Globular proteins are compact, roughly spherical proteins that usually perform metabolic or signalling functions, whereas fibrous proteins are long, strand-like proteins that usually provide structural support. For IB Biology AHL, the key principle is that each protein’s three-dimensional form enables its function.
| Feature | Globular proteins | Fibrous proteins |
|---|---|---|
| Overall shape | Compact and approximately spherical | Long, narrow and strand-like |
| Solubility | Usually soluble in water | Usually insoluble in water |
| Amino acid arrangement | Polypeptide chains fold into complex three-dimensional conformations | Polypeptide chains form elongated structures, often with repeating patterns |
| Typical role | Metabolism, transport, signalling or defence | Strength, support and protection |
| IB example | Insulin | Collagen |
The different shapes result from how polypeptide chains fold and interact. In water-soluble globular proteins, many hydrophilic R-groups face outward and interact with water, while hydrophobic R-groups tend to cluster inside. This produces a compact structure with specific binding regions.
Insulin is a globular protein hormone. Its precise three-dimensional conformation allows it to circulate in blood and bind specifically to insulin receptors on target cells, initiating a response that contributes to blood glucose regulation.
Collagen is a fibrous protein formed from three polypeptide chains wound together into a triple helix. Collagen molecules assemble into fibrils, producing high tensile strength. This makes collagen suitable for structural roles in tissues such as tendons, skin and connective tissue.
A common misconception is that “fibrous” means a protein contains dietary fibre. It does not: the term describes the protein’s elongated molecular form. Likewise, not every globular protein is an enzyme; insulin is globular but functions as a signalling molecule.
Exam technique: This is AHL content in subtopic B1.2 Proteins. For a “distinguish” question, state paired differences involving both protein types. For an “explain” question, link form directly to function: compact, soluble insulin enables specific receptor binding, while elongated collagen fibrils provide tensile strength.
