biological membranes

• why organisms need membranes

  -control entry and exit of substances

  -separate cell contents from outside cell

  -separate cell components from cytoplasm

  -cell recognition and cell signalling

  -holding the components of metabolic reactions in place
• partially permeable


  -cell membranes that allow water and other solutes to cross

  -bilayer- 2 layers of phospholipids and proteins embedded in this layer
• differentiation


  -bilayer itself is incapable of all functions- too fragile

  -other components needed

  -number and type of components vary- part of differentiation for cell
• stability and fluidity


  -cholesterol gives membranes in some eukaryotic cells some stability

  -fits between fatty acid tails- barrier more complete

  -this means water and ions can't pass through easily
• communication


  -receptor sites- some allow hormones to bind to cell to allow a cell response to be carried out

  -cell can respond to hormone only if it has specific receptor on its plasma membrane

  -glycolipids and glycoproteins may be involved- allow recognition by the immune system

  -glycoproteins can bind together in tissues
• metabolic processes


  -enzymes and co- enzymes

  -some reactions of photosynthesis happen in membranes of chloroplasts

  -respiration- membranes in mitochondria

  -enzymes and co- enzymes may bind to these membranes

  -more membrane there is, the more enzymes it can hold
• more cholesterol means a decreased permeability
• proportion of saturated and unsaturated fatty acids affect permeability
• temperature


  -increased temperature means increased fluidity and increased permeability- to a point

  -bilayer can begin to melt and proteins begin to denature

  -water expands and puts pressure on plasma membrane
• temperature on membranes


  -high temperatures denature proteins on membrane and phospholipid molecules have more kinetic energy so move apart
• ph on membranes


  -changes tertiary structure of membrane proteins
• use of solvents on membranes


  -may dissolve membrane
• role of membranes within cells


  -surround organelle and separate it from cytoplasm

  -site of chemical reactions

  -provide attachment sites for enzymes
• importance of membrane on rough endoplasmic reticulum


  -hold ribosomes in place

  -separate synthesised proteins from cytoplasm
• partially permeable membrane


  -membrane with holes small enough for only water molecules to go through it

  -large molecules like glucose cannot fit
• why is it called the fluid mosaic model?

  -phospholipid molecules can move freely laterally and makes membrane fluid

  -proteins are distributed throughout the membrane unevenly and in a mosaic pattern

  -agreed structure is based off chemical and experimental evidence- model
• high temperatures on membrane permeability

  -increased fluidity

  -more kinetic energy

  -phospholipids become less tightly packed

  -increase in permeability
• lower temperatures on membrane permeability

  -decreased fluidity

  -less kinetic energy

  -decreased permeability- phospholipids closely packed
• organisms in cold environment need more unsaturated fatty acid chains in the bilayers- increases membrane fluidity
• affect of solvents on membrane permeability

  -organic solvents- dissolve phospholipids in the bilayer

  -loses its structure

  -more permeable

  -different solvents have different effects

  -increase in solvent, increase in membrane permeability
• ways membrane permeability is affected

  -cholesterol concentration

  -proportion of unsaturated fatty acid tails in the bilayer

  -solvents

  -temperature