A multicellular organism can be composed of hundreds to trillions of cells. Each of these cells contains the necessary organelles and other components to carry out all its vital functions. However, cells do not function in isolation. Cells must be able to bind together. It is essential for the organism that its cells be able to communicate with, and respond to, each other, too. Gap junctions and plasmodesmata help to fulfill this goal in animal and plant cells, respectively.
Animal and Plant Cell Structure
Animal and plant cells are eukaryotic cells and share many of the same components. Both are surrounded by a plasma membrane or lipid bilayer. In addition to the typical cell components, like a nucleus, mitochondria, endoplasmic reticulum, ribosomes, or Golgi apparatus, plant cells also contain chloroplasts and a rigid cell wall. The cell wall is composed of cellulose, hemicellulose and other polysaccharides, and exists exterior to the plasma membrane. The cell wall can be 0.1 micrometers or more in thickness.
Connections Between Cells
In animal cells, several different types of cell connections, or junctions, exist. These junctions have different and sometimes opposing roles. Some junctions serve to seal cells together and prevent the exchange of material between them. Other junctions help cells attach to each other or to the extracellular matrix. Yet other junctions actually allow for the exchange of chemical and electrical signals directly between cells. Gap junctions are such communicating junctions. The plasmodesmata of plant cells serve much the same role.
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A gap junction is a protein channel that forms between the plasma membranes of two cells. One subunit of the protein, called a connexon, penetrates the plasma membrane of each cell, and the connexons then join together. A continuous channel then exists between the cytoplasm of one cell and the cytoplasm of the other. This protein channel is large enough for ions and small molecules to pass directly from one cell to the other.
Plasmodesmata are structurally different from gap junctions but serve essentially the same function. Each plasmodesma is a channel between two plant cells that passes through the cell wall. Unlike in gap junctions, in a plasmodesma the plasma membrane of one cell is continuous with the plasma membrane of the other, forming a narrow passageway between them. The cytoplasm of both cells is thus connected and small molecules can pass from one cell to the other. A structure called a desmotubule is also usually found in a plasmodesma - it connects, and it continuous with, the smooth endoplasmic reticula of both cells.