Body tissues that consist of cells with adjacent plasma membranes are usually those in which a high degree of fast communication is required to facilitate tissue function. There are various types of junctions that exist between cells, depending on the function of the tissue itself.
Tight junctions connect the plasma membranes of adjacent cells in a sheet, preventing small molecules from leaking between the cells. This allows the sheet of cells to act as a wall within the tissue. These kinds of junctions are important in the cells that line an animal’s digestive tract to force nutrients from food to pass directly into the blood vessels (because they cannot pass through the spaces between the cells).
Anchoring junctions, also known as demosomes, mechanically attach the cytoskeleton of a cell to the cytoskeleton of another cell, or to its extracellular matrix. These kinds of cell-to-cell junctions are important in tissues and organs that experience a high degree of mechanical stress, such as the skin and muscle.
Some cell membranes that touch one another must rapidly exchange material and information. The junctions that form between these cells are called communicating junctions. In these junctions, a chemical or electrical signal passes directly from one cell to another adjacent one. In animals, these kinds of junctions are called gap junctions, and occur between virtually all tissues in the body. In plants, these communicating junctions are called plasmodesmata, and they allow nutrients to be exchanged between plant cells that are separated by a lamina.