RP4: Permeability

Question 1

What are the 2 factors that can change the permeability of a phospholipid bilayer?

Answer 1

1.) Temperature.
2.) Concentration of solvents (such as ethanol.)

Question 2

What is the pigment contained in beetroot cells called?

Answer 2

• Betalain.

Question 3

If the cell- surface membrane of a beetroot cell has a higher permeability, what does this mean for the amount of pigment that leaks out of the cells into the test-tube?

Answer 3

• More pigment will leak out of cells into test-tube.

Question 4

How can the permeability of beetrot cell membranes (in different concentrations of ethanol) be measured?

Answer 4

• Can be measured by the amount of pigment leaked from beetroot cells into solution (using a colorimeter.)

Question 5

What is the method for making different colour standards (of betalain extract - first step in this practical?)

4 main steps

Answer 5

1.) Mix known volumes of betroot extract/ water to prepare series of 6 boiling tubes containing 5cm3 of each concentration of extract.
2.) Label the standars : 0,20,40,60,80,100.
3.) Complete table showing volume of water/ extract to make concentrations.
4.) Use a colorimeter to work out absorbance for each concentration and plot calibration curve of absorbance against concentration.

Question 6

How do you prepare different concentrations of betalain extract (from to 0- 100%) in the first part of this practical? Ie. what volumes of betalain extract/ water for each concentration?

Answer 6

2.) 0%: 5ml of water 0ml betalain.
3.) 20% 4ml of water, 1ml betalain.
4.) 40%: 3ml of water, 2ml betalain.
5.) 60%: 2ml of water, 3ml betalain.
6.) 80%: 1ml water, 4ml betalain.
7.) 100%: 0 ml water, 5 ml betalain

Question 7

True or False

Betalin extract would leak out cells on its own.

Answer 7

• False.
• Betalin extract cannot pass through cell- surface membranes unless the membrane/ cells are damaged.
• Vacuole is burst open (where the betalain is found) so, pigment can leak out of cells.

Question 8

What is a qualitative observation that could be made in this practical? What is the issue with this though? What should be done instead?

Answer 8

• Observations of colour.
• Ie. different concentrations have different visisble intensity of purple colour.
• Doesn’t give accurate concentration.
• SHOULD BE DONE: Use a colorimeter and compare the absorbances.

Question 9

What COULD be done instead of using a colorimeter to determine absorbances of unknwon concentrations and determining concentration? Issue?

Answer 9

• Simply observe the intensity of purple in unknwon concentration of betalin extract and compare this to intensity of purple in known concentrations of betalin.
• Issue: doesn’t give you accurate value for concentration.

Question 10

What are the labels for the x and y axis on calibration curve you draw?

Answer 10

• x: concentration of extract (percentage.)
• y: absorbance (from colorimeter.)

Question 11

What is the method for stage 2 of this practical?

6 main steps.

Answer 11

1.) Place 2cm3 of each concentration of ethanol in 6 separate boiling tubes (from 0-100%)
2.) Blot the beetroot. Cut beetroot into 12 equal discs (in length) - using scalpel.
3.) Blot beetroot discs using paper towel/ place 2 discs into each boiling tube.
4.) Place bungs on boiling tubes/ heat in water bath for 5 mins.
5.) Place solutions into separate cuvettes/ work out absorbance using colorimeter - record results in table.
6.) Use your absorbances to determine the concentration of betalin extract leaked from cells (using 1st calibration curve.)

Question 12

Why do we place boiling tubes in water bath at 30 degrees celcius?

Answer 12

• Controls the temperature.
• As temp affects the permeability of cell membranes.
• This ensures that the only factor affecting the permeability is the ethanol concentration.

Question 13

Why do we place the boiling tubes (with different concentrations of ethanol/ a beetroot cylinder) in water bath at 30 degrees celcius and not any higher?

Answer 13

• Would damage the proteins within the plasma membrane.
• They would denature and this would cause MORE pigment to be released.

Question 14

Why do we place a bung on the boiling tubes with different concentrations of ethanol when heating the solutions to 30 degrees celcius?

Answer 14

• Prevents the evaporation of ethanol (this could affect the concentrations of ethnaol in each boiling tube.)

Question 15

Why do we place two discs of beetroot in each of the six boiling tubes?

Answer 15

• Same surface area of cell membrane.
• Same potential maximum amount of betalin that could move out of the cell.
• So, the only factor affecting betalin moving out of cell is the concentration of ethanol.

Question 16

Why do we blot the beetrot with a paper towel?

Answer 16

• To remove any excess water/ to remove any betalain that has been released through cutting of beetroot.

Question 17

What do you use to calibrate the colorimeter in this practical?

Answer 17

• Water.

Question 18

What are the solutions (in boiling tubes) placed into before their absorbance is measured by colorimeter?

Answer 18

• Placed in cuvettes.

Question 19

If beetroot was placed in a high temperature, why would more pigment be released?

Answer 19

• Cause membrane proteins to denature.
• So, more pigment would be released.

Question 20

If beetroot is placed in alcohol, why would more pigment be released?

Answer 20

• Lipids are soluble in alcohol.
• Alcohol will dissolve phospholipid bilayer/ cause damage.
• So, more pigment will be released.

Question 21

If beetroot is placed in an acid, why would more pigment be released?

Answer 21

• Causes membrane proteins to denature.
• So, the pigment will be released.