mock (membranes)

• what are the functions of the plasma membrane

  - barrier between the cell and environment, controlling which substances enter and leave the cell

  - partially permeable: let some molecules through but not others

  - allow cell communication (cell signalling)
• what are the function of membranes within cells

  - membranes around organelles act as barriers, dividing the cell into compartments

  - they can form vesicles to transport substances

  - they control which substances enter and leave the organelle

  - membranes within organelles act as barriers

  - site of chemical reactions e.g. inner mitochondria membrane contains enzymes for respiration
• what model describes plasma membranes

  fluid-mosaic model
• How does the "fluid mosaic model" describe the structure of the plasma membrane?

  fluid: phospholipid molecules form a double bilayer which is fluid as the phospholipids are constantly moving

  mosaic: protein molecules are scattered through the bilayer
• what are the two glyco molecules in the bilyaer

  - glycoproteins: proteins attached to a carbohydrate

  - glycolipids: lipids attached to a carbohydrate
• why does the membrane arrange as a bilyer

  the head is hydroplhilic so face out towards the water on either side of the membrane and the tails are hydrophobic so face in towards each other
• which substances can dissolve in the bilayer and which ones can't and why

  water-soluble substances e.g. ions can't dissolve as the centre is hydrophobic but fat-soluble substances can
• what is cholesterol's role in the bilayer

  restricts the lateral movement of the membrane:

  it fits between the phospholipids and binds to the hydrophobic tails causing them to pack more closely together making it less fluid and more rigid
• how does cholesterol's role change at a) high temps and b) low temps

  a) makes it less fluid at high temps and prevents substances leaking in and out

  b) prevents them packing too close together so increases the fluidity
• what are extrinisic proteins + example of type

  proteins that attach to one side of the membrane e.g. cell receptors
• what are intrinsic proteins + example of type

  proteins that fit across the whole membrane e.g. protein carriers and channels
• what is the role of glycoproteins and glycolipids

  - stabilise the membrane by forming hydrogen bonds with surrounding water molecules

  - sites where drugs, hormones and antibodies bind

  - receptors for cell signalling

  - antigens involved in the immune response
• what are proteins called that act as receptors for messenger molecules called

  membrane-bound receptors
• explain the role of cell membrane receptors in cell siganlling

  receptor proteins have specific shapes - only messenger molecules with a complementary shape can bind to them
• what is a cell that responds to a particular messenger molecule called

  target cell
• what are two types of messenger molecules

  hormones and drugs
• how do drugs work in cell signalling

  they bind to receptors in cell membranes and trigger a response in the cell, or block the receptor to prevent it from working
• what are the two factors that affect the permeability of cell membranes

  temperature and solvents
• explain the permeability of cell membranes at temps below 0°C

  the channel and carrier proteins deform increasing the permeability

  ice crystals may form and pierce the membrane, making it highly permeable when it thaws
• explain the permeability of cell membranes at temps 0-45°C

  the phospholipids can move around and aren't as tightly packed together so the membrane is partially permeable
• explain the permeability of cell membranes at temps above 45°C

  the kinetic energy of the phospholipids increases so they move more which increases the permeability

  the carrier and channel proteins denature so more permeable

  water inside the cell expands, putting pressure on the membrane
• how does changing the solvent affect the permeability of membranes

  a solvent such as ethanol will dissolve the lipids in the bilayer which increases the permeability

  increasing the concentration of the solvent increases the permeability
• describe a method to measure the effect of temperature on permeability

  1) cut 5 equal sized pieces of beetroot and rinse to remove any pigment released during cutting

  2) place the pieces in 5 different test tubes with 5cm³ of water

  3) place the test tubes in a water bath at different temperatures

  for the same length of time

  4) remove the pieces of beetroot from the tubes

  5) use a colorimeter to measure the percentage absorbance --> the higher the permeability, the more pigment released, so the higher the absorbance of the liquid
• how to change the method to measure the effect of solvents

  use different concentrations of solvents
• define diffusion

  passive net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient until equilibrium is reached
• which molecules can diffuse across cell membranes + example

  small, non-polar, lipid-soluble molecules such as oxygen
• which polar molecule can still diffuse across the membrane + why

  water because it's small enough to fir between the phospholipids
• what factors affect the rate of diffusion

  concentration gradient

  thickness of exchange surface

  surface area

  temperature
• describe a method to measure the rate of diffusion across a cell membrane

  1) make up some agar jelly with phenolphthalein and dilute sodium hydroxide

  2) fill a beaker with dilute hydrochloric acid, using a scalpel, cut out a few cubes from the jelly and put them in the beaker

  3) leave the cubes and they'll turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide
• how to adjust the diffusion method to measure how temp, concentration and surface are affects

  change the factors and time how long it takes for the cubes to go colourless
• by what process do larger, polar molecules travel through the membrane

  facilitated diffusion
• Define facilitated diffusion

  passive net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient, through carrier or channel proteins until equilibrium is reached
• how do carrier proteins work

  1) A large molecule attaches to a carrier protein in the membrane

  2) the protein changes shape

  3) This releases the molecule on the opposite side of the membrane
• how do channel proteins work

  They form pores in the membrane for charged particles to diffuse through
• what is active transport

  the movement of particles against a concentration gradient, using carrier proteins and ATP
• by what process to particles that are too big to fit through carrier proteins travel through the membrane (e.g. proteins and lipids)

  endocytosis
• is endocytosis passive or active

  active, so uses ATP
• describe endocytosis

  1) the membrane bends to surround the substance

  2) it then pinches off to form a vesicle inside the cell
• what process do cells use to secrete substances

  exocytosis
• describe exocytosis

  1) vesicles containing the substance pinch off from the sacs of the golgi and move towards the membrane

  2) the vesicles fuse with the membrane and release their contents outside the cell