AS Biology - Cell Structure - Cell Membranes - Part 2

The permeability of cell membranes can be determined by factors such as :

• temperature
• alcohol concentration

There is an experiment that is used to determine cell membrane permeability using beetroot...

• Beetroot cells contain a coloured pigment that leaks out
• The higher the permeability, the more pigment leaks out

How temperature affects beetroot membrane permeability:

(1) Cut 5 equal sized pieces of beetroot and rinse to remove any pigment released during cutting

(2) Place each piece in five different test tubes with 5cm3 of water

(3) Place each test tube in a water bath at a different temperature, e.g. 10 degrees Celsius -----> 50 degrees Celsius

(4) Remove the beetrot from the tubes leaving only the coloured liquid

(5) Use a colorimeter - a machine that passes light through the liquid and measures how much of that light is absorbed. 
-The higher the absorbance -----> more pigment released -----> higher membrane permeability 

Temperatures below 0 degrees Celsius:

• Phospholipids don't have much energy so thay can't move very much
• They're packed closely together and the membrane is rigid
• Channel proteins and carrier proteins denature, increasing membrane permeability
• Ice crystals may form and pierce the membrane making it highly permeable when it thaws

Temperatures between 0 degrees Celsius and 45 degrees Celsius:

• Phospholipids move around and are not packed as close together - membrane's partially permeable
• As temperature increases the phospholipids move more because they've more energy - increases membrane permeability

Temperatures above 45 degrees Celsius:

• The phospholipid bilayer starts to melt so the membrane becomes more permeable
• Water inside the cell expands putting pressure on the membrane
• Channel proteins and carrier proteins in the membrane denature so can't control what enters or leaves the cell - increasing permeability.

How does alcohol concentration affect cell membrane permeability?

• As the alcohol concentration increases, so does the cell membrane permeability
• This is because alcohol dissolves lipids in the cell membrane
• This then causes the membrane to lose its structure

Drugs and cell membrane receptors

• Many drugs work by binding to receptors in the cell membrane
• They either trigger a response in the cell, or block the receptor and prevent it from working

• Cells need to communicate to control processes inside the body and respond to changes in the environment
• One cell releases a messenger molecule (e.g. a hormone)
• This molecule travels to another cell (e.g. in the blood)
• The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

Cell Membrane Receptors

• Membrane-bound proteins act as receptors for messenger molecules
• Receptor proteins have specific shapes
• Only messenger molecules with a complimentary shape can bind to them
• Different cells have different types of receptors - they respond to different messenger molecules
• A cell that responds to a particular messenger molecule is called a target cell