The organs in a multicellular organism’s body are made up of tissues formed by cells. To work together cohesively, cells must communicate. One way that cells communicate is through direct contact with other cells. The points of contact that connect adjacent cells are called intercellular junctions.
Intercellular junctions are a feature of fungal, plant, and animal cells alike. However, different types of junctions are found in different kinds of cells. Intercellular junctions found in animal cells include tight junctions, gap junctions, and desmosomes. The junctions connecting plant cells are called plasmodesmata. Of the junctions found in animal cells, gap junctions are the most similar to plasmodesmata.
Plasmodesmata are passageways that connect adjacent plant cells. Just as two rooms connected by a doorway share a wall, two plant cells connected by a plasmodesma share a cell wall.
The plasmodesma “doorway” creates a continuous network of cytoplasm—like air flowing between rooms. It is through this cytoplasmic network—called the symplast—that most nutrients and molecules are transferred among plant cells.
A single plant cell has thousands of plasmodesmata perforating its cell wall, although the number and structure of plasmodesmata can vary across cells and change in individual cells. The continuum of cytoplasm created by plasmodesmata unifies most of a plant.
Most of the water and nutrients that move through a plant are transported by vascular tissue—xylem and phloem. However, plasmodesmata also transport these materials among cells and ultimately throughout the plant.
Plasmodesmata are versatile, and continuously alter their permeability. In addition to water and small molecules, they can also transport certain macromolecules, such as receptor-like protein kinases, signaling molecules, transcription factors, and RNA-protein complexes.
As cells grow, their density of plasmodesmata decreases unless they produce secondary plasmodesmata. Certain parasitic plants develop secondary plasmodesmata that connect them to hosts, allowing them to extract nutrients.