Re: Why are cells so small?

Dear Katie,

Thanks for your question - you are right, cells come in a wide variety of 
shapes and sizes but most are tiny. The largest single cells are found 
in single-celled organisms called Protozoa and these are just about 
visible with the human eye (approx 2-3 mm), the most familiar of these is 
Amoeba. Why then do cells not get any bigger than this?


A single cell carries out all of the functions necessary for life, it takes 
in oxygen and gets rid of carbon dioxide and other wastes, it has to 
regulate its water absorbtion, take in food, move about, be sensitive to 
its environment and reproduce.  The way a cell exchanges gases (oxygen and 
carbon dioxide) with the outside world is through diffusion.  That means 
that the gases move into the cell if the concentration of the gas is lower 
in the cell than in the outside world and the opposite for gases moving out 
of the cell.  This is fine for a small cell which has a large surface area 
to volume ratio so there is plenty of "surface" for the gases to move 
across.  For large cells however this is not an efficient way of respiring 
as it takes far too long for gases to move across the cell membrane and 
deep inside the cell where it is needed.



Yvonne Buckley