Question

Human epithelial cells can be grown in a culture dish, where they undergo many rounds of cell division. During most of the cell cycle, they adhere tightly to the surface of the dish, spreading out into a rectangular solid shape that approximates a White Castle burger, 65 μm x 65 μm x 1 μm thick. But during cell division they lose most of their attachment to the surface of the dish and round up into a spherical shape with a radius of 10 μm. In both shapes the cell has about the same volume (check it for yourself), but the surface area of the cell goes down by about 7-fold when it rounds up (you can check this, too). How can a cell quickly decrease its S/V by decreasing its surface area, while keeping its volume about the same?

Answer 1

To first understand the answer we must look to have the ratio volume: area or Surface Area to Volume ratio (SA:V) behaves. SA:V has different units has volume is expressed in a cubic unit and area in and square unit, this means that as one increase or decrease the other does too, but the difference in the units makes that both have a different rate. So when the size increases, the area will increase slower than the volume, and vice versa, when the size decreases, the area will decrease slower than the volume.

In an sphere, particularly, has the diameter goes down the SA:V goes up, because it will loss more volume than area. Then, the question is when a cell produces sphere like structures that allow it to adjust its SA:V? The answer is exocytosis, as this process is the creation of vesicles, small spherical structures, made from the membrane that causes a greater volume than area loss. But this only answer one way of the process, how to pass from a sphere to a cubic form, the reverse process is achieved by the reverse cellular process. Endocytosis is the incorporation of vesicles from the medium, this incorporate a higher volume than area.