Biomolecules — Part 2: Proteins, enzymes, vitamins, nucleic acids and hormones: visual lesson, CBSE Class 12 Chemistry

Lesson

Proteins and other biomolecules

Part 2 — the molecules that build, run, inherit and signal in living cells.

In this lesson you will

Describe amino acids, the peptide bond and the four levels of protein structure, and explain denaturation
Describe enzymes as biocatalysts
Classify vitamins as fat- or water-soluble and link deficiencies to diseases
Distinguish DNA and RNA in structure and function, and state the roles of important hormones

Concept

Amino acids

α-Amino acids have –NH₂ and –COOH on the same carbon, with a side chain R. Of about 20 in proteins, those the body cannot make are essential (must be in food). In water they exist as zwitterions (H₃N⁺–CHR–COO⁻), so they are crystalline, water-soluble and high-melting, and amphoteric.

Table of five amino acids with their side chains and notes
Amino acid	R group	Note
Glycine	–H	Only one that is not optically active
Alanine	–CH₃	Non-essential
Valine	–CH(CH₃)₂	Essential
Lysine	–(CH₂)₄NH₂	Essential, basic
Aspartic acid	–CH₂COOH	Acidic

Diagram

The peptide bond

The –COOH of one amino acid and the –NH₂ of another condense with loss of water, forming a peptide bond (–CO–NH–, an amide link). Two amino acids give a dipeptide; many give a polypeptide; a protein has more than about 100 residues.

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