Proteins do most of the work in a cell: they speed up reactions, carry oxygen, build hair, send signals and fight germs. All of them are chains of the same 20 kinds of amino acids, linked by the dehydration synthesis of Monomers and Polymers.
Key idea
A protein is a chain of amino acids (20 kinds) that folds into a precise shape. The order of amino acids decides the shape, and the shape decides the job.
Every amino acid has the same core (an amino group, a carboxyl group and a hydrogen around a central carbon) plus a side chain called the R group. The 20 R groups differ: some are polar or charged and like water; others are nonpolar and avoid it. The bond that joins two amino acids is a peptide bond, so a chain is a polypeptide.
| Level | What it is | Held by |
|---|---|---|
| Primary | The order (sequence) of amino acids | Peptide bonds |
| Secondary | Coils (α helix) and folds (β pleated sheet) in parts of the chain | Hydrogen bonds along the backbone |
| Tertiary | The whole chain folded into a 3-D shape | Interactions between R groups |
| Quaternary | Two or more chains fitted together (hemoglobin has four) | The same kinds of interactions |
Why it works
Folding is driven by the R groups. Nonpolar R groups cluster inside, away from water; polar and charged ones end up on the surface. Change one amino acid and the fold can change. In sickle cell disease, one amino acid in each of hemoglobin's beta chains is swapped (valine for glutamic acid). That one change lets the hemoglobin molecules stick together into stiff rods, which bend red blood cells out of shape.
Worked example: Name the job
What is the main job of each protein? Amylase, hemoglobin, collagen, antibodies.
- Amylase speeds up the breakdown of starch: an enzyme.
- Hemoglobin carries oxygen in red blood cells: transport.
- Collagen gives skin and tendons strength: structure.
- Antibodies bind to germs: defense.
Answer: Enzyme, transport, structure, defense
Worked example: Count the bonds
A short protein (a peptide) has 30 amino acids. How many peptide bonds does it have, and how many water molecules were released making it?
- A chain of $n$ units has $n - 1$ links.
- $30 - 1 = 29$ peptide bonds.
- Each bond released one water: 29 water molecules.
Answer: $29$ and $29$
Common mistake
Thinking a protein is just a list of amino acids. The list (primary structure) matters because it decides how the chain folds, and only the folded shape can do the job. Check: heat an egg white and its proteins unfold. The amino acid list has not changed, but the proteins no longer work the same way.