PRACTICAL 5 -

  • PRE LAB -

    • 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 the cells

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

  • EQUIPMENT -

    • water baths

    • thermometer

    • distilled water

    • syringe

    • beetroot

    • cork borer

    • white tile

    • knife

    • pipette

    • test tubes and holder

    • colorimeter

    • cuvettes

    • forceps

  • METHOD -

    • cut a beetroot into 8 identical cylinders using a cork borer and wipe/rinse to clean off any pigment released as a result

    • place each of the cylinders of beetroot in 10ml of distilled water, place each test tube in a water bath at a range of temperatures between 0 degrees celsius and 70 degrees celsius

    • leave the sample for 15 minutes, pigment will leak out of the beetroot, record the exact temperature of the water bath using a temperature

    • remove the test tubes from the water baths and remove the cylinders of beetroot from them, and move the liquid into clean cuvettes

    • calibrate the colorimeter using a control sample of distilled water, add the sample and record the absorbance value, renter the control cuvette and then repeat using the other samples

    • a higher absorbance indicates higher pigment concentration so a more permeable membrane

  • RISK ASSESMENT -

  • SCALPEL -

    • RISK - cuts from sharp objects

    • SAFETY PRECAUTION - cut away from fingers, use forceps to hold sample, keep edge away from edge of desk

    • EMERGENCY PROTOCOL - elevate and apply pressure to cuts and seek assistance

    • RISK LEVEL - low

  • BROKEN GLASS -

    • RISK - cuts from sharp objects

    • SAFETY PRECAUTION - take care when handling glassware and keep away from desk edge

    • EMERGENCY PROTOCOL - elevate and apply pressure to cuts, dont remove glass from wounds and seek medical help

    • RISK LEVEL - low

  • HOT LIQUIDS -

    • RISK - scalding

    • SAFETY PRECAUTION - handle with care, use tongs to remove boiling tubes from water baths and wear eye protection

    • EMERGENCY PROTOCOL - run under cold water and seek medical assistance

    • RISK LEVEL - low

  • GRAPH -

    • plot a graph of absorbance (Abs) against temperature (degrees celsius)

  • CONCLUSION -

    • as temperature increases, the permeability of the cell-surface membrane will also increase, this is because the proteins in the membrane denature as the heat damages the bonds in their tertiary structure

    • this creates gaps in the membrane for the molecules to pass through

    • at low temperatures, phospholipids have little energy and are packed tightly together which makes the membrane rigid, this causes a decrease in permeability and restricts molecules from crossing the membrane

    • but at very low temperatures, ice crystals can form which can pierce the cell membrane and increase the permeability