Re: What purpose does Pinocytosis serve?

All cells are constantly internalizing membrane and extracellular fluid, as 
you know.  The rates vary from cell to cell - a macrophage, for example, 
internalizes about 1/4th of its volume every hour, fibroblasts less, 
amoebae even more.  The purpose is to take in a wide range of extracellular 
molecules and atoms.  As you guessed, minerals, like iron and copper, are 
taken up in this way.  Also, cholesterol-containing particles called LDLs, 
composed of cholesterol and proteins, the way cholesterol is transported in 
the blood, are taken up by pinocytosis.  The list goes on.

The other purpose of pinocytosis is so important and is less obvious.  In order 
for cells to communicate with each other - hormones, growth factors, 
neurotransmitters for nervous system function, etc., they have to be able 
to constantly secrete.  Secretion requires a source of membrane.  Some of 
this membrane is synthesized and stored inside the cell but some of it 
comes from the cell surface and is internalized by the constant pinocytosis 
taking place.  The constant pinocytosis means that you have a lot of 
membrane coming in to recycle fast to the surface for secretion.  It's a 
case of what goes up must come down.  In this case the cell uses the 
membrane productively for bringing stuff in and also for sending stuff out.

So not only are you getting in a lot of good stuff from the outside, but 
you are also sucking in membrane that you can use to send other stuff out.  
Hope this helps