2.1.5c Plasma Membrane Permeability Flashcards
Practical for investigating the effect of different conditions on permeability
Beetroot cells contain a purple pigment (betalain) that leaks out - the higher the permeability of the membrane, the more pigment leaks out the cell
Method for testing effect of temperature on beetroot membrane permeability
- Cut 5 equal sized pieces of beetroot & rinse to remove any pigment released during cutting
- Place the 5 pieces in 5 diff test tubes, each with 5cm^3 of water
- Place each test tube in a water bath at a different temp (10, 20, 30, etc) for the same length of time
- Remove the pieces of beetroot from the tubes, leaving just the coloured liquid
- Use a colorimeter - a machine that passes light through the liquid & measures how much of that light is absorbed. The higher the permeability of the membrane, the more pigment is released, so the higher the absorbance of the liquid.
Results of the beetroot experiment
- As temp increases, permeability also increases
- As temp increases, the phospholipids within the cell membrane move more bc they have more energy
- Increased movement means the phospholipids are not as tightly packed tg, increasing permeability
- At high temps, the phospholipid bilayer may even start to melt & break down, further increasing permeability
- Vol of water inside the cells expands, putting pressure on membrane, causing channel & carrier proteins to deform so they can no longer control what enters/leaves the cell
- Temp also affects the conformation of proteins bc at high temps the intermolecular forces between amino acids are broken which affects the protein’s specificity & function
See pg53 for membrane permeability at each degree celsius
Factors affecting membrane permeability
- Temperature
- Solvents
Relationship between temperature & membrane permeability
Increasing temperate increases membrane permeability
Membrane permeability at temperatures below 0
- The phospholipids dont have much energy, so cant move very much
- Theyre packed closely tg & the membrane is rigid
- But channel proteins & carrier proteins in the membrane deform, increasing membrane permeability
- Ice crystals may form & pierce the membrane making it **highly permeable* when it thaws
Membrane permeability at temperatures 0-45
- The phospholipids can move around & arent packed as tightly tg - the membrane is partially permeable
- As the temp increases the phospholipids move more bc they have more energy - this increases the permeability of the membrane
Membrane permeability at temperatures above 45
- The phospholipid bilayer starts to break down & the membrane becomes more permeable
- Water inside the cell expands, putting pressure on the membrane
- Channel proteins & carrier proteins deform so they cant control what enters/leves the cell - this increases permeability
Affect of different solvents on membrane permeability
- Surrounding cells in a solvent (sa ethanol) increases the permeability of their cell membranes
- This is bc solvents dissolve the lipids in a cell membrane, so the membrane loses its structure
- Some solvents increase permeability more than others, eg. ethanol increases permeability more than methanol
Relationship between solvent concentration & membrane permeability
Increasing the concentration of the solvent will also increase membrane permeability
What is a colorimeter
An instrument that can be used to determine the concentration of a solute in a solutionqq
How exactly does a colorimeter work
By shining a light through a solution & measuring either the:
- transmission (% of light that passes through the solution)
OR the
- absorption (the light absorbed by the solution)
The concentration of the solute is proportional to the absorbance of the solution, so a more concentrated solution will have a higher absorbance
Colorimeter filters
- Colorimeters have coloured filters & can be adjusted to select a suitable wavelength of light
- You should use the colour that is complementary to the colour of your solution, as this will give the maximum absorption
What is absorption
A ratio of the amount of light that passes through a solution compared with the amount of light that passes into the solution
