Membrane-bounded channels allow passage of molecules across cell or organelle membranes. For example, the Nuclear Pore Complex (NPC; 1.I.1) allows the flow of small molecules and macromolecules between the nucleus and the cytoplasm. The Plant Plasmodesmata (PPD; 1.I.2) allows the flow of similar molecules between adjacent plant cells. Both are complex with many protein constituents. Planctomycetes bacteria have been reported to have a nuclear pore complex (see Family 1.I.3).  The round window is one of the two openings into the cochlea from the middle ear (see Family 1.I.4).