Re: explain how single cell organisms can survive?

Here are three issues which allow single-celled organisms to survive:

1) Single-celled organisms have a high surface to volume ratio.  This
allows  the rate of chemical exchange with the extracellular environment to
be high.  This means that the cell can acquire adequate nutrients and get
rid of waste products efficiently.

2) Multicellular organisms are complex.  Cells in a multicellular organism
have become specialized.  There are cells which get rid of body waste. 
There are cells which produce offspring.  There are cells which mediate
locomotion.  Because of this specialization, when you isolate cells from a
multicellular organism, individual cells do not survive.

Single-celled organisms, however, lack any specialization.  All of the
necessary functions for survival can be carried out by the single cell. 
The trade-off is that single-celled organisms cannot achieve the complexity
or diversity seen in multicellular organisms.  


3) Single-celled organisms can proliferate.  That means they can reproduce
by replicating their DNA and dividing the cell in two.  The specialization
achieved by multicellular organisms comes with the sacrifice of many cells'
ability to reproduce.  Very few cell types in a multicellular organism
can produce functional new organisms.

Thanks for an interesting question!