Both gap junction and ion channel are channels between two adjacent cells. Gap junction permit the transfer of any small molecules like nutrient monomers but ion channels permit only ion molecules like K+, Na+ etc. 

Hi,

Gap junctions are the connection places for cytoplasm of two adjacent cells that allow transportation of various molecules but ion channels are the proteins responsible for carrying molecules.

Hi

The ion channels  have narrow pores and are moderately to highly selective in regard to which ions are permeant. They are closed in the resting state and have short lifetimes in the open state, typically a millisecond, that enable them to transmit highly frequent neural signals. 

Gap junctions, also known as cell-to-cell channels, serve as passageways between the interiors of contiguous cells. Gap junctions are clustered in discrete regions of the plasma membranes of apposed cells. 

The answers have been clarifying, but I am not sure about the correction of the statement that ion channels carry molecules. I think that they carry ionic populations in solution, not as molecules - but I am no expert in these details.

Gap junctions are composed of two hemi-channels (one per cell), each with six connexin subunits. They enable the passive diffusion of compounds up to 1 kDa between the two cells connected by the gap junction. So, they are rather unspecific. There are also reports that hemi-channels can act on their own, without another hemi-channel attached. 

As Cory J. Poole already explained very well, Ion Channels allow the flow of ions in and out of the cell, depending on the electrochemical gradient. Most ion channels are specific for ions, e.g. K+, or for a type of ion, e.g. kations. They do not, to my knowledge, allow the flow of molecules. Also, ion channels do not connect two cells directly, they enable the rapid flow of ions from the extra- to the intracellular space and vice versa.

I hope I could help you. The links are articles I found with a quick search, that might introduce you a little bit to the topic of gap junctions. 

http://www.sciencedirect.com/science/article/pii/S0092867400812829

http://www.eurekaselect.com/63825/article

Hi, Luis

we recently discovered (and published) that gap junctions between astrocytes in  brain are permeable for but selective to large molecules, however this permeation strongly depends on polyamine spermine content (Benedikt et al., 2012; see attached).

In addition, when hydrogen ions (acidity) close connexin-43 gap junctions the ionic current through the pores can be restored by spermine (Skatchkov et al., 2015 in press, NeuroReport).

Hemichannels, as follow from our additional experiments, behave similar. So, these are large pores with certain ion selectivity (for biophysical details please check works of Veenstra and Bukauskas groups) that have been in attention of biologists a half of century and you can find hundreds of article on the subject, however little attention has being paid to polyamines and their role to regulate both: ionic currents and molecular transfer through the pores.

Cordially, Serguei. 

Thank you very much to all of you! I have now a clear understanding of what gap junctions and ion channels are. 

Best wishes, 

Pocholo

Hi, Pocholo Luis

ion channels and gap-junctions are both ion and molecular  channels that allow molecules to move through and there are many examples of it,  for brief: AMPA receptor channels (Bahring et al., 1997, J.Physiol., 502: 575-589), Kir4.1 channels (Kuchweryavykh et al., 2007, Channels, 1: 172-178), etc. So, if these ion channels of ~6-7 angstrom pore diameter allow molecules of ~> 0.2KDa permeate (polyamines for example), the larger channels, such as connexin Gap junctions (~15 angstrom pore diameter) allow molecules (charged or neutral!) permeate up to ~1.5 KDa molecular weight. We tested from 0.5KDA to 1.2KDa with different charges, means cations and anions.

Such Gap-junctions are modulated by different molecules and in addition, when hydrogen ions (acidity) close connexin-43 gap junctions the ionic current through the pores can be restored by spermine (Skatchkov et al., 2015) I will attached the PDF that now is in press, NeuroReport, later. Thank you and everybody very much for very good question and discussion on it to evaluate some dogmas.

Cordially, Serguei. 

Gap junctions are made up of connexin (Cx) proteins intercellular channels through which molecules with

Hello Everyone,

Can gap junctions allow the passage of nutrients (molecules like glucose) from the capillary to the cells? In a tissue like pancreas, how does the nutrient molecule enter the cell from the blood vessel?Could someone explain?