It is possible to determine the distribution of cell sizes (i.e., volumes) in a random population of cells.
Because cell size is related to cell age, an age distribution is also obtained. If cells were produced in a manner like the commercial production of small spherical objects (such as ball bearings or marbles), then their sizes would be distributed normally about some mean size.
However, in a random, exponentially expanding population of cells, the distribution of size or age is not normal.
Instead, a distribution exists in which there are more young (small) cells than old (large) cells.
In an ideal population (i.e., one in which a cell divides equally into two daughter cells, each of which grows to the size of the parent cell before dividing), the age or size distribution takes the form of the solid red curve seen in Figure 2-7.
This curve indicates that there is an exponential decrease in the relative number of cells with age or size. This is so because the absolute number of cells entering division is constantly increasing in an exponential manner, so that each new generation of cells is exponentially greater in number than the one that preceded it. It should be emphasized that the curve of Figure 2-7 is derived mathematically and applies the assumption that the cells have identical doubling times.
The distribution described above probably does not exist in nature, because cells display variations in individual generation times and do not divide into progeny of exactly equal volume. Thus, the distribution of cell size ranges beyond V and 2 V (where 2 V is the volume of the ideal parent cell and V is the volume of each daughter cell). Furthermore, it has been shown that the small cells produced by unequal or early division do not, in turn, give rise to smaller progeny; instead, there is a random fluctuation about some inherited average size. A size distribution typical for a random population of cells is shown in Figure 2-8.
Although several possibilities exist for the rate of cell growth between divisions, most studies of cell volume distributions in either random or “synchronized” cultures (see below) indicate that cells grow either linearly or exponentially. For example, individual Tetrahymena pyriformis cells grow exponentially in volume between divisions, whereas Chlorella ellipsoidea cells grow linearly.
In many cell cultures, the cell volume distribution remains constant throughout the exponential phase of the population growth curve, but in some cultures (such as cultures of the flagellate Poly-tomella agilis and the yeast Schizosaccharomyces pombe) the cell volume distribution shifts toward smaller size during the exponential phase. The latter observation indicates that in some cases the growth of individual cells occurs less rapidly than the growth (in numbers of cells) of the entire population.
The sizes attained by cells in culture also depend on other factors, such as the temperature at which the cells are maintained. In T. pyriformis, average cell size increases with increasing culture temperature between 28° and 34°C. In P. agilis, cells cultured at lower temperatures have a greater average size than cells cultured at higher temperatures (in the range 9° to 25°C). It is apparent that, depending on the conditions of the culture, the events that result in cell division may occur either more or less rapidly than the events that result in individual cell growth.