Cell Junctions And ECM

Question 1

Tight junctions
Adherens junctions
Gap junctions
Desmosomes
Hemidesmosomes
Plasmodesmata

Answer 1

Tight junctions limit diffusion of water-soluble solutes between cells, and restrict movement of lipids and proteins from the basolaterial to apical surface

Adherens junctions link adjacent cells via the actin cytoskeleton

Gap junctions (plasmodesmata in plants) allow the transfer of molecules via diffusion . Allows the passage of ions, low MW precursors, small signalling molecules composed of connexin proteins

Desmosomes link adjacent cells via intermediate filaments

Hemidesmosomes attach cells to connective tissue e.g. beneath the basal lamina

Plasmodesmata are communication bridges between plant cells, bypassing the cell wall. Permits the transfer of signalling molecules and viruses

Question 2

Adherens junctions role in apoptosis

How are Adherens junctions formed

Answer 2

Loss of Adherens junctions and the contraction of the actin ring both trigger apoptosis

Formed via trans-membrane proteins mediating assembly. Called cadherins which are calcium dependent adhesion molecules that link to actin cytoskeleton

Question 3

Desmosomes formation and their role in disease

Answer 3

1. Mediated assembly by cadherins to links intermidate filament netweork to plasma membrane
2. Intermediate filaments attached to Desmosomes via plakoglobin
3. Skin blistering, heart conditions

Question 4

Tight junctions adaptations

Answer 4

1. Have short extracellular linking domains that form strong interactions with adjacent membranes
2. Leads to strong and close membrane interactions
3. Different tight junctions have different extracellular domains that leads to varying levels of tightness (permeability) through the para cellular pathway
4. Distinct protein compositions of the apical and basal membranes due to different requirements for molecule transportation/impermeability

Question 5

Give the two pathways of transport across epithelial cells

Answer 5

1. Parcellular pathway is the restricted movement of substances through the tight junctions
2. Transcellular pathway moves substances from the exoplasmic face into the cytoplasm, and into the blood stream

Question 6

The ECM described

Answer 6

1. The ECM are the molecules arranged between cells, mostly connective tissue such as keratin, fibrin, collagen, elastic fibre, reticular fibre
2. Connective tissue provides strength and structure, stores growth factors, and involved in tissue repair
3. Dense connective tissue = cartilage, loose connective tissue = adipose tissue, special connective tissue = bone/blood
4. Cells secrete ECM
5. Hemidesmosomes links cytoplasmic intermediate filaments to ECM via integrin linkage to basal lamina, which is connected to ECM via laminins

Question 7

Basal lamina description and function

Answer 7

1. Also known as the basement membrane - Sheets of laminin and collagen network that anchors epithelial cell to the ECM
2. Attached to a cell via hemidesmosomes + integrins
3. Integrins attached to ECM via laminins
4. Role in physical support, cell development and filtering

Question 8

What is the most abundant ECM protein, explaining its structure and function
Explain how vitamin C is linked to collagen

Answer 8

1. The 27 types of triply-helical collagen
2. Synthesised in the ER, modified via hydroxylation, glycosylation and disulphide bonds and then trafficked through the Golgi and to the plasma membrane whilst laterally associated with procollagen
3. Secreted collagen crosslinks with other collagens and proteoglycans in the ECM
4. Vitamin C (ascorbic acid) involved in stabilising collagens triple helix
5. Lack of vitamin C results in collagen degradation
6. Blood vessels become fragile, bruising occurs easily, gums bleed