Phospholipids and triglycerides are two types of lipid compounds, where "lipid" is the biochemical term for what we generally call "fat." There are some marked structural similarities between phospholipids and triglycerides, but their structural differences result in very disparate physical properties and different uses in the body and cells.
Phospholipids
Phospholipids are large molecules made up of four smaller components. They have a glycerol backbone, where a glycerol is an alcohol-like molecule. Attached to this are a phosphate group -- a combination of phosphorus and oxygen -- and two fatty acids. Fatty acids are long chains of carbon and hydrogen with two oxygen atoms at the end closest to where they bond to the glycerol backbone. While the fatty acids and glycerol backbone of phospholipids dissolve poorly in water, the phosphate group dissolves well in water.
Triglycerides
Like phospholipids, triglycerides have four separate parts, one of which is a glycerol molecule. Instead of two fatty acids bonded to the glycerol backbone, however, triglycerides have three. The entire triglyceride molecule dissolves poorly in water because of its chemical makeup. The physical properties of triglycerides depend upon the identity of the fatty acids they contain. Triglycerides with fatty acids that have no carbon-carbon double bonds are saturated fats and have high melting points. Those with carbon-carbon double bonds are unsaturated fats, and have lower melting points.
Membranes
One of the most important properties of phospholipids is their ability to dissolve partially in water and partially in fat. The fatty acid tails of phospholipids have an attraction to the tails of other phospholipids and tend to clump together, while the phosphate heads have an attraction to water. As a result, phospholipids spontaneously assemble into two-layered membranes when exposed to water, where all the tails point in toward the middle and the heads point out. This is how cell membranes structure themselves.
Diet and Fat
The fats you take in as part of your diet are triglycerides. Triglycerides are also storage fats that you keep in adipose tissue for periods of fasting. You can burn dietary triglycerides for energy -- they contain nine calories per gram. You can also make triglycerides from excess protein and carbohydrate that you consume. While the caloric content of saturated and unsaturated fat is identical, saturated fat is less heart healthy than unsaturated fat.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
