What limits a cell's size?
One factor (and I'll explore others in an upcoming essay) is its surface area-to-volume ratio. A semi-spherical cell's internal volume grows proportionally to the cube of its radius, and its surface area grows proportionally to the square of that radius. A cell’s volume thus grows much more swiftly than its surface area.
This ratio has serious consequences for cellular survival, though. The cell’s membrane funnels nutrients into the cell and secretes waste. So if the interior grows too large relative to the cell membrane, the cell’s metabolic processes slow to a crawl.
A new study reveals that some mammalian cells have evolved a mechanism to keep their surface area-to-volume ratio CONSTANT even as the cell grows. They do this by folding their plasma membranes hundreds of times to increase *effective* surface area, thus helping them maintain high levels of nutrient uptake.
There are other ways to get around this limit, too. Case in point: a giant bacterium called Thiomargarita magnifica can exceed one centimeter in diameter, so large that it is visible by the naked eye. It does so by filling between 65-80 percent of its internal volume with an empty vacuole. In other words, it pushes most of its “working” molecules to the cell periphery, thus shortening diffusion distances.
All this, and much more, in a forthcoming essay for
@AsimovPress.