Lipid is the biochemical name for the fats we eat. Lipids are waxy, insoluble hydrophobic macromolecules. Lipids make up fats, oils, steroids and waxes. Lipids also make up a part of the cell membrane in the form of phospholipids, and they are metabolized in the body to store and create energy for cells. Lipids are a major source of nutrition, and they have a specific structure that forms the essential fatty acids that are used for chemical reactions in the body.
The main biomolecule responsible for the head or beginning of the lipid chain is glycerol. Glycerol is a small alcohol molecule that attaches to the second part of the lipid structure--the fatty acid chains. Using dehydration, glycerol attaches to three fatty acids, making up one, simple lipid molecule. The fatty acid chains contain different number of carbons, which creates different kinds of lipid molecules.
Fatty acids are the important part of the lipid molecule for the body. The fatty acids are the components removed from the glycerol head and used for energy and storage in the cell. The fatty acid chains are also what make lipids hydrophobic and insoluble in water. However, the glycerol head is hydrophilic. This is particularly useful in soaps, where the fatty acids dissolve with oil and dirt, and the hydrophilic head combines with water to wash it away. This process is called emulsification.
Most people have heard of the terms "saturated" and "unsaturated" fats. This term is used to describe the fatty acids that make up the lipid structure. Saturated fats are lipid molecules with no double bonds in their fatty acid tails. They pack tightly together to form solid foods like butter. Conversely, an unsaturated lipid has double bonds in the tails which cause kinks and bends in the long fatty acid chain. These unsaturated molecules form the liquid fats like oils.
Another common term used in the food industry is trans fats. The term "trans" refers to the bond structure of the lipid molecule. The alternate bond formation from trans is cis, although cis is not commonly used to describe lipids in foods. These bond formations refer to the position of the hydrogen molecules across the double bond. Cis bonds are formed when the hydrogen molecules lie on the same side of the carbon double bond, and they are associated with the good fats. Trans bonds are formed when the hydrogen molecules lie on opposite sides of the double bond. These bonds are carcinogenic, which make them dangerous in foods.
Phospholipids are an important lipid structure since it makes up the outer membrane of cells. These lipids are comparable to a normal lipid structure, but in place of one fatty acid chain is a phosphate group. This type of lipid is different from normal structures in that it is soluble in both water and oils. Their structure is used as a cell membrane by placing the hydrophilic phosphate group on the outer portion of the cell, allowing it to interact with the interstitial fluid. The inner fatty acid tails of the membrane are hydrophobic, and keep a balance of ions allowed in and out of the cell.