietary), so a question can probe either property.
The Zwitterion & Amphoteric Behaviour
Here lies the conceptual heart of the subtopic. Amino acids are colourless, crystalline solids that NCERT says behave like salts rather than like simple amines or carboxylic acids. The reason is that an acidic $\ce{-COOH}$ and a basic $\ce{-NH2}$ coexist in one molecule. In aqueous solution the carboxyl group loses a proton and the amino group accepts a proton, producing a dipolar ion called the zwitterion: neutral overall, yet carrying both a positive and a negative charge.
For glycine the internal proton transfer is written as an equilibrium:
$\ce{H2N-CH2-COOH <=> H3N^{+}-CH2-COO^{-}}$
Schematic of the zwitterion: an internal proton hop converts the neutral form into a dipolar ion with $\ce{-NH3^+}$ and $\ce{-COO^-}$.
In its zwitterionic form an amino acid is amphoteric — it reacts with both acids and bases. In acidic medium the carboxylate accepts a proton to give the cation; in basic medium the ammonium loses a proton to give the anion:
$\ce{H3N^{+}-CH2-COO^{-} + H^{+} -> H3N^{+}-CH2-COOH}$
$\ce{H3N^{+}-CH2-COO^{-} + OH^{-} -> H2N-CH2-COO^{-} + H2O}$
These $\alpha$-amino acids polymerise through peptide bonds into proteins. See how it scales up in Protein Structure: Primary to Quaternary.
Isoelectric Point
Because the charge on an amino acid responds to pH, there is a specific pH at which the molecule exists predominantly as the neutral zwitterion and carries no net charge. This pH is called the isoelectric point. At a pH below the isoelectric point the molecule picks up a proton and becomes a net cation; at a pH above it, the molecule loses a proton and becomes a net anion. Exactly at the isoelectric point the amino acid does not move toward either electrode in an electric field, and its solubility is at a minimum.
In strongly acidic solution, which species of glycine dominates and why?
Below the isoelectric pH, excess $\ce{H+}$ protonates the carboxylate of the zwitterion. The dominant species is the cation $\ce{H3N^{+}-CH2-COOH}$, which migrates to the cathode. Raising the pH past the isoelectric point would instead favour the anion $\ce{H2N-CH2-COO^{-}}$.
Dipolar Nature, Melting Point & Solubility
The zwitterion explains two physical properties NEET likes to ask about together. NCERT states that amino acids are water-soluble, high-melting solids that behave like salts. Both observations flow from the dipolar character.
| Observation | Cause via the zwitterion |
|---|---|
| High melting point | The crystal is held by strong electrostatic attraction between $\ce{-NH3^+}$ and $\ce{-COO^-}$ ions, like an ionic salt; a large energy input is needed to melt it. |
| Good water solubility | The charged dipolar ion interacts strongly with polar water molecules, so it dissolves readily. |
| Salt-like behaviour | The species in the crystal and in solution is ionic, not the neutral $\ce{-NH2}/\ce{-COOH}$ form, so amino acids behave unlike simple amines or carboxylic acids. |
NCERT Intext Question 10.4 makes this comparison explicit: the melting points and water solubility of amino acids are generally higher than those of the corresponding halo acids, precisely because halo acids cannot form a zwitterion and remain ordinary covalent molecules.
Optical Activity & the Glycine Exception
The $\alpha$-carbon of a typical amino acid is bonded to four different groups — $\ce{-NH2}$, $\ce{-COOH}$, $\ce{-H}$ and the side chain R. Four different substituents make the $\alpha$-carbon an asymmetric (chiral) centre, so the molecule is optically active. NCERT states that, except glycine, all naturally occurring $\alpha$-amino acids are optically active because the $\alpha$-carbon is asymmetric, and they exist in both D and L forms. Most naturally occurring amino acids carry the L-configuration, drawn with the $\ce{-NH2}$ group on the left.
Glycine is the lone optically inactive amino acid
In glycine the side chain R is $\ce{-H}$. The $\alpha$-carbon then carries $\ce{-NH2}$, $\ce{-COOH}$ and two hydrogen atoms — only three distinct groups. With no four-different-group centre, glycine has no chirality and shows no optical activity.
Remember the cause (asymmetric $\alpha$-carbon), not just the exception. That distinguishes a guess from understanding.
Amino acids in ten lines
- Amino acids carry an $\ce{-NH2}$ and a $\ce{-COOH}$ group; in $\alpha$-amino acids both sit on the same ($\alpha$) carbon, which also holds H and side chain R.
- Only $\alpha$-amino acids are obtained on hydrolysis of proteins; about twenty occur in nature, all in L-configuration.
- Acidic = more $\ce{-COOH}$ (glutamic, aspartic); basic = more $\ce{-NH2}$ (lysine, arginine, histidine); neutral = equal numbers.
- Non-essential = made in the body; essential = ten acids that must come from diet.
- In water the $\ce{-COOH}$ loses and the $\ce{-NH2}$ gains a proton, forming the dipolar, neutral zwitterion.
- The zwitterion makes amino acids amphoteric — they react with both acids and bases.
- The isoelectric point is the pH at which the net charge is zero and the acid does not move in an electric field.
- Dipolar ionic character explains the high melting points and water solubility, higher than the corresponding halo acids.
- The asymmetric $\alpha$-carbon makes amino acids optically active and able to exist in D and L forms.
- Glycine (R = H) has no asymmetric carbon and is the sole optically inactive amino acid.