• What is plasma membrane


  a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell.
• State 5 roles of membranes at the surface of the cells

  At the surface of the cell:

  -Separate cell from environment.

  barrier between the cell and its environment controlling which substances enter and leave the cell.

  For example phospholipid bilayers allow small , non polar molecules e.g oxygen, carbon dioxide to diffuse across

  doesn't allow large molecules e.g glucose

  - Regulate passage of substances in and out of the cell, by exo/endocytosis, diffusion through the lipid bilayer, facilitated diffusion by channel/carrier proteins & active transport by carrier proteins. Examples include aquaporins, sodium/potassium pump, glucose co-transport.

  -proteins help with cell recognition by other cells e.g the cells of the immune system.
• What is phospholipid bilayer?

  a two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic fatty acid tails facing inward and the hydrophilic phosphate heads facing outward. hydrophilic phosphate form both inner and outer surface of membrane.

  
• What is allowed across the phospholipid bilayer membrane

  only lipid-soluble molecules and some small molecules are able to freely pass through the lipid bilayer. large molecules cannot pass through the lipid bilayer e.g glucose.

  Small polar molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly.
• What roles does the plasma membrane have inside the cell

  compartmentalisation.

  -can form vesicles to transport substances between different areas of the cell

  -controls what substance enter and leave the organelle e.g nuclear pore permits RNA to leave nucleus.

  -protects cells from contents of lysosomes
• what is cell recognition


  one cell specifically binds to another cell of a certain type
• define the term 'compartmentalisation' and explain why compartmentalisation is useful to cells

  this is the formation of separate membrane bound areas in a cell. keeps incompatible reactions separate e.g lysosomes containing hydrolytic enzymes. maintains necessary chemical conditions for different reactions and protects other organelles
• Define partially permeable


  membrane that allows some substances to cross but not other
• define cell signalling


  Cells communicate with one another by signals. (intracellular communication) molecule binds to the protein (receptor) to trigger chemical response inside the cell
• Explain what it is meant by 'fluid' and 'mosaic' in the fluid mosaic model of membrane structure

  phospholipids within the bilayer are free to move (causes the moments of the other components) (fluid) and proteins of various shapes and sizes are embedded through the bilayer like tiles in a mosaic.See an expert-written answer!We have an expert-written solution to this problem!
• What is the fluid mosaic structure

  phospholipid molecules form a continuous, double layer (bilayer) the bilayer is 'fluid' because the phospholipids are constantly moving.

  cholesterol molecules are present within the bilayer

  protein molecules are scattered through the bilayer like tiles in a mosaic. some proteins have a polysaccharide (carb) chain attached-these are called glycoproteins

  some lipids also have a polysaccharide chain attached- called glycolipids
• what is the size of phospholipid bilayer

  7nm thick
• what are glycoproteins? + roles

  intrinsic proteins embedded in the cell-surface membrane with attached carbohydrate chain.

  Roles:

  -cell adhesion

  -cell recognition

  -receptor for messenger molecules

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

  -receptors for cell signalling. this is where a chemical binds to the receptor and produces a response e.g receptors for neurotransmitters. binding triggers an impulse.

  -receptor for drugs, hormones and antibodies to bind

  -act as antigens; cell surface molecules involved in the immune response.
• What are glycolipids? + roles


  lipid with a carbohydrate chain attached. similar to glycoproteins.

  Roles:

  -receptors for messenger molecules

  -cell markers

  -antigens and can be recognised by the cells of immune system as self, or non-self (foreign) immune system can make this distinction by the identification molecules on their surface
• What are phospholipid?


  phosphate head and 2 fatty acid tails. form bilayer that can move around each other.
• What is cholesterol? + roles

  type of lipid. it is present in all cell membranes (except bacterial cell membranes. cholesterol molecules fit between the phospholipids. it REGULATES MEMBRANE FLUIDITY.

  Depending on temperature

  at high temperatures, it binds to the hydrophobic tails of the phospholipid causing them to pack more closely together. makes membrane LESS FLUID + MORE RIGID this stabilizes the membrane and raises its melting point,

  At low temperatures it prevents them from clustering together and stiffening (becoming too solid) by stopping them from grouping too closely and crystallising
• What are channel proteins? + roles

  Intrinsic proteins

  provide hydrophilic channel through membrane so small polar molecules and ions (charged) can diffuse through down a concentration gradient. spans an entire membrane.

  they are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.
• What are carrier proteins? + roles

  Intrinsic proteins

  Role in passive transport, facilitated diffusion (down a conc gradient) and active transport (against a conc gradient) into cells. this often involves the shape of the protein changing (for active)
• What are intrinsic proteins?

  transmembrane proteins that are embedded through both layers of a bilayer

  
• What are extrinsic proteins?


  present in one side of the bilayer only.
• Draw and label a diagram of the fluid mosaic model of a cell membrane

  
• Draw a diagram of a phospholipid, label its components and annotate w/ their properties

  + what is allowed to diffuse across

  Phospholipids form a barrier to dissolved substances

  -head and tail

  -head is hydrophilic-it attracts water

  -the tail is hydrophobic-it repels water

  the molecules arrange in bilayer. heads face out. centre of the bilayer is hydrophobic so the membrane doesn't allow water-soluble substances (like ions) through it-acts as a barrier to these dissolved substances.

  does allow fat-soluble substances e.g fat soluble vitamins, can dissolve in the bilayer and pass directly through the membrane

  
• Define cell adhesion


  when cells join together to form tight junctions in certain tissues
• Explain how cholesterol affects the fluidity and stability of membranes

  it REGULATES MEMBRANE FLUIDITY.

  Depending on temperature

  at high temperatures, it binds to the hydrophobic tails of the phospholipid causing them to pack more closely together. makes membrane LESS FLUID + MORE RIGID this stabilizes the membrane and raises its melting point,

  At low temperatures it prevents them from clustering together and stiffening (becoming too solid) by stopping them from grouping too closely and crystallising

  (intercalates between the phospholipids)
• what is a membrane bound protein


  it is a protein attached to a membrane
• give examples of cell signalling

  receptors for neurotransmitters e.g actelycholine at nerve cell synapses. the binding of the neurotransmitters triggers/prevents impulse in next neurone

  
  

  receptors for peptide hormones e.g insulin and glucagon

  glucagon is a hormone that is released when there isn't enough glucose in the blood. it binds to receptors on liver cells, causing the liver cells to break down stores of glycogen to glucose

  (i.e affect the uptake and storage of glucose by cells)
• Give an example of a drug binding to cell membrane receptors

  many drugs work by binding to cell receptors. they either trigger a response in the cell or block the receptor and prevent it from working

  e.g antihistamines: cell damage causes the release of histamine. histamine binds to receptors on the surface of other cells and causes inflammation. antihistamines work by blocking histamine receptors on cell surface. prevents histamine from binding to the cell and stops inflammation.
• What are the factors that affect permeability of membrane

  increasing temperature

  changing the solvent (can be concentration)
• Describe and explain the effect of changing temperature on the permeability of cell membranes

  Increased temperature. this increases the permeability of cell as phospholipids have more kinetic energy therefore move more. this more fluid membrane loses structure and at high enough temperature will break down completely.

  the loss of structure increases permeability of the membrane making it easier for particles to cross it.
• what happens to the carrier and channel proteins in the membrane at high temperatures?

  they will become denatured i.e change shape because hydrogen bonds break and tertiary structure unravels. these proteins are involved in transport across the membrane so as they denature, membrane permeability will be affected -(causing the membrane to rapidly increase in permeability)- they can't control what enters or leaves the cell
• Describe and explain the effect of solvent concentration on the permeability of cell membranes

  Organic solvents will dissolve membranes, disrupting cells-they are less polar than water. Pure alcohol solutions are toxic as they destroy cells in the body. It can dissolve the membrane making them more permeable as the structure is disrupted.

  less concentrated solutions of alcohols such as membranes will not dissolve membranes but will still cause damage

  
  

  The non-polar alcohol molecules can enter the cell membrane and the presence of these molecules between phospholipids disrupts the membrane. Disrupted->more fluid and more permeable. some cells need intact cell membranes for specific functions e.g transmission fo nerve impulses by neurones. when neuronal membranes are disrupted nerve impulses are no longer transmitted as normal.

  this happens when you drink alcohol leaving in a change in behaviour
• How does ethanol increase permeability of cell membranes + how does concentration of solvent affect permeability

  it is a solvent and dissolves the cell membrane (lipids) causing the membrane to lose its structure . some solvents increase cell permeability more than other e.g ethanol increases cell permeability more than others. increasing the concentration of solvent will also increase membrane permeability, because as there is more solvent molecules , there is more disruption and dissolving of the membrane ->increasing fluidity
• Describe a method using a colorimeter to investigate the effect of temperature on the permeability of cell membranes

  Cut five equal sized pieces of beetroot (discs) and rinse them to remove any pigment released during cutting

  Label five test tubes 20,35,45,60 and 80.

  Place the 5 beetroot discs into each of the 5 test tubes using forceps.

  Add 15cm^3 of distilled water into each.

  Set up a water bath using cold tap water in 500ml beaker. this will be used for heating your tubes.

  place all 5 test tubes into the waterbath and a thermometer in the tube labelled 80 degrees.

  Place the beaker with the test tubes in it on a tripod and gauze

  Using the bunsen burner heat the water bath

  When the thermometer reads 20 degrees, remove the tube labelled 20 degrees and place into a 250ml beaker of cold water. this is the cooling water bath.

  As the temperature continues to rise, remove each tube at its labelled temperature and place into the cooling water bath

  Wait for 3 minutes to allow the final tube to cool down, and then swirl each tube well to make sure any pigment is well mixed into the water.

  Use a dropper pipette to transfer each solution to 5 cuvettes. transfer enough to fill the cuvette.

  Check the colorimeter reading with distilled water. it should be 0 absorbance.

  use the colorimeter to measure absorbance for each temperature and record this information. plot a graph of absorbance against temperature

  colorimeter responds to blue/green filter
• Why were you given discs of beetrot?

  big surface area- more space for it to work and particles to diffuse across
• why were the discs cut to the same thickness

  same volume: surface area ratio. controlling over surface area having any effect
• Why did you wash and blot the discs before adding to the test tubes in the water bath?

  pigment may leak so it stops it from
• Why did you place the tubes in cold water once they had reached the desired temperature

  stops the experiment carrying on. want to see effective temperature on protein. stops the temperature rising further- critically for experiment to measure exact temperature effect on permeability.
• why did you agitate the test tube before removing?

  to make sure all the pigment well mixed into the water- any colour is equally disturbed so you get a representative sample.
• Why do you use water first in colorimeter

  you can zero or reference your other samples against this. act as a control
• What is diffusion?


  Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.

  continues until particles are evenly distributed throughout the liquid or gas. (movements are equal in both direction)