biological membranes

• cell membranes are partially permeable barriers

  • cell membranes form a barrier and separate the cell form contents from the cells exterior environment , or separate organelles from cytoplasm
• permeability refers to the ability to let substances pass through

  • some very small molecules simply diffuse through the cell membrane in between structural molecules
  • some substances pass through special protein channels or are carried by carrier proteins
  • some substances dissolve in the lipid layer and pass through
• the roles of membranes (surface of the cell)

  • the plasma membrane separates the cells components from its external environment
  • regulate transport of materials into and out of cell
  • may be the site for chemical reactions
  • contains receptors for cell signalling
  • has antigens so that the organisms immune system can recognise the cell being 'self' and not attack it
• within cells

  • the membranes around many organelles present in the eukaryotic cells separate organelle content from the cell cytoplasm
  • eg inner membrane of chloroplasts called thylakoid membranes, house chlorophyll , on these membranes some of the reactions of photosynthesis occurs
• fluid mosaic model

  • consists of the phospholipid bilayer with proteins floating in it
  • lipids can move and change place with each other and some proteins may move giving it fluidity
  • made up out of their hydrophilic head and hydrophobic head
• cholesterol

  • regulates the fluidity of the membrane maintain chemical stability and resists the effect of temperature change on the structure of the membrane
• • small and non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids
  • water is also small enough to fit between phospholipids, so its able to diffuse across plasma membrane even though it is polar = osmosis
• intrinsic (integral)proteins

  • extend through both layer
  • channel proteins = allow diffusion of ions and polar molecules across the membrane
  • carrier proteins = can change shape, used in active transport and facilitated diffusion
• glycoproteins

  • protein molecules with carbohydrate chain attached which project outwards from the cell
  • help stabilise the cell by forming bonds with surrounding water molecules
  • act as receptor sites for hormones, neurotransmitters and some drugs = cell signalling
• extrinsic proteins

  • only present on one side of the bilayer
  • important for controlling metabolic processes, especially important in membranes surrounding organelles such as mitochondria and chloroplast
• glycolipids

  • this are lipid molecules with carbohydrate chain act as markers or antigen identifying cells that are in the body
• diffusion (passive)

  • the movement of lipid soluble particles down a [] gradient directly through the plasma membrane
  • eg O2, CO2, steroid hormones
• facilitated diffusion (passive)

  • via channel protein (charged particles) or carrier proteins (larger particles)
  • Na+, Cl-, K+
  • glucose
• osmosis (passive)

  • movement of water molecules from high water potential to low water potential across a partially permeable membrane
  • water
• active transport (active)

  • movement of substances against a [] gradient vias carrier proteins , process uses ATP
  • uptake of minerals by roots
  • absorption of glucose by villi in the small intestine
• endocytosis (active)

  • take in
  • engulfing pathogen
• exocytosis (releasing) passive


  release of hormones such as insulin
• turgid

  • plant cell wall prevents bursting, membrane pushes against the cell wall
• plasmolysed

  • cell membrane pulls away from the cell as water leaves - the cell is plasmolysed
  • plant tissues with plasmolysed cells are = flaccid
• factors affecting membrane structure and permeability

  • temperature
  • solvents on phospholipids
• when temp drops

  • saturated fatty acids become compressed, kinks in their tail push the adjacent phospholipid molecule away - maintaining the membrane fluidity
  • proportion of unsaturated and saturated fatty acids within cell membrane determine the membrane fluidity at cold temp
  • cholesterol acts as a buffer - prevents in the membranes fluidity
  • does this by preventing the phospholipids molecule packing too closely - in between groups of phospholipid molecules
• when temp increases

  • more kinetic energy, increases membranes fluidity
  • permeability increases cholesterol molecule buffers, to some extent the effect of increasing heat is reduces the increase in membrane fluidity
• proteins and temperature

  • high temperatures causes the atoms within the large molecule to vibrate, H bonds and ionic bonds that hold the structure together unfold
  • tertiary structure changes and cannot change back again - denatured
• effects of solvent on phospholipid

  • organic solvents such as acetone and ethanol will damage cell membranes as they dissolve lipid