So, the fence junction has 2 roles: To prevent the molecules from just passing through the gap between the cells. This means that it has to enter the cells to through diffusion/osmosis/active transport. This further means that there's more control over what molecules enter the cell. The other function is to prevent the 2 molecules mixing. This happens as the fence junction is joint together so they are touching. If you look at the barrier function diagram that you have shown, the top arrow shows an apical molecule as it can enter the cells like that. In the barrier function, there is no control as to what molecules enter as they can go in the gap, like before, but now, they have to enter the cell so the apical molecule has to go through the top of the cell into it and then down the cell and then out of it. The cell doesn't have to release it of it doesn't want to. I hope this answers your question
Also, in the diagram where it says " prevent intermixing of molecules between the apical and lateral membrane" - I feel like this statement is really invalid and makes so sense whatsoever.. I mean first off (Ik stupid but), what are the molecules between the apical and lateral membranes..as in where are they located..is it molecules outside of the cell..to me, it's just such a weird way to put it?..
Right, so the apical molecules don't have to go through both cells in the barrier. I can just squeeze through the small gap in the middle and just enter the one it wants to. The membrane is the little gap between the lateral parts of the cell. I'm barrier cells, both apical and lateral molecules will be in the same place, (the space in between the 2 lateral parts). In the fence function, if an apical molecule wants to go into the left cell, it would have to enter the right cell first which prevents the apical and lateral molecules mixing as they will enter from the top. As it has to travel through the cell, there will be more control over what molecules enter as the cell membrane doesn't have to let it in if it doesn't need it.
Sorry for all the headache - could you please let me know what you think about this below.
About the fence function
Ok, so the triangle represents an apical molecule. The circle represents a lateral molecule.
In the diagram below, I have drawn two red arrows from a triangle. This triangle can just go from the left cell to the right cell horizontally (first arrow) and so not mix with the lateral molecules OR it can go through the left cell and then after passing the tight junction, go into the right cell (second arrow). Now, if the triangle goes down the second arrow route, then that means that the apical and lateral molecules intermix - this is shown by the green squiggly bit right under the tight junction... So how does the fence function prevent intermixing of molecules between the apical and lateral membrane?
I am so sorry for bothering again - just needed to write this out as would otherwise go mad overthinking about it lol.