Cellular Dimensions Are Limited by Oxygen Diffusion

Most cells are microscopic, invisible to the unaided eye. Animal and plant cells are typically 5 to 100 μm in diameter, and many bacteria are only 1 to 2 μm long (see the inside back cover for information on units and their abbreviations). What limits the dimensions of a cell? The lower limit is probably set by the minimum number of each type of biomolecule required by the cell. The smallest cells, certain bacteria known as mycoplasmas, are 300 nm in diameter and have a volume of about 10^-14 mL. A single bacterial ribosome is about 20 nm in its longest dimension, so a few ribosomes take up a substantial fraction of the volume in a mycoplasmal cell. The upper limit of cell size is probably set by the rate of diffusion of solute molecules in aqueous systems.

For example, a bacterial cell that depends upon oxygen consuming reactions for energy production must obtain molecular oxygen by diffusion from the surrounding medium through its plasma membrane. The cell is so small, and the ratio of its surface area to its volume is so large, that every part of its cytoplasm is easily reached by O₂ diffusing into the cell. As cell size increases, however, surface-to-volume ratio decreases, until metabolism consumes O₂ faster than diffusion can supply it. Metabolism that requires O₂ thus becomes impossible as cell size increases beyond a certain point, placing a theoretical upper limit on the size of the cell.