Final answer:
Gap junctions are protein structures that create channels for ions and small molecules to move directly between cells. Channel proteins, like aquaporins and ion channels, facilitate the movement of water and ions across the cell membrane, with some channels able to open or close in response to stimuli.
Step-by-step explanation:
Gap junctions are proteins that form channels between cells, allowing ions and small molecules to diffuse directly from one cell to another. These junctions are composed of connecting protein rings that are embedded in the cell membrane of adjacent cells. They allow the passage of water and small solutes, but not larger molecules like proteins and nucleic acids. The channel proteins in gap junctions provide a pore or passageway for molecules such as ions, which are charged atoms, making it difficult for them to cross the hydrophobic interior of the cell membrane without assistance.
There are also other types of channel proteins, like aquaporins and ion channels, the former allowing water to diffuse across the membrane at a high rate, and the latter enabling the transport of specific ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-). Some ion channels are 'gated' and can open or close in response to various stimuli.