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2.3.42 · Biological Molecules

Globular and Fibrous Proteins — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 6 parts Lesson 19 of 25 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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2.3.4 Globular and Fibrous Proteins — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Compare globular and fibrous proteins in structure, solubility and function.Shape — compact and roughly spherical.Folding — hydrophobic R groups point towards the centre and hydrophi…

  • Specific shape
  • Conjugated proteins
  • Globular protein
  • Prosthetic group
  • Immunoglobulin
  • Folding — hydrophobic R groups point towards the centre and hydrophilic R groups point outwards

Part 1 of 4Globular proteins

Shape
  • Shape compact and roughly spherical.
Folding
  • Folding hydrophobic R groups point towards the centre and hydrophilic R groups point outwards.
Solubility
  • Solubility water molecules surround the outer hydrophilic groups, so the protein is generally soluble.
Roles
  • Roles easy transport and metabolic roles, e.g. enzymes catalysing specific reactions and immunoglobulins responding to antigens.
Specific shape
  • Specific shape folding by R-group interactions produces specific shapes.
Conjugated proteins
  • Conjugated proteins some contain a prosthetic group, e.g. haem in haemoglobin.

Part 2 of 4Fibrous proteins

1Shape
  • Shape long strands of polypeptide with cross-linkages of hydrogen bonds.
2Structure
  • Structure little or no tertiary structure.
3Solubility
  • Solubility many hydrophobic R groups make them insoluble.
4Sequence
  • Sequence a limited number of amino acids in a highly repetitive sequence.
5Function
  • Function strong, organised, insoluble structures suit structural roles, e.g. keratin in hair and nails and collagen in skin, tendons and ligaments.

Part 3 of 4Comparison using SAFES

FeatureGlobularFibrous
ShapeRoughly sphericalLong strands
Amino acid sequenceIrregular, wide range of R groupsRepetitive, limited R groups
FunctionPhysiologicalStructural
ExamplesHaemoglobin, insulin, immunoglobulinsCollagen, keratin, myosin, actin, fibrin
SolubilityGenerally solubleGenerally insoluble

Part 4 of 4More from the specification

More from the specification
Primary and Secondary Protein Conformations
  • The primary structure of a protein is a polypeptide chain of amino acids bonded together by covalent peptide bonds between an amine group (-NH₂) and a carboxyl carbon;
    • secondary structure forms when hydrogen bonds between carbonyl oxygen and amide hydrogen atoms fold the chain into an α-helix or β-pleated sheet.
Tertiary Bonds and Disulfide Bridges
  • The tertiary structure of a polypeptide chain involves 3D folding stabilized by interactions between R-groups,
  • including hydrogen bonds, ionic interactions, hydrophobic attractions,
  • and strong covalent disulfide bridges between cysteine residues.
Globular vs Fibrous Polypeptide Chains
  • Globular proteins like haemoglobin consist of folded spherical chains with hydrophilic exteriors making them water-soluble,
    • (which binds oxygen)
  • whereas fibrous proteins like collagen consist of repetitive, elongated chains forming insoluble structural ropes with high tensile strength.
Primary and Secondary Protein Conformations
  • The primary structure of a protein is a polypeptide chain of amino acids bonded together by covalent peptide bonds between an amine group (-NH₂) and a carboxyl carbon;
    • secondary structure forms when hydrogen bonds between carbonyl oxygen and amide hydrogen atoms fold the chain into an α-helix or β-pleated sheet.
Tertiary Bonds and Disulfide Bridges
  • The tertiary structure of a polypeptide chain involves 3D folding stabilized by interactions between R-groups,
  • including hydrogen bonds, ionic interactions, hydrophobic attractions,
  • and strong covalent disulfide bridges between cysteine residues.
Globular vs Fibrous Polypeptide Chains
  • Globular proteins like haemoglobin consist of folded spherical chains with hydrophilic exteriors making them water-soluble,
    • (which binds oxygen)
  • whereas fibrous proteins like collagen consist of repetitive, elongated chains forming insoluble structural ropes with high tensile strength.
Primary structure
  • The sequence of amino acids in a polypeptide.
Secondary structure
  • Α-helix and β-pleated sheet held by backbone hydrogen bonds.
Quaternary structure
  • Arrangement of multiple polypeptide subunits.
Amine group
  • The –NH2 functional group.
Triple helix
  • Three polypeptides wound together.

Key terms

TermMeaning
Globular proteinA compact, soluble protein with a specific shape.
Fibrous proteinA long, insoluble structural protein.
Conjugated proteinA protein with a non-protein prosthetic group.
Prosthetic groupA permanent non-protein part of a protein.
KeratinA fibrous structural protein in hair, nails and feathers.
ImmunoglobulinA globular protein that binds specific antigens.

Avoid these mistakes

1
Saying fibrous proteins are soluble — most are insoluble.
2
Putting hydrophobic R groups on the outside of globular proteins — they are in the centre.
3
Saying fibrous proteins have complex tertiary structure — they have little or none.
4
Saying all globular proteins lack prosthetic groups — some, like haemoglobin, have them.

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