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Cell Membrane/Plasma Membrane • plasma membrane is the boundary that separates the living cell from its surroundings • functions: • • • • 1. integrity of the cell – size and shape 2. controls transport = “selectively permeable” 3. excludes unwanted materials from entering the cell 4. maintains the ionic concentration of the cell & osmotic pressure of the cytosol • 5. forms contacts with neighboring cells = tissue • 6. sensitivity - first part of cell that is affected by changes in the extracellular environment Cell Membrane/Plasma Membrane • comprised of a phospholipid bilayer • phospholipids are the most abundant lipid in the plasma membrane – about 75% of lipids • phospholipid: – glycerol + 2 fatty acids – addition of a phosphate group Hydrophilic head WATER Hydrophobic tail WATER • phospholipids are amphipathic molecules – – – – containing hydrophobic and hydrophilic regions hydrophobic fatty acid “tails” hydrophilic phosphate “head group” makes the phospholipid both non-polar and polar http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf Plasma Membrane Composition: polar heads out Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol non-polar tails in Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE • • • the polar and non-polar attributes of the lipids results in a bilayer arrangement inserted into the membrane will also be cholesterol - which is also polar (OH group and non-polar steroid rings) -so the OH group faces out and the steroid rings face inward -gives the membrane fluidity also associated with membrane proteins and sugars Plasma Membrane Fluidity & Membrane proteins • plasma membrane also contains embedded proteins • many lipids and proteins are also modified through the addition of carbohydrates – glycoproteins – glycolipids • membrane proteins must be able to change shape to function • some of them even laterally diffuse through the PM • so the membrane must be fluid Plasma Membrane Fluidity • fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it • membrane fluidity is due to several factors – – – – temperature - lipids move around more with increased temp lipid packing – lipids with shorter fatty acid tails are less stiff saturation of fatty acids – more C=C bonds (more unsaturated) increase fluidity cholesterol – decreases fluidity at warmer temps; increases fluidity at lower temps Lateral movement occurs 107 times per second. Flip-flopping across the membrane is rare ( once per month). Membrane Proteins and Their Functions • membrane proteins determine most of the membrane’s specific functions • proteins link on the extracellular side to an extracellular matrix of proteins – support the cells within a tissue • proteins link on the cytoplasmic side to the cytoskeleton - via adaptor proteins Membrane Proteins and Their Functions EXTRACELLULAR SIDE two kinds of membrane proteins: N-terminus 1. Extrinsic or Peripheral: bound to the surface of the membrane ◦ often are linked to integral membrane proteins ◦ function mainly as enzymes 2. Intrinsic or Integral: penetrate the hydrophobic core ◦ those that span the membrane are called transmembrane proteins helix C-terminus CYTOPLASMIC SIDE Membrane Proteins and Their Functions 6 major functions of integral membrane proteins: ◦ ◦ ◦ ◦ 1. transport – channel proteins & transporters 2. enzymatic activity 3. signal transduction – receptor proteins 4. cell-cell recognition – cell identity markers ◦ ◦ ◦ e.g. ABO antigens 5. intercellular joining - linkers 6. attachment- to the cytoskeleton and extracellular matrix (ECM) Enzymes Glycoprotein Enzymatic activity Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular Membrane structure results in selective permeability a cell must exchange materials with its surroundings ◦ controlled by the plasma membrane plasma membranes are selectively permeable ◦ regulating the cell’s molecular traffic permeability = property that determines the effectiveness of the PM as a barrier hydrophobic (nonpolar) molecules dissolve in the phospholipid bilayer and pass through the membrane rapidly ◦ must be small enough to diffuse across the PM ionic and polar molecules do not cross the membrane easily ◦ require transport mechanisms ◦ provided by transport proteins Membrane Gradients selective permeability of the PM allows the cells to control the concentration of ions within the cell (in the ICF) and outside the cell (in the ECF) this results in a distinct distribution of positive and negative ions inside and outside the cell ◦ typically the inside of the cell is more negatively charged this difference in electrical charge between inside and outside = electrical gradient because it occurs across the PM – we call this difference in charge = membrane potential can be measured with tiny glass electrodes varies from cell to cell very important in the functioning of neurons and muscle cells Transport Proteins transport proteins allow passage of hydrophilic substances across the membrane ◦ specific for the substance it moves numerous types: 1. channel proteins – have a hydrophilic channel that certain molecules or ions can use as a tunnel to move across the membrane 2. carrier proteins - bind to molecules and change shape to shuttle them across the membrane 3. pumps – carrier proteins that require the hydrolysis of ATP (or GTP) to move substances What determines the direction of transport?? two basic things 1. concentration of what is being moved 2. available energy passive transport diffusion osmosis facilitated active transport primary AT secondary AT endocytosis exocytosis http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html A. Diffusion = movement of materials from [high] to [low] -random movement, no energy needs to be synthesized -the movement is driven by the inherent kinetic energy of the particles moving down their concentration gradient -three ways to diffuse: 1. through the lipid bilayer: lipid soluble (non-polar), alcohol, gases, ammonia, fat-soluble vitamins 2. through a channel: charged ions or small polar molecules -some channels are “gated” – open and close 3. facilitated diffusion: larger, polar molecules too big for channels B. Osmosis = diffusion of water from [high] to [low] OR movement of water from [low solute] to [high solute] Lower concentration of solute (sugar) in osmosis – the membrane is permeable to water and NOT to the solutes BUT it is the concentration of solutes that causes the water to move Higher concentration of solute Sugar molecule H2O the solutes are surrounded by a hydration shell of water molecules Selectively permeable membrane this decreases the amount of free water molecules available to move so increased solute concentration decreases the concentration of free water molecules water movement is affected by this drop in free water molecule concentration Osmosis Same concentration of solute B. Osmosis = diffusion of water from [high] to [low] OR movement of water from [low solute] to [high solute] in osmosis – it is the concentration of solutes that causes the water to move -known as tonicity experiment – U shaped tube divided by a membrane permeable to water only -increase the solute concentration in the right half of the tube -this increases the pressure caused by the increase in solutes = osmotic pressure (OP) -therefore increasing solute concentration increases osmotic pressure -water will move in to decrease this OP OP is important in determining how much fluid remains in your blood and how much leaves to surround the cells in your tissues Osmosis is controlled by tonicity = degree to which a the concentration of a specific solute surrounding a cell causes water to enter or leave the cell hypertonic e.g. isotonic = [S]in = [S]out hypotonic = [S]in > [S]out hypertonic = [S]in < [S]out water enters cell water exits cell no water movement C. Facilitated transport = molecules move by a carrier protein from [high] to [low] -binds to a carrier protein on the plasma membrane -transported by the carrier protein -no energy required – transported down the concentration gradient by the carrier protein -but there is a limit to the amount of facilitated diffusion cells can undergo and it has to do with the number of carrier proteins on the PM -molecules that are insoluble, too polar or too large e.g. glucose amino acids Active transport = transport requires the expenditure of energy -usually provided through the hydrolysis of ATP ADP+Pi -cell is moving a substance against its concentration gradient -cell is forming transport vesicles -cell is internalizing something Several kinds: 1. primary active transport = molecules are moved against its concentration gradient i.e. from [low] to [high] -directly uses the energy of ATP hydrolysis for this 2. secondary active transport = molecules are moved against its concentration gradient i.e. from [low] to [high] -movement is dependent upon another ion’s concentration gradient 3. Exocytosis – cell secretion 4. Endocytosis – cell internalization -pinocytosis Exocytosis and Endocytosis -phagocytosis are referred to as -receptor-mediated endocytosis Bulk Transport (Active process) A. Primary Active Transport = • • requires a protein carrier called a pump - pump that hydrolyzes ATP (ATPase) e.g. sodium/potassium ATPse pump : maintains a specific concentration of Na+ and K+ in and out of the cell – • • • • • • • three Na+ are pumped out of a cell and 2 K+ are pumped into the cell 3 Na+ ions bind to the pump ATP binds to the pump and is hydrolyzed a phosphate group remains bound to the pump = phosphorylation phosphorylation changes the activity of the pump by altering its shape Na+ is expelled out of the cell – against its concentration gradient 2 K+ ions then bind the pump and causes the release of the P group another shape change by the pump - releases the K+ into the cell http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html# Animation: Active Transport Right-click slide / select “Play” © 2011 Pearson Education, Inc. 2. secondary active transport: -the energy stored in a concentration gradient is used to drive the transport of other materials – primary active transport establishes high [Na] outside the cell - creates a Na gradient -diffusion of Na back into the cell allows the movement of a second ion – either in the same direction as the Na+ (symporter) or in the opposite direction (antiporter) e.g. Na+/Ca2+ antiporter – opposite direction for Na+ and Ca2+ movement – the Na+ concentration gradient creates potential energy which is used by an antiporter pump - as Na leaks back in through this antiporter – the potential energy is converted into kinetic energy which drives the movement of a Ca2+ ion against its gradient -most of our cells use the energy created by the Na+ gradients to power the movement of other ions in and out of our cells diffusion diffusion Na pump diffusion diffusion http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html# B. Exocytosis = secretion of a substance outside the cell -products made within the cell, packaged by the Golgi into transport vesicles fusion with the plasma membrane and release outside the cell e.g. nerve cells - neurotransmitter release http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/animations.html# C. Endocytosis = reverse of exocytosis, internalization of substances -3 forms: 1. pinocytosis = “cell drinking” 2. phagocytosis = “cell eating” 3. receptor-mediated endocytosis (RME)= internalization of specific substances -binding of a ligand with its receptor internalization into the cell -occurs at specific sites within the PM called clathrin-coated pits -proteins accumulate at these clathrin-coated pits -internalization clathrin-coated vesicle -CC vesicle fuses with endosomes – eventually fuse to the lysosome for processing http://sumanasinc.com/webcontent/animations/content/endocytosis.html