Suppose you are studying the transport of a certain polar molecule across the plasma membrane of cells in culture. over a period of time, you measure the concentration of a polar molecule inside and outside of the cells. you find that the concentration of the molecule is lower in the cell but is gradually increasing. you also measure the atp concentration inside the cell and find that it is not changing. which of the following is probably responsible for the transport of this polar substance into the cell? a) active transport
b) endocytosis
c) facilitated diffusion
d) secondary active transport
e) simple diffusion

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link:

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!

it is called the mitotic spindle