1. Cell Communication

2. The Cellular “Internet”
Within multicellular organisms, cells must communicate with one another to coordinate their activities
A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response
Signal transduction pathways are very similar in all organisms, even organisms as different as unicellular yeasts and multicellular mammals

3. Local (Short-Distance) Signaling
Cells may communicate by direct contact
Plasmodesmata in plant cells
Gap junctions in animal cells
Animal cells can also use cell-cell recognition
Membrane-bound surface molecules can interact and communicate

4. Local (Short-Distance) Signaling
Messenger molecules can also be secreted by the signaling cell
Paracrine signaling:
One cell secretes (releases) molecules that act on nearby “target” cells
Example: growth factors
Synaptic Signaling:
Nerve cells release chemical messengers (neurotransmitters) that stimulate the target cell

5. Long-Distance Signaling
Endocrine (hormone) signaling
Specialized cells release hormone molecules, which travel (usually by diffusion through cells or through the circulatory system) to target cells elsewhere in the organism

6. The Three Stages of Cell Signaling
There are 3 stages at the “receiving end” of a cellular conversation:
Reception
Transduction
Response

7. Stage 1: Reception
The target cell “detects” that there is a signal molecule coming from outside the cell
The signal is detected when it binds to a protein on the cell’s surface or inside the cell
The signal molecule “searches out” specific receptor proteins
The signal molecule is a ligand
It is a molecule that specifically binds to another one (think enzymes!)

8. Stage 2: Transduction
This stage converts the signal into a form that can bring about a specific cellular response
One signal-activated receptor activates another protein, which activates another molecule, etc., etc.
These act as relay molecules
Often the message is transferred using protein kinases, which transfer phosphate groups from ATP molecules to proteins

9. Stage 2: Transduction

10. Stage 3: Response
The signal that was passed through the signal transduction pathway triggers a specific cellular response
Examples: enzyme action, cytoskeleton rearrangement, activation of genes, etc., etc.
Diagram example: transcription of mRNA

11. The Specificity of Cell Signaling
The particular proteins that a cell possesses determine which signal molecules it will respond to and how it will respond to them
Liver cells and heart cells, for example, do not respond in the same way to epinephrine because they have different collections of proteins