Membrane Structure and Function Chapter 7

Plasma Membrane  The boundary that separates the living cell from its nonliving surroundings.  Surrounds the cell and controls chemical traffic into and out of the cell.  Selectively permeable: allows some substances to cross more easily than others.

Phospholipid Bilayer  Polar (hydrophilic) heads of phospholipids are oriented towards the outside of the membrane.  The nonpolar (hydrophobic) tails are oriented towards the inside of the membrane, away from the cell contents and the outside environment.  Proteins are imbedded in the bilayer.

Fluid Mosaic Model  Membranes must be fluid to work properly.  Membranes are held together by weak hydrophobic interactions.  Lipids and proteins drift and bob within the bilayer.  Membranes can solidify if the temperature drops too low. Critical temperature is lower in membranes with a greater concentration of unsaturated phospholipids.

Fluid Mosaic Model Cont.  Unsaturated hydrocarbon tails enhance membrane fluidity, because kinks in the hydrocarbon tails hinder close packing of phospholipids.  Cholesterol, found in plasma membranes of eukaryotes, makes the membrane: Less fluid at warmer temperatures by restraining phospholipid movement. Less fluid at warmer temperatures by restraining phospholipid movement. More fluid at lower temperatures by preventing close packing of phospholipids. More fluid at lower temperatures by preventing close packing of phospholipids.

Fluid Mosaic Model Cont.  A membrane is a mosaic of different proteins embedded and dispersed in the phospholipid bilayer.  1. Integral proteins: inserted into the membrane so their hydrophobic regions are surrounded by hydrocarbon portions of phospholipids. They may be: Unilateral, reaching only partway across the membrane. Unilateral, reaching only partway across the membrane. Transmembrane, with hydrophobic midsections between hydrophilic ends exposed on both sides of the membrane. Transmembrane, with hydrophobic midsections between hydrophilic ends exposed on both sides of the membrane.  2. Peripheral proteins: not embedded but attached to the membrane's surface. May be attached to integral proteins or held by filaments of cytoskeleton.

Fluid Mosaic Model Cont.  Additional cell markers (carbohydrates) are found on the external surface of the plasma membrane.  Usually branched oligosaccharides (< 15 monomers).  Some covalently bonded to lipids (glycolipids).  Most covalently bonded to proteins (glycoproteins).

Fluid Mosaic Model Cont.  Cell-cell recognition -- Ability of a cell to determine if other cells it encounters are alike or different from itself.  Cell-cell recognition is crucial in the functioning of an organism.  Sorting of an animal embryo's cells into tissues and organs.  Rejection of foreign cells by the immune system.  Variation from species to species, between individuals of the same species, and among cells in the same individual.

Inner Life of the Cell Inner Life of the Cell

Permeability of the Membrane  Nonpolar (Hydrophobic) Molecules  Dissolve in the bilayer and cross it with ease. (hydrocarbons, O 2 ).  Smaller molecules will cross the membrane faster.  Polar (Hydrophilic) Molecules  Small, polar molecules (H 2 O, CO 2 ) are small enough to pass slowly between membrane lipids.  Larger, polar molecules (glucose) will not easily pass through.  Ions (Na+, H+) have difficulty penetrating the hydrophobic layer due to charge.

Permeability of the Membrane cont  Biological membranes are permeable to specific ions and certain polar molecules of moderate size. These hydrophilic substances avoid the hydrophobic core of the bilayer by passing through transport proteins.  Transport proteins:  May provide a hydrophilic tunnel through the membrane.  May bind to a substance and physically move it across the membrane.  Are specific for the substance they transport.

Passive Movement Across the Membrane  Concentration gradient -- concentration change over a distance in a particular direction.  Diffusion -- The net movement of a substance down a concentration gradient (from area of high conc to area of low conc).  Passive transport – Spontaneous diffusion of a substance across a biological membrane; does not require the cell to expend energy; driven by potential energy stored in a concentration gradient.

Osmosis is the passive transport of water  Osmosis -- Diffusion of water across a selectively permeable membrane from an area of higher water potential to an area of lower water potential.  Hypertonic solution -- (hyperosmotic) A solution with a greater solute concentration than that inside a cell; water will move out from cell and it may shrivel and die.  Hypotonic solution – (hypoosmotic) A solution with a lower solute concentration compared to that inside a cell; water will move into the cell and it may burst.  Isotonic solution – (isosomotic) A solution with an equal solute concentration compared to that inside a cell; equal water flow in both directions.

Water Potential / Osmotic Pressure  Pure water = 0.  The more solute is dissolved, the lower the water potential (negative #).  Equals 0 if both sides of membrane have equal solute concentration.  Greater solute conc (lower water potential) = greater osmotic pressure.

Water Balance of Cells With Walls  Prokaryotes, some protists, fungi and plants have cell walls outside the plasma membrane.  Plasmolysis -- a walled cell shrivels and the plasma membrane pulls away from the cell wall as the cell loses water to a hypertonic environment; usually lethal.  Turgid -- Firmness or tension found in walled cells that are in a hypoosmotic environment. Turgid cells provide mechanical support for plants. Turgid cells provide mechanical support for plants.  In an isotonic environment, there is no net movement of water into or out of the cell; loss of structural support from turgor pressure causes plants to wilt.

Facilitated Diffusion  Movement of solutes across a membrane with the help of transport proteins (permeases).  Transport proteins are specific for the solutes they transport. Specific binding site analogous to an enzyme's active site.  Transport proteins can be inhibited by molecules that resemble the solute normally carried by the protein (similar to competitive inhibition in enzymes).  Uniport Transport Protein – carries a single molecule across.  Symport – moves 2 different molecules in same direction.  Antiport – moves 2 molecules in opposite directions.

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Active Transport  Energy-requiring process during which a transport protein pumps a molecule across a membrane, against its concentration gradient.  Transport proteins involved in active transport harness energy from ATP to pump molecules “uphill”.  An example is the sodium-potassium pump.  ATP powers the shape change in the protein from Na-receptive to K-receptive.  Three Na+ ions out of the cell for every two K+ ions pumped into the cell.

 mplates/student_resources/shared_resour ces/animations/ion_pump/ionpump.html mplates/student_resources/shared_resour ces/animations/ion_pump/ionpump.html mplates/student_resources/shared_resour ces/animations/ion_pump/ionpump.html  hill.com/olcweb/cgi/pluginpop.cgi?it=swf::5 35::535::/sites/dl/free/ / /bio03.swf::Sodium- Potassium%20Exchange%20Pump hill.com/olcweb/cgi/pluginpop.cgi?it=swf::5 35::535::/sites/dl/free/ / /bio03.swf::Sodium- Potassium%20Exchange%20Pump hill.com/olcweb/cgi/pluginpop.cgi?it=swf::5 35::535::/sites/dl/free/ / /bio03.swf::Sodium- Potassium%20Exchange%20Pump

Vesicle-Mediated Transport  Large molecules (proteins and polysaccharides) cross membranes by the processes of exocytosis and endocytosis.  Exocytosis -- Process of exporting materials from a cell; vesicle usually budded from the ER or Golgi and migrates to and fuses with plasma membrane.  Used by secretory cells (insulin from pancreas, or neuro-transmitter from neuron).

Vesicle-Mediated Transport cont Vesicle-Mediated Transport cont  Endocytosis -- Process of importing materials into a cell; vesicle forms from a region of plasma membrane that sinks inward and pinches off.  Phagocytosis -- (cell eating) Endocytosis of solid particles; forms a food vacuole which fuses with a lysosome.  Pinocytosis -- (cell drinking) Endocytosis of fluid droplets.  Receptor-mediated endocytosis – Large molecules attach to specific receptors on the cell's surface; causes vesicle to form around the substance. Membrane-embedded proteins cluster in regions called coated pits. Membrane-embedded proteins cluster in regions called coated pits. Molecule that binds to the receptor site is called a ligand. Molecule that binds to the receptor site is called a ligand.

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Kidney Function (osmoregulation and excretion of nitrogenous wastes – homeostasis)  / cw/content/index.html / cw/content/index.html / cw/content/index.html  Chapter 44  nimations/content/kidney.html nimations/content/kidney.html nimations/content/kidney.html