11.3 HOW DO DISTANT CELLS COMMUNICATE?Cell-Cell Signaling in Multicellular Organisms-Hormones: information-carrying molecules that can act on distant target cells because they are secreted by plant and animal cells into bodily fluids.-Hormones are small molecules usually present in small concentrations.-Despite their size they have a large impact on the activity of target cells and the condition of the body as a whole.-Important point of signaling molecules is whether or not they are lipid soluble.-Ability of a signaling molecule to pass through lipid bilayers determines how a target cell recognizes it.-Most lipid-soluble signaling molecules are able to diffuse across the hydrophobic region of the plasma membrane and enter the cytoplasm of their target cells.-Large or hydrophilic signaling molecules are lipid insoluble and do not cross the plasma membrane. Must be recognized at the cell surfaceSignal Reception-Hormones and other cell-cell signaling molecules deliver their message by binding to receptor molecules.-Presence of an appropriate receptor dictates which cells respond to hormone-Cells in different tissues can respond to the same signaling molecule-Identical receptors in diverse cells and tissues allow long-distance signals to coordinate the activities of cells throughout a multicellular organism.1. Receptors are dynamic: high hormonal stimulation over long periods can cause the number of receptors to decline. 2. Receptors can be blocked. -Most signal receptors are located in the plasma membrane, where they bind to signaling molecules that can’t cross the membrane. -Other signal receptors exist inside the cell where they respond to lipid-soluble signaling molecules that readily diffuse through the plasma membrane.-Signal receptors change their shape when a hormone binds to them-Signal receptor: a protein that changes its shape and activity after binding to a signaling molecule.Signal ProcessingProcessing Lipid-Soluble Signals-When lipid-soluble signals enter a cell, the info they carry is processed directly without any intermediate steps.Processing Lipid-Insoluble Signals-When a signaling molecule binds at the cell surface, it triggers signal transduction-Signal transduction- the conversion of a signal from one form to another. A longand often complex series of events ensues, collectively called a signal transduction pathway.-In a cell, signal transduction converts an extracellular signal to an intracellular signal.Intracellular Signals May Be Amplified-Signal amplification is one reason that hormones are present in small concentrations but trigger a large response from cells. -Signal transduction begins at the plasma membrane and amplification occurs inside the cell. 1. G-protein coupled receptors initiate the production of intracellular or “second” messengers that then amplify the signal.2. Enzyme-linked receptors amplify the signal by triggering the activation of a series of proteins inside the cell, through the addition of phosphate groupsSecond Messengers:1. Second messengers aren’t restricted to a single role or single cell type—thesame second messenger can initiate dramatically different events in differentcell types2. It is common for more than one second messenger to be involved in triggering a cell’s response to the same extracellular signaling moleculeSignal Response-Second messengers or a cascade of protein phosphorylation events also may altergene expression, or activate or deactivate existing proteins in the target cellSignal Deactivation-Phosphorylation cascades are also sensitive to the presence of the external signal.-Phosphates- an enzyme that removes phosphate groups from cascade proteins are always present in the cell-Signal transduction systems trigger a rapid response and can be shut down quickly.-Sensitive to small changes in concentration of hormones or in the numberand activity of signal receptors.Crosstalk: Synthesizing Input from Many Signals-Signal transduction pathways form a network. It allows cells to respond to many different signals in an integrated way.*****LECTURE ADD IN:-DNA is always same- same code-Cells make cells- mitosis- occurs through signaling-Stem cells are not terminally differentiated-Stem cell needs to rejuvenate. Always need to keep a stem cell after differentiation-Phosphorylation- idea that a phosphate can get stuck onto a protein (kinase)-Effect of this is to create a switch- depending on whether or not a phosphate is