Permeability

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Permeability: The ability of the cell membrane to allow substances to pass through it.

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• The permeability of a cell is affected by
  • pH
  • Temperature
• The effect of such factors can be tested using beetroot
  • Beetroot cells contain a dark purple-red pigment
  • The higher the permeability of the beetroot cell membrane, the more this pigment leaks out of the cell
• Pigment is found in the vacuole of the cell, which is surrounded by a membrane called the tonoplast, While ions are found in the cell cytoplasm
  • Meaning if a chemical was added to the outside of a cell and it was to damage the cell membranes, the ions would leak first and then later followed by the pigment
  • This is because the pigment will take longer to leak out of the vacuole as the chemical damages the cell membrane then the tonoplast allowing pigment to leak
• Remember that membranes contain -
  • Phospholipids
  • Glycoproteins
  • Cholesterol

Water Potential

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Water potential is a measure of how easily water can move from one place to another. Water always moves from an area of higher water potential to an area of lower water potential

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Hydrophilia

• You need to remember that hydrophobic (non-polar) molecules pass through the cell membrane, way easier than hydrophilic (polar) molecules
• Remember that Hydrophobic tails point inwards, Hydrophilic heads point outwards
  • When hydrophobic molecules like Oxygen or Carbon dioxide want to pass through, they simply dissolve in the lipid layer and pass easily
  • When hydrophilic molecules like ions or glucose want to pass through, they have a harder time passing through and usually require carrier proteins to help with it
• Simply - Membranes are more permeable to hydrophobic substances that are non-polar

Beetroot Test - Temperature

• We use a colorimeter to test the amount of pigment in a sample

Method

1. Use a scalpel to cut 4 equal pieces of beetroot from the same beetroot, and rinse each piece with water to remove any pigment already released from cutting
   1. This is a control variable, having the pieces the same size
2. Use a pipette to fill 4 test tubes with 5cm^3 of water, and place a piece of beetroot in each test tube
3. Place each of the test tubes in a separate water bath, ranging from 0 ℃, 20 ℃, 40 ℃, 60 ℃
4. Remove the test tubes after 5 minutes, and remove the beetroot pieces leaving the liquid
5. Use a colorimeter by giving it 5 minutes to stabilise