Re: How can these larger cells function efficiently enough to overcome the f

I'm VERY sorry about the delay. I missed your message and only spotted it
when cleaning out my inbox!

The main problem a high volume-to-surface ratio causes is that the
consumption of nutrients and the production of waste are all proportional,
roughly speaking, to volume, while the cell can presumably only take in
nutrients through its surface.

There are a few ways a cell can overcome this problem.

1: Develop more efficient methods for moving waste and nutrients around.

Large cells have well-developed internal membrane systems, which package
waste and nutrients into vesicles that are sorted by a complex, and still
incompletely understood, mechanism. If a cell can use nutrients
efficiently, it does not need to have as great an absorptive surface area.
The nerves, which are pretty large as cells go, need to traffic nutrients
up and down their long axons continuously to survive. Likewise,
single-celled protozoa have rudimentary digestive systems.

2: Don't be spherical.

A cell can grow larger without necessarily increasing its volume-to-surface
ratio if it extends thin, absorptive filaments from its surface. This
method, which we see quite a bit of in cells of the digestive system that
need to absorb nutrients quickly, increases its absorptive area without
adding too much volume. 

3: Store nutrients and waste inside the cell.

If nutrient supply and waste disposal are the main barriers to large cell
size, we'd expect the largest cells to be those that store nutrients inside
themselves. This is the case; eggs in the single-celled oocyte stage,
before fertilization, can be visible to the naked eye in many species. They
do not need to import much in the way of nutrients, though, as they store
what they need internally.

I hope this answers your questions. The question of cell size is actually
still an open one. If there is some ideal ratio of surface-to-volume for
any given type of cell, how does the cell find it? What keeps cells from
growing too large or too small? We know quite a bit about cell cycle
regulation, but very little about cell size regulation.

Paul Nagami
Undergraduate