REQUIRED PRACTICAL 4 - PERMEABILITY OF CELL MEMBRANES

Question 1

what are cell surface membranes made up of

Answer 1

• phospholipid bilayer
• makes them selectively permeable

Question 2

how can the permeability of the cell surface membrane be changed

Answer 2

• different variables, like temperature and conc of solvents like ethanol

Question 3

how can the permeability of a membrane be measured

Answer 3

• by using BEETROOT cells
  as they contain a purple pigment called BETALAIN
• when the cell surface membrane has a HIGHER PERMEABILITY, MORE pigment leaks out of cells
• permeability can therefore be measured by the AMOUNT OF PIGMENT LEAKED OUT FROM BEETROOT CELLD into an aqueous solution by using a colourimeter

Question 4

what is the equipment needed for this practical

Answer 4

• beetroot
• scalpel
• forceps
• cutting board
• ruler
• tongs
• distilled water
• boiling tubes
• boiling tube rack
• colourimeter
• cuvettes
• filter paper
• timer
• water bath
• thermometer
• ethanol

Question 5

describe the method for this practical

Answer 5

• cut the beetroot into 6-10 identical cubes using a scalpel
• wipe/rinse the beetroot cubes to clean off any pigment released as a result of cutting

for investigation of temperature
- place each of the cubes of beetroot in an equal volume of distilled water, ex 10ml
- place each test tube in a water bath at a range of temperatures, ex 10, 20, 30, 40, 50 and 60 degrees C

for investigation of solvents
- create a dilution series of ethanol using distilled water
- ethanol concentrations should range from 0-100% ethanol

• leave the samples for 20 mins, this will allow enough time for the pigment to leak out of the beetroot
• set the colourimeter to a BLUE FILTER and zero using a cuvette with distilled water
• filter each sample into a cuvette using filter paper
• measure the absorbance for each solution
• a HIGH absorbance indicates a HIGHER pigment conc and therefore a MORE permeable membrane

Question 6

name the potential risks for this practical

Answer 6

• scalpel
• broken glass
• ethanol
• hot liquids

Question 7

what is the risk regarding the scalpel

Answer 7

cuts from a sharp object

Question 8

what is the safety precaution regarding the scalpel

Answer 8

• cut away from fingers
• use forceps to hold sample whilst cutting
• keep the scalpel away from the edge of the desk

Question 9

what is the risk regarding the broken glass

Answer 9

cuts from a sharp object

Question 10

what is the safety precaution regarding the broken glass

Answer 10

• take care when handling glassware
• keep away from the edge of the desk

Question 11

what is the risk regarding the ethanol

Answer 11

irritant/flammable

Question 12

what is the safety precaution regarding the ethanol

Answer 12

• wear eye protection
• keep away from naked flames

Question 13

what is the risk regarding the hot liquids

Answer 13

scalding/burning of skin

Question 14

what is the safety precaution regarding the hot liquids

Answer 14

• handle with care
• use tongs to remove boiling tubes from water bath
• wear eye protection
• keep away from the edge of the desk

Question 15

what are the conclusions for TEMP as the named variable

Answer 15

• as temp INCREASES, the permeability of the CSM INCREASES
• due to proteins in the CSM denaturing as the heat damages the bonds in their tertiary structure, this creates gaps in the CSM which molecules can pass through easily
• at LOW temps, phospholipids have LITTLE energy and are packed CLOSELY to make the CSM rigid, this causes a DECREASE in permeability and RESTRICTS molecules from crossing the CSM
• at VERY LOW temps, ice crystals can form which pierce the CSM and INCREASE permeability as molecules can pass through easily

Question 16

what are conclusions for CONC OF SOLVENTS as the named variable

Answer 16

• ethanol = alcohol, therefore it is LIPID SOLUBLE, meaning it may dissolve the phospholipid bilayer
• ethanol causes to CSM to RUPTURE which releases the pigment from the cell
• HIGHER conc of ethanol will cause MORE disruption to the membrane, meaning MORE gaps will form
• therefore, as the conc of ethanol INCREASES, the permeability of the CSM also INCREASES

Question 17

what are the issues comparing to a colour standard

Answer 17

• subjective
• colour obtained may not match any of the colour standards

Question 18

why do the beetroot slices need to be washed

Answer 18

• wash off any pigment on the surface
• show release is only due to the effect of the variable, temp or solvents

Question 19

why is there the need to regularly shake each test tube containing the cubes of beetroot

Answer 19

• ensures all surfaces of cubes remain in contact with liquid
• maintain a conc gradient for diffusion

Question 20

why is there the need to control the volume of water

Answer 20

• too much water would DILUTE the pigment, therefore the solution would appear lighter/ more light passes through in the colourimeter than expected
• so results are comparable

Question 21

how could it be ensured that the beetroot cubes were kept at the same temp throughout the experiment

Answer 21

• take readings in intervals throughout the experiment of the temp in the tube by using a digital thermometer/ temp sensor
• use corrective measure if the temp has fluctuated

Question 22

what does a high absorbance suggest about the cell membrane

Answer 22

• more permeable/damaged
• more pigment leaking out = surrounding solution is more concentrated

Question 23

explain how pH affects permeability

Answer 23

• high or low pH INCREASES permeability
• transport proteins denature as H/ionic bonds break which CHANGES the tertiary structure