Amino Acids | Chemistry

📖 In-Depth Theory

Structure of Amino Acids

AMINO ACIDS are the monomers of proteins — natural condensation polymers.

EVERY AMINO ACID has TWO functional groups:

–NH₂ (amine group) — basic

–COOH (carboxylic acid group) — acidic

General structure: H₂N–CH(R)–COOH

where R = side chain (different for each amino acid).

There are 20 different amino acids in living systems — they differ only in their R group.

Examples:

Glycine: R = H (simplest amino acid)

Alanine: R = CH₃

Cysteine: R = CH₂SH

PROPERTIES:

Amino acids are AMPHOTERIC — contain both acidic (–COOH) and basic (–NH₂) groups.

At neutral pH, exist as ZWITTERIONS — both groups ionised: –NH₃⁺ and –COO⁻.

Soluble in water — ionic charges attract water molecules.

Peptide Bonds and Protein Formation

Amino acids polymerise by CONDENSATION POLYMERISATION.

PEPTIDE BOND formation:

The –COOH of one amino acid reacts with the –NH₂ of the next.

Water is eliminated.

A PEPTIDE BOND (–CO–NH–) forms — this is an amide link.

H₂N–CH(R₁)–COOH + H₂N–CH(R₂)–COOH → H₂N–CH(R₁)–CO–NH–CH(R₂)–COOH + H₂O

DIPEPTIDE: two amino acids joined by one peptide bond.

POLYPEPTIDE: many amino acids joined by peptide bonds.

PROTEIN: one or more polypeptide chains folded into a specific 3D structure.

The sequence of amino acids (determined by DNA) determines:

The protein's PRIMARY STRUCTURE.

How it folds → SECONDARY and TERTIARY structure.

The protein's function — enzymes, structural proteins, antibodies, hormones.

Hydrolysis of Proteins

HYDROLYSIS: the reverse of condensation — breaking peptide bonds by adding water.

PROTEIN + H₂O → amino acid mixture (with acid catalyst or enzyme)

INDUSTRIAL HYDROLYSIS:

Soap production: triglycerides (esters) hydrolysed by NaOH → glycerol + fatty acid salts (soap).

Food processing: protein hydrolysates used in sauces, flavourings.

BIOLOGICAL HYDROLYSIS:

Digestion: protease enzymes break peptide bonds in dietary proteins → amino acids absorbed.

Lysosomes: break down old proteins within cells.

WHY PROTEINS ARE BIODEGRADABLE:

Peptide bonds can be hydrolysed enzymatically.

Amino acids re-enter metabolic cycles.

This makes proteins more environmentally friendly than synthetic addition polymers (which resist hydrolysis).

LINK TO DNA:

DNA sequence → mRNA → protein sequence via genetic code.

A change in DNA base sequence → different amino acid → different protein → possibly different function (mutation).

⚠️ Common Mistake

Peptide bonds are AMIDE links (–CO–NH–) — the same type as in nylon. The bond forms between the –COOH of one amino acid and the –NH₂ of the next — water is lost. All proteins are condensation polymers of amino acids. Hydrolysis breaks peptide bonds — the reverse of their formation.

📐 Key Equations

–COOH + H₂N– → –CO–NH– + H₂O (peptide bond formation)

Protein + H₂O → amino acids (hydrolysis)

📌 Key Note

Amino acids: –NH₂ and –COOH groups, differ in R group. 20 different amino acids. Peptide bond: –CO–NH– (amide link), water lost. Proteins = condensation polymers of amino acids. Hydrolysis breaks proteins back to amino acids. DNA sequence → protein sequence. Proteins biodegradable.

🎯 Matching Activity — Amino Acids and Proteins

Match each term to its correct description. — drag the symbols on the right to match the component names on the left.

Amino acid

Drop here

Peptide bond

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Dipeptide

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Protein hydrolysis

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Monomer with both –NH₂ and –COOH groups — 20 different types differing in R group

Two amino acids joined by one peptide bond

–CO–NH– amide link formed between –COOH and –NH₂ with loss of water

Addition of water breaks peptide bonds — digestion by protease enzymes

⭐ Higher Tier Only

HT only — draw the condensation reaction between two amino acids showing the peptide bond formed and water molecule released. Explain the relationship between DNA base sequence, amino acid sequence and protein structure. Explain why changing one amino acid can alter protein function.

🔬 Triple Science Only

Amino acids (4.7.3.3) is chemistry-only AND HT only — not in Combined Science at any tier.

🎯 Test Yourself

Question 1 of 2

1. What type of bond forms between amino acids in a protein, and what small molecule is released?

2. How many different amino acids are used in living organisms to build proteins?

⭐ How Well Do You Understand This Topic?

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🤖 Ask MrBadmus

Stuck? Just ask! 💬

I'll use CFIFA for calculations and flag Higher/Triple content clearly.