Re: Where in the cell are the carrier proteins located?

First a wee bit of introduction. A cell membrane is built like a sandwich 
(a so-called lipid bilayer) with lots and lots of proteins stuck in the 
sandwich and through it. Over 80% of a membrane can actually be protein. 
For me, the easiest image of what a bilayer is, is this: you know the 
little printed magnetic sheets of plastic you can stick on the fridge 
door? If you take two of these and place them black (magnetic) side 
towards each other, they can stick together: you just made a bilayer. 
Now, you know that if you pour water and oil in a glass, they don’t mix. 
So if you have a zillion molecules that are fat (hydrophobic/lipophilic) 
at one end and water-like (hydrophilic/lipophobic) at the other, they 
will want to order themselves so that the fatty bits stick together and 
the hydrophilic ends point out into the water. A lipid bilayer is made of 
two sheets of such molecules called phospholipids. See how this compares 
to the two magnets?

OK, so if we had a membrane made only of such phospholipids, we would 
have a problem making molecules that are hydrophilic cross it, right? 
Within the surfaces of phosphates (and other hydrophilic parts) you have 
an effective shield of fat (the inside of the sandwich). So these 
molecules need help. A carrier protein binds specifically one kind of 
compound on one side of a membrane and transfers it towards the other 
side when the compound cannot traverse the membrane unaided.

So carrier proteins are found in the cell membranes: plasma membrane, 
endoplasmic reticulum, mitochondrial inner membrane, plant thylakoid 
membrane etc. etc. I would guess that they should be present everywhere. 
Even if the membrane does have large pores, there are compounds that need 
help to flip across the lipid bilayer: not least the phospholipids 
themselves.

I hope this answers your question.
Kind regards,

Erik