Steroids



 

Learning Objective

  • Describe some functions of steroids

Key Points

    • Steroids are lipids because they are hydrophobic and insoluble in water, but they do not resemble lipids since they have a structure composed of four fused rings.
    • Cholesterol is the most common steroid and is the precursor to vitamin D, testosterone, estrogen, progesterone, aldosterone, cortisol, and bile salts.
    • Cholesterol is a component of the phospholipid bilayer and plays a role in the structure and function of membranes.
    • Steroids are found in the brain and alter electrical activity in the brain.
    • Because they can tone down receptors that communicate messages from neurotransmitters, steroids are often used in anesthetic medicines.

Terms

  • osmoregulationthe homeostatic regulation of osmotic pressure in the body in order to maintain a constant water content
  • neurotransmitterany substance, such as acetylcholine or dopamine, responsible for sending nerve signals across a synapse between two neurons
  • hormoneany substance produced by one tissue and conveyed by the bloodstream to another to affect physiological activity

Structure of Steroid Molecules

Unlike phospholipids and fats, steroids have a fused ring structure. Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings, and many of them, like cholesterol, have a short tail. Many steroids also have the –OH functional group, and these steroids are classified as alcohols called sterols.

Steroid StructuresSteroids, such as cholesterol and cortisol, are composed of four fused hydrocarbon rings.

Cholesterol

Cholesterol is the most common steroid and is mainly synthesized in the liver; it is the precursor to vitamin D. Cholesterol is also a precursor to many important steroid hormones like estrogen, testosterone, and progesterone, which are secreted by the gonads and endocrine glands. Therefore, steroids play very important roles in the body’s reproductive system. Cholesterol also plays a role in synthesizing the steroid hormones aldosterone, which is used for osmoregulation, and cortisol, which plays a role in metabolism.

Cholesterol is also the precursor to bile salts, which help in the emulsification of fats and their absorption by cells. It is a component of the plasma membrane of animal cells and the phospholipid bilayer. Being the outermost structure in animal cells, the plasma membrane is responsible for the transport of materials and cellular recognition; and it is involved in cell-to-cell communication. Thus, steroids also play an important role in the structure and function of membranes.

It has also been discovered that steroids can be active in the brain where they affect the nervous system, These neurosteroids alter electrical activity in the brain. They can either activate or tone down receptors that communicate messages from neurotransmitters. Since these neurosteroids can tone down receptors and decrease brain activity, steroids are often used in anesthetic medicines.