Chapter 7: The Cell Membrane Phospholipids! Amphipathic Molecules: " Phosphate head! hydrophilic " Fatty acid tails! Hydrophobic! Arranged as a bilayer Phosphate attracted to water Fatty acid repelled by water 2007-2008 Aaaah, one of those structure function examples Arranged as a Phospholipid bilayer! Selectively Permeable bilayer: polar hydrophilic heads nonpolar hydrophobic tails " Allows some substances to cross it more easily than others.! Serves as a cellular barrier / border sugar H 2 O salt Largely impermeable to polar molecules Cell defines cell! Cell separates living cell from aqueous environment " thin barrier = 8nm thick! Controls traffic in & out of the cell " allows some substances to cross more easily than others! hydrophobic (nonpolar) pass much easier than hydrophilic (polar) substances polar hydrophilic heads waste lipids
Cell is more than lipids Today we know that there are peripheral and trans proteins associated with and embedded in phospholipid bilayer! But this wasn t always the case! Gorter & Grendel: Structure must be phospholipid bilayer! Davson and Danielli: Proposed the Sandwich Model! Problems? 1. All cells are not identical. Different functioning must have different chemical structure and composition. 2. Membrane proteins often amphipathic so not very soluble in water. SANDWICH MODEL: lipid bilayer with outer layer of proteins FLUID-MOSAIC MODEL: Proteins in or attached to lipid bilayer (Nicolson & Singer) Cell must be more than lipids! In 1972, S.J. Singer & G. Nicolson proposed that proteins are inserted into the phospholipid bilayer It s like a fluid It s like a mosaic It s the Fluid Mosaic Model! Movement & Fluidity of Membranes! Hydrophobic interactions hold the phospholipids in the formation of a bilayer " Phospholipids! Float around laterally rapidly! Can flip from one side of the bilayer to the other but this is rare " Proteins! Some float around in the! Some move in specific directions along cytoskeletal fibers driven by motor proteins! Others are anchored and hardly move at all " Fluidity! Saturated hydrocarbons #Higher % of phospholipids with unsaturated hydrocarbons keeps the more fluid at lower temperatures than those with higher % of phospholipids with saturated hydrocarbons! Cholesterol #Helps resist changes in fluidity #Restrains phospholipid movement in high temp #Hinders close packing of phospholipids in low temp Membrane lipid composition varies! s must remain fluid & flexible! about as fluid as thick salad oil " altering the % of unsaturated fatty acids in phospholipids! keeps less viscous (thick)! cold-adapted organisms, like winter wheat # increase % in autumn
Why are proteins the perfect molecule to build structures in the cell? Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids 2007-2008 Peripheral protein Cholesterol Cytoplasm Trans proteins Filaments of cytoskeleton Membrane carbohydrates (glycolipids & glycoproteins - short branched! Play a key role in cell-cell recognition: Classes of amino acids What do these amino acids have in common? " ability of a cell to distinguish one cell from another " important in organ & tissue development " basis for rejection of foreign cells by immune system! Ex: Antigens # Molecules that elicits an immune response # What is bound by antibodies that mark that cell as an invader for destruction by nonpolar & hydrophobic
Classes of amino acids What do these amino acids have in common? I like the polar ones the best! Integral & Peripheral Proteins! Integral proteins penetrate the hydrophobic core of the lipid bilayer.! Trans proteins span the! Other integral proteins extend only partly into the lipid core polar & hydrophilic! Peripheral proteins are not embedded in the lipid bilayer at all and are loosely bound to the surface of the Proteins domains anchor molecule! Within : " nonpolar amino acids! span hydrophobic region! anchors protein into because water does not attract these R groups! On outer surfaces of in fluid: " polar amino acids! hydrophilic properties! these amino acids extend into extracellular fluid & into the cytosol Polar areas of protein Nonpolar areas of protein Examples aquaporin = water channel in bacteria Porin monomer H 2 O H 2 O β-pleated sheets Bacterial outer Retinal chromophore H + H + Nonpolar (hydrophobic) COOH α-helices in the cell H + NH 2 Cytoplasm proton pump channel in photosynthetic bacteria function through conformational change = protein changes shape
Aquaporin-1 in Action Types of associated proteins http://www.erin.utoronto.ca/~w3bio315/picts/lectures/lecture2/membraneproteinassoc1.jpg Many Functions of Membrane Proteins Many Functions of Membrane Proteins Outside Plasma Inside Transport Span the May provide selective hydrophilic channel May shuttles substances through changing shape May hydrolyze for energy to actively pump substances Enzymatic activity Active site faces aqueous solution May be found alone Many enzymes may be organized into teams: multi-enzyme complexes Signal Transduction Membrane Protein is the receptor Has binding site for a chemical messenger Binding of the signal molecule may cause shape change in the protein which passes the message to the inside of the cell Cell-Cell Recognition Glycoproteins serve as I.D. tags These glycoproteins are recognied by proteins of other cells Intercellular Joining Membrane proteins of neighboring cells hook together in types of junctions. Examples include Gap Junctions and Tight Junctions Cytoskeleton & ECM attachment Microfilaments noncovalently bind to proteins Maintains cell shape Stabilizes location of some proteins Through ECM, the cell can cooridinate extracellular and intracellular changes
Diffusion through phospholipid bilayer! What molecules can get through directly? " fats & other lipids Movement across the Cell Membrane lipid inside cell NH 3 sugar outside cell aa salt H 2 O! What molecules can NOT get through directly? " polar molecules! H 2 O " ions (charged)! salts, ammonia " large molecules! starches, proteins 2007-2008 Lipid Bilayer permeability Diffusion across cell! Hydrophic Substances: dissolve in the lipid bilayer of the and cross it without the help of proteins " O 2, lipids, CO 2! Hydrophilic Substances: cannot pass through the hydrophobic core of the bilayer " Ions and polar molecules like glucose PROTEINS ARE THE KEY TO MAKING CELL MEMBRANES PERMEABLE TO HYDROPHILIC and CHARGED SUBSTANCE!! Cell is the boundary between inside & outside " separates cell from its environment Can it be an impenetrable boundary? IN food carbohydrates sugars, proteins amino acids lipids salts, O 2, H 2 O IN OUT OUT waste ammonia salts CO 2 H 2 O products cell needs materials in & products or waste out NO!
Lipid Bilayer permeability! TRANSPORT PROTEINS SPAN THE MEMBRANE: # CHANNEL PROTEINS: HYDROPHILIC CHANNELS (pores) Diffusion - the movement of any substance so that! 2nd Law of Thermodynamics governs biological systems " universe tends towards disorder ( entropy) $ EX: AQUAPORINS # CARREIR PROTEINS: HOLD ONTO SUBSTANCES, CHANGE SHAPE THEMSELVES, AND SHUTTLES SUBSTANCES ACROSS. $ EX: Glucose Transporter! TRANSPORT PROTEINS CAN BE HIGHLY SPECIFIC ESPECIALLY CARRIER PROTEINS!!!! Diffusion " Each substance moves down ITS OWN concentration gradient. " HIGH LOW concentration Diffusion Simple Diffusion! Move from HIGH to LOW concentration " Diffusion is a form of passive transport! no energy needed " In Simple Diffusion:! no proteins required Type of Simple Diffusion: Movement of water Movement continues until a state of dynamic equilibrium is reached: Solutes continue to cross the but at equal rates back and forth (no more change in concentration) diffusion osmosis
The Special Case of Water Movement of water across the cell 2007-2008 Osmosis is just diffusion of water! Some water is bound by inter-molecular bonds to other hydrophilic substances! Diffusion of water occurs from: " HIGH concentration of free water to LOW concentration of free water! Water will thus move across a semi-permeable from a region of low solute concentration to that of higher solute concentration Concentration of water! Direction of osmosis is determined by comparing TOTAL solute concentrations Managing water balance! Cell survival depends on balancing water uptake & loss! Tonicity: The ability of a solution to cause a cell to gain or lose water " Hypertonic - more solute, less free water " Hypotonic - less solute, more free water " Isotonic - equal solute, equal free water water hypotonic hypertonic net movement of water freshwater balanced saltwater
1 Managing water balance! Hypotonic (hypo=less) " a cell in fresh water " high concentration of water around cell! Cell gains water, swells & can burst! Ex: Paramecium # Problem: Water continually enters Paramecium cell # Solution: Contractile vacuole $ pumps water out of cell KABOOM! $ No problem, " plant cells here! turgid = firm! cell wall protects from bursting freshwater Pumping water out - adaptation for osmoregulation! Contractile vacuole in Paramecium 2 Managing water balance 3 Managing water balance! Hypertonic (hyper=more) " a cell in salt water " low concentration of water around cell! Cell loses water & can die # Ex: Shellfish # Solution: take up water or pump out salt " Plant cells! plasmolysis = wilting # can sometimes recover if placed back in more ideal conditions I m shrinking, I m shrinking! I think I will survive! saltwater! Isotonic (iso=same) " animal cell immersed in mild salt solution " no difference in concentration of water between cell & environment! problem: none # no net movement of water $ flows across equally, in both directions # cell in equilibrium # volume of cell is stable! example: That s perfect! I could be better blood cells in blood plasma must remain stable [not implode or explode] # IV solutions in hospital are slightly salty to match solute concentration of blood cells balanced
Aquaporins 1991 2003 Do you understand Osmosis?! Allow water to move rapidly into & out of cells " evidence that there were water channels! protein channels allowing steady flow of water across cell.05 M.03 M Cell (compared to beaker) hypertonic or hypotonic Peter Agre John Hopkins Roderick MacKinnon Rockefeller Beaker (compared to cell) hypertonic or hypotonic Which way does the water flow? in or out of cell Do you understand Osmosis? Assume the is impermeable to sucrose but permeable to fructose and glucose. Any Questions??.05 M.03 M What way does glucose move? into or out of cell What way does sucrose move? into Neither or out of cell What way does fructose move? into or out of cell
Facilitated Diffusion! Diffusion through protein channels and carriers " These proteins move specific molecules across cell " NO energy needed!!! " The diffusion that occurs normally is just HELPED along since the particle is charged and cannot otherwise pass through the. facilitated = with help open channel = fast transport HIGH! Membrane becomes semi-permeable to polar molecules via protein channels and carriers: inside cell " Channel proteins can be Ion Channels! Tunnels for specific ions to pass through " Some are Gated-Ion Channels! Tunnels that open and close in response to a stimulus # Stimuli can be signal molecules that bind to the trans protein on the extracellular side of the # Stimuli can be electrical signals like in the nervous system H 2 O aa sugar LOW The Bouncer NH 3 salt outside cell Active Transport! Cells may need to move molecules against concentration gradient " conformational shape changes transport solutes from one side of to another like a pump " costs energy = conformational change uses up LOW Active transport is directional! Uniports: Move a single substance in one direction. Ca 2+ carriers! Symports: Move two substances in the same direction. Taking up a.a. from intestine requires simultaneous binding of Na+ by same transport protein.! Antiports: Move two substances in opposite directions, on in and one out of the cell. Sodium-potassium pump. HIGH The Doorman
Primary & Secondary Active Transport Secondary Active Transport In this scenario, prior to these pictures, was used to actively pump Na+ out of the cell! Primary Transport: Requires the direct participation of the energy-rich molecule! Secondary Transport: Does NOT use DIRECTLY. Rather, its energy is supplied by an ion concentration gradient established by a primary active transport that DOES use DIRECTLY. X 1st pumped against concentration gradient Next, X diffuses down its concentration gradient and the energy released is used to pump S against its concentration gradient The high concentration of Na+ outside stores potential energy Getting through cell! Passive Transport " Simple diffusion! diffusion of non-polar, hydrophobic molecules # lipids # HIGH LOW concentration gradient " Facilitated transport! diffusion of polar, hydrophilic molecules! through a protein channel # HIGH LOW concentration gradient Transport summary simple diffusion facilitated diffusion! Active transport " diffusion against concentration gradient! LOW HIGH " uses a protein pump " requires active transport
How about large molecules?! Moving large molecules into & out of cell " Through vesicles use exocytosis " Exocytosis - secretion of molecules through the fusion of vesicles with the plasma " Endocytosis - cell takes in biological molecules and particles by forming vesicles from the plasma! Phagocytosis = cellular eating! Pinocytosis = cellular drinking! Receptor-mediated endocytosis = enables the cell to acquire bulk quantities of specific substances. $ Receptor proteins are clustered in regions of the called coated pits $ Specific substances to be transported inward bind to these receptors $ Plasma with receptors and bound ligands then form a vesicle, moving into the cytoplasm Endocytosis phagocytosis pinocytosis receptor-mediated endocytosis Fuse vacuole with lysosome for digestion non-specific process Specific: ligands bind receptors