2.1.5 Biological Membranes

• Phospholipid bilayers

  • 'Fluid Mosaic' structure
  • 'Fluid" - phospholipids are constantly moving
  • 'Mosaic" - protein molecules scattered
  • around 7nm thick
• Phospholipids

  • hydrophobic tail
  • hydrophilic head
  • heads face outwards towards water
  • tails face inwards towards other tails forming the centre of the bilayer which is hydrophobic
• Glycoprotein


  • proteins with a polysaccharide chain attached
• Glycolipids


  • lipids with a polysaccharide chain attached
• Glycolipids and glycoproteins

  • stabilising - form H bonds with water molecules
  • binding sites for drugs, hormones and anti-bodies
  • receptors for cell signalling
  • antigens - cell surface molecules involved with the immune response
• plasma (surface) membranes

  • barriers between cell and environment
  • control what can go in and out of the cell
  • PARTIALLY PERMEABLE
  • allow recognition and communication (signalling)\ of other cells
• surface membranes within cells

  • Compartmentalisation - functions are more efficient
  • vesicles - for transport
  • control what goes in and out of an organelle
  • membranes within organelles - barriers between membrane contents
  • enclose sites of chemical reactions
• cholesterol - a type of lipid (sterol)

  • small, flattened - fit easily between phospholipids
  • bind to HYDROPHOBIC TAILS
  • at LOWER temperatures cholesterol prevents phospholipids from packing closely together, INCREASING membrane fluidity
  • at HIGHER temperatures, cholesterol bind to hydrophobic tails, causing phospholipids to bind closer together, REDUCING membrane fluidity
• cell signalling

  1. a cell releases a messenger molecule (e.g. a hormone)
  2. this molecule travels (e.g. via the blood) to another cell
  3. the messenger molecule is detected by a cell as it binds to a receptor on its cell membrane
• Proteins that control what goes in and out of the cell

  • channel proteins
  • carrier protiens
• Cell membrane receptors (signalling)

  • proteins in cell membrane act as receptors for messenger molecules
  • receptor proteins have specific shapes - messenger molecules are complementary in shape
  • different cells have different types of receptors that respond to different messenger molecules
  • target cell - a cell that responds to a particular messenger molecule
  • e.g. glucagon (hormone)- binds to receptors on liver cells so they break down glycogen into glucose when levels are low
• Drugs - cell signalling

  • bind to receptors in cell membranes
  • either trigger a response or block the receptor
  • e.g. antihistamines - block histamine receptors, preventing histamine from binding and causing inflammation
• Investigating permeability of the cell membrane - temperature

  1. cut equal sizes of beetroot, rinse to remove pigment released when cutting
  2. place in separate test tubes with 5cm3 of water
  3. place in a water baths at different temperatures for the same measure of time
  4. remove pieces from the tubes, leaving coloured liquid
  5. use a colorimeter to find absorbance, a higher absorbance, more pigment was released, higher permeability of the membrane
• Permeability changes - below 0C

  • little energy - phospholipids do not move, rigid structure
  • channel and carrier proteins deform, increasing permeability
  • ice crystals may form and pierce the membrane, making it highly permeable when it thaws
• Permeability changes - between 0-45C

  • energy - phospholipids can move around, aren't tightly packed, so partially permeable
  • further temp increase - phospholipids move more, increases permeability
• Permeability changes - above 45C

  • phospholipid bilayer starts to melt (break down), even more permeable
  • water inside cell expands, increased pressure on the membrane
  • Channel and carrier proteins deform, increasing permeability
• Permeability changes - solvents

  • a solvent (e.g. ethanol) can increase permeability
  • can dissolve lipids in the cell membrane, loses its structure
  • different solvents will increase permeability more than others
  • increasing concentration of solvent will also increase membrane permeability
• Diffusion - definition

  Net movement of particles from an area of higher concentration to an area of lower concentration.
• Diffusion

  • molecules diffuse both ways but net movement is to a lower conc
  • concentration gradient - path from an area of high conc to low conc
  • particles diffuse DOWN a concentration gradient
  • a PASSIVE process - no energy required
• RATE OF DIFFUSION

  1. high concentration gradient
  2. a thin exchange surface - a short diffusion distance
  3. a large surface area
  4. a warmer temperature - to provide kinetic energy for movement
• Investigating diffusion - general method

  1. agar jelly with phenolphthalein and a solution of dilute sodium hydroxide - turns agar jelly pink (alkali)
  2. fill a beaker with dilute hydrochloric acid
  3. use a scalpel to cut a few small cubes of jelly and put them in the beaker of acid
  4. leave the cubes, they will eventually turn colourless
• Investigating diffusion - SURFACE AREA

  • cut jelly into different sized cubes - work out SA:V
  • time how long it takes for each cube to turn colourless, in the same conc and vol of HCL
  • larger SA:V (smaller cubes) will go colourless first
• Investigating diffusion - CONCENTRATION GRADIENT

  • test tubes containing different concentrations of HCL
  • place equal sized cubes of jelly in test tube
  • time how long it takes for each cube to turn colourless
  • higher concs will go colourless first
• Investigating diffusion - TEMPERATURE

  • several boiling tubes of same conc and vol of HCL
  • place into water baths of varying temperatures
  • put equal sizes of jelly cubes into each tube
  • time how long it takes for each cube to turn colourless
  • higher temps should turn colourless first
• Diffusion through the CELL MEMBRANE

  • small, non-polar molecules
  • e.g. oxygen and carbon dioxide
  • can easily diffuse through spaces between phospholipids
• Facilitated diffusion through proteins

  • large, charged and polar molecules
  • e.g. ions, amino acids, glucose
  • do not diffuse directly through the bilayer
• Facilitated diffusion through proteins

  • particles move down a concentration gradient
  • a passive process - does not require energy
  • through carrier or channel proteins
• Carrier proteins

  Facilitate diffusion for LARGE MOLECULES
• Channel proteins

  Facilitate diffusion for CHARGED PARTICLES
• Carrier proteins

  • different carrier proteins facilitate diffusion for different molecules
  • process

  1. a large molecule attaches to a carrier protein in the membrane
  2. the protein changes shape
  3. this releases the molecule on the other side of the membrane
• Channel proteins

  • different channel proteins facilitate diffusion of different charged particles
  • Process - channel proteins form pores that allow charged particles to diffuse through
• ACTIVE TRANSPORT

  • moving molecules and ions against a concentration gradient
  • requires ENERGY
  • uses carrier proteins
• Carrier proteins

  • use in active transport
  • energy from ATP
  • moves it against the concentration gradient
  • process

  1. a large molecule attaches to a carrier protein in the membrane
  2. the protein changes shape
  3. this releases the molecule on the other side of the membrane
• ENDOCYTOSIS

  • for molecules that are too large for carrier proteins to take INTO the cell (e.g. proteins, lipids, some carbohydrates)
  • the cell surrounds a substance with a section of its plasma membrane and it pinches off to form a vesicle that contains the ingested substance
  • white blood cells (mainly phagocytes) - use this to take in microorganisms and dead cells to destroy them
  • requires energy from ATP
• EXOCYTOSIS

  • substances need to be released from the cell (e.g. digestive enzymes, hormones and lipids)
  • vesicles containing substances pinch off from sacs of the Golgi body and move towards the plasma membrane
  • some substances aren't released and are implanted into the plasma membrane (e.g. membrane proteins)
  • requires energy from ATP
• OSMOSIS

  • movement of water molecules from an area of high water potential to an area of low water potential, down a water potential gradient
• Pure water has the highest water potential - all solutions have a lower water potential
• Water potential

  The potential of water molecules to diffuse in or out of a solution
• hypOtonic solution

  Solution with a higher water potential (and lower conc of solute) compared to the cell
• hypERtonic solution

  Solution with a lower water potential (and high conc of solute) compared to the cell