## Big and Small

Just how big is a cell? The human body is composed of 60-100 trillion individual cells. Each cell is too small to see with the naked eye. Our tendency is to think of a cell as a very small place.

However, that's our "species bias" speaking. In the spectrum of sizes of living things, humans are very large. At least 99% of the world's species are smaller than we are — most much smaller.

While a cell may seem small to us, in terms of the important functions of life it is a very large place. Biological functions are essentially chemical functions, and for the purposes of chemistry, a cell is as vast as a warehouse — or a factory.

An interesting question to ask is why, when larger organisms evolved, the small, unicellular organisms already in existence didn't simply evolve into larger and larger cells, rather than adopting a multicellular construction. There are at least two sensible reasons for this.

First, there's the surface area to volume ratio problem. This is one of the most important functional problems of life. You'll investigate this in one of your lab exercises. The problem is that surface area is a function of the square of the length of an object, while volume is a function of the cube of the length. That means that, as an object gets larger without changing its shape, its volume increases at a faster rate than its surface area. If you consider one simple task of survival, it becomes obvious why this is a problem. Cells get nutrition by absorbing it directly through their surfaces. Thus, the speed at which a cell can feed itself is directly related to how much surface area it has. But the amount of cell that must be fed is determined by its volume. If the volume gets too big, and the surface area doesn't keep up, the cell will starve because it won't be able to get enough nutrition to feed all of itself.

There are essentially three ways around this difficulty. One is to change shape by flattening the object. We'd do fine if we were a millimeter thick and had the length and width of a football field. Personally, I think that sounds a bit awkward. Another way is to have a very complicated surface area instead of a smooth one. Animal bodies (and some cellular organelles) actually make use of this strategy. The interiors of our lungs, the linings of our small intestines, and the interior membranes of mitochondria and chloroplasts all reflect this solution to the problem of limited surface area. The third way is to be subdivided into many small, interconnected structures — like a multicellular organism. To a certain extent, each cell of a multicellular organism is an independent agent. Each has to manufacture its own energy and manage its own waste products; each contains a complete set of genes and manages its own genetic activity. And each uses its own surface for absorption, secretion and excretion, thus avoiding the surface area to volume problem of large objects.

The second reason for large organisms to be composed of multiple cells, rather than one huge one, is for purposes of specialization. The production of specialized cells, tissues, organs and organ systems is a particular feature of particularly the animal kingdom, though plants do these things as well, though generally less extensively.