2.3.5—Fibrous proteins
- Syllabus
- 9700–2028–2029
- Objective
- 2.3.5
- Level
- AS
Globular and fibrous proteins trade solubility for organised function
Globular and fibrous proteins are broad structural and functional classes, not mutually exclusive chemical categories. Globular proteins are generally compact and soluble; fibrous proteins are generally long, organised and insoluble. Their shape and organisation help explain their typical roles.
| Feature | Globular proteins | Fibrous proteins |
|---|---|---|
| Overall form | Compact, roughly spherical fold | Long strands or extended fibres |
| Sequence/organisation | Often varied sequences that fold into a specific 3-D shape | Often repetitive sequence with organised repeating structure |
| Solubility | Generally soluble in water because hydrophilic groups can face the surface | Generally insoluble because of extended organisation and exposed hydrophobic character |
| Typical role | Physiological or functional roles, such as enzymes, antibodies or transport proteins | Structural roles, such as collagen or keratin |
| Function link | Specific fold creates binding or active surfaces | Alignment and stabilisation support resistance to pulling forces |
Globular folding tends to bury hydrophobic R groups and expose hydrophilic R groups, helping the protein remain dispersed in water and form specific working sites. Fibrous proteins use long, repetitive or aligned organisation with stabilising interactions to form strong structures, so insolubility is compatible with their structural role.
Use “generally” rather than treating the classes as absolute rules: globular describes a typical compact shape and solubility pattern, while fibrous describes a typical extended structural pattern. “Fibrous” does not mean unstructured, and a protein’s class is not a separate chemical type defined by one bond.
ConceptA-Level CAIE Biology AS