Lipids
What's a lipid?
Lipids are a diverse group of molecules with one major common trait - they are largely nonpolar and thus (for the most part) insoluble in water. Unlike other biomolecules, lipids are not a polymer and therefore have no monomer. Despite not being a polymer, lipids do group together and aggregate, and it is in this aggregated state that they do a majority of their major functions.
They exhibit a lot of structural variety as well as functional variety. The three major functions of this category of biomolecule are: making up membranes, storing energy, and cell signaling.
Types of Lipids
Fatty Acids
Fatty acids are long hydrocarbon chains with a carboxyl group at one end.
The hydrocarbon chain can be saturated (has no double bonds and a straight chain) or unsaturated (has double bonds and a bent or "kinked" chain). If you think about the word saturated, in its common use it usually means completely soaked to the point where something can't hold anymore water. In this sense, the fatty acid is "full" of hydrogens and can't hold any more; a carbon atom can form four bonds, if it forms a double bond with another carbon atom, that means there will be one fewer bond to a hydrogen atom.
An increase in unsaturation causes a decrease in "effective chain length," which lowers the melting point of the fatty acid. Likewise, an increase in carbon number would make the hydrocarbon chain longer, which would increase the melting point.
Phospholipids
Phospholipids are composed of two fatty acid tails, a phosphate group, and an alcohol that usually joins them together (typically a glycerol). The fatty acid tails are hydrophobic while the phosphate head is hydrophilic.
Phospholipids form a bilayer and are the major component of membranes within cells.
The Hydrophobic Effect
Many types of lipids, such as phospholipids and fatty acids, are amphipathic - they have a hydrophilic region and a hydrophobic region. When in a mixed system of both lipid and water, they will spontaneously assemble into organized structures based on the hydrophobic effect.
The hydrophobic parts will move to avoid the water, while the hydrophilic parts will move to be near the water. This will maximize polar-polar and nonpolar-nonpolar interactions and minimize the free energy of the system, making for a more favorable state.
Phospholipids will spontaneously form bilayers, where the hydrophobic tails are on the inside with each other, while the hydrophilic heads are on the outside in contact with water. Fatty acids will form micelles, spheres that have the hydrophobic parts on the inside of the sphere and the hydrophilic parts on the outside, in contact with water.
Steroids
Steroids a type of lipid with four hydrocarbon rings that have two main jobs: cell communication and as a component of cell membranes. Steroid hormones, such as androgens, estrogens, corticosteroids, and progestogens, serve as chemical messengers that carry messages between tissues in an organism.
Sterols are a subgroup of steroids. They have four hydrocarbon rings that are nonpolar, as well as a polar hydroxyl group. While there are a variety of sterols, such as phytosterols in plants and ergosterol in yeast and other fungi, the predominant sterol in animals is cholesterol (which is the sterol pictured to the left).
Cholesterol is extremely important to cell membranes, working as a regulator of membrane fluidity; it constricts the movement of the phospholipids at higher temperatures and prevents clustering and stiffening and lower temperatures.
In addition, cholesterol functions as a precursor to fat-soluble steroid hormones and vitamins.
Other Types of Lipids
There are a wide variety of other types of lipids, each with its own functions. We will not go super in-depth with them, but will briefly discuss the structure and function of some of these other types.
Triacylglycerols consist of glycerol and three fatty acid chains. Their primary function is to provide stored energy (storing more than double the energy when compared to carbohydrates) as well as insulation. Due to being nonpolar, triacylglycerols do not readily dissolve in water, which makes them better for long-term energy storage than short-term quick sources of energy. They are stored in fat cells (adipocytes) in vertebrates, as well as oil in plant seeds.
Glycosphingolipids, also known as simply "glycolipids," have fatty acid tails with a "head" consisting of one or more carbohydrates. They function in cell signaling by working as sites of biological recognition.
Waxes serve as energy stores and also work to repel water, due to their hydrophobic properties. They typically have relatively high melting points and are composed of long fatty acid chains with long-chain alcohols. The most well-known animal wax is beeswax.