Lipids

Lipids are an extremely important class of molecules for all life that we know of. Lipids are fats, but they do much more than you might know. Lipids form the major component of every cell membrane. They form smaller compartments in cells such as vesicles. They form the crucial insulation around neurons that allow them to transmit nerve signals at high speed.

The simplest lipids: Fatty acids

The simplest kind lipids are the fatty acids (shown). They're called that because of the carboxylic acid (COOH) "head" and the fatty or aliphatic (CH-rich) "tail".

A molecule that is aliphatic contains long chains of hydrocarbons, usually saturated with all the hydrogens the chain can incorporate.

The acidic head group is soluble in water because it contains elements that form hydrogen bonds (H-bonds) with water. The head group is also called hydrophilic (water loving).

The aliphatic tail, on the other hand is hydrophobic (water fearing). The tail does not dissolve in water and tends to bury itself in substances, like other aliphatic tails, that are similar to it: Like dissolves like. This will lead to some profound effects that determine much of the structure of cells, organelles and tissues, as we shall see.

Double bonds in the fatty-acid tail

Lipids in water – micelles

When placed in aqueous (water) solution, fatty acids and other lipids tend to form structures that minimize the exposure of the aliphatic tails to water. One such structure is the micelle shown here.

We'll use this little abbreviation for a fatty acid. The blue circle is the hydrophilic head and the curvy line is the hydrophobic tail:

Fatty acid hydrophilic head groups orient themselves out toward the solvent, protecting the aliphatic tails on the inside, where much of the water of solution can be excluded.

Micelles form spontaneously whenever lipids are dissolved in water.

Liposomes

In a similar manner, lipids can also spontaneously arrange into this configuration (cross section shown), a liposome. The core is filled with aqueous solution, so the head groups are still exposed to water, and the tails are clearly buried in a region protected from water. This arrangement is called a lipid bilayer ("two layers").

The membranes around cells are a more complicated kind of liposome, generally made of a different kind of lipid called a phospholipid, but the idea is the same.

Phospholipids

Phospholipids are the most abundant kind of lipid in cells. They form the bulk of cell membranes and do a few other jobs.

Phospholipids have two main characteristics that distinguish them from the simple lipids we looked at above. First, the head group contains a phosphate (OPO₃) group as the hydrophilic portion of the molecule. Second, phospholipids have two hydrophobic tails.

Phospholipid bilayers

The diagram on the right shows a common shorthand drawing for phospholipids, a circle with a couple of wavy lines sticking out from it. To the right is a schematic of a phospholipid bilayer, which forms the major part of most cell membranes.

It is thought that the presence of a kink in one of the tails causes the membrane to have a little more space—be a little looser—so that water and other molecules can move more freely through it.

Note: This sketch is highly schematic. Phospholipid membranes don't look quite this organized, and they contain many other structures like trans-membrane ("across the membrane") proteins and other molecules.

Cholesterol

Cholesterol is another very important class of lipid molecule. The head group of cholesterol is a flat and relatively rigid system of four cyclic hydrocarbons which, when inserted into a phospholipid membrane, serve to stiffen it.

While cholesterol is a key component of many (but not all) kinds of cell membranes and other structures that involve lipids, it can also be a problem, particularly for older humans with high-fat diets. Because cholesterol is hydrophobic, it tends to clump to itself in aqueous solution, like blood. Cholesterol in the blood system (serum cholesterol) can stick to itself in clumps and "plaques" large enough to block important arteries, causing cardiovascular disease.

Trans-membrane proteins

The cells of more complicated organisms, eukaryotes, have membranes that are selectively permeable to most substances. That means no chemical substance can just cross the membrane boundary freely.

Instead eukaryotic cells have a wide variety of trans-membrane (literally "across the membrane") proteins that form specific channels for specific substances. For example, the potassium (K)-channel protein is shown schematically below.

The alpha helices of the K-channel protein are hydrophobic, so being buried in among the

Lipids as electrical insulators

Just as one more example of the importance of lipids to higher organisms, consider neurons, the long cells that form the wiring (nerves) of the body and brain. Electrical signals, in the form of a wave of ion movement across membranes, carry nervous impulses between the brain & spinal chord and the other control, motor and sensory neurons. These signals would not propagate at the high speeds needed if those "wires" – the long axons of the neurons weren't electrically insulated from their surroundings.

The insulation of neurons is in the form of special cells called Schwann cells that produce a lipid mixture called myelin. Myelin consists of several lipids, but mainly the galactoceramide molecule, shown below, and cholesterol.