5 - Temperature & membrane permeability

What is the effect of the cell-surface membrane being made up of a phospholipid bilayer?

It is selectively permeable

What happens when the cell-surface membrane has a higher permeability?

More pigment leaks out of cells:

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

What is the equipment required for this practical?

• Water baths & thermometer

• Distilled water & syringe

• Beetroot & cork borer

• White tile & knife

• Pipette & test tubes

• Colorimeter & cuvettes

• Forceps

What is the method for this practical?

1. Cut a beetroot into 8 identical cylinders using a cork borer & rinse to clean off any pigment released

2. Place each cylinder in a test tube of 10ml of distilled water. Place each test tube in a water bath at a range of temperatures between 0 - 70°C

3. Leave the samples for 15 minutes & record the exact temperature of the water bath using a thermometer

4. Remove the test tubes from the water baths & remove the cylinders of beetroot from the test tubes. Decant the liquid into clean test tubes

5. Set the colorimeter to a blue filter & zero using a cuvette with distilled water. Filter each sample into a cuvette using filter paper

6. Measure the absorbance for each solution

What is the risk, safety precaution & in emergency for the hazard: scalpel

Risk:

• cuts from sharp object

Safety precaution:

• cut away from fingers & use forceps to hold sample whilst cutting

• keep away from edge of desk

In emergency:

• elevate cuts & apply pressure

• seek medical assistance

What is the risk, safety precaution & in emergency for the hazard: broken glass

Risk:

• cuts from sharp object

Safety precaution:

• take care when handling glassware

• keep away from edge of desk

In emergency:

• elevate cuts & apply pressure

• don’t remove glass from wound & seek medical assistance

What is the risk, safety precaution & in emergency for the hazard: hot liquids

Risk:

• scalding

Safety precaution:

• handle with care & use tongs to remove boiling tubes from water bath

• wear eye protection

• keep away from edge of desk

In emergency:

• run burn under cold water

• seek medical assistance

What is the risk, safety precaution & in emergency for the hazard: ethanol

Risk:

• irritant/flammable

Safety precaution:

• wear eye protection & keep away from naked flames

In emergency:

• wash eyes & skin with cold water

What graph should be plotted for this practical?

Absorbance (y-axis) against ethanol concentration/temperature (y-axis)

What is the conclusion from this practical?

As temperature increases, the permeability of the cell-surface membrane increases:

• the proteins in the membrane denature as the heat damages the bonds in the tertiary structure (creates gaps in the membrane)

• at low temperatures, phospholipids have little energy & are packed closely together to make the membrane rigid (decreases permeability)

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