Active transport describes what happens when a cell uses energy to transport something. We’re not talking about phagocytosis (cell eating) or pinocytosis (cell drinking) in this section. We’re talking about the movement of individual molecules across the cell membrane. The liquids inside and outside of cells have different substances. Sometimes a cell has to work and use some energy to maintain a proper balance of ions and molecules.

Proteins in the Membrane

Active transport usually happens across the cell membrane. There are thousands of proteins embedded in the cell’s lipid bilayer. Those proteins do much of the work in active transport. They are positioned to cross the membrane so one part is on the inside of the cell and one part is on the outside. Only when they cross the bilayer are they able to move molecules and ions in and out of the cell. The membrane proteins are very specific. One protein that moves glucose will not move calcium (Ca) ions. There are hundreds of types of these membrane proteins in the many cells of your body.

Many times, proteins have to work against a concentration gradient. That term means they are pumping something (usually ions) from areas of lower to higher concentration. This happens a lot in neurons. The membrane proteins are constantly pumping ions in and out to get the membrane of the neuron ready to transmit electrical impulses.

Stopping the Transport

Even though these proteins are working to keep the cell alive, their activity can be stopped. There are poisons that stop the membrane proteins from transporting their molecules. Those poisons are called inhibitors. Sometimes the proteins are destroyed and other times they are just plugged up.

Imagine that you are a cell and have ten proteins working to pump calcium into the cell. What if a poison came along and blocked eight of them? You could not survive with just two pumps working and would slowly die. It would be like expecting you to breathe with your mouth and nose plugged up.