Glossary - L

lipid

Any of a group of organic compounds that are

• 1.greasy to the touch,
• 2.insoluble in water
• 3.soluble in alchohols and ether.

Lipids are the main constitieunts of fats and other esters . They are important as major fuels in cell metabolism. Along with water, proteins, nucleic acids and carbohydrates, lipids are essential biomolecules in the structure and function of living matter.
Polar lipids have amphiphillic properties and are the building blocks of cell membranes and liposomes.

liposomes (lipid vesicles)

Spherical closed structures of curved lipid bilayers (vesicles) which entrap in their interior a part of the solvent in which they freely float. They may be composed of one or several concentric bilayer membranes.

Their sizes ranges in the range from 20nm to several dozens micrometers, while the thickness of their membranes is about 4nm.
Lecithin is a common example.

lyotropic

Systems (usually in the context of liquid crystals ) whose properties (e.g. phase) are dependent on their concentration rather than on the temperature.

How a change in the concentration of the molecules affects the shape and type of structures formed may naively be understood by realising that a change in the concentration not only effects how a structure (e.g. micelle) interacts with another structure but also affects the forces between molecules within each aggregate thereby modifyiong the size and shape of the structures. This is because the forces that hold the amphiphillic molecules together in these structures are not covalent bonds but are the weaker hydrophobic, hydrogen-bonding and electrostatic screening interactions.