Cells signal to each other to survive, even bacteria.

Target Cell: The cell that is being communicated to.

Ligand: The signal

Receptor: The thing that receives the signal and gives it to the target cell.

Autocrine (auto = self):
A cell communicates with itself 🙁

Paracrine (para = nearby):
Nearby cells are communicated with.

Juxtracrine (juxta = beside, next to, touching):
Touching cells communicate.

Endocrine (endo = within):
Usually, the ligand travels through the bloodstream

There are many different kinds of ligands and receptors:
Certain ligands and receptors do different things, that is just how it is.

The Pathway of converting external signals into responses with the cell:

  1. RECEPTION: The ligand activates the receptor
  2. TRANSDUCTION (Protein Modification): The receptor phosphorylates (gives ATP) the transduction enzymes so it changes into something that can be used.
  3. RESPONSE: The enzyme then activates a transcription factor, that then creates some mRNA, which creates protein.
    Can turn on/off genes, etc.

Second Messengers:
If a G-protein is used it activates a protein which then activates another protein, which is a secondary messenger in a way.

Steroids go RIGHT THROUGH into the nucleus, whereas proteins need a MEMBRANE RECEPTOR.

Transduction / Phosphorylation cascade:
They use ATP for the enzymes that amplify the protein, so it works better for the response.

Apoptosis:
Programmed/planed cell death (so it doesn’t become cancer or be taken over by a virus.

Cell Proliferation: cell division