That we experience life as we know it is partially due to the evolution of biological membranes. They allowed to define an inner and outer part of cells and provided to protect life from a partly hostile environment. The so called plasma membrane surrounds cells and inside of each cell biological membranes confine regions of various functional properties. Areas as such are for example the endoplasmic reticulum, the Golgi apparatus or the mitochondria. The latter one is responsible for the energy production of biological cells, whereas the other ones are related to the synthesize and the distribution of important macromolecules called proteins.

Biological membranes, besides playing a role in the structuring of cells have also a functional role. They allow to maintain electrochemical gradients, provide a mean of communication between the inner and outer and they are involved in capturing and releasing energy.

Shown is a DMPC lipid. This lipid belongs to the family of phospholipids. This means that in the head group one finds phosphorus. This particular lipid has two fatty acid chains with each including 14 carbon atoms. Pic: Avanti Polar Lipids Inc.

Lipids are amphiphilic molecules which means that they consist of a part which likes water and a part which does not like it. From our daily experience we know a subgroup of lipids very well. These are fats. The head group of lipids likes to be exposed to water, it is hydrophilic. Fatty acid chains which accompany the head group are hydrophobic, meaning that this is the part of a lipid which does not want to be exposed to water. The fatty acid chains consist of a series of CH bonds. There are many different kinds of lipids which might differ in head group, in chain length or in the number of double bonds in the fatty acid chains. The most important lipid species belong to the groups of phospholipids and sterols. An example of the latter one is cholesterol. Phospholipids are characterized by the existence of  phosphorus in the head group.

Displayed are the protein aquaporin in a membrane. This protein is believed to play an important role in the permeation of water through biolgical membranes.

If one would take a biological membrane apart and one would weight each of the three main components one would find that concerning the membrane weight 30-80 percent of the weight comes from the lipids, 20-60 percent from the proteins and 0-10 percent from the sugars.

Biological membranes are self-organizing structures. The assembling process is an interplay of various interactions. To understand this at first we need to learn about the self-aggregation of lipids in water and about some interesting physical effects of lipid aggregates. This will follow in a separate article.

Contact the author: hseeger (at) xscience.info