Steroids are another class of lipids aside from the triglycerides. Unlike triglycerides, steroids are not formed as polymers; rather they are a class of biologically active hydrophobic molecules with a specific structure based on four connected carbon rings (Figure 1). This structure is the uniform structure of any steroid, to which different groups can be added to in order to produce different molecules.

Figure 1: The structure of the steroid molecule, showing the four carbon rings. Steroid molecules have considerable variability, but all of them have this consistent structure of seventeen carbon atoms, forming four rings. Because of this uniform structure, many steroids can be synthesized from other steroids in various biological pathways and there is some degree of convertibility between them.

Steroids have lots of functions in biological organisms. One well known use of steroids is in hormones (Figure 2). Some performance enhancing drugs known as anabolic steroids are as the name suggests steroid hormones, which have this structure and have anabolic effects that increase muscle mass. These molecules are often artificial, but have similar properties to testosterone, a naturally occurring steroid hormone which promotes the development of male sexual characteristics. Hormones like estradiol and progesterone are also steroids, but they have different structures and are associated with female sexual characteristics, although men and women produce both hormones, as they are involved in various biological functions. Another type of steroid hormones are called corticosteroids. These hormones regulate processes like the immune system, inflammation and stress responses.

Figure 2: Some examples of steroid hormones found in humans. Top left: Testosterone, a sex hormone that promotes the development of male sex characteristics. Top right: Estradiol, a sex hormone that promotes the development of female sex characteristics. Bottom right: Cortisol, a corticosteroid produced by the adrenal cortex of the adrenal glands. Bottom left: Dexamethasone, an artificial corticosteroid that is used as a treatment for some inflammatory diseases. These molecules all have the basic structure of a steroid, but the different groups attached to the steroid rings cause the different functions of each molecule. Dexamethasone, the artificial corticosteroid has a fluoride atom attached to it, unlike the othere steroids, which contain carbon, hydrogen and oxygen atoms only.

Another notable use for steroids in biology is in cell membranes structures. Cholesterol, a compound which has been highlighted for its hazards for cardiac health, is a vital constituent of the cell membranes in animals, ensuring their integrity and stability, allowing the phospholipid bilayer to maintain fluidity and flexibility. Plants and fungi though do not contain cholesterol in their cell membranes. Rather they use phytosterols and ergosterols respectively. Thus an animal free diet is by definition cholesterol free! These molecules are all steroids, and have the same carbon ring structure (Figure 3). Collectively, they are called sterols.

Figure 3: Structures of cholesterol (left), phytosterol (middle) and ergosterol (right), which are all different types of sterol molecules found in different organisms. These are important steroid molecules that ensure the integrity of cell walls in animals, plants and fungi. They all have the consistent four carbon ring structure of steroids, and have the same function, but are found in different organisms. As some fungi are important in human diseases, biological analogues of ergosterol have been developed which compete with the ergosterol and specifically disrupt fungi cell membranes but not the human cell membranes.

There are also other steroids which have other functions, and it would be excessive to write an exhaustive discussion of all of them. So in conclusion, steroids are fascinating and diverse range of lipids that have a wide range of functions in biological systems, while maintaining a single common structure.