Chapter 5 Biological Membranes 5 1 The Structure of Biological Membranes biological membranes are made of lipid and protein molecules They regulate the passage of materials divide the cell into compartments serve as sur faces for chemical reactions adhere to communicate with other cells and transmit signals be tween the environment and the interior cell phospholipids are responsible for the physical properties of the biological molecules phospholipids form bilayers because the molecules have 1 two distinct regions one strongly hydrophobic and the other hydrophilic 2 cylindrical shapes that allow them to asso ciate with mater most easily as a bilayer Hugh Daveson and James Danielli proposed the lipid bilayer concept sandwich Plasma membrane is no more than 10 nm thick The Davison Danielli Model In 1972 Jonathan Singer and Garth Nicolson proposed a model of membrane structure that represented a synthesis of know properties of biological membranes fluid mosaic model according to their fluid mosaic model a cell membrane consists of a fluid bilayer of phospholipid molecules in which the proteins are embedded or otherwise associated con stantly changing mosaic pattern Phospholipid bilayers act like liquid crystals The molecules are free to rotate and can move laterally within their single layer This gives the bilayer a property of a two dimensional fluid Organism have regulatory mechanisms for maintaining cell membranes in an optimally fluid state Some organism compensate for temperature changes by altering the fatty acid con tent of their membrane lipids and some lipids stabilize membrane fluidity within certain limits Ex cholesterol biological membranes fuse and form closed vesicles lipid bilayers are flexible two major classes of membrane proteins integral proteins and peripheral proteins integral membrane proteins are firmly bound to the membrane amphipathic peripheral proteins proteins that are located on the inner and outer surface of the plasma membrane usually bound to exposed regions of integral proteins by non covalent interactions transmembrane proteins proteins that extend completely through the membrane membrane protein molecules are asymmetrically oriented 5 2 Overview of Membrane Protein Functions anchoring passive transport active transport enzymatic activity signal transduction cell recognition and intercellular junction by signal transduction information may be transmitted from proteins in the plasma membrane to the cell interior 5 3 Cell Membrane Structure and Permeability selectively permeable membranes biological membranes present a barries to polar molecules the lipid bilayer of the plasma membrane is relatively impermeable to charged ions of any size so ions and most large polar molecules pass through the bilayer slowly transport proteins proteins that move ions amino acids sugars and other needed polar molecules through membranes two main types are carrier proteins and channel proteins carrier proteins transporters bind the ion or molecule and undergo changes in shape resulting in the movement of the molecule across the membrane carrier mediated the transfer of solutes by carrier proteins located within the membrane ABC transporters ATP binding cassette use energy donated by ATP to transport certain ions sugars and polypeptides across the cell membrane About 48 ABC transporters in human cells channel proteins form tunnels called pores which many are gated cells regulate the pas sage of materials through the channels by opening and closing the gates in response to electri cal changes chemical stimuli or mechanical stimuli porins are transmembrane channel proteins that allow various solutes or water to pass through membranes They are rolled up barrel shaped B pleated sheets that form pores Peter Agre identified transmembrane proteins called aquaporins that function as gated water channels aquaporins help prevent dehydration by returning water form the kidney tubules into the blood 5 4 Passive Transport diffusion the net movement of particles atoms molecules or ions from a region of higher concentration to a region of lower concentration resulting from random motion concentration gradient a difference in the concentration of a substance from one point to another osmosis the net movement of water through a selectively permeable membrane from a region of higher concentration to a region of lower concentration isotonic a fluid contains equal solute concentration hypertonic a term referring to a solution having an osmotic pressure or solute concentra tion greater than that of the solution with which it is compared hypotonic a term referring to a solution having an osmotic pressure or solute concentra tion less than that of the solution with which it is compared plasmolysis cell loses water to its surrounding and its contents shrink and the plasma membrane separates from the cell wall facilitated diffusion a specific transport protein makes the membrane permeable to a particular solute channel can facilitate transport only down a concentration gradient facilitated diffusion is powered by the concentration gradient 5 5 Active Transport high concentration sodium potassium pump active transport system can pump materials from a region of low concentration to a region of