The phospholipid bilayer which the cell membrane is an example of, is composed of various cholesterol, phospholipids, glycolipids and proteins. Below is an example of a simple phospholipid bilayer.

The smaller molecules between the phospholids is Cholesterol which helps to give rigidity or stability to the membrane. The phospholipids are the hydrophilic circles with hydrophobic tails. And since most of the cell and area surrounding the cell is made up of water, these fatty acid tails always 'push' away from the water. Causing either a bilayer as above or a 'micelle' which is a single layer circle of phospholipids with the tails pointing in.

There are different are 10 main types of lipids in cell membranes. Each type of cell or organelle will have a differing percentage of each lipid, protein, and carbohydrate. The main types of lipids are:

2.1.1. Phospholipids:

Phospholipids are made up of a hydrophilic head and a hydrophobic tail. The head group has a 'special' region that changes between various phospholipids. This head group will differ between cell membranes [types of cells] or different concentrations of specific 'head groups'. The fatty acid tails call also differ, but there is always one saturated and one unsaturated 'leg' of the tail.

Phospholipids are 2 fatty acids one saturated and one unsaturated (shown by the double bond) that are linked to a glycerol.

2.1.2. Cholesterol:

I have symbolized cholesterol as: Cholesterol is a major component of cell membranes and serves many other functions as well. Cholesterol helps to 'pack' phospholipids in the membranes, thus giving more rigidity to the membranes. Also cholesterol serves diverse functions such as: it is converted to vitamin D (if irradiated with Ultra Violet light, modified to form steroid hormones, and is modified to bile acids to digest fats.

2.1.3. Semi-permiable/Osmosis

The membranes of cells are a fluid, they are semi-permeable, which means some things can pass through the membrane through osmosis or diffusion. The rate of diffusion will vary depending on the its: size, polarity, charge and concentration on the inside of the membrane versus the concentration on the outside of the membrane. When something is permeable it means that something can spread throughout, like (The perfume is permeating the room.). Here is a list of some molecules and how they relate to passing through the membrane without assistance, in other words, through osmosis:

Hydrophobic Molecules:

Small uncharged Polar Molecules:

Large Uncharged Polar Molecules:

Ions:

Various substances will pass through the membranes at varying rates through osmosis.

2.1.4. Membrane proteins:

One role of proteins in cells is for transport of molecules/ions into or out of cells. Three methods of doing this are through active, facilitated or passive transport. Other roles are in cell recognition, receptors, cell to cell communication. There is more information on membrane proteins and other proteins in later sections.

2.1.5. Hydrophobic/Hydrophilic

A very simplistic idea of what these characteristics are is:

Hydrophilic and hydrophobic are, respectively, the like and dislike. Hydrophilic areas of a phospholipid, or a protein are 'attracted' to water, and hydrophobic regions are repelled by water.

2.1.6. Self-assembly

Self-assembly occurs due to the thermodynamics, if the phospholipids are in a water (or other polar solution) the tails will want to be 'away' from the solution. The could all go to the top (like oil on water), or they could have the tails point toward each other. With the tails pointing toward each other, this could form 2 different formations.

First would be a micelle which would like like a ball with the phospholipid heads on the outside and the tails pointing together like this or in the form of a lipid bilayer:

Please send questions/comments/suggestions to: Mark Dalton at mwd@cray.com