Lipids are a structurally heterogeneous group of compounds.

Their common feature is their insolubility in water, due to the presence of large hydrophobic groups.

For example steric acid has a hydrophobic "tail" of 17 carbon atoms and their associated hydrogens attached to carboxylic acid (COOH) group.

This generates a molecule with two distinct domains.

The hydrocarbon tail is extremely hydrophobic and so insoluble in water.

The carboxylic acid is extremely hydrophilic.

Molecules with both hydrophobic and hydrophilic groups are termed amphipathic.

One part hates water the other part loves it.

In aqueous solution, lipids tend to self-assemble into micelles, bilayers and other more complex structures.

In these structures, the lipid's hydrophobic tail is sequestered away from contact with water.

Its hydrophilic head is immersed in water.

a bilayer

The cell boundary

The common feature of all living cells is a distinct, lipid-rich barrier, the plasma membrane.

This membrane defines the boundary between the outside and the inside of the cell.

The difference between the two is profound.

Outside is mostly water, with few complex molecules.

Inside is a concentrated solution of proteins, nucleic acids, and smaller molecules - the cytoplasm.

This bounded system, or cell, has the properties of life. It can reproduce itself using energy taken from beyond the boundary and exporting disorder.

Cells are capable of directed movement and complex behaviors.

In multicellular organisms, the membrane is the site of cell-cell coordination and communication.

Visualizing the plasma membrane

The plasma membrane is invisible in the light microscope, cause it is so thin, only ~5-10 x 10^-9 meters or nanometers (nm) across.

Current light microscopes have a resolution limit of ~300 nm.

To see the details of the plasma membrane we need to use an electron microscope.

Looking at the surface of a cell by electron microscopy, we typically see two distinct structures.

The outer layer is the glycocalyx. It is composed primarily of carbohydrates and proteins.

In animal cells, it often appears amorphous, that is unstructured. It is also quite flexible.

In bacteria and archaea, fungi and plants, the glycocalyx is thicker and forms a rigid cell wall that provides structural support.

Building a plasma membrane:

The plasma membrane is built on a foundation of lipids. All earthly organisms use lipids built on glycerol.

Glycerol has three hydroxy groups. In the phospholipids, one of these groups is linked to a phosphate group.

Phosphate is derived from phosphoric acid. The phosphate group is added to one of glycerol's terminal hydroxy groups in a standard condensation reaction.

Phosphoglycerol even more hydrophilic than glycerol itself.

The other two glycerol -OH groups are available to be coupled to other groups.

Typically this involves condensation reactions and the release of water.

In the Archaea, branched isoprene chains are attached to the glycerol group via ether linkages.

In the bacteria and eukarya, unbranched fatty acids are linked to glycerol via an ester linkages.

In both cases, the resulting lipids organize themselves to minimize the exposure of their hydrophobic groups to water.

This leads to the formation of a bilayer membrane.

The structure of a lipid bilayer changes with temperature. At low temperature the membrane solidifies; the lipid tails pack closely with one another.

This is because the motions of the molecules are not vigourous enough to disrupt the van der Waals interactions between the hydrophobic side chains.

As temperature increases, the membrane melts. The lipids tails become increasingly disordered.

Membrane fluidity is critical for the correct functioning of the proteins embedded within the membrane.

Cells control membrane fluidity by regulating membrane composition, particularly whether it uses lipids whose hydrocarbon chains are saturated or unsaturated.

In a saturated hydrocarbon, the carbons are together linked by single bonds. The hydrocarbon chain is straight.

In an unsaturated hydrocarbon chain, some of the carbons are linked by double-bonds.

Instead of two hydrogens attached to each doubled-bonded carbon, there is only one.

When a lipid is dehydrogenated, hydrogens are removed and -C=C- bonds are introduced.

The presence of a -C=C- bond leads to a kink in the hydrocarbon chain. Such kinked chains cannot pack together as regularly as can saturated hydrocarbon chains.

Compare the saturated fatty acid stearic acid to the unsaturated fatty acid oleic acid. Both have the same number of carbons in their hydrocarbon chains.

While stearic acid melts at 69°C oleic acid melts at 13°C.

Cholesterol is another major lipid component of eukaryotic membranes.

In many eukaryotic cells, the membrane contains one cholesterol molecule per molecule of phospholipid.

Cholesterol is a relatively rigid molecule with single hydrophilic -OH group and a large and rigid sterol-derived hydrophobic group.

Cholesterol acts to maintain membrane fluidity and stiffeness. It also decreases the permeability of membranes to water and other small molecules.

It is essential for normal membrane function.