5CELL MEMBRANECELL MEMBRANE - is a 2-dimensional fluid mosaic of lipids and proteins and also is in constant motion.- It’s a thin, elastic structure, ~7.5 nanometers thick and envelops the cell.- Composed mainly of proteins and lipids.- Basic structure is a lipid bilayer - a thin film of ~ 2 molecules thick and continuous over the entire cell surface.
62. CHOLESTEROL 3. GLYCOLIPID. CELL MEMBRANE Lipid bilayer is made of 3 types of molecules ;1. PHOSPHOLIPIDS2. CHOLESTEROL3. GLYCOLIPID.
8iii) GLYCOLIPIDs -are lipids with attached carbohydrate groups ~ 5% of membrane lipids- Non-polar parts - are the fatty acid “tails”Polar parts - are the attached carbohydrate groupsOnly appear on membrane layer that faces extra-cellular fluid (one reason for the asymmetry of the two sides).
9Asymetrical Distribution of Lipids ExtracellularIntracellularModified from Figure 11-17, Page 355 from: Essential Cell Biology by Alberts et al. 1997, Garland Publishing Inc. New York, NY
10Cholesterol can fill gaps between phospholipids Saturated FA’s = increase in fluidityModified from Figure and Panel 2-4 from: Essential Cell Biology: An introduction to the Molecular Biology of the Cell by Alberts, Bray, Johnson, Lewis, Raff, Roberts and Walter 1997, Garland Publishing Inc. New York, NY
13MEMBRANE PROTEINS 1. PERIPHERAL PROTEINS Are membrane proteins that are located on the periphery of membranes and they are either on the cell surface or on the inside of cellThey associate with membrane lipids or integral proteins at inner or outer surface of the membrane.They can be stripped away from membrane by methods that do not disrupt membrane integrityFUNCTION1. As enzymes on cell surfaces2. As regulatory portions of ion channels and transmembrane receptors3. Roles in cell signaling – by some reversible attachment of proteins oncell surface
182. SELECTIVE PERMEABILITY PROPERTIES OF CELL MEMBRANES2 main properties are;1. FLUIDITY2. SELECTIVE PERMEABILITY
19PROPERTIES OF CELL MEMBRANES 1. FLUIDITY – (2-dimensional fluid in constant motion)SIGNIFICANCE :Allows for fusion of membranes (e.g. fusion of vesicles with organelles)Allows for diffusion of new lipids and new proteins laterally, so they are equally distributionAllows for diffusion of proteins and other molecules laterally across the membrane in signaling/reactionsAllows for proper separation of membranes during cell division
20MEMBRANE FLUIDlTY is determined by; PROPERTIES OF CELL MEMBRANES - FLUIDITYMEMBRANE FLUIDlTY is determined by;1. LIPID COMPOSITION2. TEMPERATURE
24PROPERTIES OF CELL MEMBRANES - FLUIDITY 2. TEMPERATUREOrganisms regulate lipid composition (thus membrane fluidity) in response to temperature.Cold/ low temperatures – membranes “gel” and are not fluid.Hot/ high temperature – membranes are too fluid and become “leaky” allowing ions to cross.Hibernating animals incorporate more unsaturated fatty acids (fatty acids with double bonds) to prepare for drop in their body temperature .
25PROPERTIES OF CELL MEMBRANES - SELECTIVE PERMEABILITY SELECTIVE PERMEABILITY OF MEMBRANE – they are selective for the movement of molecules across the membrane.MOVEMENT OF MOLECULE across membrane is limited by;i) SIZE - small molecules cross membrane and large molecules do notE.g. Water, O2, CO2, ethanol (46 MW) and glycerol (92 MW) can cross the membrane- Glucose (180 MW) can NOT cross membraneii) POLARITY - Hydrophobic molecules can “dissolve” in the lipid bilayer, not polar molecules.E.g. Ethanol is more hydrophobic than glycerol so crosses membrane fasteriii) IONIC CHARGE-– membranes are highly impermeable to ionsBut ions and large molecules do pass across biologicalmembranes – through proteins that pass through membranes (integral proteins – channel/transport protein)
36Ion Channels -small aqueous holes Propertiesselectivefastpassivegated - open or closed
37Ion Channels -small aqueous holes Propertiesselectivefastpassivegated - open or closed
38CHANNELS The pore in some channels can be opened or closed. Opening/ closing of channels are controlled/gated by a specific stimulus.Example of a specific stimulus:i) Voltage; - VOLATGE - GATED CHANNELii) Ligand; - LIGAND -GATED CHANNELSiii) Specific stress; - STRESS – ACTIVATED CHANNELS
39VOLTAGE - GATED CHANNEL Example;Na+ voltage gated channels opens when the membrane potentialdepolarizes (i.e. becomes more positive). It has activation and inactivationgates.
40LIGAND- GATED CHANNELBinding of a chemical (ligand) to a specific site on the receptor causes a change in membrane potential and causes it to allow a specific ion to pass through the channel in the membrane.
41STRESS - ACTIVATED GATED ION CHANNEL The channels open/close when a physical stress is applied to the channel proteinE.g. Auditory hair cells converts a physical stress to an electrical signal.
64(extracellular fluid) pinocytosis(extracellular fluid)1332vesicle containingextracellularfluid(cytoplasm)cell(b)Figure: 03-07Title:Two types of endocytosis.Caption:(a) To capture drops of liquid, a dimple in the plasma membrane deepens and eventually pinches off as a fluid-filled vesicle, which contains a random sampling of the extracellular fluid. (b) Pseudopodia encircle an extracellular particle. The ends of the pseudopodia fuse, forming a large vesicle that contains the engulfed particle.phagocytosisfood particlepseudopod123particleenclosed in vesicle
65Endocytosis and Exocytosis: VacuoleTransport Figure 5-28: Receptor-mediated endocytosis and exocytosis