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 roughly thirty trillion co-operating cells, each of which is as complicated as the organism as a whole,2 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.
~ Carl Sagan
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.)
The nature of cells
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.
About Paul Ingraham
I am a science writer, former massage therapist, and I was the assistant editor at ScienceBasedMedicine.org for several years. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook or Twitter.
This article is part of the Biological Literacy series — fun explorations of how the human body works, what I think of as “owner’s manual stuff.” Here are ten of the most popular articles on this theme:
- When To Worry About Shortness of Breath … and When Not To — Three minor causes of a scary symptom that might be treatable
- Micro Muscles and the Dance of the Sarcomeres — A mental picture of muscle knot physiology helps to explain four familiar features of muscle pain
- Why Do We Get Sick? — The curious and tangled connections between pain, poor health, and the lives we lead
- Why Does Pain Hurt? — How an evolutionary wrong turn led to a biological glitch that condemned the animal kingdom — you included — to much louder, longer pain
- Does Fascia Matter? — A detailed critical analysis of the clinical relevance of fascia science and fascia properties
- The Unstretchables — Eleven muscles you can’t actually stretch hard (but wish you could)
- Organ Health Does Not Depend on Spinal Nerves! — One of the key selling points for chiropractic care is the anatomically impossible premise that your spinal nerve roots are important to your general health
- We Are Full of Critters — The human body is a colony of ten trillion co-operating cells
- Healing Time — Can healing be hurried? Would we even notice if it was?
- You Might Just Be Weird — The clinical significance of normal — and not so normal — anatomical variations
- 34 Surprising Causes of Pain — Trying to understand pain when there is no obvious explanation
- Although I have repeated the myth, I have frowned at it suspiciously several times. I’ve always thought it was obvious that mass had to be considered for it to be meaningful, which is why I particularly like the chart. I also always assumed that most of the bacteria surely had to be in the poop chute, which isn’t such a fun fact. The idea that we have more bacteria than cells sort of implies symbiosis on a vast scale, bacteria everywhere, which is true in a way … but the bacterial populations outside the gut are really small compared to our own cell populations. ⤻
- Well, not quite. By numbers, we have a lot more red blood cells and platelets than all the other types combined, and those cell are relatively simple. But all the other cells are rather complex, and there’s still trillions of ‘em. ⤻