Membranes and Receptors: Lecture Notes PDF

Membranes and Receptors Session 2 LECTURE 2.1 ROLE OF MEMBRANES AS PERMEABILITY BARRIERS Dr. Ahmed Faisal Obed  Main objectives 1- describe the role membrane as permeability barriers to small hydrophilic molecules. 2- explore the protein mediated mechanism that allow the uptake...

Membranes and Receptors Session 2 LECTURE 2.1 ROLE OF MEMBRANES AS PERMEABILITY BARRIERS Dr. Ahmed Faisal Obed  Main objectives 1- describe the role membrane as permeability barriers to small hydrophilic molecules. 2- explore the protein mediated mechanism that allow the uptake and extrusion of specific water molecules and ions Body fluids Intracellular Extracellular Intravascular Interstitial  The Cell Membrane 2007-2008  Phospholipids Phosphate “attracted to water” Phosphate head – hydrophilic Fatty acid tails Fatty acid – hydrophobic Arranged as a bilayer “repelled by water” Aaaah, one of those structure–function examples  Arranged as a Phospholipid bilayer Serves as a cellular barrier / border sugar H2O salt polar hydrophilic heads nonpolar hydrophobic impermeable to polar molecules tails polar hydrophilic heads waste lipids Cell membrane defines cell Cell membrane separates living cell from aqueous environment – thin barrier = 8nm thick Controls traffic in & out of the cell – allows some substances to cross more easily than others hydrophobic (nonpolar) vs. hydrophilic (polar)  Permeability to polar molecules? Membrane becomes semi- permeable via protein channels – specific channels allow specific material across cell membrane inside cell H2O aa sugar NH3 salt outside cell Cell membrane is more than lipids… Transmembrane proteins embedded in phospholipid bilayer – create semi-permeabe channels lipid bilayer protein channels membrane in lipid bilyer membrane  + H H Examples + Retinal chromophore NH2 aquaporin = water channel in bacteria Porin monomer H2O -pleated sheets Bacterial Nonpolar outer (hydrophobic) COOH membrane -helices in the cell membrane H+ Cytoplasm H + proton pump channel in photosynthetic bacteria function through conformational change = H2O protein changes shape Movement across the Cell Membrane 2007-2008  Diffusion 2nd Law of Thermodynamics governs biological systems  Diffusion  movement from HIGH LOW concentration  Simple Diffusion Move from HIGH to LOW concentration – “passive transport” – no energy needed movement of water diffusion osmosis  Facilitated Diffusion Diffusion through protein channels – channels move specific molecules across cell membrane facilitated = with help – no energy needed open channel = fast transport HIGH LOW “The Bouncer” Figure 7.17 EXTRACELLULAR FLUID (a) A channel protein Channel protein Solute CYTOPLASM Carrier protein Solute (b) A carrier protein  Active Transport Cells may need to move molecules against concentration gradient – conformational shape change transports solute from one side of membrane to other – protein “pump” – “costs” energy = ATP LOW conformational change ATP HIGH “The Doorman” Two types of active transport Primary the energy is derived directly from breakdown of adenosine triphosphate (ATP) or of some other high-energy phosphate compound. Secondary the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport. In both instances, transport depends on carrier proteins that penetrate through the cell  Carrier proteins used in active transport include – Uniporters – move one molecule at a time – Symporters – move two molecules in the same direction – Antiporters – move two molecules in opposite directions – Terms can also be used to describe facilitated diffusion carriers 19  Active transport Many models & mechanisms ATP ATP antiport symport  Uniport pore One type of molecule transported Change of configuration P P P Phosphorylation Dephosophorylation ATP + H2O ADP + Pi © 2016 Paul Billiet ODWS  Coupled pores Two molecules transported together Symport: Both molecules move in the same direction Change of configuration P P P Phosphorylation Dephosophorylation © 2016 Paul Billiet ODWS ATP + H2O ADP + Pi  Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of other solutes Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell © 2011 Pearson Education, Inc. Examples of Cotransport systems Na+- glucose co-transport system of the small intestine and kidney (symport). Entry of sodium provides the energy for the entry of glucose. Na+- Ca2+ - exchange - Inward flow of sodium down its concentration gradient drives outward flow of Ca2+ up its concentration gradient (antiport).  Getting through cell membrane Passive Transport – Simple diffusion diffusion of nonpolar, hydrophobic molecules – lipids – HIGH LOW concentration gradient – Facilitated transport diffusion of polar, hydrophilic molecules through a protein channel – HIGH LOW concentration gradient Active transport – diffusion against concentration gradient LOW HIGH – uses a protein pump ATP – requires ATP Transport summary simple diffusion facilitated diffusion active ATP transport  1991 | 2003 Aquaporins Water moves rapidly into & out of cells – evidence that there were water channels protein channels allowing flow of water across cell membrane Peter Agre Roderick MacKinnon John Hopkins Rockefeller