REQUIRED PRACTICAL 4 - PERMEABILITY OF CELL MEMBRANES Flashcards
investigation into the effect of a named variable on the permeability of cell surface membranes
what are cell surface membranes made up of
- phospholipid bilayer
- makes them selectively permeable
how can the permeability of the cell surface membrane be changed
- different variables, like temperature and conc of solvents like ethanol
how can the permeability of a membrane be measured
- 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
what is the equipment needed for this practical
- beetroot
- scalpel
- forceps
- cutting board
- ruler
- tongs
- distilled water
- boiling tubes
- boiling tube rack
- colourimeter
- cuvettes
- filter paper
- timer
- water bath
- thermometer
- ethanol
describe the method for this practical
- 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
name the potential risks for this practical
- scalpel
- broken glass
- ethanol
- hot liquids
what is the risk regarding the scalpel
cuts from a sharp object
what is the safety precaution regarding the scalpel
- cut away from fingers
- use forceps to hold sample whilst cutting
- keep the scalpel away from the edge of the desk
what is the risk regarding the broken glass
cuts from a sharp object
what is the safety precaution regarding the broken glass
- take care when handling glassware
- keep away from the edge of the desk
what is the risk regarding the ethanol
irritant/flammable
what is the safety precaution regarding the ethanol
- wear eye protection
- keep away from naked flames
what is the risk regarding the hot liquids
scalding/burning of skin
what is the safety precaution regarding the hot liquids
- handle with care
- use tongs to remove boiling tubes from water bath
- wear eye protection
- keep away from the edge of the desk
what are the conclusions for TEMP as the named variable
- 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
what are conclusions for CONC OF SOLVENTS as the named variable
- 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
what are the issues comparing to a colour standard
- subjective
- colour obtained may not match any of the colour standards
why do the beetroot slices need to be washed
- wash off any pigment on the surface
- show release is only due to the effect of the variable, temp or solvents
why is there the need to regularly shake each test tube containing the cubes of beetroot
- ensures all surfaces of cubes remain in contact with liquid
- maintain a conc gradient for diffusion
why is there the need to control the volume of water
- 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
how could it be ensured that the beetroot cubes were kept at the same temp throughout the experiment
- 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
what does a high absorbance suggest about the cell membrane
- more permeable/damaged
- more pigment leaking out = surrounding solution is more concentrated
explain how pH affects permeability
- high or low pH INCREASES permeability
- transport proteins denature as H/ionic bonds break which CHANGES the tertiary structure
