Cell Junctions Flashcards
What 5 things are cell junctions essential for?
maintaining positions with the body
giving strength to the tissues
cell-cell communication (signalling)
ECM-cell signalling (info about environment)
sharing of cytoplasm between neighbouring cells
What does the structure of a cell junction depend on?
function and environment
what are the 6 types of cell junctions?
tight junctions
gap junctions
plasmodesmata
adherens junctions
desmosomes
hemidesmosomes
Where do tight junctions form?
at the apex of animal cells between neighbouring epithelial cell
What type of cells have tight junctions and where?
at the apex of animal epithelial cells
What is the purpose of tight junctions?
they form a water-tight barrier between compartments
How do tight junctions form on the cell?
as a series of pockets in a belt around the cell
What are the series of pockets made by tight junctions?
intercellular spaces each surrounded by a network of proteins
What do tight junctions prevent?
the movement of molecules between cells (paracellular movement) and make it so that only transcellular pathways are available
What is an analogy for tight junctions?
gaskets or belts
Describe the structure of tight junctions
the integral proteins of tight junctions form continuous fibrils that encircle the cell like a gasket to prevent paracellular movement of molecules between epithelial animal cells
What do the integral proteins of tight junctions make contact with?
the neighbouring cells on all sides
What are the two families of proteins important to tight junctions?
claudins and occludins
What type of cell junction is essential in forming the blood/brain barrier?
tight junctions in endothelial cells of the blood vessels in the brain
T or F: most capillaries do not have tight junctions
true except the ones in endothelial cells in blood vessels of the brain
What is the purpose of the tight junctions in the blood/brain barrier?
it prevents the passage of molecules into the brain such as ions and water
What can still get through the blood brain barrier? Why?
immune cells can send out protein ligands that interact with the tight junction complexes and open them
T or F: tight junctions also maintain structural polarity of epithelial cells
True
How do tight junctions help maintain the structural polarity of epithelial cells?
the thick band keeps apical membrane proteins from moving too far down the sides of the cells aaand it keeps basolateral membrane proteins from reaching the apical side
What is the zonula occludens?
the band of tight junctions around epithelial cells
Describe gap junctions
sites of communication between adjoining animal cells
What is the function of gap junctions?
intercellular communication
exchange of materials between the two cells
T or F: gap junction proteins in 2 cells are not attached
false they are attached to allow intercellular communication and exchange materials between them
Are gap junctions in animal cells or plant cells?
animal cells
T or F: there is direct contact between 2 cell membranes because of gap junctions
false
What are gap junctions between cells formed by?
channels called connexons
How selective are connexons relative to other transport channels
less selective - anything small can pass through (ex. second messengers cAMP and IP3)
What kind of molecules can move through connexons? What cannot pass?
small molecules like second messengers cAMP and IP3
proteins and RNA are too large to pass
What kind of cells are connexons most often found in?
cells that need to act in unison (ex. cardiac muscle or esophagus smooth muscle)
T or F: if even one cell (for cells that need to act in unison) is exposed to a hormone, it can help the stimulus be rapidly transmitted to all cells of the tissue because of gap junctions
true
What are connexon channels made of?
hexamers of integral membrane proteins called connexins
Describe connexins
integral membrane proteins that form hexamers to make connexion channels
What are gap junctions analogous to?
ion channels
T or F: connexon channels remain open all the time
false! they are like ion channels and they do not remain open all the time
how do connexon channels open?
their pore is triggered by stimuli such as voltage change, ligand change, or a pH change
What is the function of plasmodesmata?
they are sites of communication between adjoining plant cells that directly connect the two cytoplasms
What is the singular word for plasmodesmata?
plasmodesma
Are plasmodesmata in animal or plant cells?
plant
When are the plasmodesmata formed?
while the cell wall is being built during cytokinesis
What animal cell junctions are plasmodesmata similar to?
gap junctions
What size particles can pass through plasmodesmata?
small particles
T or F: the channels of plasmodesmata are lined with cell membrane
true
T or F: the plasmodesmata allows the plasma membrane between two cells to be continuous
true
How does the plasmodesmata allow the plasma membrane to be continuous between 2 cells?
the channels are lined with cell membrane
What is the desmotubule?
a specialized membrane tube that connects the smooth ER of both cells and widens the pore
What are the 3 components of the plasmodesmata?
annulus
desmotubule
plasmam membrane
Describe the annulus of the plasmodesmata
the space around the desmotubule where the cytoplasm and solutes can move freely through the open space around the desmotubule
Where are adherens junctions most common?
in intestinal epithelial cells of animals
What structure do adherens junctions form on a cell?
a belt of actin filaments called the zonula adherens just below the zonula occludens
What is the zonula adherens?
a belt of actin filaments formed by adherens junctions that wraps around the cell just under the zonula of occludins
Are adherens junctions in plant or animal cells?
animal
What is the purpose of adherens junctions?
to connect the EC environment to the intracellular actin cytoskeleton
Why are adherens junctions important?
the connection between the extra- and intercellular environment allow:
cells to maintain their shape within tissue
the actin filaments of one cell to be indirectly linked to the actin filaments in a neighbouring cell - this allows for tissues to use their actin cytoskeleton in a coordinated way
send signals to cell about environment
Describe cadherins
calcium-binding glycoproteins in the plasma membrane that maintain adherens junctions
What are adherens junctions maintained by?
calcium-dependent adhesion proteins called cadherins
How do cadherins function?
they bind to each other like velcro and calcium ions will form bridges between domains of adjacent cadherins
Where does calcium bind on cadherins?
between the extracellular cadherins domains
What does the binding of calcium to cadherins cause?
cadherins will become more rigid so that they can stick together like velcro
What structure do cadherins have when there is no calcium bound?
they are floppy
When will cadherins be more likely to bind together or to a cell like velcro?
when they are bound to Ca2+ and rigid
What does the cytoplasmic domain of cadherin interact with on the inside of the cell membrane?
beta catenin
What does beta catenin do after it binds with cadherin in the cytosol?
it binds to the actin skeleton and can send signals to the nucleus about cell attachment status
What does beta catenin do after it binds with cadherin in the cytosol?
it binds to the actin skeleton and can send signals to the nucleus about cell attachment status
T or F: it doesn’t matter if cells are attached
false!! cell attachment is a survival signal!
Why is cell attachment a survival signal?
if cells become detached from each other, they could become sources of cancer so cell detachment triggers apoptosis
Describe desmosomes including their function
they are strong junctions in animal cells that form disc-shaped sites of attachment and transmit strength between cells
What do desmosomes involve?
calcium-dependent attachment between cells via cadherin-like proteins
What do desmosomes form?
disc-shaped localized sites of attachments (like rivets) that are attached to intermediate filaments
T or F: desmosomes, unlike adherens junctions, do not involve calcium-dependent attachment between cells
FALSE they do and they use cadherin-like proteins too
Which junction is the strongest animal cell junction?
desmosomes
What is the main purpose of desmosomes?
to transmit strength between cells
What kind of cells are desmosomes most common in?
cells in animal tissues that undergo mechanical stress (they require strength)
What is a key difference between desmosomes and adherens junctions?
desmosomes connect to intermediate filaments whereas adherens junctions connect to microfilaments (actin filaments)
How is calcium involved in desmosome function?
Ca2+ binds and makes the EC domains of cadherin-like proteins rigid
What happens when Ca2+ binds to the cadherin-like proteins of desmosomes? Where do they bind?
on the cytoplasmic side, the cadherin-like proteins attach to a large, dense, button-shaped adaptor protein
What do the cadherin-like proteins attach to on the cytoplasmic side of desmosomes? (after calcium has bonded)
large, dense, button-shaped adaptor proteins
What are the adaptor proteins attached to when the cadherin-like proteins bind to them?
intermediate filaments
What is the order of the junctions in animal cells from apex to basal lamina?
tight junctions
adherens junctions
desmosomes
gap junctions
hemidesmosomes
What junction type are hemidesmosomes similar to?
desmosomes
What is the main difference between hemidesmosomes and desmosomes?
hemidesmosomes are between cell and substrate (ECM) whereas desmosomes are between cells
Where exactly are hemidesmosomes found?
between the basal lamina/basement membrane and the basal surface of epithelial cells
Are hemidesmosomes in plant or animal cells?
animal epithelial cells
What is the main function of hemidesmosomes?
anchor epithelial cell to basement membrane
What is the tightest attachment between cells and the ECM in the body?
hemidesmosomes
What proteins are involved with hemidesmosomes?
integrins
What cytoskeleton component are involved with hemidesmosomes?
intermediate filaments
What is the function of the integrin proteins involved with hemidesmosomes?
they link a button-shaped adaptor protein to the basement membrane through the basal plasma membrane
Which junctions are found in animal cells?
tight junctions gap junctions adherens junctions desmosomes hemidesmosomes
Which junctions are found only in plant cells?
plasmodesmata
