Chapter 5A_filled Plasma Membrane Functions PDF

1 STRUCTURE AND FUNCTION OF THE PLASMA MEMBRANE Chapter 5  2 BE ABLE TO… Sketch a cell membrane according to the fluid mosaic model. Indicate positions and orientations of...

1 STRUCTURE AND FUNCTION OF THE PLASMA MEMBRANE Chapter 5  2 BE ABLE TO… Sketch a cell membrane according to the fluid mosaic model. Indicate positions and orientations of phospholipids, cholesterol and integral and peripheral membrane proteins. Explain why membranes are asymmetrical. Describe at least 3 different factors that affect membrane fluidity. Define diffusion, osmosis, amphipathic and electrogenic Describe the characteristics of molecules that can easily pass through a phospholipid bilayer. Explain what happens to an animal cell placed into hypotonic, isotonic and hypertonic solutions. Compare/contrast simple diffusion and facilitated diffusion. Compare/contrast passive and active transport. Explain how co-transport works to transport molecules against their concentration gradient.  3 COMPONENTS AND STRUCTURE  4 PLASMA MEMBRANE FUNCTIONS 1. defines outer border of all cells and organelles 2. manages what enters and exits cell 3. receives external signals and initials cellular responses 4. adherence to neighboring cells  5 FLUID MOSAIC MODEL 5-10 nanometers (nm) thick A mosaic of components (phospholipids, cholesterol, proteins, and carbohydrates) that give the membrane a fluid character. Proposed in 1972 by S.J. Singer and G.L. Nicolson  6 PHOSPHOLIPIDS primary component of PM 2 fatty acid chains glycerol phosphate Each FA can be either saturated or unsaturated Amphiphilic  7 THE PHOSPHOLIPID BILAYER Phospholipids arrange themselves in a bilayer polar heads face outward hydrophobic tails face inward. (credit: modification of work by Mariana Ruiz Villareal)  8 PROTEINS Image Credit: sciencepics/Shutterstock.com 2nd major component of membranes Function as transporters, receptors, enzymes, or in binding and adhesion Integral proteins – integrated completely into the bilayer Peripheral proteins – occur only on the surfaces  9 INTEGRAL PROTEINS Integral membrane proteins (IMPs) have 1 or more hydrophobic regions (hydrophobic amino acids) and others that are hydrophilic. Locations and number of regions determine how they arrange within bilayer.  10 PERIPHERAL PROTEINS Attached to either phospholipid or integral protein Can carry out similar functions to IMPs  11 CARBOHYDRATES AND THE GLYCOCALYX Credit: V. Summersby/Springer Nature Limited 3rd major component of PM 2-60 monosaccharides, branched or unbranched Located on exterior surface of PM, bound to either proteins (forming glycoproteins) or to lipids (forming glycolipids). Function in cell-cell recognition & attachment.  12 MEMBRANE FLUIDITY Membrane needs to be flexible but maintain structure Fluidity is affected by: Phospholipid type –saturated FAs pack together more closely than unsaturated FAs fluidity greater with more unsaturated FAs Temperature – cold temperatures compress molecules making membranes more rigid Cholesterol - acts as a fluidity buffer keeps membranes fluid when cold but not too fluid when hot.  13 MEMBRANE FLUIDITY https://www.youtube.com/watch?v=LKN5sq5dtW4  14 COMPONENTS AND STRUCTURE SUMMARY  15 PASSIVE TRANSPORT  16 Membranes are asymmetric inner surface differs from outer surface Interior proteins anchor fibers of cytoskeleton to membrane exterior proteins bind extracellular matrix glycoproteins bind to substances cell needs to import  17 MEMBRANES ARE SELECTIVELY PERMEABLE allows some molecules to pass but not others this allows cytosol solutions to differ from extracellular fluids Ex) all cells maintain an imbalance of Na+ and K+ ions between interior and exterior environments Transport across a membrane can be either Passive – requires no energy Active – requires energy (ATP) Concentration gradient established substances accumulate along a range  18 PASSIVE TRANSPORT - DIFFUSION Simplest type of passive transport is diffusion occurs when a substance moves from area of high concentration down its concentration gradient In membranes this occurs through the lipid bilayer Net movement ceases once equilibrium achieved Only small nonpolar molecules (O2, CO2) & lipid hormones can diffuse through membrane (credit: modification of work by Mariana Ruiz Villareal)  19 FACTORS THAT AFFECT DIFFUSION RATES Concentration gradients - greater difference = faster diffusion Mass of the molecules - smaller molecules = faster diffusion Temperature – higher temperature = faster molecular movement Solvent density – dehydration = increased cytoplasm density = slower diffusion rates Solubility – more nonpolar (lipid-soluble) = faster diffusion Surface area – increased surface area = faster diffusion Distance travelled –greater distance = slower rate important factor affecting upper limit of cell size Pressure – pressure forces solutions through membranes faster ex) kidney cell filtration rates affected by blood pressure  20 FACILITATED PASSIVE TRANSPORT Facilitated diffusion moves substances down their concentration gradients through integral transmembrane proteins Ions and small polar molecules 2 types of facilitated transport proteins: 1. Channel proteins 2. Carrier proteins  21 CHANNEL PROTEINS Top, bottom, and inner core are composed of hydrophilic AA – attract ions &/or polar molecules some open all the time Others are gated open when signaled Ex) Aquaporins - specific to H2O (credit: modification Muscle cells have gated ion of work by Mariana Ruiz channels allowing muscle Villareal) contraction when opened tens of millions of molecules per second  22 FACILITATED DIFFUSION – CHANNEL PROTEINS Credit: Rao, A., Ryan, K., Tag, A. and Fletcher, S. Department of Biology, Texas A&M University.  23 CARRIER PROTEINS All carrier proteins are specific to a single substance bind to substance, change shape & “carry it” to the other side many allow movement in either direction concentration gradient-dependent ex) glucose transport proteins (credit: modification of work by one thousand to a million Mariana Ruiz Villareal) molecules per second  24 FACILITATED DIFFUSION – CARRIER PROTEINS Credit: Rao, A., Tag, A. and Fletcher, S. Department of Biology, Texas A&M University.  25 PASSIVE TRANSPORT - OSMOSIS diffusion of water across a membrane water always moves from an area of higher water concentration to one of lower water concentration. [water] inversely related to [solute] water moves from low [solute] to high [solute] differences in [water] occur when a solute cannot pass through a selectively permeable membrane  26 TONICITY how an extracellular solution can change the volume of a cell by affecting osmosis often correlated to osmolarity of a solution total solute concentration of a solution (permeable and non-permeable solutes) When solutions with different osmolarities are separate by a membrane permeable to water but not the solute: water moves from the solution with lower osmolarity through the membrane  27 TONICITY – CELL VS. SOLUTION A hypotonic extracellular fluid has lower osmolarity than the cytoplasm – water enters the cell An isotonic extracellular fluid has the same osmolarity as the cytoplasm – no net water movement (water still moves) A hypertonic extracellular fluid has higher osmolarity than the cytoplasm – water exits the cell Animal cells function best when extracellular fluids are isotonic. (credit: Mariana Ruiz Villareal)  28 OSMOREGULATION Organisms with cell walls (plants, fungi, bacteria some protists) prefer hypotonic solutions The pressure exerted by the PM against the CW (turgor pressure) is critical for cell function Hypertonic solution causes plasmolysis – PM detaches from the CW (wilting in plants) (credit: modification of work by Mariana Ruiz Villareal)  29 OSMOREGULATION IN PLANTS Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting; the turgor pressure is restored by watering it (right). (credit: Victor M. Vicente Selvas)  OSMOREGULATION BY OTHER ORGANISMS 30 Freshwater protists use contractile vacuoles, to pump water out of the cell so they do not burst (credit: modification of work by NIH; scale-bar data from Matt Russell) Marine invertebrates have internal salt concentrations that match their environment Fishes excrete diluted urine to get rid of excess H2O or salts Osmoreceptors of brain cells monitor solute concentrations in our blood Albumin in blood helps control extracellular osmotic pressure