Tuesday, March 15, 2011
Mostly Small is a Good Thing
Whatever org tree you or I may favor (personally, I'm a Carl Woese 3-domain man), everything living in its branches either is a cell or constructed of cells. These cells, of course, are mostly small—too small to see with the unaided eye. This is so obviously the case, that I can state with some confidence of impunity that any living thing you may haphazardly observe standing, lying, sitting, running, wriggling or oozing across your field of view consists of a great quantity of compact, highly organized cells—though exceptions exist and we'll get to them later. There are a lot of limiting factors, related to size, that have strongly influenced evolution to keep cells small. Some of these considerations include: chemical energetic factors, limited strength v functionality of phospholipid membranes, transport of molecules across membranes, metabolic requirements, etc, etc. Yes, there are many factors to take into account, but one little statement sums up the basis for any discussion of this subject:
Or, more crudely and more to the point: the bigger a cell is, the more difficult it becomes for the cell to stay in one piece, feed itself and dispose of wastes.
Cells are magnificently complex and have millions (and billions) of molecules popping in out of the organelles, zipping around in the cytoplasm, roaring along the cytoskeleton and being transported back and forth across the external membrane at a rate that has yet to be adequately described or illustrated (though XVIVO makes a serious dent). A cell is NOT SIMPLY a little water balloon (even an enucleated red blood cell is far more complex than that). It is NOT JUST a tiny bag filled with amazing chemicals and long, tangled strands of DNA. It is NOT ONLY an engine, a factory or a power plant. A cell is a CITY in miniature...perhaps the analogous equivalent to New York, Moscow and Shanghai rolled into one massively complex micro-megalopolis. It is a tiny kingdom peppered with power generators, crisscrossed with girders, flowing with lines of chemical communication, supported by water and sewage infrastructure, thousands of factories, untold numbers of public works crews and an efficient parcel delivery service integrally linked to a lightning fast import/export industrial enterprise centrally controlled by a bustling civic center. And, in the case of some bacterial cells, it can make a doppelgänger of itself every twenty minutes.
Generally speaking, cells are marvels of miniaturization, and they use their smallness to their molecular advantage. Everything inside the cell is close to everything else. No molecule has to travel far, because there just isn't far to travel. In contrast, the impractical consequences of having really large individual cells quickly demonstrates numerous reasons for cell size limitation. If most humans cells were, say three millimeters in diameter, serious negative results are immediately apparent: Internal organs would have to be larger (a heart as big as a beach ball and lungs the size of mattresses) and simply would not fit in a human body as we know it. In the case of injury, a simple cut in skin of such over-sized cells would leave a huge swath of destruction that would be very slow to heal. Connective tissue built up of these huge cells would lack strength and stability. The crystal clarity of an eye's lens would be impossible if made from layers of big, plump cells. Many precise anatomical components, such as blood vessels, the inner ear, and kidneys would be clumsy and unlikely to function. Most importantly, the formerly speedy transport of molecules from outside each cell to the center, or from the center out would never be sufficiently fast across the vast interior of bb sized cells. Most molecules would have to swim or be carried across a veritable ocean to reach their destinations. Therein lies the surface-to-volume ratio challenge.