Cell junctions Flashcards
What are cell junctions?
Multiprotein complexes forming intercellular bridges or cell contacts with extracellular matrix
What are the main examples of cell junction?
Apical junction complex
Desmosomes and hemidesmosomes
Gap junctions
Where is the apical membrane found?
At the apex of the cell
Where is the basal membrane found?
At the base of the cell
What is the apical junction complex made up of?
An occluding zone (tight junction) and an adherent zone
Describe the location and function of the apical junction complex
Near apex of cell only in epithelial tissues only. Enables cells to form barrier (e.g. blood/brain, gut lining).
Describe the structure of the occluding zone
Adjacent membranes are glued close together by membrane proteins in outer layer, which controls permeability
Give two features of tight junctions
- Zonula occludens that hold cells together but also form a permeability barrier preventing passage between cells
- Zonula adherens that hold the cells together
Describe the structure and function of the adherent zone
Actin filaments from each cell connect to transmembrane proteins. It can form a contractile adhesion belt in a sheet of epithelial cells
What’s another name for desmosomes?
Macula adherens
What’s the function of desmosomes?
They’re local rivets between epithelial cells that attach them together
What’s the function of hemidesmosomes?
Hemidesmosomes attach epithelial cells to the basement membrane, which anchors the cells.
Describe the structure of desmosomes
Desmosomes are made up of transmembrane linker glycoproteins (cadherin proteins). Intermediate filaments run from the desmosome into the cytoplasm.
Describe the structure of hemidesmosomes
Hemidesmosomes connect the basal surface of epithelial cells via intermediate filaments. The transmembrane proteins of hemidesmosomes are integrin.
Why are hemidesmosomes and desmosomes crucial to tissues that experience mechanical stress?
They are anchoring junctions, so the cells stay where they’re meant to
What is the function of gap junction?
Gap junctions have pores, which allow molecules to pass from cell to cell for communication
Give two examples of how gap junctions are controlled
Calcium ion levels and the pH
Why are gap junctions useful? (2)
The movement of calcium ions allow cell synchrony in smooth muscle tissue
They enable cells to communicate with each other directly
How are the pores in gap junctions formed?
A group of protein molecules called connexins form a structure called a connexon. When connexons from two adjacent cells align, they form a continuous channel between them
What type of molecules are able to pass through the pores?
Small molecules, like inorganic ions, and other small water soluble molecules (smaller than 1000kDa)
Why aren’t proteins, nucleic acids or sugars able to pass through the pores?
The pores are too small to allow these large molecules to pass through
How do gap junctions couple cells together?
Both electrically and metabolically, which allows them to have direct communication
Which type of synapse involves gap junctions?
Electronic synapses
What is the most widespread of cell junctions in animal tissues?
Gap junctions
