TABLE OF CONTENTS In biophysics and biochemistry, it makes little sense to discuss lipids except within the context of their function in biological membranes. Biological membranes are organized, sheet-like, self-assembled units that consist mainly of proteins and lipids. Organic chemists define a lipid as a substance that is essentially insoluble in water and that can be extracted from cells by low-polarity organic solvents such as ether or chloroform. It is sufficient for our purposes to treat a lipid as a fat. This definition is far too broad to serve us for long but is sufficient to get us started. Membranes are important because they are highly selective permeable barriers that give cells their individuality. Membranes also contain receptors and signal generators for biological communication. Finally, energy conversion takes place within some types of biological membranes.
Stryer lists seven common features of all biological membranes. These are:
Membranes are sheetlike structures that form closed boundaries between regions of different composition. Typically, they are only a few molecules thick roughly 6 to 10 nm thick.
Membranes consist mainly of proteins and lipids. The composition varies from 80% lipid to 80% protein in most membranes. There are also carbohydrates attached to the lipids and proteins.
Membrane lipids are relatively small and are amphiphilic. Because they have both hydrophobic and hydrophilic portions, they spontaneously form closed bilayer structures in water, with polar surfaces and hydrophobic cores.
Specific proteins mediate various functions of the membrane by acting as gates, pumps, transducers, and other similar functions.
Membranes are...
