BISC 307L 2nd Edition Lecture 4 Current Lecture Cellular Communication There are two basic types of physiological signaling electrical signals changes in a cell s membrane potential and chemical signals molecules secreted by cells into the extracellular fluid 4 basic methods of cellular communication 1 Gap junctions direct cytoplasmic passage of metabolites 2 nd messenger molecules or ions between adjacent cells 2 Contact dependent signaling surface molecules on one cell membrane bind to surface molecules on another cell s membrane 3 Long distance communication combination of electrical signals carried by nerve cells and chemical signals transported in the blood a Endocrine signaling cell secretes substances which can travel through blood circulation to a distant location to bind a target cell with a specific receptor ex hormones b Pheromone chemical signal that triggers a natural response in another individual of the same species 4 Local communication chemicals diffuse through ECF limited in distance a Paracrine signaling cell secretes substances which diffuse through the ECF and affect nearby cells ex metabolite or neurotransmitters b Autocrine signaling signal acts on the same cell that secreted it in the first place Synaptic transmission is a special case that is a combination of long distance and local paracrine in this case signaling electrical signals travel along a neuron until it reaches the end where it is translated into a chemical signal called a neurotransmitter that is secreted and diffuses across a narrow extracellular space to target a cell and incite a rapid cellular response Mechanisms of Hormone Action A hormone is a chemical released by a cell or gland in one part of the body that sends out messages that affect cells in other parts of the organism They are classified as hydrophilic water soluble or lipophilic lipid soluble 3 pathways 1 Hydrophilic hormone which can t cross the membrane has to bind to plasma membrane receptors which is activated and in turn activates an intracellular second messenger system which alters enzyme activity causing a response directly or via a change in gene expression 2 Lipophilic hormone which is not freely soluble in the plasma needs carrier proteins in the plasma and the ECF to transport it around The unbound portion can diffuse directly through the plasma membrane and bind to a cytoplasmic or nuclear receptor The hormone receptor complex then moves into the nucleus and binds directly to DNA becoming a transcription factor that activates other transcription factors and directly affects gene expression For most lipophilic hormones There are specific carrier proteins usually globular that bind it with much higher affinity than any other hormone There also exist carrier proteins that can bind indiscriminately to many lipid hormones ex the most abundant protein in the plasma albumin 3 Lipophilic hormone that binds to plasma membrane receptors and activate 2 nd messenger systems Discovered while studying progesterone a lipophilic hormone they found plasma membrane receptors for it There are similar receptors for testosterone and estrogen Herrera Side note Brown fat highly metabolically active its in babies because they use it to thermoregulate themselves Hibernating mammals use it too Brown fat uses up a lot of energy a lot of ATP Discovery of this hormone important Constitutes a previously unknown mechanism for the benefits of exercise on reducing all kinds of bad things Signal Transduction General mechanism of signal transduction An extracellular signal molecule binds to and activates a plasma membrane receptor that activates a 2ndary messenger system to create a response Important Amplification occurs such that a single molecule binding to a receptor can cause a physiological response through an amplifier enzyme enzyme that activates several more molecules and secondary messengers Types of Membrane Receptors Here are the types of general membrane receptors four general types 1 Ion channel can be opened and closed and have gates which can swing closed or be opened This is a ball and chain type which actually exists Ligand activates or inactivates the gate so it is called a ligand activated channel 2 Receptor enzyme within the same molecule it has receptor and enzymatic functions Has two regions a receptor region on the extracellular side of the cell membrane and an enzyme region on the cytoplasmic side The enzymes of receptor enzymes are either tyrosine kinase or guanylyl cyclase the amplifier enzyme that converts GTP to cyclic GMP 3 G protein coupled receptors large and complex family of membrane spanning proteins that cross the phospholipid bilayer seven times So it is characterized by having 7 transmembrane segments and the cytoplasmic tail of the receptor protein is linked to a three part membrane transducer molecule known as a G protein The asterisk tells us that progesterone receptor is a G protein coupled receptor 4 Integrin receptors which are a little different Mediate blood clotting wound repair cell adhesion and recognition in the immune response and cell movement during development Will talk about it more later Ionic Basis of Resting Membrane Potential 3 things you need to remember about cells 1 Na K pump is vitally important 2 Because of the action of this pump we have low Na inside and high Na outside creating concentration gradient 3 Almost all cells have open potassium selective ion channels in their membranes that s the green block K pump is vital in all cells there has to be a way for potassium to get out Take those 3 conditions and you will see that almost all animal cells have a negative membrane potential Bottom line is if there is an unequal distribution of ions across the cell membrane and the membrane has open ion channels for one or more of these ions then an electrical potential difference across the membrane will develop Ligand Gated Ion Channels Channel on left potassium selective open at all times so it is called a leakage ion channels Leakage potassium channels are ubiquitous all animal cells have it The open K channel rapidly comes into equilibrium and internal negativity develops across the membrane Gated channels use the ball and chain mechanism shown on the right If the binding of a substance usually to the EC side but sometimes the IC side controls whether the gate is open or closed then it is called a ligand gated ion channels There also exist voltage gated and mechanically gated ion channels