Question: Why are cells small?
There are three arguments that are used to explain why cells are small.
1) Surface Area : Volume Ratio
The surface area of the cell determines the area over which materials can get into
and out of the cell. Materials can be things like nutrients, gases, waste products
and so on. The volume of the cell determines how much material needs to get in
and out. And so the relationship between surface area and volume becomes very
important. This is the relationship between how much material needs to get into
and out of cell and how easily this can occur. The problem is that as cells get
larger, the surface area to volume ratio falls. In the example, there’s a cell of
dimension L and another one of dimension 2L. In the smaller box, the surface
area is 6 times the area of one side, so 6L . The volume is L . In the larger cell, the
surface area is 24L and the volume is 8L . If you work out the surface area to
volume ratio for each of those, so for the smaller box, it’s 6:1, and for the lower
one it is 3:1. As the cell is getting larger, the surface area to volume ratio is
falling. We can put this into a common example. The volume of this room
determines how many people can seat in here and the surface area is the entrances
and exits of the room. If there is a mismatch of people to the number of exits then
there will be a very long line up of people waiting to get out of the room. And for
a cell that can be a real problem. For a cell it will not get the nutrients it needs and
won’t be able to get rid of the waste products. The solution to this is rather than
creating really large cells, to get larger organisms into the cell, they subdivide. So
multicellular organism solve this problem by being collections of very small cells.
So individual cells have a favorable surface area to volume ratio.
Extensions of the plasma membrane increase surface area and decrease volume.
2) Rates of Diffusion
In the absence of a transport system, molecules and structures move around the
cell by diffusion (random gradient motion). This process is good but is not fast. In
fact, the time required for diffusion, is proportional to distance squared to the
distance over which diffusion has to occur squared. So if we consider a molecule
of oxygen for example: for it to travel 01mm it takes 0.067sec. That’s really
nothing. So oxygen can get into and out of the cell relatively easily. On the other
hand for a molecule of oxygen to travel 1 m, it takes 78 days by diffusion. This is
far too long to support the cells inside an organism such as an elephant.
Diffusion distances that are long result in rates of diffusion that are too slow to
sustain life. So in the absence of some kind of a transport system, diffusion
distances have to be kept small in order for the rates of transport to be efficient to
sustain the processes of life.
3) Adequate concentration or synthetic capacity.
For a biochemical reaction to occur, you need adequate concentrations of
substrates to proceed. As cells get larger they need more molecules to achieve that
adequate concentration. For example: as the linear dimension of the cell doubles,
the volume goes from L to 8L . The cell will require 8x as many molecules for a
given reaction in the larger cell as in the smaller cell. And this can become a