Chapter 11 Cell Communication Cell to cell communication is essential for multicellular organisms The combined effects of multiple signals determine cell response 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 Local and Long Distance Signaling Cells in a multicellular organism communicate by chemical messengers Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells In local signaling animal cells may communicate by direct contact or cell cell recognition Figure 11 4 Communication by direct contact between cells In many other cases animal cells communicate using messenger molecules that travel only short distances In long distance signaling plants and animals use chemicals called hormones Figure 11 5 Local and long distance cell communication in animals o Paracrine signaling 2 cells right next to each other o Synaptic 2 neurons right next to each other o Hormones enter the bloodstream Cells receiving signals go through three processes o Reception o Transduction o Response Reception the binding between a signal molecule ligand and receptor is highly specific The receptor changes shape which is often the initial transduction of the signal Most signal receptors are plasma membrane proteins membrane receptors MUST be on the plasma membrane outside of cell There are three main types of membrane receptors o G protein coupled receptors o Receptor tyrosine kinases o Ion channel receptors A G protein coupled receptor is a plasma membrane receptor that works with the help of a G protein Most water soluble polar signal molecules bind to specific sites on receptor proteins in the plasma membrane These ligands cannot cross the cell membrane Polar ligands are very hydrophilic H2O soluble can t cross the cell G proteins are enzymes GTPases GTP GDP Pi ON OFF o The G protein acts as an on off switch If GDP is bound to the G protein the G protein is inactive If GTP is bound to the G protein the G protein is active Receptor tyrosine kinases are membrane receptors that attach phosphates to amino acids called tyrosines o RTK s the receptors themselves are enzymes o Figure 11 7c Membrane receptors receptor tyrosine kinases o Can trigger multiple signal transduction pathways at once A ligand gated ion channel receptor acts as a gate when the receptor changes shape o When a signal molecule binds as a ligand to the receptor the gate allows specific ions such as Na or Ca2 through a channel in the receptor o Figure 11 7d Membrane receptors ion channel receptors Intracellular Receptors Nonpolar ligands are hydrophobic CAN cross the plasma membrane receptors are intracellular Small or hydrophobic ligands have intracellular receptors found in the cytosol or nucleus of target cells Examples of hydrophobic messengers are the steroid and thyroid hormones of animals An activated hormone receptor complex can act as a transcription factor turning on specific genes Transcription factors can turn gene expression ON or OFF Figure 11 8 Steroid hormone interacting with an intracellular receptor Signal transduction usually involves multiple steps Can amplify a signal A few molecules can produce a large cellular response Can provide more opportunities for regulation of the cellular response Like falling dominoes the receptor activates another protein which activates another and so on until the protein producing the response is activated At each step the signal is transduced into a different form usually a shape change in a protein In many pathways the signal is transmitted by a cascade of protein phosphorylations Protein kinases transfer phosphates from ATP to protein a process called phosphorylation Turns the protein ON Protein phosphatases remove the phosphates from proteins a process called dephosphorylation Turns the protein OFF The extracellular signal molecule ligand that binds to the receptor is a pathway s first messenger Second messengers are small non protein water soluble molecules or ions that spread throughout a cell by diffusion Cyclic AMP and calcium ions are common second messengers Cyclic AMP cAMP is one of the most widely used second messengers Adenylyl cyclase an enzyme in the plasma membrane converts ATP to cAMP in response to an extracellular signal Calcium ions Ca2 act as a second messenger in many pathways Calcium is an important second messenger because cells can regulate its concentration Figure 11 12 The maintenance of calcium ion concentrations in an animal cell Nuclear and Cytoplasmic Responses cellular activities Ultimately a signal transduction pathway leads to regulation of one or more The response may occur in the cytoplasm or may involve action in the nucleus
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