intercellular communication - lacking in most prokaryotes/protists 

• uses many different molecules to communicate
• dissolved gasses like nitric oxide can also be used as signals
• signal molecules either attached to surface, secreted through plasma membrane, or released by exocytosis
• receptor proteins - have 3D shapes that fit the shape of a specific signal molecule
  • signal molecule and receptor protein bind, changing the shape of the protein
  • change in protein shape >> response within the cell
  • hard to find, can make up less than 0.01% of a cell's mass
• immunochemistry - uses antibodies to target/isolate specific molecules/proteins
• molecular genetics - intentionally creates mutations in genes
  • receptor malfunction is very evident, more easily seen
  • determines relationship between protein structures and cellular functions

types of cell signaling - 4 basic mechanisms for communication between cells 

• autocrine signaling - cells sending signals to themselves; may reinforce developmental changes
• direct contact - when cells are actually close enough to touch each other
• paracrine signaling - released molecules that only influence cells in close vicinity
• endocrine signaling - uses hormones, which lasts longer in the circulatory system
• synaptic signaling - used by animals' nervous systems
  • neurotransmitters - don't travel through the circulatory system; released by nerve cells to very close target cells
  • chemical synapse - association of a neuron and its target cell
  • neurotransmitters pass across the synaptic gap, last very briefly

second messengers - substances used to relay message from receptors to inside the cytoplasm 

• alter the behavior of certain proteins by binding to them, changing their shape
• cyclic AMP (cAMP) - used by all animal cells
  • produced by adenylyl cyclase when started by G-protein
  • activates the alpha-kinase enzyme, adding phosphates to certain proteins
  • works in muscle cells to make more glucose available
• calcium ion - serves as 2nd messengers though found in low levels inside the cell
  • levels are much higher outside the cell
  • gated channels controlled by G-proteins allow Ca++ in to start certain activities
  • IP3 made from phospholipids and phospholipase binds to ER to let Ca++ into the cytoplasm
  • binds to calmodulin (148-amino-acid protein w/ 4 binding sites for C++) to activate other proteins

protein kinase cascades - chains of protein messengers used to relay messages to the nucleus 

• usually starts w/ phosphorylating a stage 1 protein
• each stage protein activates a large number of proteins in the next stage, and so forth
• different signals may use some of the same messengers, but ultimately have different targets
• vision amplification cascade - starts w/ light activating rhodopsin (a G-protein)
  • rhodopsin activates hundreds of transducin (another G-protein)
  • each transducin causes phosphodiesterase enzyme to change thousands of cyclic GMP
  • human rod cells sensitive enough to detect brief flashes of just 5 photons
• cell division amplification cascade - starts w/ phosphorylating ras (a protein kinase)
  • ras proteins activate series of phosphorylation, leading to division
  • 1/3 of cancers involve a mutation in the ras protein gene, causing unrestrained growth