Receiving the Signal (HL) (HL IB Biology)

Transmembrane receptor proteins

• Receptors that are located in the cell membrane have an external binding site and an internal region which extends into the cytoplasm
  • These are transmembrane proteins as they extend across the width of the membrane
• Transmembrane receptors are characterised by:
  • Hydrophilic amino acid regions at either end of the protein that are contact with the aqueous solution inside and outside the cell
  • A hydrophobic amino acid region within the membrane that is in contact with the hydrophobic tails of the phospholipids inside the cell membrane
• Some ligands bind to these receptors instead of entering the cell cytoplasm

Transmembrane receptor diagram

The cell surface membrane has many different components, including transmembrane proteins, e.g. glycoproteins, that function as receptors

Intracellular receptors

• Non polar, hydrophobic, ligands, e.g. steroid hormones, can diffuse through the phospholipid bilayer
• These ligands bind to receptors in the cytoplasm or on the DNA in the nucleus of the cell
  • Steroid hormones such as oestradiol will bind to the receptor molecule and activate it so that protein synthesis is initiated

Intracellular receptor diagram

Oestradiol is a steroid hormone that diffuses across the cell membrane and binds to an intracellular receptor in the cytoplasm of target cells

• When a ligand binds to either the transmembrane receptor or the intracellular receptor, a cascade of events follows which leads to a resulting change in cell activity
• The sequence of events is called the signal transduction pathway
  • Different ligands and different receptors trigger different signal transduction pathways
• All signal transduction pathways follow the same basic process
  • Binding with the receptor
    • For a transmembrane receptor:
      • A ligand binds to the extracellular region of the transmembrane receptor protein, causing a change in shape of the internal region of the protein
    • For an intracellular receptor:
      • A ligand binds to an intracellular receptor, forming a ligand-receptor complex
  • Signal transduction through a multistep pathway of events
    • For a transmembrane receptor:
      • Transmembrane proteins initiate a signal transduction pathway which involves phosphorylation events and a second messenger
    • For an intracellular receptor:
      • Intracellular ligand-receptor complexes are activated to follow a signal transduction pathway
  • Cellular responses which may include:
    • Regulation of gene expression through control of transcription or translation
    • Change in metabolic activity
    • Regulation of enzyme activity
    • Cell death
    • Rearrangement of the cytoplasm of the cell
    • Regulation of proteins, e.g. channels in the plasma membrane

Signal transduction diagram

Ligands can bind to transmembrane receptors, resulting in signal transduction and a cellular response