Our great enemies are the viruses. These pesky little parasites have no mitochondria - they are unable to produce their own power for living. From time to time, our membrane guardians fall down on the job and a virus penetrates a cell. With power now available, these terrors start reproducing. Overwhelmed by virus particles, the unfortunate cell perishes. Then the released virus attacks other cells. In even the mildest virus infections, millions of cells perish. If it were not for a variety of body defences, the viruses would take over and you would not be long for this world.
Hormones are also part of the communications system, acting as chemical messengers. For example : your blood sugar starts rising. Your pancreas steps up production of insulin, the hormone that says, “Speed up burning of sugar”. The bloodstream carries this work order around and the cells respond. Or, you may decide to chop some wood. You will need extra energy. In this case your thyroid sends the hormonal work order to cells : “Speed up production of ATP”.
My membrane has a sophisticated recognition system. Each of us carries an identification tag, recognized by other cell membranes. Any foreigner or intruder is simply chased away from our individual colonies. Imagine what would happen if we tolerated strangers. A hair cell might wander into my area and hair would soon sprout from your eyes. Warts might start growing in your kidney, liver cells on your eyelids. The membrane also seems to have communications system to talk to other cells. How it functions I don’t know - enzymes again, may be? Anyway, if you take a heart apart, separating it into individual cells, those cells will pulse at random but soon they will be beating in unison again!! Somehow the word gets around.
Just as remarkable as our internal structure is our external wall. My membrane is a bare .0000001 millimetre thick. Until very recently, scientists thought of this gossamer covering as little more than a kind of tight cellophane bag. Thanks to the electron microscope, they now realize that it is one of my most important components. Acting as gatekeeper, the cellular membrane decides what shall be admitted, what excluded. It controls the cell’s internal environment - keeping in exact balance salts, organic materials, water and other substance. Life is absolutely dependent on this.
We cells manufacture upward of 600 enzymes - most remarkable - substances. On order from RNA, these master chemists instantly and effortlessly synthesize proteins - taking protein from a piece of fish, breaking it down into its components and rearranging the amino acids to make the human protein needed for, say, your thumbnail. Cellular enzymes also build bafflingly complex hormones and disease-fighting antibodies and perform many tasks beyond the capabilities of the world’s most gifted chemists.