Cells communicate by sending and receiving signals.

1. Cells communicate by sending and receiving signals. Signal is sent  received  signal transduction pathway --series of steps that elicit a specific cellular response Cells close together can send signals directly by direct contact or through gap junctions or plasmodesmata.

2. Local signaling—cells send molecules that are received by neighboring cells Long-distance signaling (endocrine signaling) —use of hormones that are carried to other cells through the bloodstream

3. Signal Transduction • Reception—signal molecule binds to receptor protein on cell surface or inside cell • Transduction—receptor’s conformation changes • Response—pathway triggers cellular response

4. Most receptors are on the cell surface, but steroid hormone receptors are inside the cell since steroids can pass through the cell membrane. Usually steroid hormones are transcription factors; they turn genes on or off to produce new proteins.

5. Peptide hormones bind to membrane-bound receptors and change the structure of existing proteins. • 3 main types of membrane-bound receptors: • G-Protein Linked Receptors • Receptor Tyrosine Kinases • Ligand-Gated Ion Channel Receptors

6. G-Protein Linked Receptors have 7 transmembrane a helices G protein—on when bound to GTP—can activate other proteins --off when bound to GDP --is a GTPase so changes don’t last long

7. Receptor Tyrosine Kinases have 2 domains that form a dimer when a signal binds. Tyrosines become phosphorylated and activate relay proteins that cause cellular responses.

8. Ligand-Gated Ion Channel Receptors open when a ligand (signal) binds to the receptor • ions pass through • change in ion concentration triggers cellular response

9. Signals can be relayed by a cascade of molecular interactions (usually phosphorylation).

10. Kinases phosphorylate molecules. Phosphatases dephosphorylate molecules.

11. Small molecules like cAMP can be 2nd messengers. (1st messenger is the signal itself) Can amplify signal

12. Ca2+ is a common second messenger. • There is usually more Ca2+ outside the cell than in the cytosol. An increase in [Ca2+] can cause responses including: • Muscle contraction • Cell division • Secretion of substances

13. Ca2+ is released from the ER when the cell receives a signal.

14. Cytoplasmic Response—activation of existing proteins Nuclear Response—Transcription of new proteins

15. The proteins in a cell determine what signal molecules can bind to that cell. Each cell type responds differently to the same signal because each type of cell has different sets of proteins.