Chapter 5 Plasma Membranes - Biology 2E PDF

BIOLOGY 2E Chapter 5 STRUCTURE AND FUNCTION OF PLASMA MEMBRANES Lecture PowerPoint Slides This work is licensed under a Creative Commons Attribution-NonCommercial- ShareAlike 4.0 International License. CHAPTER 5 PLASMA MEMBRANES...

BIOLOGY 2E Chapter 5 STRUCTURE AND FUNCTION OF PLASMA MEMBRANES Lecture PowerPoint Slides This work is licensed under a Creative Commons Attribution-NonCommercial- ShareAlike 4.0 International License. CHAPTER 5 PLASMA MEMBRANES 5.1 Phospholipids 5.2 Passive Transport 5.3 Active Transport 5.4 Bulk Transport PLASMA MEMBRANE FUNCTIONS Defining the outer border of all cells and organelles Managing what enters and exits the cell Receiving external signals and initiating cellular responses Adhering to neighboring cells 5.1 MEMBRANE COMPONENTS AND STRUCTURE Learning Objectives By the end of this section, you will be able to do the following: Understand the cell membrane fluid mosaic model Describe phospholipid, protein, and carbohydrate functions in membranes Discuss membrane fluidity FLUID MOSAIC MODEL 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 5.1 – PHOSPHOLIPIDS The main fabric of cell membranes is composed of phospholipids, which are amphipathic --a glycerol molecule Hydrophilic --a phosphate group head (polar). --2 fatty acid chains Hydrophobic (nonpolar) tails Each fatty acid can be either saturated or unsaturated Carbons are saturated with H: all single C-C bonds Unsaturated when at least one double C=C bond occurs  PHOSPHOLIPID BILAYER Phospholipids arrange themselves in a bilayer polar heads face outward hydrophobic tails face inward. (credit: modification of work by Mariana Ruiz Villareal)  PROTEINS Proteins are the second major component of membranes. Proteins can be transporters, receptors, enzymes, or can function in binding and adhesion. Integral proteins – integrated completely into the bilayer Transmembrane proteins are integral proteins that pass completely through the phospholipid bilayer Peripheral proteins – occur only on the surfaces INTEGRAL PROTEINS Integral membrane proteins have one or more regions that are hydrophobic (composed of hydrophobic amino acids) and others that are hydrophilic. The locations and number of regions determine how they arrange within the bilayer.  CARBOHYDRATES The third major component are carbohydrates. Located on the exterior surface of the plasma membrane, bound to either proteins (forming glycoproteins) or to lipids (forming glycolipids). Function in cell-cell recognition & attachment. MEMBRANE FLUIDITY The membrane needs to be flexible but not so fluid that it cannot maintain its structure. Fluidity is affected by: Phospholipid type – phospholipids with saturated fatty acids can pack together more closely than those with unsaturated FA; therefore, more SFA, more rigid Temperature – cold temperatures compress molecules making membranes more rigid Cholesterol, located within the fatty acid layer, acts as a fluidity buffer; keeping membranes fluid when cold and from not getting too fluid when hot. MEMBRANE FLUIDITY https://www.youtube.com/watch?v=LKN5sq5dtW4 Plasma membranes are asymmetric; the inner surface differs from the outer surface. For example: Interior proteins anchor fibers of the cytoskeleton to the membrane Exterior proteins bind to the extracellular matrix Glycoproteins bind to substances the cell needs to import 5.1 MEMBRANE COMPONENTS AND STRUCTURE Learning Objectives You should now be able to do the following: Understand the cell membrane fluid mosaic model Describe phospholipid, protein, and carbohydrate functions in membranes Discuss membrane fluidity 5.2 PASSIVE TRANSPORT Learning Objectives By the end of this section, you will be able to do the following: Explain why and how passive transport occurs Understand the osmosis and diffusion processes Define tonicity and its relevance to passive transport TRANSPORT The plasma membrane is selectively permeable, which means it allows some molecules to pass through, but not others. Molecules that can cross the phospholipid bilayer are said to be permeant. This feature allows cytosol solutions to differ from extracellular fluids. Example: all cells maintain an imbalance of sodium and potassium ions between the interior and exterior environments. WHAT CAN PASS THROUGH THE PHOSPHOLIPID BILAYER? Gases: N2, O2, CO2 Yes Hydrophobic/nonpolar molecules Yes (e.g. large hydrocarbons) Small polar molecules (e.g. H2O, glycerol, urea) Yes Large polar molecules (e.g. glucose & other uncharged mono & disaccharides) No Ions/electrically charged molecules (e.g. AA’s, H+, HCO3-, Na+, K+, No Ca2+, Cl-, Mg2+)  MOVING SUBSTANCES ACROSS THE MEMBRANE Transport across a membrane can be either Passive – requiring no added energy, or Active – requiring energy (ATP or a coupled electrochemical gradient) PASSIVE TRANSPORT The simplest type of passive transport is diffusion. Diffusion occurs when a substance from an area of high concentration moves down its concentration gradient. In membranes this occurs through the lipid bilayer. Net movement ceases once equilibrium is achieved. (credit: modification of work by Mariana Ruiz Villareal)  FACTORS THAT AFFECT DIFFUSION RATES Concentration gradients - Greater difference, faster diffusion Mass of the molecules - Smaller molecules diffuse more quickly Temperature - Molecules move faster when temperatures are higher Surface area – increased surface area speeds up diffusion rates Pressure – in some cells (i.e. kidney cells), blood pressure forces solutions through membranes, speeding up diffusion rates FACILITATED PASSIVE TRANSPORT Facilitated transport, aka facilitated diffusion, moves substances down their concentration gradients through transmembrane, integral membrane proteins. Ions and small polar molecules diffuse this way Two types of facilitated transport proteins: Channel proteins Carrier proteins CHANNEL PROTEINS The top, bottom, and inner core of channel proteins are composed of hydrophilic amino acids: attract ions &/or polar molecules Some are open all the time Others are gated, only opening when a signal is received (credit: modification of work by Mariana Ruiz Villareal)  CARRIER PROTEINS All carrier proteins are specific to a single substance Bind to that substance, change shape & “carry it” to the other side. Many allow movement in either direction, as concentration gradients change Important example: Glucose transport proteins (GLUTS) (credit: modification of work by Mariana Ruiz Villareal)  OSMOSIS Osmosis is the diffusion of water across a membrane. Water always moves from an area of higher water concentration to one of lower water concentration. Differences in water concentration occur when a solute cannot pass through the selectively permeable membrane.  OSMOSIS The circles represent solute molecules. The red dotted line represents a membrane permeable to water but not solute. Which side of this beaker has a greater concentration of water? In which direction will water diffuse? TONICITY Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. When solutions are separated by a membrane permeable to water but not the solute, water moves through the membrane and down its concentration gradient. Hypertonic, isotonic, and hypotonic describe the osmolarity of the cell to that of its extracellular fluid. TONICITY A hypertonic extracellular fluid has lower osmolarity than the cytosol – water leaves the cell. An isotonic extracellular fluid has the same osmolarity than the cytosol – water does not move. A hypotonic extracellular fluid has higher osmolarity than the cytosol – water enters the cell. Animal cells function best when extracellular fluids are isotonic. (credit: Mariana Ruiz Villareal) OSMOREGULATION Organisms whose cells have cell walls (such as plants, fungi, and bacteria) prefer hypotonic extracellular solutions. The pressure exerted by the plasma membrane against the cell wall (turgor pressure) is critical to organismal growth & functions. Hypertonic solutions causes plasmolysis – plasma membrane detaches from the cell wall (credit: modification of work by Mariana Ruiz Villareal) 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 Freshwater protists use contractile vacuoles, to pump water out of their cells 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 5.2 PASSIVE TRANSPORT Learning Objectives You should now be able to do the following: Explain why and how passive transport occurs Understand the osmosis and diffusion processes Define tonicity and its relevance to passive transport 5.3 ACTIVE TRANSPORT Learning Objectives By the end of this section, you will be able to do the following: Understand how electrochemical gradients affect ions Distinguish between primary active transport and secondary active transport 5.3 ACTIVE TRANSPORT Active transport is needed any time an ion or molecule is transported through a membrane protein: against its concentration gradient (from low to high concentration) or against its electrochemical gradient (ex. H+ ions to a solution that is more positive). Energy is always required for active transport. Two types of active transport Primary – where ATP provides the energy Secondary – where an electrochemical gradient provides the energy  ELECTROCHEMICAL GRADIENTS Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. An electrical gradient, where the cytoplasm contains more negatively charged molecules (more neg ions & proteins) than the extracellular fluid, is critical for proper cell functioning. (credit: “Synaptitude”/Wikimedia Commons) ELECTROCHEMICAL GRADIENTS (credit: “Synaptitude”/Wikimedia Commons) PROTON GRADIENT CARRIER PROTEINS (credit: modification of work by “Lupask”/ Wikimedia Commons) Active transport occurs through transmembrane, integral carrier proteins called pumps. There are 3 types of pumps. Uniporter carries one molecule or ion Symporter carries two different molecules or ions, in the same direction Antiporter carries two different molecules or ions, in different directions  PRIMARY ACTIVE TRANSPORT Primary active transport moves an ion or molecule up its concentration gradient using energy from ATP hydrolysis. (credit: modification of work by Mariana Ruiz Villareal) Example – Na+-K+ pump Moves 3 Na+ out and 2 K+ in using 1 ATP Questions: Is this pump a uniporter, symporter or antiporter? This pump is an electrogenic pump. What do you think that means? SECONDARY ACTIVE TRANSPORT Secondary active transport uses an electrochemical gradient created by primary active transport to move a different substance against its concentration gradient. (credit: modification of work by Mariana Ruiz Villareal) Many amino acids and glucose enter the cell this way. Question: How can this be called active transport when ATP is not used? 5.3 ACTIVE TRANSPORT Learning Objectives You should now be able to do the following: Understand how electrochemical gradients affect ions Distinguish between primary active transport and secondary active transport 5.4 BULK TRANSPORT Learning Objectives By the end of this section, you will be able to do the following: Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis Understand the process of exocytosis 5.4 BULK TRANSPORT Sometimes cells need to import or export molecules/particles that are too large to pass through a transport protein. Bulk transport is a type of active transport. Energy is required. 5.4 BULK TRANSPORT Importing by bulk transport is called endocytosis and exporting is called exocytosis. There are types of endocytosis: Phagocytosis Pinocytosis Receptor mediated endocytosis ENDOCYTOSIS In phagocytosis (cellular eating), the cell membrane surrounds a particle and engulfs it. In pinocytosis (cellular drinking), the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. In receptor-mediated endocytosis, uptake of a specific substance is targeted by binding to receptors on the external surface of the membrane.  PHAGOCYTOSIS Video of phagocytosis: https://www.youtube.com/ watch?v=hacbn_xcZdU (credit: modification of work by Mariana Ruiz Villareal) PINOCYTOSIS (credit: modification of work by Mariana Ruiz Villareal) RECEPTOR-MEDIATED ENDOCYTOSIS (credit: modification of work by Mariana Ruiz Villareal) EXOCYTOSIS In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents are then released to the exterior of the cell. (credit: modification of work by Mariana Ruiz Villareal) 5.4 BULK TRANSPORT Learning Objectives You should now be able to do the following: Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis Understand the process of exocytosis