(c) factors effecting membrane permeability

What affects membrane permeability?

• Anything that affects the structure of a cell membrane can affect its permeability
• You need to know the effects that solvents and temperature can have on cell membranes

How do solvents affect membrane permeability?

• The permeability of cell membranes depends on the solvent surrounding them
• Some solvents (such as ethanol) dissolve the lipids in a cell membrane
• The membrane loses its structure
• This increases membrane permeability

How do different solvents and concentrations affect permeability?

• Some solvents increase membrane permeability more than others
• Ethanol increases membrane permeability more than methanol
• Increasing the concentration of the solvent will increase membrane permeability

What is a solvent?

• A solvent is any substance (usually a liquid) that can dissolve other substances

How does temperature affect membrane permeability?

• Cell membranes are affected by temperature
• Temperature affects how much the phospholipids in the bilayer can move
• This affects membrane structure and permeability

What happens to membranes at temperatures below 0°C?

• The phospholipids don’t have much energy
• They can’t move very much
• They’re packed closely together
• The membrane is rigid

What happens to proteins at temperatures below 0°C?

• Channel proteins and carrier proteins denature (lose structure and function)
• This increases the permeability of the membrane

What other effects occur below 0°C?

• Ice crystals may form
• These pierce the membrane
• This makes it highly permeable when it thaws

What happens to membranes between 0°C and 45°C?

• The phospholipids can move around
• They aren’t packed as tightly together
• The membrane is partially permeable

How does increasing temperature (0–45°C) affect permeability?

• As the temperature increases the phospholipids move more
• This is because they have more energy
• This increases the permeability of the membrane

What happens to membranes above 45°C?

• The phospholipid bilayer starts to melt (break down)
• The membrane becomes more permeable

What happens inside the cell at high temperatures?

• Water inside the cell expands
• This puts pressure on the membrane

What happens to proteins above 45°C?

• Channel proteins and carrier proteins denature
• They can’t control what enters or leaves the cell
• This increases the permeability of the membrane

Can proteins denature at low temperatures?

• Proteins (e.g. enzymes) denature at high temperatures
• Very cold temperatures (below 0°C) can also cause proteins to denature

Real-life example of membrane permeability and heat?

• When you cook fruit or vegetables, the food softens and liquid is released
• This is partly because the cell membranes start to break down
• They become more permeable

How can membrane permeability be investigated?

• You can investigate how variables (e.g. solvent concentration and temperature) affect membrane permeability
• This is done using beetroot

Why is beetroot used in permeability experiments?

• Beetroot cells contain a coloured pigment
• The pigment leaks out when membranes become permeable
• The higher the permeability, the more pigment leaks out

How do you prepare beetroot samples for the experiment?

• Use a scalpel to carefully cut five equal sized pieces of beetroot
• Make sure you cut on a cutting board
• Rinse the pieces to remove any pigment released during cutting

How do you set up the beetroot experiment?

• Add the five pieces to five different test tubes
• Each test tube should contain 5 cm³ of water
• Use a measuring cylinder or pipette to measure the water

How do you apply temperature in the experiment?

• Place each test tube in a water bath at a different temperature
• Example temperatures: 10°C, 20°C, 30°C, 40°C, 50°C
• Leave them for the same length of time
• Use a stopwatch to measure time

What do you do after heating the beetroot?

• Remove the pieces of beetroot from the test tubes
• Leave just the coloured liquid

How do you use a colorimeter in this experiment?

• Switch the colorimeter on
• Allow five minutes for it to stabilise
• Set it to a blue filter (or wavelength of about 470 nm)

How do you calibrate the colorimeter?

• Add distilled water to a cuvette
• Place it in the colorimeter
• Calibrate the machine to zero

How do you measure absorbance?

• Use a pipette to transfer liquid from a test tube to a clean cuvette
• Place the cuvette in the colorimeter
• Read and record the absorbance

How do you repeat measurements?

• Repeat for all remaining test tubes
• Use a clean pipette and cuvette each time

How do you interpret the results?

• The higher the absorbance reading, the more pigment released
• The more pigment released, the higher the membrane permeability

What can be done with the results?

• The colorimeter can be connected to a computer
• Software can be used to collect data
• A graph of the results can be drawn

What are key safety precautions in this experiment?

• Assess all safety risks before starting
• Be careful when using a scalpel
• Cut away from yourself
• Ensure the blade is clean and sharp
• Place test tubes gently into the water bath
• Use tongs to remove them as they may be hot

What are important practical tips?

• Fill cuvettes about three quarters full
• Avoid splashing hot water when using a water bath