The Cell Membrane Usman Sumo Friend Tambunan Arli Aditya Parikesit Bioinformatics Group Faculty of Mathematics and Science University of Indonesia Overview • Cell membrane separates living cell from nonliving surroundings • thin barrier = 8nm thick • Controls traffic in & out of the cell • selectively permeable • allows some substances to cross more easily than others • hydrophobic vs hydrophilic • Made of phospholipids, proteins & other macromolecules Phospholipids Phosphate • Fatty acid tails • hydrophobic • Phosphate group head Fatty acid • hydrophilic • Arranged as a bilayer Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads More than lipids… • In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Transmembrane proteins Peripheral protein Cytoplasm Filaments of cytoskeleton Membrane fat composition varies • Fat composition affects flexibility • membrane must be fluid & flexible • about as fluid as thick salad oil • % unsaturated fatty acids in phospholipids • keep membrane less viscous • cold-adapted organisms, like winter wheat • increase % in autumn • cholesterol in membrane Membrane Proteins • Proteins determine membrane’s specific functions • cell membrane & organelle membranes each have unique collections of proteins • Membrane proteins: • peripheral proteins • loosely bound to surface of membrane • cell surface identity marker (antigens) • integral proteins • penetrate lipid bilayer, usually across whole membrane • transmembrane protein • transport proteins • channels, permeases (pumps) Proteins domains anchor molecule • Within membrane Polar areas of protein • nonpolar amino acids • hydrophobic • anchors protein into membrane • On outer surfaces of membrane • polar amino acids • hydrophilic • extend into extracellular fluid & into cytosol Nonpolar areas of protein H+ Retinal Examples chromophore NH2 water channel in bacteria Porin monomer b-pleated sheets Bacterial Nonpolar outer (hydrophobic) COOH membrane a-helices in the Cytoplasm cell membrane H+ proton pump channel in photosynthetic bacteria function through conformational change = shape change Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Enzyme Cell surface activity receptor Cell surface Cell adhesion Attachment to the identity marker cytoskeleton Membrane carbohydrates • Play a key role in cell-cell recognition • ability of a cell to distinguish one cell from another • antigens • important in organ & tissue development • basis for rejection of foreign cells by immune system The Cell Membrane Active and Passive Transport Cell Membrane • Phospholipid bilayer: hydrophilic heads and hydrophobic tails • Semi-permeability • Transport proteins (passive transport channels) • Ion pumps (active transport pumps) • Receptor proteins (neurons, hormones, immune system) • Carbohydrate chains ( identification cards) •What is passive transport? What are the three types of passive transport? Passive Transport • Diffusion - process by which molecules tend to move from an area where they are more concentrated to an area where they are less concentrated Passive Transport • In Passive Transport - Molecules move down the concentration gradient (no energy required) • 1) simple diffusion -molecules are small enough and soluble can pass directly through the lipid bilayer • 2) facilitated diffusion – need transport proteins (molecules are either to large or can’t pass through the lipid bilayer themselves) • 3) osmosis – molecules can’t pass through lipid bilayer at all, but water can(movement of water) • Depends on molecule size, lipid solubility, and concentration gradient Passive Diffusion • http://www.youtube.com/watch?v=JShwXBWGMyY • http://www.youtube.com/watch?v=s0p1ztrbXPY Solutions • A mixture of solid particles (solute) and liquid (solvent) • Most common solvent is water • Cells are surrounded by solutions • Three types: isotonic, hypotonic, and hypertonic solutions Isotonic Solutions • Particles are equal inside and outside of the cell • Water molecules are equal inside and outside of the cell Hypotonic Solutions • There are more particles inside than outside • There is more water outside the cell • Water will move INTO the cell Hypertonic Solutions • There are more particles outside • There is more water inside • Water will move OUT OF the cell Active Transport • Molecules move against the concentration gradient • Requires energy • 1. Solute Pumps (Na+-K+-ATPase pump) • 2. Bulk Transport • Na+-K+-ATPase pump • https://mail.xavierhs.org/exchange/UngerJ/Inbox/No%20Subject-748.EML/07_16ActiveTransport_A.swf/C58EA28C-18C0-4a97-9AF2-036E93DDAFB3/07_16ActiveTransport_A.swf?attach=1 Transport Across The Membrane • Endocytosis (pinocytosis, phagocytosis): phagocytes (macrophages)- molecules are coming into cells • Exocytosis: release of proteins, hormones, neurotransmitters- release of proteins, hormones, neurotransmitters • http://www.youtube.com/watch?v=4gLtk8Yc1Zc&NR=1 Transport summary simple diffusion facilitated diffusion active ATP transport How about large molecules? • Moving large molecules into & out of cell • through vesicles & vacuoles • endocytosis • phagocytosis = “cellular eating” • pinocytosis = “cellular drinking” • exocytosis exocytosis Endocytosis fuse with phagocytosis lysosome for digestion pinocytosis non-specific process triggered by receptor-mediated molecular endocytosis signal The Special Case of Water Movement of water across the cell membrane 2007-2008 Osmosis is diffusion of water • Water is very important to life, so we talk about water separately • Diffusion of water from high concentration of water to low concentration of water • across a semi-permeable membrane Concentration of water • Direction of osmosis is determined by comparing total solute concentrations • Hypertonic - more solute, less water • Hypotonic - less solute, more water • Isotonic - equal solute, equal water water hypotonic hypertonic net movement of water Managing water balance • Cell survival depends on balancing water uptake & loss freshwater balanced saltwater Managing water balance • Isotonic • animal cell immersed in mild salt solution • example: blood cells in blood plasma • problem: none • no net movement of water • flows across membrane equally, in both directions • volume of cell is stable balanced Managing water balance • Hypotonic • a cell in fresh water • example: Paramecium • problem: gains water, swells & can burst • water continually enters Paramecium cell • solution: contractile vacuole ATP • pumps water out of cell • ATP • plant cells • turgid freshwater Water regulation • Contractile vacuole in Paramecium ATP Managing water balance • Hypertonic • a cell in salt water • example: shellfish • problem: lose water & die • solution: take up water or pump out salt • plant cells • plasmolysis = wilt saltwater 1991 | 2003 Aquaporins • Water moves rapidly into & out of cells • evidence that there were water channels Peter Agre Roderick MacKinnon John HopkinsRockefeller Osmosis… .05 M .03 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic Which way does the water flow? in or out of cell References • http:// bioap.wikispaces.com/file/view/Ch%207%20Cell%20Membrane.ppt • http:// www.xavierhs.org/s/717/images/editor_documents/faculty/tanzmanj/Cell%20Membrane%20Transport.ppt • http:// www.lf3.cuni.cz/opencms/export/sites/www.lf3.cuni.cz/en/applicant/premedical/study-materials/biology/8_Premedical_Cell_communication.ppt.