Functions<br />Protects the cell<br />Semi-permeable barrier: selects incoming and outgoing substances (passive/active transport)<br />Maintain concentration of various substances<br />Allows cell recognition/communication<br />Allows receptivity (hormones, etc)<br />Allows the cell to maintain its shape<br />Helps compartmentalize subcellular domains<br />Phospholipids are a major component of cell membranes.<br />
Fluid Mosaic Model (1972)<br />- Phospholipid bilayer with proteins forming a mosaic pattern as they float/penetrate through the membrane<br />Hydrophilic end: phosphate = polar<br />Hydrophobic end: fatty acid = non-polar<br />
Transport through membranes<br />Active: ATP needed / proteins are used as pumps to move substances against the concentration gradient.<br />Passive: no ATP needed / small particles cross the membrane through special proteins or lipid bilayer<br />
Passive Transport<br />Diffusion: <br />movement of particles through the lipid bilayer <br />from an area of high concentration to an area of low concentration <br />O2, H2O, urea, glycerol, CO2<br />Facilitated Diffusion: <br />diffusion of particles through special transport proteins <br />charged molecules (even small ones) and large uncharged molecules<br />Na+, K+, glucose<br />
Passive Transport<br />Osmosis: diffusion of water across a semi-permeable membrane: plasma membrane permeable to water but not to solute <br />Solute = dissolved particle <br />Solvent = liquid medium in which particles may be dissolved<br /><ul><li>Movement:
from low solute concentration to high solute concentration</li></li></ul><li>Credit: Stephen Taylor<br />
Solutions can be...<br /><ul><li>Isotonic: The solutions being compared have equal concentration of solutes.
Hypertonic: The solution with the higher concentration of solutes.
Hypotonic: The solution with the lower concentration of solutes.
http://cc.hccs.edu/Biology/AllStudyPages/Diffusion_Osmosis/Elodeagif.swf</li></li></ul><li>Active Transport<br />Movement across membrane against concentration or electrochemical gradient (from low to high concentrations )<br />Requires:<br />specific integral membrane proteins <br />requires energy to overcome the concentration and electrochemical gradient <br />It is used to pump specific compounds in or out of the cell <br />Example: sodium, potassium, calcium<br />It allows cells to accumulate needed substances even when the concentration is lower outside<br />
The K+/Na+ Pump: An example of Active Transport<br />Essential for the nerve impulse to be produced<br />The pump is an integral membrane protein<br />Cellular [K+] is high and [Na+] is low<br />Protein pumps K+ in and Na+ out<br />This pump works independent of concentration gradient <br />K+/Na+ Pump<br />