Cell junctions

Question 1

What are cell junctions?

Answer 1

Multiprotein complexes forming intercellular bridges or cell contacts with extracellular matrix

Question 2

What are the main examples of cell junction?

Answer 2

Apical junction complex
Desmosomes and hemidesmosomes
Gap junctions

Question 3

Where is the apical membrane found?

Answer 3

At the apex of the cell

Question 4

Where is the basal membrane found?

Answer 4

At the base of the cell

Question 5

What is the apical junction complex made up of?

Answer 5

An occluding zone (tight junction) and an adherent zone

Question 6

Describe the location and function of the apical junction complex

Answer 6

Near apex of cell only in epithelial tissues only. Enables cells to form barrier (e.g. blood/brain, gut lining).

Question 7

Describe the structure of the occluding zone

Answer 7

Adjacent membranes are glued close together by membrane proteins in outer layer, which controls permeability

Question 8

Give two features of tight junctions

Answer 8

• Zonula occludens that hold cells together but also form a permeability barrier preventing passage between cells
• Zonula adherens that hold the cells together

Question 9

Describe the structure and function of the adherent zone

Answer 9

Actin filaments from each cell connect to transmembrane proteins. It can form a contractile adhesion belt in a sheet of epithelial cells

Question 10

What’s another name for desmosomes?

Answer 10

Macula adherens

Question 11

What’s the function of desmosomes?

Answer 11

They’re local rivets between epithelial cells that attach them together

Question 12

What’s the function of hemidesmosomes?

Answer 12

Hemidesmosomes attach epithelial cells to the basement membrane, which anchors the cells.

Question 13

Describe the structure of desmosomes

Answer 13

Desmosomes are made up of transmembrane linker glycoproteins (cadherin proteins). Intermediate filaments run from the desmosome into the cytoplasm.

Question 14

Describe the structure of hemidesmosomes

Answer 14

Hemidesmosomes connect the basal surface of epithelial cells via intermediate filaments. The transmembrane proteins of hemidesmosomes are integrin.

Question 15

Why are hemidesmosomes and desmosomes crucial to tissues that experience mechanical stress?

Answer 15

They are anchoring junctions, so the cells stay where they’re meant to

Question 16

What is the function of gap junction?

Answer 16

Gap junctions have pores, which allow molecules to pass from cell to cell for communication

Question 17

Give two examples of how gap junctions are controlled

Answer 17

Calcium ion levels and the pH

Question 18

Why are gap junctions useful? (2)

Answer 18

The movement of calcium ions allow cell synchrony in smooth muscle tissue
They enable cells to communicate with each other directly

Question 19

How are the pores in gap junctions formed?

Answer 19

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

Question 20

What type of molecules are able to pass through the pores?

Answer 20

Small molecules, like inorganic ions, and other small water soluble molecules (smaller than 1000kDa)

Question 21

Why aren’t proteins, nucleic acids or sugars able to pass through the pores?

Answer 21

The pores are too small to allow these large molecules to pass through

Question 22

How do gap junctions couple cells together?

Answer 22

Both electrically and metabolically, which allows them to have direct communication

Question 23

Which type of synapse involves gap junctions?

Answer 23

Electronic synapses

Question 24

What is the most widespread of cell junctions in animal tissues?

Answer 24

Gap junctions

Question 25

Why are desmosomes also known as spot desmosome or macula adherens (macula = Latin for spot)?

Answer 25

They are circular or spot like in outline, and not belt- or band shaped like adherens junctions.