Cell junctions Flashcards
What are the 3 types of cell junctions?
- Occluding junctions
- Anchoring junctions
- Communicating junctions
What is the function of occluding junctions?
To seal the cells of the epithelium, preventing the leakage of small molecules
What are the characteristics of occluding junctions?
- Polarised with different transport proteins at different locations
- Selective permeability barriers such as for aplha alpha and small sugars
What is the composition of tight junctions?
- Row of opposing transmembrane and adhesion proteins (claudins and occludin)
- Specific claudins necessary for specific solutes
- Extracellular domains interact with eachother
- Other accessory proteins interact with the cytoskeleton
What is the function of anchoring junctions?
Mechanically attaching cells to each other/the matrix
What are the 2 types of actin filament attachment sites?
- cell-cell junctions (adherens junctions)
- cell-matrix junctions (focal adhesion)
What are the 2 types of intermediate filament attachment sites?
- cell-cell junctions (demosomes)
- cell-matrix (hemidemosomes)
Where are anchoring junctions found?
In tissues that undergo mechanical stress
What is the function of adherens junctions and demosomes?
To hold cells together using transmembrane proteins, cadherins
What is the function of focal adhesion and hemidomosomes?
bind to extracellular matrix using transmembrane proteins, integrins
What do all attachment sites have in common?
intracellular anchoring proteins that connect junction to cytoskeleton
Describe and example of a cell-cell junction
- Adhesion belts in epithelial cells which are found below the tight junctions encircling the cell and form before the tight junctions.
- Formed from contractile bundle of actin and uses anchoring proteins
How do focal adhesions work?
Use transmembrane protein integrins, and actin and use same intracellular anchoring proteins as adherens junctions
How do hemidemosomes function?
attach epithelial cells to the matrix using intermediate filaments and anchoring proteins called plectin
What is the function of communicating junctions?
Mediate the passage of chemical/electrical signals between cells
What are the 3 types of communicating junctions?
- Gap junctions
- Chemical synapses
- Plasmodesmata (plants)
What is the function of gap junctions?
Allows cells to communicate directly via the cytoplasm by allowing small molecules and inorganic ions to pass freely
Describe the structure of gap junctions
- 1.5nm wide
- Made from connexons with 6 on each side of the gap, each being multi-spanning (*4) transmembrane proteins
- Different types of connexins form homotypic/heterotypic junctions
Name 5 functions of gap junctions
- Direct reliable transfer of electrical signals
- Synchronising of tissue
- Allows like cells to respond in the same manner
- Importance in embryogenesis
- Controllable by pH and Ca2+
What are plasmodesmata?
- Directly connect the cytoplasms of different cells
- Fine and thin, extensions of ER (desmotubule)
- Some plants allow mRNA/viral RNA to pass through
