Proteins

Revise Proteins for Biology 1 (IAL) WBI11 (AS Level) — revision notes and instant AI marking.

📖 Revision notes · preview

Edexcel IAL Biology · Proteins

Proteins: Structure, Enzymes & Nucleic Acids

Big idea: A protein's entire job — whether it's an enzyme, a hormone, or a structural fibre — comes down to one thing: the exact 3D shape its amino acid chain folds into, and that shape controls everything the protein can and can't do.

Summary — What This Chapter Covers

Amino acids are the monomers of proteins — a central carbon bonded to an amine group, a carboxylic acid group, a hydrogen, and a variable R group.
Peptide bonds form between amino acids via condensation reactions (releasing water); hydrolysis breaks them back down (adding water).
Proteins have four levels of structure: primary (sequence), secondary (α-helix / β-pleated sheet via H-bonds on the backbone), tertiary (3D folding via R-group interactions), and quaternary (multiple polypeptide chains together).
Globular proteins (e.g. haemoglobin, enzymes) are compact, soluble, and functional. Fibrous proteins (e.g. collagen, keratin) are long, insoluble, and structural.
Enzymes are globular proteins that act as biological catalysts, lowering activation energy via a specific active site (lock-and-key vs. induced-fit models).
Enzyme reaction rate is affected by temperature, pH, enzyme concentration, and substrate concentration — each in a distinct, explainable way.
Nucleotides (pentose sugar + phosphate + nitrogenous base) are the monomers of DNA and RNA, joined by phosphodiester bonds into a sugar-phosphate backbone.
DNA is a double-stranded, antiparallel double helix held together by complementary base pairing (A-T, G-C) via hydrogen bonds; RNA is single-stranded, uses ribose and uracil instead of deoxyribose and thymine.

1. Amino Acids, Proteins & Protein Structure

1.1 What is an amino acid?

Think of an amino acid like a Lego brick with four connectors sticking off a central carbon atom. Every single amino acid — all 20 of them found in living things — shares the exact same three connectors, and differs only in the fourth:

Amine group (–NH₂) — always the same
Carboxylic acid group (–COOH) — always the same
A hydrogen atom — always the same
R group — this is the ONE part that changes between amino acids, and it's what gives each amino acid its personality (acidic, basic, polar, non-polar)

H | H₂N — C — COOH | R ← this is the only variable part

Because the R group is the only thing that varies, it's the R group that ultimately decides how a protein folds and what it can do — this idea comes back again and again in this chapter, so lock it in now.

Key Fact

20 amino acids exist in nature. They all share the same core structure (amine + carboxyl + H on a central carbon) — only the R group differs between them.

1.2 Peptide bonds — how amino acids link up

When two amino acids join, it's not magic — it's a straightforward condensation reaction: the –OH from one amino acid's carboxyl group and an –H from the next amino acid's amine group are kicked out together as a water molecule (H₂O), and the two amino acids fuse where those atoms used to be. That new bond, between the carbon of one amino acid and the nitrogen of the next, is called a peptide bond.