ch.16 Flashcards
Where are most animal cells embedded?
in an extracellular matrix
What are basal laminae?
thin layers on which epithelial cells rest
What do basal laminae surround?
muscle cells, adipose cells, and peripheral nerves
Where is extracellular matrix most abundant?
in connective tissues
Bone, tendon, and cartilage is considered ____ tissue.
connective
What do extracellular matrices have?
tough fibrous proteins embedded in a gel-like polysaccharides
What do adhesion proteins do?
link components of the matrix to one another and to attached cells
tendons have high proportion of _____ _______
fibrous proteins
cartilage has high level of _______ that form a compression-resistant gel.
polysaccharides
Bone matrix is hardened by what?
calcium phosphate crystals
What is the major structural protein?
collagen
What does collagen form?
triple helicies
Three polypeptide chains are wound together to form _______
collagen
What do the triple helix domains consist of?
repeats of the amino acid sequences Gly-X-Y (a glycine in every third position)
what is the smallest amino acid that allows polypeptides to pack closely together?
glycine
What amino acid is frequently found in the X position?
proline
what is frequently found in the Y position?
hydroxyproline
What amino acid stabilizes the helices?
hydroxyl groups (by forming hydrogen bonds)
where is hydroxyproline formed?
in the ER by modification of proline in collagen polypeptide chains
What is the most abundant type of collagen?
type I
What does type I collagen form?
collagen fibrils in which the triple helical molecules form regular staggered arrays
Assembly of fibrils occurs outside the cell from soluble precursor _______.
procollagens
Covalent cross-links between side chains of _____ and hydroxylysine residues help strengthen the fibrils.
lysine
______ can come together to form collagen fibers, which can be several μm in diameter
Fibrils
What type of collagen is basal laminae?
type IV
Gly-X-Y repeats are interrupted by short nonhelical sequences, making them more ______.
flexible
What are 2 other functions of collagen?
1) link some basal laminae to underlying connective tissues
2) participate in cell-matrix interactions
Connective tissues with elastic fibers are common where?
in organs that stretch and return to shape (etc. lungs)
Elastic fibers are made of ______ which is cross-linked into a network.
elastin
Extracellular matrix gels are formed from ______
polysaccharides
what are glycosaminoglycans (GAGs)?
repeating units of disaccharides
What is hyaluronan?
the only GAG that is a single long polysaccharide chain
Where is hyaluronan synthesized?
plasma membrane
How is hyaluronan synthesized?
by a transmembrane hyaluronan synthase
all other GAGs are linked to proteins to form __________
proteoglycans
What is the function of matrix adhesion proteins?
link matrix components to one another and to cell surfaces
what is fibronectin?
the main adhesion protein of connective tissues
Fibronectin is often cross-linked into ______
fibrils
What has binding sites for both collagen and GAGs?
fibronectin
what consists of 3 polypeptide chains, each with rod-like domains and interspersed globular domains?
basal laminae
Basal laminae subunits have binding sites for surface receptors and _______
proteoglycans
Can laminins self-assemble into networks?
yes
what are integrins?
transmembrane proteins that attach cells to the extracellular matrix
how many different integrins are there?
24
what do integrins bind to?
various components of the extracellular matrix, including collagen, fibronectin, and laminin
Integrins also anchor the ______ to the extracellular matrix.
cytoskeleton
what are the 2 types of cell-matrix junctions?
focal adhesions and hemidesmosomes
what are focal adhesions?
bundles of actin filaments anchored to beta subunits of integrins via other proteins
what proteins anchor actin filaments to beta subunits of integrins?
alpha-actinin, talin, and vinculin
what do hemidesmosomes do?
anchor epithelial cells to basal laminae
what links the basal lamina layer of the extracellular matrix to intermediate filaments via plectin?
alpha6 and beta4 integrins
What do alpha6 and beta4 integrins also bind to?
laminins
what cell adhesion molecules mediate cell-cell adhesion?
selectins, integrins, immunoglobulin (Ig) suer family, and cadherins
many cell-cell adhesions depend on divalent cations requiring ____, _____, or _____
Ca2+, Mg2+, or Mn 2+
what do selectins specifically mediate?
transient interactions between leukocytes and endothelial cells or blood platelets
______ leave the circulation at sites of tissue inflammation by interacting with the endothelial cells of capillaries.
leukocytes
What is an example of a heterophilic interaction?
binding of ICAMs to integrins (where an adhesion molecule on one cell recognizes a different molecule on another cell)
What are homophilic interactions?
an adhesion molecule on one cell binds to the same molecule on another cell
The homophilic binding between N-CAMs contributes to what?
associations between nerve cells during development
what mediates homophilic interactions?
cadherins
What are 2 stable adhesion junctions linking the cytoskeletons of adjacent cells?
adherens junctions + desmosomes
Adherens junctions do what?
link actin filaments of adjacent cells
Beta-catenin and p120 are members of the _____ protein family
armadillo
What do beta-catenin and p120 do?
bind to cadherin and help maintain stability
what do desmosomes do?
link intermediate filament cytoskeletons of adjacent cells
What are the 2 cadherins that bind across the junction?
desmoglein + desmocollin
Plakoglobin + plakophilin bind to the cadherins and link to _________
desmoplakin
desmoplakin binds what?
the intermediate filaments
Tight junctions in epithelial cell sheets do what?
form a seal that prevents free passage of molecules and ions between cells
What do tight junctions separate?
apical and basolateral domains of the plasma membrane
Tight junctions provide minimal _______ ______ between the cells.
adhesive strength
Tight junctions are usually associated with what?
adherens junctions and desmosomes in a junctional complex
How are tight junctions formed?
by a network of protein strands that continues around the entire cell
What are transmembrane proteins in the network?
occuludin, claudin, and junctional adhesion molecules (JAM)
What do the cytosolic tails link?
the tight junction complex to the actin cytoskeleton
Within a tissue, cells are often linked by _____ ______.
gap junctions
In electrically excitable cells, passage of ions through gap junctions couples does what?
synchronizes contractions of neighboring cells
Gap junctions also allow passage of _____ _____.
signaling molecules
Gap junctions consist of transmembrane proteins in the _______ family
connexin
how many connexins form a connexon?
6
______ of adjacent cells align, forming open channels between the two cytoplasms.
Connexons
Specialized gap junctions occur on some nerve cells and form an _______ _________.
electrical synapse
When specialized gap junctions are open, what do they allow?
the rapid passage of ions between the 2 nerve cells
Several human diseases are associated with _____ mutations.
connexon
Adhesion between plant cells is mediated by the _____ _______.
cell walls
The ______ ________, a pectin-rich region, acts as a glue to hold adjacent cells together
middle lamella
Adjacent plant cells communicate through cytoplasmic connections called ________
plasmodesmata
What does plasmodesmata allow the regulated passage of?
macromolecules and regulatory molecules such as transcription factors
______ and ______ can be targeted to plasmodesmata in response to specific signals.
proteins and lipids
