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. Hormones in Plants In plants hormones can travel in vessels or diffuse through the air as a gas Ethylene = gas that helps regulate growth and promotes ripening

7. The Three Stages of Cell Signaling There are 3 stages at the “receiving end” of a cellular conversation: 1.Reception 2.Transduction 3.Response

8. 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!)

9. 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

10. Stage 2: Transduction

11. 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

12. 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