Plasma Membrane

• Fluid Mosaic Model


  the fluid mosaic model of the plasma membrane = bilayer of lipid mols w protein mols dispersed in it

  the proteins, many of which float in the fluid lipid bilayer, form a constantly changing mosaic pattern
• Membrane Lipids


  the lipid bilayer forms the basic fabric of the membrane

  it is constructed largely of phospholipids, w smaller amounts of cholesterol
• Phospholipids - Phosphate Heads


  polar hydrophilic phosphate heads - attracted to water

  nonpolar hydrophobic fatty acid tails - repel from water

  the phosphate heads r the main constituent of both the intracellular + extracellular fluids

  as a result they lie on both the inner + outer surfaces of the membrane
• Phospholipids - Fatty Acid Tails


  the nonpolar hydrophobic tails avoid water + line up in the centre of the membrane

  
• Phospholipid Bilayer


  2 parallel sheers of phospolipid mols lying tail to tail, w their polar head bathed in water on either side of the membrane

  
• Cholesterol


  20% of membrane lipid is cholesterol

  like phospholipids, cholesterol has a polar region + a nonpolar region

  it wedges its platelike hydrocarbon rings between the phospholipid tails which stiffens the membrane

  
• Membrane Proteins


  a cell's plasma membrane bristles w proteins that allow it to communicate w its enviro

  proteins make up half of the plasma emebrane by mass + r responsible for most of the specialised membrane functions

  some proteins drift freely along the membrane surface

  others r tethered to intra/extracellular strucutres + restricted in movement
• Integral Proteins


  intergral proteins r firmly inserted into the lipid bilayer

  most r transmembrane proteins that span the entire membrane + protrude on both sides

  all integral proteins have both hydrophobic + hydrophilic regions

  this structural feature allows them to interact w both the nonpolar lipid tails buried in the membrane + the water inside + outside the cell
• Channel Proteins


  some transmembrane proteins r involved in transport + form channels

  small, water-soluble mols/ions can move thru these pores, bypassing the lipid part of the membrane
• Carrier Proteins


  other transmembrane proteins act as carriers that bind to a substance + then move it thru the membrane

  they hydrolyse ATP as an energy source to actively pump substances across the membrane
• Peripheral Proteins


  unlike integral proteins, peripheral proteins r not embedded in the lipd bilayer

  instead they either attach loosely to integral proteins/have a hydrophobic region that anchors them into the membrane
• Peripheral Proteins - Structural Support


  peripheral proteins inc a network of filaments that help support the membrane from its cytoplasmic side

  others r motor proteins involved in mechanical functions, like changing cell shape during cell division + muscle cell contraction
• Simple Diffusion


  substancs diffuse directly thru the lipid bilyaer

  such sunbstances r usually small nonpolar mols that readily dissolves in lipds (lipid soluble)

  these inc gases like O2 + CO2, steroid hormones + fatty acids
• Facilitated Diffusion


  certain mols, notably glucoses + other sugars, some amino acids + ions r transported passively even tho they r inable to pass thru the lipid bilayer

  instead they move thru the membrane by a passive transport process = facilitated diffusion in which the transported substance either binds to carrier proteins in the membrane + is ferried across/ moves thru water-filled channel proteins
• Carrier-Mediated Facilitated Diffusion


  carriers r transmembrane proteins that r specific for transporting certain polar mols or classes of mols, like sugars + amino acids that r too large to pass thru membrane channels

  alterations in the shape of the carrier allow it to first envelop + then release the transported substance, allowing it to by pass the nonpolar regions of the memsbrane
• Channel-Mediated Facilitated Diffusion


  channels r transmembrane proteins that transport substances, usually ions/water thru aqueous channels from one side of the membrane to the other

  channels r selective due to pore size + the charges of the amino acids lining the pore
• Leakage Channels


  leakage channels r always open + simply allow ions or water to move according to concentration gradients
• Gated Channels


  gated channels r controlled usually by chemical/electrical signals

  like carriers, many channels can be inhibited by certain mols. show saturation + tend to be specific