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 surface Signal 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 Processing Processing 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 long and 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 groups Second Messengers 1 Second messengers aren t restricted to a single role or single cell type the same second messenger can initiate dramatically different events in different cell types 2 It is common for more than one second messenger to be involved in triggering a cell s response to the same extracellular signaling molecule Signal Response Second messengers or a cascade of protein phosphorylation events also may alter gene expression or activate or deactivate existing proteins in the target cell Signal 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 number and 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 there