Cell junctions

Question 1

What are the 3 types of cell junctions?

Answer 1

1. Occluding junctions
2. Anchoring junctions
3. Communicating junctions

Question 2

What is the function of occluding junctions?

Answer 2

To seal the cells of the epithelium, preventing the leakage of small molecules

Question 3

What are the characteristics of occluding junctions?

Answer 3

• Polarised with different transport proteins at different locations
• Selective permeability barriers such as for aplha alpha and small sugars

Question 4

What is the composition of tight junctions?

Answer 4

• 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

Question 5

What is the function of anchoring junctions?

Answer 5

Mechanically attaching cells to each other/the matrix

Question 6

What are the 2 types of actin filament attachment sites?

Answer 6

• cell-cell junctions (adherens junctions)

- cell-matrix junctions (focal adhesion)

Question 7

What are the 2 types of intermediate filament attachment sites?

Answer 7

• cell-cell junctions (demosomes)

- cell-matrix (hemidemosomes)

Question 8

Where are anchoring junctions found?

Answer 8

In tissues that undergo mechanical stress

Question 9

What is the function of adherens junctions and demosomes?

Answer 9

To hold cells together using transmembrane proteins, cadherins

Question 10

What is the function of focal adhesion and hemidomosomes?

Answer 10

bind to extracellular matrix using transmembrane proteins, integrins

Question 11

What do all attachment sites have in common?

Answer 11

intracellular anchoring proteins that connect junction to cytoskeleton

Question 12

Describe and example of a cell-cell junction

Answer 12

• 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

Question 13

How do focal adhesions work?

Answer 13

Use transmembrane protein integrins, and actin and use same intracellular anchoring proteins as adherens junctions

Question 14

How do hemidemosomes function?

Answer 14

attach epithelial cells to the matrix using intermediate filaments and anchoring proteins called plectin

Question 15

What is the function of communicating junctions?

Answer 15

Mediate the passage of chemical/electrical signals between cells

Question 16

What are the 3 types of communicating junctions?

Answer 16

1. Gap junctions
2. Chemical synapses
3. Plasmodesmata (plants)

Question 17

What is the function of gap junctions?

Answer 17

Allows cells to communicate directly via the cytoplasm by allowing small molecules and inorganic ions to pass freely

Question 18

Describe the structure of gap junctions

Answer 18

• 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

Question 19

Name 5 functions of gap junctions

Answer 19

1. Direct reliable transfer of electrical signals
2. Synchronising of tissue
3. Allows like cells to respond in the same manner
4. Importance in embryogenesis
5. Controllable by pH and Ca2+

Question 20

What are plasmodesmata?

Answer 20

• Directly connect the cytoplasms of different cells
• Fine and thin, extensions of ER (desmotubule)
• Some plants allow mRNA/viral RNA to pass through