Lipids
Lipids are one of the four major building blocks of life, along with carbohydrates, proteins, and nucleic acids. Lipids essentially are any complex biological molecules that are hydrophobic. Lipids can take the form of fats, corticosteroids, phospholipids, waxes, pigments, and more. The three most biologically important forms of lipids are fats, phospholipids, and corticosteroids.
Sections
Fats
Fats serve the two main functions of long-term energy storage and cushioning for vital organs. Fats consist of four parts, one glycerol, and three fatty acid chains. Glycerol is of the formula C3H8O3. A fat is essentially a three-length carbon chain, with hydrogens on one side, and hydroxylgroups on the other, with hydrogens on the end. Since it has hydroxyl groups it is an alcohol. Fatty acids are long chains of carbons usually 16 or 18 carbons in length. On one end of the fatty acid there is a carboxyl group. After the carboxyl group there is a simple chain of carbons, filling their valences by bonding with the previous and next carbon, and bonding to two hydrogens. Sometimes, instead the carbon will bond with only one hydrogen and have a double bond with the previous or next carbon in the chain. Since there are just hydrogens and carbons, atoms with relatively equal electronegativity, the molecule is not polarized. This accounts for fat not being soluble in water.
Another important distinction in fats, especially from a nutritional standpoint, is between saturated fats and unsaturated fats. As was mentioned above, for each carbon in a chain, the carbon can be single bonded to two other carbons and two hydrogens, or sometimes it will be instead bonded to only one other hydrogen, and double bonded to one of the other carbons. A saturated fat is a one in which all of the carbons are bonded to two carbons and two hydrogens (excluding the end carbon of the chain, which is bonded to only one carbon and to three hydrogens). An unsaturated fat is one in which there is at least one double bond between carbons, usually more then one.
The main distinctions between the two types of fats can be drawn to molecular structural differences. For saturated fats, the fats are in flat lines (2D perspective from the side) or plates (3D perspective), and this allows them to stack, and move tightly together. Thus saturated fats tend to form solid substances at room temperature, butter being one common example. Unsaturated fats instead appear to be crooked and crinkled when looked at from the various perspectives. This is because the double bond(s) leave a space with no matter, where the hydrogens are missing, and the molecule angles itself, to fill the space. Since the molecules are angled in all sorts of ways, they cannot pack too tightly, and thus form liquids at room temperatures, an example being various oils. Saturated fats have been known to contribute to heart disease, by building up plaques on vessel linings and impeding blood flow.
A triglyceride with unsaturated fatty acid tales. Note how the tails are spread out from each other, preveting it from solidifying at room temperature. (Rusconi) |
A triglyceride with saturated hydrocarbon tails. There are no kinks in the tails so the tails are closely packed allowing it to solidify at room temperature. (Rusconi) |
Phospholipids
The main function of phospholipids is to form membranes, the cell membranes and other membranes inside the cell. Phospholipids are similar to fats in that they contain fatty acids and glycerol, yet they also contain an extra section, a phosphate group and some associated carbons, hydrogens, and nitrogens. Phospholipids are composed of a glycerol, with two fatty acids, and a phosphate group bonded to the third hydroxyl. The structure of phospholipids is such that on one end they have the fatty acids acting as hydrophobic tails, the tails often being crinkled in various ways due to double carbon bonds. On the other head the phosphate group has a negative charge, and an affinity for water.
Because of these structural attributes, phospholipids exhibit several interesting abilities that make them ideal for use in cell membranes. Phospholipids have a tendency to form a variety of structure that will allow them to "guard" their hydrophobic tails from water. One such structure is the Micelle, where the heads all face out and the tails all face in, covered from interaction with water by the heads. In the case of cell walls, they pack tight together with a double layer of phospholipids, with the heads facing out on both layers, and the tales facing in next to each other. These combinations of hydrophilic and hydrophobic areas make it hard for large molecules to pass through the membranes of cells. Phospholipids are also similarly used in the membranes for organelles inside the cell.
A phospholipid. Two fatty acids and a phosphate groupare bonded to the glycerol molecule(Rusconi) .
Corticosteroids (Steroids)
The final major group of lipids, steroids, are molecules known for their carbon skeleton of four fused rings. The variation in steroids is a question of the various functional groups attached at various place around the skeleton. One noteworthy steroid is cholesterol. Cholesterol is a common component in many foods with animal fats. Cholesterol is naturally found in the membranes of animal cells. Cholesterol ends up being the basis from which many important hormones are produced. Of the same token, if one individual has too much cholesterol in their blood it may contribute to heart diseases and/or attacks.
Additional Resources and Links
References
Campbell, N., & Reece, J. (2002). Biology: Fifth edition. San Francisco: Benjamin Cummings.
Rusconi, J. (n.d.). The Virtual Cell Web Page Chapter 2: The Biomolecules. Retrieved September 18, 2004, from http://personal.tmlp.com/Jimr57/textbook/chapter2/chapter2.htm