3.2.3

• All cells are


  surrounded by membranes
• In eukaryotic cells


  many oftheorganellesaresurrounded by membranes
• Cell surface membranes


  surround cells
• Cell Surface Membranes are a barrier


  betweenthecellandits environment—controllingwhich substances enterandleavethecell
• Cell surface membranes are

  partially permeable—let some molecules throughbutnot others
• Substances can move across the cell surface membrane by


  ●diffusion●osmosis●active transport
• The cell - surface membrane sometimes called

  theplasma membrane
• The membranes around organelles


  dividethecellintodifferent compartments— theyactas abarrier betweentheorganelleandthecytoplasm
• Example of membranes within cells

  ● Thesubstances neededforrespiration—like enzymes● arekept together insideamitochondrion bythemembrane surroundingthemitochondrion
• The membranes around organelles are also partially permeable and control


  what substances enterandleavetheorganelle
• Example of membranes around organelles


  ●RNA leavesthenucleus viathenuclear membrane— also called thenuclear envelope●DNAistoo largetopass throughthepartially permeable membrane— soremains inthenucleus
• The basic structure of all cell membranes is that they are composed of


  ●lipids—mainly phospholipids●proteins●carbohydrates—attached to proteinsorlipids
• In 1972 to describe the arrangement of molecules in the membrane


  thefluid mosaic modelwassuggested

  
• In the fluid mosaic model the

  ●phospholipid molecules formacontinuous,double layer— called abilayer●bilayeris 'fluid' — because the phospholipids are constantly moving

  
• In the fluid mosaic model the

  bilayeris 'fluid' —becausethephospholipidsareconstantly moving

  
• bilayer


  double layer

  
• In the fluid mosaic model the

  Proteinsarescattered throughthebilayer like tiles in a mosaic—include channel proteinsandcarrier proteins— whichallow large moleculesandionstopass throughthemembrane

  
• In the fluid mosaic model, the channel proteins and carrier protiens

  allow large moleculesandionstopass throughthemembrane

  
• In the fluid mosaic model : the receptor protiens on the cell surface membrane allow

  celltodetect chemicals released from other cells— thechemicalssignalto thecelltorespondinsome way— e.g thehormone insulin bindstoreceptor proteinsonliver cellswhichtellsthecellstoabsorb glucose

  
• The phospholipid bilayer is about

  7 nm thick

  
• In the fluid mosaic model, some proteins are

  ●able to move sideways throughthebilayerwhileothersarefixed in position●haveacarbohydrate attached— calledglycoproteins●Some lipidsalso havecarbohydrate attached— calledglycolipids●Cholesterol moleculesare alsopresent withinthebilayer

  
• glycoproteins


  proteinsthathaveacarbohydrate attached

  
• glycolipids


  lipidsthathaveacarbohydrate attached

  
• Transport proteins create hydrophilic channels to


  allow ions and polar molecules to travel through the membrane T
• The cell surface membrane creates


  an enclosed space separating the internal cell environment from the external environment and intracellular membranes form compartments within the cell such nucleus mitochondria and RER
• Internal cell membranes


  surrounding organelles e.g. the mitochondria
• Transport proteins There are two types

  channel pore proteins carrier proteins
• Membranes


  separate different areas control the exchange of material across them acting as an interface for communication
• The rate of diffusion across both external and internal cell membranes can


  vary Some specialised cells are adapted for rapid transport across their membranes
• Each transport protein is specific to

  a particular ion or molecule
• Cellular membranes formed from bilayer of phospholipids roughly 7nm wide therefore just


  just visible under an electron microscope at very high magnifications
• The rate of diffusion depends on :

  The concentration gradient - The thickness of the exchange surface The surface area -
• Transport proteins allow the cell to control

  control which substances enter or leave
• fluid mosaic model also helps to explain

  Passive and active movement between cells and their surroundings Cell - to cell interactions Cell signalling
• The rate of diffusion depends on The concentration gradient

  the higher it is the faster the rate of diffusion As diffusion takes place , the difference in concentration between the two sides of the membrane decreases until reaches an equilibrium i.e. the concentration on both sides is equal ) . This means that diffusion slows down over time .
• Membranes become less fluid when there is :

  An increased proportion of saturated fatty acid chains as the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains
• Phospholipids structurally contain two distinct regions

  a polar head and two nonpolar tails
• The rate of diffusion depends on : The thickness of the exchange surface surface

  the thinner the exchange surface i.e. the shorter the distance the particles have to travel the faster the rate of diffusion .
• Membranes become less fluid when there is

  A lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed
• The phosphate head of phospholipid is polar


  polar hydrophilic therefore soluble in water