LIPIDS HAVE MANY USES CONNECTED TO THEIR HYDROPHOBIC PROPERTIES

Of the four major organic molecule types, carbohydrates and lipids are both fairly simple.  Lipids have two major constituent molecules:  glycerol, a 3-carbon molecule with alcohol groups on each carbon;  and, attached to the carbons of the glycerol through the oxygens of the alcohol groups, two or three fatty acids,  unbranched chains of a few or many carbons with various attached groups and a carboxyl group (the acid) at the far end.  Generally the middle fatty acid points away from the top and bottom ones - although, again, in three dimensions it may not be exactly opposite.  3-fatty-acid lipid molecules are virtually nonpolar, which makes them not mix with water.  The fatty acid molecules may be saturated (no double bonds) or unsaturated.  If one of the glycerol carbons is carrying a group that is not a fatty acid, this is still a lipid-type molecule, such as the phospholipids that make up cell membranes.

Lipids have several uses in living things.  A major use is as water barriers, as happens in cell membranes, but these are usually made of lipids whose central fatty acid is a phosphate-containing chain that is hydrophilic while the other two chains are hydrophobic.  Lipids are also used for extra waterproofing, as waxes and oils, when water entry or water loss is a special problem, or as insulation around nerves, trapping a complex soup of water and ions and keeping it close to the nerve membrane where it is needed.  Lipids are often used in animals for long-term storage of energy in the form of fat:  fat in cell inclusions is close to chemically inert, which makes it ideal for long storage, and it has physical properties that can be taken advantage of as well.  When necessary, fats can be broken down into molecules similar to breakdown intermediates of respiration and fed into respiratory processes for energy.  While stored, fats can be used as cushions, as is the fat between your eyeballs and the bony sockets they are in, or as heat insulation, important in animals like us who generate and maintain a constant body temperature.  Several types of messenger molecules, including steroid hormones, are lipid-based molecules as well.