state that globular proteins are generally soluble and have physiological roles and fibrous proteins are generally insoluble and have structural roles

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Proteins

Proteins

Learning Objective

State that globular proteins are generally soluble and have physiological roles, whereas fibrous proteins are generally insoluble and have structural roles.

What is a Protein?

Proteins are polymers of amino acids linked by peptide bonds. They fold into specific three‑dimensional shapes that determine their function. Two broad structural categories are recognised:

  • Globular proteins

  • Fibrous (structural) proteins

Globular Proteins

Globular proteins adopt compact, roughly spherical shapes. Their interior is largely hydrophobic, while polar residues line the surface, making them generally soluble in aqueous environments. This solubility enables them to perform a wide range of physiological functions, such as:

  • Enzymatic catalysis (e.g., DNA polymerase)

  • Transport and storage (e.g., haemoglobin, albumin)

  • Regulation and signalling (e.g., hormones like insulin)

  • Immune defence (e.g., antibodies)

Fibrous Proteins

Fibrous proteins consist of long, repetitive polypeptide chains that assemble into extended filaments or sheets. Their side‑chains are often rich in non‑polar residues, leading to strong intermolecular interactions and a high degree of insolubility in water. Their primary role is structural, providing mechanical strength and support. Typical examples include:

  • Collagen – main component of connective tissue

  • Keratin – forms hair, nails, and the outer layer of skin

  • Elastin – provides elasticity in arteries and lungs

  • Fibrin – forms blood clots

Comparison of Globular and Fibrous Proteins

FeatureGlobular ProteinsFibrous Proteins
ShapeCompact, roughly sphericalExtended, filamentous or sheet‑like
SolubilityGenerally soluble in waterGenerally insoluble in water
Primary FunctionPhysiological (catalysis, transport, regulation, immunity)Structural (support, protection, elasticity)
Typical Secondary StructureMixture of \$α\$‑helices, \$β\$‑sheets and random coilsPredominantly \$α\$‑helices (e.g., keratin) or \$β\$‑sheet ribbons (e.g., silk fibroin)
ExamplesHemoglobin, enzymes, antibodies, insulinCollagen, keratin, elastin, fibrin

Key Points to Remember

  1. Solubility is a practical indicator of protein class: soluble → globular, insoluble → fibrous.

  2. Physiological roles require proteins to be mobile in the cytosol or plasma, favouring globular structures.

  3. Structural roles demand rigidity and resistance to mechanical stress, favouring fibrous structures.

  4. Both classes are built from the same 20 amino acids; differences arise from sequence patterns and higher‑order folding.

Suggested diagram: Schematic showing a soluble globular protein (e.g., enzyme) alongside an insoluble fibrous protein (e.g., collagen fibril) with labels for solubility and functional role.