A class of organic compounds called "steroids" is critical for keeping the body running smoothly. Different steroids perform important roles in the body's reproductive system, boost strength and enhance physical performance, and control the body's ability to combat allergic reactions. Although steroids are normally produced inside the human body, sometimes the quantities are insufficient or their production mechanisms are flawed. In these cases, steroids manufactured outside of the body can be administered to produce desired physiological effects.

By definition, steroids are substances that have a characteristic chemical structure consisting of chemical rings of connected atoms—specifically, 17 carbon atoms arranged over four fused rings, three of which are six-sided and one of which is five-sided. This steroid nucleus can be modified in an unlimited number of ways by adding, removing, or replacing atoms. The slightest change, such as the shift of an oxygen atom from one location to another on the molecule, will produce a compound that has different effects.

Chemists in the 1930s discovered that plants had steroids with the same carbon-ring structure as that of animal steroids. Further, chemists learned that they could convert plant steroids into animal steroids. Because isolating steroid compounds from animal sources required lots of raw materials that yielded small amounts of steroids, steroids from plant sources held great promise.

To make, or synthesize, steroids from sources outside the body, chemists take known starter compounds—called precursors—and shape them into a desired final compound through a series of chemical reactions. The number of steps and types of chemical reactions it takes to transform a precursor into a target compound varies from steroid to steroid. For example, going from diosgenin, a natural steroid compound found in Mexican yams, to the male sex hormone testosterone can be done in eight steps. Making the female sex hormone progesterone requires only five.

To get atoms to bond in specific ways, chemists draw from several techniques. Heating a molecule to a high temperature causes atoms to vibrate, which makes new bonds possible. Oxidation adds oxygen to a molecule, while reduction removes oxygen from it. Exposing the molecule to pressure or light can also produce chemical change. Delivering atoms to precise locations on a molecule can require extreme creativity and persistence. For example, chemists working on converting diosgenin to cortisone searched high and low for a way to attach a single oxygen atom to a carbon atom located at position 11 on the molecule. Only after many failures did they discover that a microorganism used in fermentation could do this repeatedly and without fail, making the conversion possible.

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