A cell is loose, wet bag of chemistry about a thousandth of a millimetre wide, give or take. It is the ridiculously complicated chemical apparatus that surrounds and supports the self-replication of molecules of DNA. Not one atom of a cell would exist if it did not somehow make DNA even better at making copies of itself.
That’s so important, I’m going to put it “in other words”: every aspect of cellular biology facilitates replication.
Cells learned to play well with other cells quite early in the history of life. It wasn’t that they were neighbourly: it was just convenient. Cells that happened to work together well, due to happy accidents, inevitably started to outnumber the competition. As long as co-operation made replication more efficient, there was no reason for the co-operation not to get more and more complicated.
Therefore, the human body is a colony of ten trillion co-operating cells, each of which is as complicated as the organism as a whole, and as varied in appearance and behaviour as all the animals of a jungle. Yet each one is taking instructions from the same master set of 46 enormous molecules of DNA hiding at the center of every cell like the Wizard of Oz. Somehow (and this is one of the great mysteries of biology), the DNA tells each cell what kind of cell to be: a toe cell, a lung cell, or a blood cell.
We are, each of us, a multitude. Within us is a little universe.
Although some cells are couch potatoes and spend their entire lives anchored to the same moist patch of biological real estate, it is important to understand that cells are by no means passive or inert.
Somehow (and this is one the great mysteries of biology), cells can be extremely athletic and mobile. An angry immune system cell at work, for instance, floats like a butterfly and stings like a bee more than any boxer could ever imagine: their speed, reach and agility is truly astounding. Moving pictures of them are always startling: we are full of critters.
To see what I mean, watch this movie of a neutrophil hunting and killing a bacteria. (This is actually quite an old film, using old technology: I have seen modern video microscopy that is much more impressive, but unfortunately could not find any of that footage to include with this article or even anywhere else on the web.)
Although defined by a membrane of molecules, cells are actually as permeable as a kitchen strainer. Various chosen atoms and molecules are allowed to flood across the membrane like a rip tide, while others are vigorously pumped one way or the other at great energy expense. For instance, a significant portion of the food we eat is burned solely for the purpose of powering sodium ion pumps in cell membranes.
All of this pumping maintains a pleasant living environment for the cells, a cell soup resembling sea water. The resemblance is not a coincidence. When organisms started emerging from the oceans some three billion years ago, they simply took the water with them, carefully packaged. A human being is a kind of Club Med for cells, a vast civilization of them all cooperating to make sure that they are perpetually swimming in a warm, fresh, oxygenated puddle of nutrient-rich water. If you understand this, much of physiology is more easily understood.
But how do ten trillion cells organize themselves into a human being … often with scarcely a single significant foul up for several decades? How do ten trillion cells even stand up? Even this fairly simple thing of rising up to a height of five or six feet or so is a fairly impressive trick for a bunch of cells who are, individually, no taller than a coffee stain. See Ten Trillion Cells Walked Into a Bar: A humourous and unusual perspective on how, exactly, a person is even able to stand up, let alone walk into a bar.