I believe a porin is a type of channel protein.
All channel proteins are transmembrane and allow passage of ions.
There are many types of channels such as ligand gated we see in nerve impulses or leak channels we see in cells to establish membrane potential that allows K+ to leak out.
All of these use diffusion opposed to active transport.
A porin I believe is just another type of channel protein that allows passage of specific molecules (like glucose) or ions.
And in the case of Aqua poring the specific ion is water.
I have found a nice essay on this ...here it is.......Hope this helps.
Dr. Romano
Porins form aqueous channels and accelerate the passive diffusion of small hydrophilic molecules across the membrane. They are found as monomers or trimers in the outer membranes of Gram-negative bacteria, as well as the mitochondria and chloroplasts of eukaryotic cells. Shown on the left is the OmpF porin side and top view. On the right is the side view of the surface of a single subunit, colored by hydrophobicity.
Solute selectivity of a porin is determined by the characteristics of the amino acid side chains at the entrance and interior lining of the pore as well as the size of the opening. Another porin, PhoE, is weakly selective for small anions due to positively charged residues, such as lysine, that attract anions towards the mouth of the porin. A specific lysine protrudes into the channel, forming a positively charged patch that draws anions into the pore, as well as constricts the pore size so that only small anions flow through.
Note the positively charged regions at the mouth of the pore and at the constriction site. This makes the pore specific for small anions.
ION CHANNELS
Ion channels are more complex than porins, generally requiring more than one subunit to form a membrane passageway. Additionally, while porins are formed from membrane-spanning b barrels, ion channels commonly span the membrane with a helices. Shown here are two different ion channels: a tetrameric bacterial potassium channel and the dipeptide antibiotic gramicidin.
Sometimes, these channel-like transporters open only when stimulated to do so, and are called gated channels. The signal could be through a ligand binding to the transporter, changes in membrane potential, changes in pH, or covalent modification by a cellular enzyme. After stimulation, the blocked gate then opens by structural changes that move a polypeptide segment out of the channel, or by a concerted conformational rotation of helices that open the pore like the iris of a camera.
TYPES OF TRANSPORT
Some transport proteins do not have a channel or pore, but instead bind molecules very selectively and change their structure to allow them to pass to the other side of the membrane. These "bind-and-release conformational transporters" can be classified as either uniport, symport, or antiport, depending on the number of types of solute molecules transported and the respective direction of transport.
One membrane transport protein found in the cells of the liver functions to shuttle glucose between the liver and bloodstream, and is an example of a uniport transporter, for it moves only one solute. The direction of movement is passive, or with its concentration gradient.
A good example of a symport conformational transport protein is the Na+–glucose transporter found in the renal epithelial cells of the kidney. This process is not passive, for the potential energy of a steep sodium gradient is dissipated and used to drive the movement of glucose against its concentration gradient.
The Na+,K+–ATPase that is responsible for maintaining the membrane potential so important for neural cell function is an example of an antiport transporter, and also demonstrates active transport. The free energy of ATP hydrolysis is used to drive the movement of sodium and potassium against their concentration gradients, maintaining a source of potential energy to be used by other cotransport proteins, such as the sodium-glucose transporter we described earlier.
