Multicellular organisms have many advantages over single celled organisms, but certainly one of the major advantages is that in a co-operative "family" of cells, each is free to specialize in ways that would be impossible if each cell had to live alone.

It is customary for groups of specialized cells to be organized into tissues, which can, in turn, be further organized in to organs and organ systems. This kind of association and co-operativity requires that similar cells be held together in close and direct physical contact with one another. Neighbors must not only work together, they must be joined together.

There are two major ways in which cells in tissues can be held together; an extracellular matrix of macromolecules can form a lattice-work that can then be used by the associated cells to move, change position and a framework in which cells can interact with one another, and cell junctions can create firm, direct, specialized points of fusion between two cells in direct physical contact.

Junctions between cells most occur on or very near the cell's plasma membrane, but can also involve the tiny space between cells and sometimes the layer of cytoplasm that lies just below the plasma membrane. These junctions mostly fall into three categories which depend on the function they serve.

• communicating junctions; these types of junctions usually help small molecules pass from one cell to the next. Examples of these are

• a chemical synapse
• a gap junction

• impermeable junctions; these junctions hold cells in contact with their neighbors, but prevent them from sharing their contents. For example

• a tight junction
• a septate junction

• adhering junctions; a simple, mechanical fastening between two cells. Examples include

• a belt desmosome
• a spot desmosome
• a hemidesmosome

Gap junctions are probably the most common type of join between two cells, and are found in almost all animal tissues. Each junction allows small, water soluble molecules to move directly between the cytoplasms of the two cells in contact, which means that both cells share metabolites and even electrical properties.

These types of junctions are made from proteins that completely cross the plasma membrane of one cell, and then make contact with an identical protein that crosses the plasma membrane of the neighbor cell. A small group of these proteins come together to form a channel or connexon through the membrane. Water soluble materials can move through the membrane using this channel, and then pass directly into a similar channel, or connexon, in the opposite membrane of the adjacent cell.

Since it is the protein connexons that are forming the junction, the membranes of the two cells remain separated by a slight gap of about 3 nm (hence the name "gap" junction!).

The united connexons cluster together in the membranes of the cells in numbers that average about 2-300 channels per cluster.

The proteins in these connexon channels and gap junction clusters are all the same and are probably located in the plasma membrane as individual subunits. When needed they come together (by moving laterally in the plasma membrane), form a channel, and connect one cell to the next.

Many surfaces of an animal body, and many linings inside an animal's body, consist of sheets of cells that act as highly selective barriers to the passage of materials, in either direction. These cell sheets separate the two sides of the organ or structure (the "inside" and the "outside"), and usually these two "sides" have very different chemical compositions, that must be kept separate and apart.

For example, the epithelial cells that line the small intestine in humans are arranged into a sheet of cells that separate the contents of the guts and the inner cavity of the organ that eventually empties into blood vessels.

While acting as a barrier that prevents the semi-digested food from mixing into the bodily fluids, this sheet of epithelial cells must also act as a very selective pump, pumping required nutrients (such as glucose) from the digesting food and into the blood.

Special tight junctions between the cells of the epithelium are very important in helping the cells stay together as a sheet of cells (i.e. joining them to one another) and also helping the cells act as a very selective barrier.

Continuous strands of "junctional proteins" form a weld between the plasma membranes of two adjacent epithelial cells in such a way as to make a very tight contact between the two sets of plasma membranes, cross the intercellular space, and then seal off the spaces between them totally and completely.

These strands of sealing proteins extend all the way around the cell forming a complete circle. At such junctions the lattice-work of junctional protein strands unites the two sets of membranes so closely that there is no intercellular space, or gap, between them.

When nutrient molecules, such as glucose, are absorbed by these cells on their outer surface (that in contact with the digesting food in the gut), they past through the epithelial cells and are then passed out of the cell once more into the blood. Tight junctions between the epithelial cells prevent the glucose molecules from diffusing around and between the adjacent cells, prevent transport proteins from moving from one surface of the cell to another, and unite the epithelial cells into a continuous sheet that prevents the gut contents from leaking into the blood.