Fluid Mosaic Model

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Structure and Function of Cell Membranes

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates. This gives the membrane a fluid character.

Function of the plasma membrane

The plasma membrane defines the borders of cells and most organelles.
The plasma membrane is partially permeable. This means that the membrane allows some materials to freely enter or leave the cell/organelle, while other materials cannot move freely.

Structure of phospholipids

A phospholipid is a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group.
The molecules arrange themselves into a bilayer which ranges from 5 to 10 nm in thickness.
The hydrophilic phospholipid head faces outwards and the hydrophobic fatty acids faces inwards.

Structure of cholesterol

Cholesterol is a lipid that sits with phospholipids in the core of the membrane.
Cholesterol is not found in bacterial cell membranes.
Cholesterol molecules make the membrane more rigid.
This explains why cholesterol helps to maintain the shape of animal cells.

The Fluid Mosaic Model

The phospholipid bilayer forms the ‘fluid’ part of the ‘fluid mosaic’ model.
The ‘mosaic’ part is made up from the various proteins, carbohydrate and lipid molecules that punctuate membranes.

Function of membrane bound proteins

Membrane bound proteins are large molecules embedded in the bilayer, they can form open pores that allow the diffusion of large molecules across the bilayer or can be transport proteins that bind to specific molecules and carry them across the membrane.
ATP-synthase is an example of a membrane bound protein and catalyses the production of ATP during oxidative phosphorylation on the inner mitochondrial membrane.

Function of glycoproteins and glycolipids

Some proteins and lipids in cell membranes have carbohydrate chains attached to them.
These chains are vital in interacting with the cell’s immediate environment.
Some glycolipids and glycoproteins are able to form hydrogen bonds with water molecules surrounding the cell, helping to stabilise the membrane.

Cell receptors

Other glycolipids and glycoproteins act as cell signalling receptors and have active binding sites for communication molecules such as hormones and drugs.
When these receptors bind a target molecule, the glycomolecule undergoes a conformational change and initiates a chain of reactions known as a cascade that can lead to a cell-level response to the binding of certain signalling molecules.

The permeability of cell membranes

The permeability of cell membranes- how easy it is for substances to pass through them – can be influenced by several factors including:

Temperature

Higher temperatures increase the fluidity of the membrane, increasing its permeability.
Using a water bath can help keep temperature constant.

Solvent concentration

The more easily the phospholipid bilayer is dissolved, the more permeable the membrane is.
Solvent concentration can be controlled by using the same solvent at the same concentration for each trial.

pH

pH affects the protein structure in the cell membrane.
Buffer solutions can be used to control the pH.

Investigating Cell Membrane Permeability

Beetroot is often used as a model because the release of the coloured pigment is easy to quantify using colorimetry. The steps involved are:

1) Collect beetroot samples

Use a cork borer to collect samples of uniform diameter.
Cut discs of a uniform depth using a sharp scalpel on a white tile and rinse in cold water. This removes excess pigment that has leaked through physically broken cell membranes.

2) Add ethanol

Prepare at least five concentrations of ethanol (e.g. 0%, 10%, 20%, 30%, 40%) in beakers.
Place the discs into the corresponding solution for 10 minutes.
Make sure the samples are completely covered by the ethanol solutions and mixed frequently throughout the 10 minutes.

3) Remove the discs

Remove the discs from the solutions to prevent further changes and allow a fair comparison between the experiments.

4) Calibrate the colorimeter

Calibrate a colorimeter by using a cuvette of distilled water at an absorbance of 520nm.
The cuvettes must be dry and the clear sides must not be touched to prevent potential errors in the readings.

5) Measure absorbance

Measure the absorbance of each solution.
Plot the results in a graph with concentration on the x-axis and absorbance on the y-axis.
The darker the solution, the more pigment has been released. This is reflected in a higher reading for absorbance.