If you were building a building, what kinds of connections might you want to put between the rooms? In some cases, you’d want people to be able to walk from one room to another, in which case you’d put in a door. In other cases, you’d want to hold two adjacent walls firmly together, in which case you might put in some strong bolts. And in still other cases, you might need to ensure that the walls were sealed very tightly together – for instance, to prevent water from dripping between them.As it turns out, cells face the same questions when they’re arranged in a tissue next to other cells. Should they put in doors that connect them directly to their neighbors? Do they need to spot-weld themselves to their neighbors to make a strong layer, or perhaps even form tight seals to prevent water from passing through the tissue? Junctions serving all of these functions can be found in cells of different types, and here, we’ll look at each of them in turn.
Plant cells, surrounded as they are by cell walls, don’t contact one another through wide stretches of plasma membrane the way animal cells can. However, they do have specialized junctions called plasmodesmata (singular, plasmodesma), places where a hole is punched in the cell wall to allow direct cytoplasmic exchange between two cells.
Plasmodesmata are lined with plasma membrane that is continuous with the membranes of the two cells. Each plasmodesma has a thread of cytoplasm extending through it, containing an even thinner thread of endoplasmic reticulum (not shown in the diagram above).Molecules below a certain size (the size exclusion limit) move freely through the plasmodesmal channel by passive diffusion. The size exclusion limit varies among plants, and even among cell types within a plant. Plasmodesmata may selectively dilate (expand) to allow the passage of certain large molecules, such as proteins, although this process is poorly understood
Functionally, gap junctions in animal cells are a lot like plasmodesmata in plant cells: they are channels between neighboring cells that allow for the transport of ions, water, and other substancescubed. Structurally, however, gap junctions and plasmodesmata are quite different.In vertebrates, gap junctions develop when a set of six membrane proteins called connexins form an elongated, donut-like structure called a connexon. When the pores, or “doughnut holes,” of connexons in adjacent animal cells align, a channel forms between the cells. (Invertebrates also form gap junctions in a similar way, but use a different set of proteins called innexins.)
Gap junctions are particularly important in cardiac muscle: the electrical signal to contract spreads rapidly between heart muscle cells as ions pass through gap junctions, allowing the cells to contract in tandem.
Not all junctions between cells produce cytoplasmic connections. Instead, tight junctions create a watertight seal between two adjacent animal cells.At the site of a tight junction, cells are held tightly against each other by many individual groups of tight junction proteins called claudins, each of which interacts with a partner group on the opposite cell membrane. The groups are arranged into strands that form a branching network, with larger numbers of strands making for a tighter seal