Cell Junctions

Cells of multicellular organisms live in the extracellular matrix, but cell junctions are what keep cells linked to each other as well as the extracellular matrix. Animal cells have three types of cell junctions: anchoring junctions, tight junctions, and gap junctions. Plants have two types of cell junctions: the middle lamella and plasmodesmata.

Animals

Anchoring junctions are strong attachments that keep cells attached to either each other or to the extracellular matrix. There are four types of anchoring junctions: Adherens junctions, desmosomes, focal adhesions, and hemidesmosomes.

Adherens juctions and desmosomes use membrane proteins called cadherins to attach cells to each other.
Focal adhesions and hemidesmosomes use membrane proteins called integrin to attach cells to the extracellular matrix.
Adherens junctions and focal adhesions are bound to actin filaments in the cytosol.
Desmosomes and hemidesmosomes are bound to intermediate filaments.

Tight junctions prevent material from oozing out into other cell layers. They are similar to the functions of gaskets on a car engine- it keeps liquids and solutes from getting into or out of compartments. For example, tight junctions are what prevents stomach acid from entering your body and wreaking havac on your innards.

Gap junctions are channels that allow small molecules and ions to be shared freely between cells that are in direct contact with each other. They are sort of the opposite of tight junctions. Membrane proteins, called connexin, on one cell line up with the connexin membrane proteins on another cell to form a tunnel, called a connexon, for certain cell products to move back and forth. These connexons are how cells can accomplish direct-contact intercellular signaling.

Plants

The middle lamella is a glue that keeps plant cells together. The middle lamella is discussed in greater detail in the article about the extracellular matrix. Here I will add an example of how the middle lamella functions to keep plant cells together. Unripe fruits are firm because all the cells are glued together in a very solid way. However, as the fruit ripens, enzymes are released that break down the middle lamella and the primary cell wall. As the strong bonds holding the plant cells together are broken, the fruit gets softer.

Plasmodesmata are the plant’s version of the animal’s gap junctions. They serve to allow molecules to be shared between adjacent plant cells. The major distinguishing feature of plasmodesmata vs. gap junctions is that the endoplasmic reticulum of adjacent plant cells are connected through the plasmodesmata via a tubule called a desmotubule.