biological membranes

• roles

  • separate cell components from environment
  • controls permeability
  • separates organelles from rest of cell
  • site for chemical reactions
  • involved in cell signalling due to proteins within membrane
  • compartmentalisation
• why is compartmentalisation vital

  • allows for specific conditions to be created
  • allows for incompatible reactions to be separated
  • allows for chemical gradient to be created
  • protect cell components
• structure of membranes


  phospholipid bilayer made of 2 layers of phospholipid molecules

  hydrophilic phospholipid head facing outwards

  hydrophobic tail facing inwards

  fluid mosaic model
• fluid mosaic model


  phospholipid bilayer with intrinsic and extrinsic proteins and cholesterol slotted between hydrophobic tails

  phospholipids constantly changing position(fluid)

  proteins distributed within(mosaic)
• cholesterol function


  regulates stability and fluidity

  too warm: PLs gain KE so move more leading to gaps in PLB-> cholesterol anchors PL in place

  too cold:PLs lose KE so move less becoming rigid-> cholesterol forces PL apart again
• glycolipid function


  acts as recognition site, receptor in cell signalling and antigens
• glycoprotein 


  acts as a recognition site for chemicals eg hormones
• transport across membranes


  small, charged/polar (hydrophilic) ->move from higher to lower concentration- facilitated diffusion via channel proteins
• transport across membranes


  small non polar (hydrophobic)-> move from higher to lower concentration- simple diffusion as dissolves into PLB
• transport across membranes


  large molecules eg glucose

  bind to carrier protein that changed its tertiary structure, can be passive(facilitated diffusion) or active(requires ATP from aerobic respiration)
• affect of solvents on membrane structure


  organic solvents will dissolve PLB by disrupting arrangement of phospholipids, separating them from eachother causing the membrane to lose its structure, break down and become more fluid
• linking solvents to affect of alcohol on body


  pure or very strong alcohols are very toxic as destroy body cells

  less concentrated such as alcoholic drinks wont dissolve membranes but do cause damage

  some cells need fully intact membranes to function eg. transmission of nerve impulses by neurones. if neuronal membranes are disrupted, nerve impulses wont transmit as normal
• affect of temperature on membrane structure


  increased temperature, increases KE of phospholipids so they move more leading to gaps in the PLB so more fluid and breaks down structure

  decreased temperatur, decreases KE of phospholipids so move less making the membrane become more rigid