Proteins, Vitamins and Nucleic Acids — Part 1: Amino acids and proteins: visual lesson, ISC Class 12 Chemistry

Lesson

Molecules of life

Part 1 — from the twenty amino acids to the proteins they build: hair, muscle, enzymes and hormones.

In this lesson you will

Classify amino acids and draw the zwitter ion of an amino acid
Explain the isoelectric point and predict the charge of an amino acid at a given pH
Show the formation of a peptide bond and define the four levels of protein structure
Explain denaturation and state the roles of enzymes and hormones

Table of five proteins, where each works and its job
Protein	Where it works	Job
Keratin	Hair, nails	Structure
Haemoglobin	Red blood cells	Carries oxygen
Pepsin	Stomach	Digests proteins
Insulin	Blood	Hormone: lowers blood glucose
Antibodies	Blood	Defence

Concept

Amino acids and their classification

Proteins are built from α-amino acids, H₂N–CH(R)–COOH: the –NH₂ and –COOH groups are on the same carbon. Except glycine (R = H), all are chiral and occur as the L-form. By diet: essential amino acids cannot be made in the body and must come from food (e.g. valine, leucine, lysine, phenylalanine, tryptophan); non-essential ones can be made (glycine, alanine, glutamic acid). By groups: neutral (one –NH₂, one –COOH), acidic (extra –COOH), basic (extra –NH₂).

Table of neutral, acidic and basic amino acids with examples and side chains
Class	Example	Side chain R
Neutral	Glycine	–H
Neutral	Alanine	–CH₃
Acidic	Aspartic acid	–CH₂COOH
Acidic	Glutamic acid	–CH₂CH₂COOH
Basic	Lysine	–(CH₂)₄NH₂

Concept

The zwitter ion

In the solid and in water, the acidic –COOH gives H⁺ to the basic –NH₂ of the same molecule, forming a dipolar ion (zwitter ion), H₃N⁺–CH(R)–COO⁻. This explains the salt-like properties: high melting points (with decomposition), solubility in water but not in non-polar solvents, and amphoteric behaviour — –COO⁻ accepts H⁺ from acids, –NH₃⁺ gives H⁺ to bases.

\ce{H3N+CH2COO- + H+ -> H3N+CH2COOH}

\ce{H3N+CH2COO- + OH- -> H2NCH2COO- + H2O}

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