required practical 4

Question 1

what is practical 4

Answer 1

investigation into the effect of a named variable on the permeability of cell-surface and membranes

Question 2

practical 4 aim

Answer 2

Cell-surface membranes are made up of a ​phospholipid bilayer​​ which makes them selectively permeable​​.

This permeability can be changed by different variables, such as ​temperature​​ and ​concentration of solvents,​​ like ethanol.

The permeability of a membrane can be measured by using beetroot cells, which contain a purple ​pigment ​​called​ betalain. ​​When the cell-surface membrane has a higher permeability, more pigment leaks out of cells.

The permeability can therefore be measured by the ​amount of pigment leaked​​ from beetroot cells into an aqueous solution using a ​colorimeter

Question 3

4 equipment list

Answer 3

● Beetroot
● Scalpel
● Forceps
● Cutting board
● Ruler
● Tongs
● Distilled water
● Boiling tubes
● Boiling tube rack
● Colorimeter
● Cuvettes
● Filter paper
● Timer
● Water bath
● Thermometer
● Ethanol

Question 4

4 method

Answer 4

1. Cut beetroot into 6-10 ​identical cubes​​ using a scalpel.
2. Wipe/rinse to ​clean off any pigment​​ released as a result.
3. If investigating temperature​​: place each of the cubes of beetroot in an equal
   volume of distilled water​ (5-15ml).
4. Place each test tube in a ​water bath​​ at a ​range of temperatures ​​(30-80​°​C).
5. If investigating concentration of solvents​​: create a ​dilution series​​ of ethanol using distilled water. Ethanol concentrations should range from ​0-100%​​ ethanol.
6. Leave the samples for ​20 minutes ​​- the pigment will leak out of the beetroot.
7. Set the colorimeter to a ​blue filter​​ and​ zero​​ using a cuvette with ​distilled water​​.
8. Filter ​​each sample into a cuvette using ​filter paper​​.
9. Measure the ​absorbance​​ for each solution. A ​higher absorbance​​ indicates
   higher pigment concentration​​, and hence a ​more permeable membrane​​.

Question 5

4 risk assessment

Answer 5

scalpel

broken glass

ethanol

hot liquids

Question 6

4 graph

Answer 6

plot a graph of absorbance against ethanol concentration / temperature

Question 7

4 conclusion

Answer 7

Temperature
● as temperature increases, permeability of the cell-surface membrane also ​increases​​ - proteins in the membrane ​denature​​ as the heat damages the bonds​​ in their ​tertiary structure​​ - ​creates gaps​​ in the membrane, so it is easier for molecules to pass through it.

● At low temperatures, phospholipids have ​little energy​​ and are ​packed closely together​​ to make the membrane​ rigid​​. This causes a ​decrease​​ in permeability and restricts molecules from crossing the membrane.

NB: At ​very low ​​temperatures, ​ice crystals​​ can form which ​pierce the cell membrane and ​increase​​ the permeability.

Concentration of ethanol

● Ethanol causes the cell-surface membrane to ​rupture​​, releasing the betalain pigment from the cell. ​Higher concentrations​​ of ethanol will cause ​more disruption​​ to the membrane and ​more gaps will form​​. Thus, as concentration of ethanol ​increases​​, the permeability of the cell-surface membrane also increases