practical: investigating membrane structure & permeability

• IV: temperature of water in water bath, in degrees celsius

  DV: absorbance for each solution from colorimeter
• volume of distilled water is controlled because it ensures consistent exposure environment for beetroot samples preventing variation in pigment diffusion
• controlling time left in water provides equal duration for pigment diffusion across samples ensuring fair comparison
• size of beetroot piece should be left the same because it standardises surface area exposed to water minimising variability in pigment release
• colorimeter used should be controlled as it maintains consistency in readings by using the same instrument and settings for all measurements
• volume of beetroot solution should be kept the same as it ensures uniform pigment concentration for accurate absorbance measurements and comparison between samples
• beetroot is used for the experiment as beetroot cells contain pigment called betalains in their vacuoles, displayed as a dark purple colour. we can observe the effect of temperature on cell membranes in beetroot by observing the leakage of this pigment, indicating the weakening of the cell membrane
• 4. record temp of water bath using thermometer, remove test tubes from water baths and remove cylinders of beetroot

  5. set colorimeter to blue filter and zero using a cuvette with distilled water, filter each sample into a cuvette using filter paper

  6. measure absorbance for each solution, higher absorbance = higher pigment conc. therefore more permeable membrane
• 1. cut beetroot into 8 identical cylinders using a cork borer and wipe clean off any pigment released as a result
  2. 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 and 70 degrees
  3. leave the samples for 15 minutes - pigment will leak out of the beetroot
• conclusion: as the temp increases = permeability of cell surface membrane increases

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