cell membrane structure

function of the plasma membrane?

• it defines the border of cells and most organells

• it is selectively permeable

  • this means it allows some materials to freely enter or leave the cell/organelle, while other materials cannot move freely

structure of phospholipids?

• a phospholipid is a molecule consisting of glycerol, 2 fatty acids, and a phosphate linked head group

• the molecules arrange themselves into a bilayer

• the hydrophilic phospholipid head faces outwards and the hydrophobic fatty acids face inwards

structure of cholesterol?

• cholesterol is a lipid that sits with phospholipids in the core of the membrane

• it is not found in bacterial cell membranes

• cholesterol molecules make the membrane more rigid

• this is why cholesterol helps to maintain the shape of animal cells

what factors affect membrane permeability?

• temperature

• solvent concentration

• pH

why does temperature increase the permeability of the cell membranes?

• higher temperatures increase the fluidity of the membrane, increasing the permeability

how does solvent concentration affect permeability?

• the more easily the phospholipid bilayer is dissolved, the more permeable the membrane is

how does pH affect the permeability?

• pH affects the protein structure in the cell membrane

what plant is used to investigate membrane permeability and why?

• beetroot

• because it contains a coloured pigment

steps to investigate membrane permeability using beetroot?

• use a cork borer to collect uniform samples of beetroot

• cut discs of a uniform depth using a scalpel on a white tile

• then rinse in water

  • this removes the excess pigment that has leaked through the broken cell membranes

• prepare at least 5 concentrations of ethanol

• place the discs into the corresponding solution for 10 minutes

  • make sure the samples are completely covered by the ethanol solutions and mixed frequently throughout the 10 mins

• remove the discs from the solution to prevent further changes and allow a fair comparison between the experiments

• calibrate the colorimeter by using a cuvette of distilled water at an absorbance of 520nm

  • the cuvettes must be dry and the clear sides must not be touched to prevent potential errors in the readings

• measure the absorbance of each solution

• plot the results in a grapj

  • the darker the solution, the more pigment has been released, this is reflected in a higher reading of the absorbance

why is it called a FLUID mosaic model?

because the phospholipids are constantly moving

why is it called a fluid MOSAIC model?

• the proteins are scattered through the bilayer, like tiles in a mosaic

what does more unsaturated fatty acids do to the membrane?

• increases fluidity and makes the membrane more fluid, allowing more phagocytosis to occur