Required Practical 4: Membrane Permeability Flashcards
A colorimeter measures the concentration of … in a solution.
It does this through shining a light through the solution and measuring the % of light that’s absorbed and how much is transmitted (more light absorbed means more coloured pigment present in the water so a higher membrane permeability)
Solute
2 variables that can change the permeability of the cell surface membrane
Temperature
Concentration of solvents (e.g ethanol)
Beetroot cells contain a red pigment (which is water-soluble) called…
Betalin
The permeability of beetroot cell membranes can be measured using a…
Colorimeter
(The colorimeter measures the % of light absorbed by the coloured pigment (betalin) in each solution. The more light absorbed, the higher the concentration of betalin in the solution. The higher the concentration of betalin in the solution then the more permeable the cell membrane is as more betalin has leaked out of the cell via the cell surface membrane)
For the investigation you cut the beetroot into 6 identical cylinders using a …
Scalpel
Which part of the beetroot cells contain betalin (a purple coloured pigment)
Vacuole
After cutting the beetroot cylinders why should you wash them before placing them in the boiling tubes
To remove excess betalin (coloured pigment) that’s leaked out from the cells across the cell surface membrane
(Otherwise this coloured pigment that’s already leaked out will end up in the solution and will increase the concentration of coloured pigment in the solution to higher than it would if it only included the coloured pigment that leaked out within the set time of the investigation (20 minutes) so the % of light absorbed will also be higher than if the solution only contained coloured pigment that had leaked out within the 20 mins)
Method for investigating the effect of temperature on the permeability of cell surface membranes using beetroot
1) Use a measuring cylinder to add 20ml of distilled water to 6 different test tubes and place each test tube in a water bath at a range of different temperatures from 30 degrees Celsius to 80 degrees Celsius
2) Leave the test tubes in the water baths to allow the water to equilibrate (reach the correct temperature)
3) Use a scalpel and ruler to cut 6 beetroot cuboids that are identical in shape and size/ use a cork borer, scalpel and ruler to cut 6 identical beetroot cylinders
4) Rinse the beetroot cylinders in distilled water to remove any excess pigment that has leaked from the cell surface membrane
5) Blot the beetroot cylinders dry with a paper towel
6) Once each of the test tubes has equilibrated use forceps to add a beetroot cylinders to each test tube, place it back in the hot water bath and set a timer for 20 minutes
7) After 20 minutes remove each test tube from the hot water bath
8) Use the forceps to remove the beetroot cylinders from each test tube
9) Decant the liquid from each test tube into a cuvette
10) Use the colorimeter to measure the percentage of light absorbed by the coloured pigment in each different solution (each different temperature) and record the results in a table
11) Plot a graph with temperature on the X axis and percentage of light absorbed on the Y axis
Once the test tubes containing beetroot cylinders have been left at different temperatures for 20 minutes, you remove the beetroot cylinders from each solution and decant the solutions to ….
A cuvette
(A small tube like container which fits into the colorimeter)
Beetroot is used to measure the permeability of cell membranes as the higher the permeability, the more…there is that leaks out into the surrounding solution in a given time. A…can be used to measure the absorbance of the coloured pigment and therefore the…of the pigment
Red coloured pigment (betalin)
Colorimeter
Concentration
How to use the colorimeter to measure the percentage absorbance of light by the coloured pigment in each different solution
1) Set the colorimeter to 0 by adding a cuvette with distilled water and pressing reset (R)
2) Add the first cuvette you are testing the coloured pigment concentration of, place in the colorimeter and press test (T)
3) Record the value for the % of light being absorbed by the colour pigment
4) Repeat for each different solution, resetting using the distilled water between each one
Ethanol is…so you should keep it away from naked flames
Flammable
When the cell surface membrane has a higher permeability, …..(more/ less) coloured pigment leaks out of the cells across the cell surface membrane
More
A …(lower/ higher) absorbance of light indicates a higher pigment concentration and a …..(more/ less) permeable membrane
Higher
More
Ethanol is an…so you should wear eye protection (protective goggles) when carrying out the investigation
Irritant
The coloured pigment moves out of the beetroot cells and into the solution by…
Diffusion
True or false, when you cut the beetroot tissue into cylinders/ cuboids, it damages the cell membranes which causes some of the red pigment to leak out
True
(This is why you wash the beetroot cylinders after you’ve cut them and then blot them dry as this leakage of pigment has had nothing to do with the temperature/ concentration of ethanol so we want to remove the pigment that’s already leaked)
When investigating the effect of ethanol concentration on the permeability of cell membranes (beetroot membranes) why should you use distilled water as a control
You can compare the control (water) with the different concentrations of ethanol to prove that it’s the ethanol concentration that’s having the effect on the permeability of the cell membranes (the leakage of the coloured pigment from the beetroot)
When investigating the effect of ethanol concentration on the permeability of beetroot membrane, the student placed each test tube in a hot water bath at 25 degrees.
How can the student ensure that the beetroot cylinders are kept at 25 degrees Celsius throughout the experiment?
Use a thermometer to measure the temperature of each tube at intervals
Use an appropriate corrective measure if the temperature has fluctuated (e.g replacing cooled water with hot water/ adding more hot water to the hot water bath/ using a thermostatically controlled water bath)
3 corrective measures to use if the temperature recorded by the thermometer of test tubes in a hot water bath has fluctuated
Add more hot water to the hot water bath
Replace cooled water with hot water
Use a thermostatically controlled water bath (it automatically ensures it stays at the correct temperature throughout)
A…water bath ensures that the water bath stays at the same set temperature throughout the investigation (and that the water doesn’t cool down and bring the temperature down etc)
This is for when investigating the effect of ethanol as temperature is a control variable
Thermostatically controlled
When investigating the effect of ethanol on the permeability of beetroot cell membranes, the test tubes should be kept in a hot water bath at…degrees celcius
25
