2.5.2 Membrane Structure & Permeability

• The permeability of cell membranes is affected by different factors or conditions, such as:
  • Temperature
  • Solvent concentration

Temperature

• Proteins and lipids (the major components in cell membranes) are both affected by temperature
• As temperature increases, lipids become more fluid
  • This increased fluidity reduces the effectiveness of the cell membrane as a barrier to polar molecules, meaning polar molecules can pass through

• At higher temperatures, any diffusion taking place through the cell membrane will also occur at a higher speed (due to increased kinetic energy)
• Changes in membrane fluidity are reversible
  • If temperatures decrease, the lipids will return to their normal levels of fluidity)

• At a certain temperature (often around 40°C) many proteins (including those in cell membranes) begin to denature
  • This disrupts the membrane structure, meaning it no longer forms an effective barrier
  • As a result, substances can pass freely through the disrupted membrane
  • This process is irreversible

Solvent concentration

• Organic solvents can increase cell membrane permeability as they dissolve the lipids in the membrane, causing the membrane to lose its structure

• You can investigate how different factors affect membrane structure and permeability using beetroot
• • Beetroot cells contain a dark purple-red pigment
  • The higher the permeability of the beetroot cell membrane, the more of this pigment leaks out of the cell

Investigating the effect of temperature on membrane permeability

Apparatus

• Scalpel
• Cork borer (optional)
• Cutting board
• Ruler
• Test tubes
• Water baths
• Stopwatch
• Colorimeter (a machine that passes light through a liquid sample and measures how much of that light is absorbed)

Method

• Using a scalpel, cut five equal-sized cubes of beetroot
  • The pieces must have the same dimensions so that they all have equal surface areas and volumes, as these factors could affect the rate at which the pigment leaks out
  • A cork borer can also be used, as long as the cores are cut to the same length

• Rinse the beetroot pieces
  • To remove any pigment released during cutting

• Add the beetroot pieces to five different test tubes, each containing the same volume of water (e.g. 5cm³)
• Put each test tube in a water bath at a different temperature (e.g. 10℃, 20℃, 30℃, 40℃, 50℃) for the same length of time
  • The time should be long enough to allow the pigment to diffuse into the water (e.g. around 30 minutes)

• Remove the beetroot pieces, leaving just the coloured liquid in the five test tubes
• Use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid
  • The higher the absorbance, the more pigment must have been released, due to a greater membrane permeability

Results

• The general pattern you would expect to see is that as temperature increases, membrane permeability also increases
  • As temperature increases, the phospholipids within the cell membrane move more because they have more energy
  • Increased movement means the phospholipids are not as tightly packed together, increasing the permeability of the membrane
  • At high temperatures, the phospholipid bilayer may even start to melt and breakdown, further increasing the permeability of the membrane
  • In addition, the volume of water inside the cells expands, putting pressure on the membrane, causing channel and carrier proteins to deform so they can no longer control what enters and leaves the cell. These factors also increase the permeability of the membrane
  • Temperature also affects the conformation (3D shape) of proteins as at high temperatures the intermolecular forces between amino acids are broken which affects the protein’s specificity and function

• If experimenting with temperatures below 0℃, membrane permeability may also be increased (once the cells have thawed again)
  • Increased permeability can be caused by channel or carrier proteins deforming at these low temperatures
  • Ice crystals that form can also pierce the cell membrane, making it highly permeable

Example results showing the effect of temperature on membrane permeability

Limitations

• Cuvettes are the small cuboid containers that hold the liquid to be measured in a colorimeter
• Cuvettes may differ in thickness (very slightly). A thicker (or scratched) cuvette will absorb slightly more light than a thinner unscratched cuvette
  • Solution: use the same cuvette for every reading, or repeat the investigation many times and find a mean

• The beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others
  • Solution: cut the discs as accurately as possible using a scalpel and ruler, and repeat each investigation several times to find a mean

• Some parts of beetroot tissue have more pigment in their cells than others
  • Solution: conduct several repeats, using different parts of the beetroot and find a mean