membranes

• membranes:

  • a barrier
  • part of what gives cell shape
  • compartmentalisation
  • controls movement in and out of cell
• intracellular means inside the cell
• extracellular means outside the cell
• hydrophobic has no charge and is non polar
• hydrophillic is charged and polar
• when a phospholipid chain is dropped in water the hydrophobic tails go on the inside and the hydrophilic head on the outside
• membranes are anywhere between 5-10 nm thick, average 7.5 nm
• fluid mosaic model: cell membranes have a consistency like oil, with sideways movement of molecules
• fluid mosaic model:

  • fluid, individual phospholipid and protein molecules move around within their layer.
  • mosaic, pattern produced by scattered protein molecules when surface membrane is viewed from above
• why is the cell membrane described as "fluid"?

  because they have a consistency like oil, with sideways movement of molecules and the individual phospholipids and protein molecules move around within their thin layer.
• why is the cell membrane described as "mosaic"?

  because of the pattern produced by scattered protein molecules when the surface membrane is viewed from above.
• the width of a membrane is 7.5 nm
• small molecules can pass through the gaps between phospholipid molecules in the membrane, such as lipid soluble molecules.
• the phospholipid bilayer is studded with proteins. these can extend all the way through the membrane (integral) or only extend half way through (peripheral)
• intrinsic proteins function as carriers and channels, allowing movement of specific molecules through the membrane through facilitated diffusion or active transport
• extrinsic proteins can either be receptors or help to stabilize the membrane. receptors have a specific shape that only a molecule of complementary shape can bind to and cause changes to take place inside the cell when activated. receptors are glycoproteins as they have a carbohydrate chain attached. these actt as recognition sites for chemicals such as hormones.
• suggest two properties that a drug must have for it to enter a cell rapidly?

  lipid soluble and small
• functions of cellular membranes:

  • providing a partially permeable barrier around or within a cell
  • compartmentalisation
  • transport of solutes
  • signal transduction for nervous transmission
  • cell to cell recognition
  • receptor site for hormones
• phospholipid:

  • cam form sheets
  • form membrane bound compartments
  • acts as a barrier to most water soluble substances
• cholesterol:

  • helps regulate fluidity of membrane
  • stabilises phospholipid bi-layer
  • prevent ions/ polar molecules passing through, important in myelin sheath around nerve cells
• intrinsic proteins = span proteins
• extrinsic proteins = embedded in one half of membrane
• channel proteins are integral proteins that form pores in the membrane
• carrier proteins are proteins that transport molecules across a membrane
• glycolipids and glycoproteins:

  • short carbohydrate chains attach to lipids and proteins.
  • chains project into watery fluid surrounding membrane, forming hydrogen bonds with the water and increasing stability of membrane structure.
  • antigens= allow cells to recognise each other eg: white blood cells (glycolipids)
  • receptor molecules= bind hormones and neurotransmitters eg: insulin receptors in liver and muscle cells (glycoproteins)
• why can phospholipids in bi-layer only move within plane of bi-layer (side to side, not up and down)?

  -because it would interrupt the process and the hydrophobic tails will be out of place (touching water)and it would effect the whole layer.

  -the phosphate head (hydrophilic) cant pass through the hydrophobic region in the centre of the bilayer.
• why do we describe cell membranes as partially permeable rather than semi permeable?

  -different membranes are permeable to a variety of substances and impermeable to a variety of others.

  -semi permeable suggests "half permeable" which is unlikely to be the case in any membrane.
• what needs to be transported across membranes?

  -ions (calcium, sodium and potassium in plants)

  -water

  -urea

  -hormones

  -glucose

  -oxygen

  -carbon dioxide
• what is diffusion?

  -the passive net movement of particles form an area of high concentration to an area of low concentration until equilibrium is met/reached.

  -due to the random movement of particles, the more kinetic energy, the more particles move, the faster diffusion occurs.

  -small, non-polar molecules can diffuse through the lipid bi-layer eg: oxygen and carbon dioxide
• facilitated diffusion:

  the same as diffusion but uses channel proteins or carrier proteins to allow substances that would normally not be able to move the membrane to do so.
• carrier proteins:

  shut one side and open on other "flip flop" and they change shape slightly eg: glucose moved across a membrane using carrier protein.
• carrier protein process:

  • carry large water soluble substances
  • specific molecules bind with proteins
  • causes it to change shape
  • molecule is released on other side of membrane (the carrier protein then converts back to its original shape)
  • no external energy is required
  • kinetic energy of the molecules themselves is all that is needed
• osmosis:

  • passive
  • down water potential gradient
  • diffusion involving only water
  • no carrier proteins needed, channel proteins used (aqua porin)
  • water movement accounts for most bulk changes in mass or volume cells
• channel proteins are always open
• water potential:

  • the potential of a solution to donate water to another cell or solution
  • measured in KPa as it is a pressure
  • the highest value is 0KPa which is pure/ distilled water.
• hypotonic= water higher up compared to cell

  isotonic= same

  hypertonic= lower down compared to cell
• How does water cross

  membranes?

  ► It is a small enough molecule to diffuse through the lipid bilayer

  ► Some membranes contain protein channels to increase their permeability

  ► Osmotic potential is determined by the number of free water molecules ie. those not bound to a solute

  ► Pure water therefore has the highest number of free water molecules
• In plant cells the pressure exerted by the cell wall also plays a part in the movement of water (called pressure potential)
• What factors affect rate of diffusion?

  -Temp, particles have more kinetic energy as temperature rises, increases rate of random movement & increases rate

  -Concentration gradient, rate is increased by having more particles in 1 side of membrane than other

  -Stirring, increases movement of particles & rate

  -Surface area, diffusion faster if theres more area to diffuse across; cells have adaptations that allow for this(red blood cells biconcave shape)

  -Distance/thickness,thicker barrier slows rate,particles have more distance to travel

  -Size of molecules, smaller diffuse faster than larger
• Lipid-based molecules

  • Because the membrane is made of phospholipids, fat-soluble molecules can simply pass through the membrane
  • e.g., steroid hormones are lipid based and diffuse through membranes into cells