Chapter 4 Cell Membranes www.cellsalive.com/ 4.1Cell membrane structure 4.1.1 Model structure 4.1.2 Composition of the Cell Membrane 4.1.3 Fluidity of membrane 4.1.4 Asymmery membrane 4.1.5 Membrane function 4.1.6 Cell cytoskeleton interaction with cellmembrane 4.1.1 stucture of Cell Membrane Every cell is encircled by a membrane and most cells contain an extensive intracellular membrane system. Membranes fence off the cell's interior from its surroundings. Membranes let in water, certain ions and substrates and they excrete waste substances. They act to protect the cell. Without a membrane the cell contents would diffuse into the surroundings, information containing molecules would be lost and many metabolic pathways would cease to work: The cell would die! www.biologie.uni-hamburg.de/b-online/e22/22.htm Cell Membranes: Surround all cells Fluid-like composition…like soap bubbles Composed of: • Lipids in a bilayer • Proteins embedded in lipid layer (called transmembrane proteins) • And, Proteins floating within the lipid sea (called integral proteins) • And Proteins associated outside the lipid bilayer (peripheral). The Cell Membrane The Cell Membrane Liposome 脂质体 artificial lipid vesicle • Model of membrane study • Carrier of drug • Use in Gene treatment Cell Membrane: At Very High Magnification & in color Cell Membrane Plasma membrane Endomembrane system • Nuclear membrane • Endoplasmic reticulum • Vessicles • Golgi apparatus Choloplast Mitochondron 4.1.2 Molecular structure model of cell membrane 1 sandwich model Danielli and Davison,1930 Protein-lipid –protein 2 Robertson,1950 suugest unit membrane model structure of dark-bright-dark under electron microscope 3 Fluid Mosaic Membrane model 1972,Singer and Nicolson suggest two-dimensional liquid, emphasize fluidity,asymmetrical非对称的membrane Fluid Mosaic Membrane Diversity of Molecules make a Mozaic Mozaic Integral proteins embedded in membrane Amphipathic! Hydrophilic Hydrophobic Hydrophilic 4.1.3 Composition of the Cell Membrane CM is composed of A) Lipids B) Proteins C) Carbohydrates 细胞膜的化学组成 Phospholipids A Membrane lipid Glycolipid Cholesterol Phospholipids The chemical nature of the Phosphate-Nitrogen chain, and the small glycerol part of the molecule are polar, while the fatty acid parts of the molecule are non polar CM is essentially a double layer (bilayer) of a type of lipid called a phospholipid The dual nature of the phospholipid molecule, the cell membrane is formed of a double later of these molecules, arranged “tail-to-tail” = phospholipid bilayer Head Tails Head Phospholipid Bilayer H H O H2O O H H H H O hydrophilic hydrophobic H2O amphipathic Organization of the cell membrane relies on water! Membrane Lipids Composed largely of phospholipids Phospholipids composed of….glycerol and two fatty acids + PO4 group P-Lipids are polar molecules… P-Lipids are represented like this Membrane lipids 膜脂 磷脂 糖脂 胆固醇 Membrane Lipids form a Bilayer Outside layer Inside Layer Phospholipid Molecule Model phosphate (hydrophilic亲水的) glycerol fatty acids (hydrophobic疏水的) Quiz If Phospholipids are polar, which end seeks out water and which avoids water? B Membrane Proteins Various shapes and sizes, depending on function in the membrane 2 main kinds: 1) Peripheral Proteins 2) Integral Proteins Membrane Proteins Integral: embedded within bilayer Peripheral: reside outside hydrophobic region of lipids Text pg. 80 Peripheral Proteins Hydrophobic (water-loving) Are attached to inner and outer surface of membrane In contact wit hydrophilic heads of phospholipids and watery interior/exterior of cell Integral Proteins Have both hydrophilic and hydrophobic areas Penetrate completely through or partially into the phospholipid bilayer Hydrophobic areas contact watery interior & exterior of cell Hydrophobic areas are embedded inside the bilayer to avoid contact with water Membrane Proteins Text pg 80 Integral membrane proteins Peripheral membrane proteins Integral Membrane Protein Functions Every cell membrane is a unique, two-sided work surface Control factor of membrane protein movement Cytoskeleton 细胞骨架Protein Congregation 蛋白的聚集 Interlink with extracelluar细胞与外侧结构 的连结 C Carbohydrates Polar – hydrophilic Found attached to outer surface of membrane only Carbohydrate + protein = glycoprotein Carobhydrate + lipid = glycolipid Review of last lesson Which component do take part in forming CM? A) Lipids B) Proteins C) Carbohydrates what is prominent point of every components? two-dimensional liquid, emphasize fluidity, asymmetrical membrane Please give conclusion of the fluid mo model The dual nature of the phospholipid molecule, the cell membrane is formed of a double later of these molecules, arranged “tail-to-tail” = phospholipid bilayer Head Tails Head Phospholipids The chemical nature of the Phosphate-Nitrogen chain, and the small glycerol part of the molecule are polar, while the fatty acid parts of the molecule are non polar CM is essentially a double layer (bilayer) of a type of lipid called a phospholipid How many types Could membrane protein divide into? What is special points of every type membrane protein? 4.1.3 Evidence for the Fluid Mosaic Model (Cell Fusion) Phospholipid Molecule Model phosphate (hydrophilic亲水的) glycerol fatty acids (hydrophobic疏水的) Experimental Observation of Fluidity Fluorescence recovery after photobleaching FRAP) Control of Membrane Fluidity Plants • unsaturated fatty acids • type of fatty acid adjusted seasonally AnimalsCholesterol • buffers membrane at hi temp • prevents membrane from crystalizing at low temp Influence factor of membrane fluidity Fatty acids length Saturation of fatty acids Temperature Cholesterol concentration 4.1.4 Asymmetry of membrane Membrane lipid asymmetry: distribution Membrane protein asymmetry :directional signal 4.1.5 membrane function In essence membranes are essential for the integrity and function of the cell. CM is material exchange gateway of the cell Functions 1) Transport raw materials into the cell from the surrounding Extra Cellular Fluid (ECF) e.g – O2,H2O,glucose,aa 2) Transport manufactured products and wastes out of the cell into the ECF e.g. – hormones, enzymes, CO2 3) Prevent entry of unwanted matter into the cell e.g. – bacteria, poisons 4) Prevents the escape of matter needed within the cell to perform cellular functions e.g. – keep cell parts, needed Membrane Functions be protective ,material exchage gateway regulate transport in and out of cell or organelle allow selective receptivity and signal transduction by providing transmembrane receptors that bind signaling molecules allow cell recognition subdivides cell into compartments with different internal conditions allows for different conditions between inside and outside of cell allows release of substances from cell via vesicle fusion with outer membrane Form special structure on cell surface http://www.emc.maricopa.edu/faculty/farabee/BIOBK/exocyt.gif Membrane function conclusion of text book provide anchoring sites for cytoskeletal components. This allows the cell to maintain its shape and perhaps move to distant sites. provide a stable site for the binding and catalysis of enzymes. regulate the fusion of the membrane with other membranes in the cell via specialized junctions provide a passageway across the membrane for certain molecules allow directed cell or organelle motility Cellular Membranes REVIEW Importance of Membranes Fluid Mosaic model Membrane Structure Membrane function