Several billion years ago, Earth was not a very friendly place. A toxic atmosphere, volcanoes everywhere and lightning flashing through the skies. That was the environment in which life began -- either through chemical synthesis on Earth or from comets or meteorites. But life couldn't begin until some barrier could be formed to protect the chemistry of life from the harsh environment. That barrier was something very close to the plasma membrane that surrounds all living cells on Earth, and lets only a few types of molecules through.
The Plasma Membrane
The plasma membrane is a phospholipid bilayer. Phospholipids are molecules with a hydrophilic "head" attached to a hydrophobic "tail." In a watery, aqueous environment, such as the ocean or the human body, a drop of phospholipids will arrange into a sphere with the heads facing the water and the tails protected on the inside.
In the plasma membrane there are two layers of phospholipids. The heads form the outer and inner faces of the membrane, because cells contain an aqueous environment on the inside and usually live in a different watery solution on the outside. The membrane structure limits the movement of molecules from one side to the other.
If you very carefully inserted a drop of red food coloring in a glass of water, you might see the drop for a few seconds, but it wouldn't take long for all the dye to spread throughout the water. That's called diffusion, and it's a random process. That means that after a while, any drop of water you pull from the glass -- from the top, bottom, sides or anywhere else -- will have just about the same number of red dye molecules in it. If the glass were full of biochemical reactions chugging along and the red dye was a disruptive chemical, or toxin, biochemical reactions all through the glass would come to a halt. Those biochemical reactions would continue if the biological molecules were inside a protective plasma membrane that didn't allow disruptive chemicals to pass though.
All else being equal, tiny molecules have an easier time passing through the plasma membrane than larger molecules. For example, the plasma membrane doesn't like polar molecules -- molecules that have an uneven distribution of electrical charge. They have a tough time making it through the plasma membrane. Water molecules are polar, so they are pushed out of the plasma membrane, but they're tiny enough to be able to slip in one side and get pushed out the other. Other tiny molecules, such as oxygen and carbon dioxide, can also sneak through. Salts such as sodium chloride are small, but in water they split into electrically charged ions, and their electrical charge keeps them from penetrating the plasma membrane.
Steroids are lipids, just like the bulk of the plasma membrane. Because lipids don't repel each other, steroids can slip through the plasma membrane unaided, even though they are much bigger molecules than water or carbon dioxide. Many steroids are important signaling molecules that are generated in one part of a complex organism and stimulate a reaction in another part of the organism. The fact that they can slip through plasma membranes with ease makes them well-suited to stimulate rapid responses. Epinephrine -- also known as adrenaline -- is an example of a steroid hormone.
- Duncan Smith/Photodisc/Getty Images