This blog post is a part of STEMTalksNC’s ever expanding General Biology Series. In the last post, we talked about the structure and function of carbohydrates. In this post, we will examine the structure and function of lipids.
Lipids are the one class of large biological molecules that does not have a true monomer, and they are not big enough to be considered macromolecules. The compounds called lipids are grouped with each other because they are hydrophobic from the C-H bonds (these bonds are non-polar). Lipids are different in form and function, and the type of lipids can be classified as: fats, phosopholipids, and steroids.
A fat is constructed from two kinds of smaller molecules: glycerol and fatty acids. Glycerol is an alcohol. A fatty acid has a long carbon skeleton, usually 16 or 18 carbon atoms in length. The carbon at one end of the skeleton has a carboxyl group, and the rest of the skeleton consists of hydrocarbon chain. In making a fat, three fatty acid molecules are each joined to glycerol by an ester linkage, a bond formed by a dehydration reaction between a hydroxyl group (from the glycerol) and a carboxyl group (from the fatty acid). The resulting fat is called a triacylglycerol (also known as triglyceride).
Saturated Fats vs Unsaturated Fats
Fats are usually classified into two groups: saturated fats and unsaturated fats. The difference between these two fats depends on the structure of the hydrocarbon chains of the fatty acid. If there are no double bonds between carbon atoms composing a chain in a fatty acid, then as many hydrogen atoms as possible are bonded to the carbon skeleton. This is a saturated fatty acid, which make up a saturated fat. If the fatty acid has 1 or more carbon double bonds (more specifically the cis double bond), then there are less hydrogen atoms attached. This is an unsaturated fatty acid, which make up saturated fats. These double bonds between the carbon atoms cause a kink and this prevents molecules from packing tightly enough to solidify at room temperature. This is why saturated fats are solid at room temperature, while unsaturated fats are liquid at room temperature.
Phospholipids have two fatty acids attached to glycerol and a phosphate group. It is an example of a “head and tail” structure. The head is hydrophillic and it contains glycerol, the phosphate group and additional smaller groups of molecules that are polar. The 2 tails are hydrophobic composing of the two fatty acids. These two fatty acids can either be saturated or unsaturated. Phospholipids assemble into a bi-layer in a cell membrane; the hydrophillic head in contact with the external solution and the hydrophobic tails in the interior of the bi-layer.
Steroids are lipids with a carbon skeleton consisting of four fused rings. Different steroids are created by varying functional groups attached to the rings.
In the next post, we will discuss proteins and the structural relation to its function.