* 7 Flashcards
amphipathic
having both a hydrophilic and a hydrophobic region
fluid mosaic model
The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.
lateral movement of phospholipids
- adjacent phospholipids switch positions about 10^7 times per second
- a phospholipid can travel about 2 nm in 1 second
- flip-flopping of phospholipids directly across from each other occurs only about once per month
integral proteins
- penetrate the hydrophobic interior of the lipid bilayer
- majority are transmembrane proteins (span the membrane); others extend only partway into the hydrophobic interior
- the hydrophobic regions of an integral protein consist of 1 or more stretches of nonpolar amino acids, usually coiled into ALPHA HELICES
- some proteins have a hydrophilic channel thru their center that allows passage of hydrophilic substances
peripheral proteins
- not embedded in lipid bilayer at all
- they are appendages loosely bound to the surface, often to exposed parts of integral proteins
membrane carbohydrates
- usually short, branched chains of <15 sugar units
- some are covalently bonded to lipids, forming glycolipids
- most are covalently bonded to proteins (glycoproteins)
channel proteins
- hydrophilic channels
- ex: aquaporins – each one allows entry of up to 3 billion water molecules per second, passing single file thru its central channel, which fits 10 at a time
carrier proteins
- transport proteins that hold onto their passengers and change shape in a way that shuttles them across the membrane
- undergo subtle change in shape that somehow translocates the solute-binding site across the membrane
- such a change may be triggered by the binding and release of the transported protein
- PASSIVE TRANSPORT! DOWN concentration gradient!
tonicity
- The ability of a solution surrounding a cell to cause that cell to gain or lose water.
- depends in part on its concentration on nonpenetrating solutes (can’t cross membrane) relative to that inside the cell
isotonic
same concentration of nonpenetrating solutes inside and outside of the cell
cholesterol and membrane fluidity
- high temps: cholesterol makes the membrane less fluid by restraining phospholipid movement
- b/c cholesterol also hinders the close packing of phospholipids, it lowers the temp required for the membrane to solidify
- cholesterol is a “fluidity buffer” – resisting changes in membrane fluidity that can be caused by changes in temp
osmoregulation
Regulation of solute concentrations and water balance by a cell or organism.
water balance of cells w/ walls
- hypotonic environment: plant cell swells as water enters by osmosis. the inelastic wall will expand only so much before it exerts a back pressure on the cell, called TURGOR PRESSURE, that opposes further water uptake. at this point, the cell is TURGID (very firm; healthy state). nonwoody plants depend on this mechanism for mechanical support
- isotonic environment: no net tendency for water to enter –> cells become flaccid
- hypertonic: cell will lose water to surroundings and shrink; as the cell shrinks, the plasma membane pulls away from the wall. this phenomenon, called plasmolysis, causes wilting and maybe death. (the walled cells of bacteria and fungi also plasmolyze in hypertonic environments).
ion channels
- channel proteins that transport ions
- many ion channels function as GATED CHANNELS
- PASSIVE TRANSPORT! DOWN concentration gradient!
gated channels
- open/close in response to a stimulus
stimuli: - electrical
- when a specific substance other than the one to be transported binds to the channel