When you look at the organization of any organism, the most simple unit of life is the cell. The cells group themselves into tissues and eventually organs and organs systems to make up an organism.[image of cell-tissue-organ]
But how do the cells know how to cooperate with each other? If we focus in on the tissue level, this is the first stage cells begin to work together or cooperate. One very important way cells can cooperate is through cell junctions.
Types of Cell Junctions
[easytable] Passage Blocker(occludins),Anchor Junctions, Communication
Tight Junction,Adherens Junction,Gap Junction
Tight junctions seal adjacent cells together to regulate the passage of water and other molecules between epithelial cells. For example, in the small intestine tight junctions allow the body the regulate what and how fast molecules enter the blood stream.
Adherens junctions allow adjacent cells to “adhere” or physically attach, but their main function is to provide contractile strength. The contractile strength allows the the folding or bending of cell sheets. This anchoring junction connects the actin filament of each cell, but does not seal off the passage of molecules between them like tight junctions do. The transmembrane proteins that are involved are cadherins and integrins.
- Cadherins: Attach membrane to membrane
- Integrins: Attach membrane to extracellular matrix
Desmosomes also are disc shaped junctions that attachment adjacent cells . Their primary function is to simply attach adjacent cells. Although keratin forms the surface of desmosomes, cadherins from the attachment between cells.
- Cadherins: membrane to membrane attachment
Hemidesmosomes anchor from intermediate filament in the cell to the extracellular matrix. These types of connections are facilitated by integrins. While desmosomes attach to other cells, hemidesmosomes attach to material outside of cells such as the basal lamina that lies underneath epithelial tissue.
- Integrins: membrane to Extracellular Matrix
Gap junctions are only found in animal cells and connect the cytoplasm of adjacent cells. The connection from the cytoplasm of one cell to another allows molecules and ions to be passed between cells. You can consider this a sharing of resources. The gap junction is made of 6 proteins known as connexons. The connexons can open or closed the gap between cells.
Plasmodesmata serve the same purpose as gap junctions except they are found in plant cells. Since plant cells have a cell wall around their plasma membrane, the plasmodesmata joins adjacent cells a little differently. The pore between plant cells is lined by the plasma membrane instead of connexons found in animal cells.
The function is still to allow the passage of molecules and ions between adjacent cells, but travel can occur exclusively through the cell wall or through the cell membrane. So there are 2 passage between adjacent plants cells in one plasmodesma.
Summary[easytable] Junction Type, Category, Main Function
Tight Junction,Occluding, Regulate passage of molecules
Adheren Junction,Anchor, Provide contractile strength
Desmosome,Anchor,Attach adjacent cells
Hemidesmosome,Anchor, Attach cells to ECM
Gap Junction,Communication,Connect cytoplasm of animal cells
Plasmodesma,Communication,Connect membrane and cell wall of plant cells