Describe experiments using marker molecules that have been done to elucidate important properties of ...

Answer:
1) When an electron-opaque tracer was added to the basolateral side of an epithelial layer, the tracer penetrated in the intracellular space between the cells, past desmosomes and the adherens junction, up to the position of the tight junction. The tracer did not pass through the tight junction. This demonstrated the barrier property of the tight junction in regulating paracellular transport, at least from the basolateral side to the apical side. A possible additional experiment would be to add the tracer to the apical side of the epithelium to assess whether the tight junction also blocks paracellular transport from that direction.
2) Microinjection of various sizes of fluorescent molecules into one cell connected to another by gap junctions revealed that gap junctions pass molecules between the cytoplasms of cells that are up to a size limitation of 1200 Daltons MW. A possible additional experiment would be to test molecules of different shapes, say long linear molecules that are >1200 D, or molecules with different chemical characteristics to determine whether size is the only limitation. Another possible experiment would be to determine what happens to gap junctions when one of the cells dies, perhaps by loading one cell with a small fluorescent probe and then puncturing the other cell to determine whether the dye leaks from the living cell gap junctions or whether the gap junctions can close.