ch 15 beyond the cell: cell adhesion, cell junctions, and extracellular structures

Question 1

epithelial tissue

Answer 1

sheets of cells that are polarized (contain discrete specializations at their opposite ends)
two ends:
apical - end of cell in contact with the external environment
basolateral - end of cell that contacts the basal lamina

Question 2

connective tissue

Answer 2

more loosely organized
cells must be attached to one another, to a mechanically rigid scaffolding or both

Question 3

cell-to-cell junctions

Answer 3

specialized connections btw the plasma membranes of adjoining cells for the purpose of adhesion, sealing, or communication
ex. adhesive junctions, tight junctions, gap junctions

Question 4

3 common junctions in animal cells

Answer 4

1. adhesive junctions - cell-cell adhesion
   - adherent junctions - found in actin microfilaments
   - desmosome - found in intermediate filaments
2. tight junctions - sealing spaces btw cells - found in transmembrane junctional proteins, actin
3. gap junctions - exchange of ions and molecules btw cells - connexins in one membrane align with those in another to form channels btw cells

Question 5

homophilic interactions

Answer 5

binding of two identical molecules to each other

Question 6

heterophilic interaction

Answer 6

binding of two different molecules to each other

Question 7

adhesion proteins

Answer 7

sites pf assembling signaling complexes and cytoskeletal structures
can assemble and disassemble in response to cellular events
continuously recycled through endocytic and exocytic pathways

Question 8

adhesive junctions rely on

Answer 8

1 intracellular attachment proteins (catenins) - link the junction to the cytoskeleton
2 cadherins - on the outer surface bind cells to each other

Question 9

adherens junction

Answer 9

• cadherin-mediated junctions that interact with actin; especially prominent in epithelial cells

Question 10

cadherins

Answer 10

any of a family of plasma membrane glycoproteins that mediate calcium-dependent adhesion btw cells
characterized by:
- repeats in their extracellular domain that are structurally similar
- a transmembrane domain
- widely varying cytosolic ends

Question 11

E-cadherin

Answer 11

best characterized by the five repeats on its extracellular domain through which two E-cadherin molecules can interact (by zipping together)
cytosolic portions interact with the cytoskeleton

Question 12

beta- and alpha-catenin

Answer 12

binds to the cytosolic tail of the cadherin
bound by alpha-catenin which recruits F-actin to the junction

Question 13

p120 catenin

Answer 13

binds the cytoplasmic tail of cadherins near the plasma membrane
regulates the stability of cadherins at the cell surface and how quickly they are endocytosed

Question 14

L cells

Answer 14

cultured fibroblasts that contain little cadherins
when purified DNA encoding E- or P-cadherins is introduced, the cells begin to produce cadherins and bind more tightly to one another

Question 15

cadherins during embryonic development

Answer 15

depletion of mRNA encoding for the main type of cadherin in the early embryo can be the cause of loos of normal embryo development

Question 16

Desmosomes

Answer 16

button-like points of strong adhesion btw adjacent cells in a tissue with structural integrity
abundant in cells that are under mechanical stress (skin, heart muscle, uterus)
rely on cadherins

Question 17

desmosome core

Answer 17

the extracellular space btw the two connected cells

Question 18

democollins and desmogleins

Answer 18

special cadherins in desmosomes
are anchored to the intermediate filaments of the cytoskeleton
plakoglobin (beta-catenin family protein) binds to desmocollin which binds to desmoplakin which attaches to intermediate filaments

Question 19

tonofilaments

Answer 19

special clusters of intermediate filaments

Question 20

lectins

Answer 20

carbohydrate-binding proteins that can be isolated from plant or animal cells and that promote cell-cell adhesion
can bind to specific sugar or sugars at the outer cell surface

Question 21

cell adhesion molecules (CAMs)

Answer 21

members of the immunoglobulin super family (IgSF)
involved it cell-cell adhesion

Question 22

selectin

Answer 22

plasma membrane glycoprotein that mediates cell-cell adhesion by binding to specific carbohydrate groups located on the surface of target cells

Question 23

leukocyte adhesion with endothelial cells

Answer 23

1 rolling of leukocytes is mediated by selectins
2 prior to invading a blood vessel, integrins are activated
3 firm attachment using ICAMs allows invasion

Question 24

tight junctions

Answer 24

prevent the movement of molecules across cell layers, leave no space btw the plasma membranes of adjacent cells
form a continuous belt around the apical ends of lateral surfaces of each cell; molecules cross the cell layer by passing through the cells
prominent in intestinal epithelial cells, abundant in ducts and cavities of glands that connect with the digestive tract (liver and pancreas), in the urinary bladder, and btw the endothelial cells linking the blood vessels of the brain

Question 25

transmembrane proteins in tight junctions

Answer 25

- occludin - junctional adhesion molecules (JAMs) - claudins

Question 26

claudins

Answer 26

interlock to form a tight seal charged amino acids in the large extracellular loop of claudins form ion-selective pores that allow passage of specific ions paracellular transport - ions move btw cells

Question 27

tight junctions block

Answer 27

- movement of lipids only in the outer monolayer - movement of integral membrane proteins completely

Question 28

gap junction

Answer 28

a region where the plasma membranes of cell are aligned and brought into contact, with a very small gap, spanned by small molecular 'pipelines' which allow small molecules and ions to pass directly btw cells abundant in tissues such as muscle and nerve, where extremely rapid communication btw cells is required (electrical synapses)

Question 29

connexon

Answer 29

assembly of six protein subunits with a hollow center that forms a channel through the plasma membrane at a gap junction

Question 30

3 types of ECM

Answer 30

1 bone 2 cartilage 3 connective tissue

Question 31

bone

Answer 31

consists mainly of a rigid extracellular matrix that contains a small number of interspersed cells

Question 32

cartilage

Answer 32

tissue constructed mostly of matrix materials that is more flexible than bones

Question 33

connective tissue

Answer 33

surrounds glands and blood vessels and is relatively gelatinous and contains interspersed fibroblast cells

Question 34

3 classes of ECM molecules

Answer 34

1 structural proteins - give strength and flexibility (collagens and elastins) 2 protein-polysaccharide complexes (proteoglycans) - provide matrix in which structural molecules are embedded 3 adhesive glycoproteins - allow cells to attach to the ECM (fibronectins and laminins)

Question 35

collagens

Answer 35

a family of closely related proteins that form high-strength fibers found in high concs in the ecm of animals secreted by several types of cells in connective tissues

Question 36

collagen fiber

Answer 36

extremely strong fibers measuring several micrometers in diameter found in the ecm; constructed from collagen fibrils that are, in turn, composed of collagen molecules lined up in a staggered array

Question 37

procollagen

Answer 37

a precursor molecule that is converted to collagen by proteolytic cleavage of sequences at both the N- and C- terminal ends

Question 38

steps in collagen assembly

Answer 38

1. collagen precursor chains are assembled in the ER lumen to form triple-helical procollagen molecules 2 after secretion from the cell, procollagen is converted to collagen in a peptide-cleaving reaction catalyzed by the enzyme procollagen peptidase 3. the molecules of collagen then bind to each other and self-assemble into collagen fibrils 4. the fibrils assemble laterally into collagen fibers. in striated collage, the 67nm repeat distance is created by packing together rows of collagen molecules in which each row is displaced by one-fourth the length of a single molecule

Question 39

the stability of collagen fibrils is reinforced by

Answer 39

hydrogen bonds involving hydroxylysine and hydroxyproline the hydrogen bonds form crosslinks within and btw collagen molecules

Question 40

elastins

Answer 40

protein subunit of the elastic fibers that impart elasticity and flexibility to the ecm rich in glycine and proline molecules are crosslinked by bonds btw lysine residues

Question 41

glycosaminoglycans (GAGs)

Answer 41

large carbs characterized by repeating disaccharide units are hydrophilic and attract water and cations, forming a gelatinous matrix where collagen and elastin are embedded

Question 42

proteoglycan

Answer 42

complex btw proteins and glycosaminoglycans found in the ecm have a number of GAG chains attached along the length of a core protein in cartilage, numerous proteoglycans become attached to long molecules of hyaluronate

Question 43

hyaluronate

Answer 43

a GAG found in high conc in the ecm where cells are actively proliferating or migrating, and in the joints btw movable bones to reduce friction

Question 44

adhesive glycoproteins

Answer 44

reinforce direct links btw the ECM and the plasma membrane laminins and fibronectins are the most common type

Question 45

fibronectins

Answer 45

adhesive glycoprotein found in the ecm and loosely associated with the cell surface binds cells the the ecm and is important in determining cell shape and guiding cell migration have two large subunits linked near the C-terminals by two disulfide bonds

Question 46

fibronectin structure

Answer 46

each fibronectin subunit is folded into a series of rodlike domains several domains bind one or more ecm macromolecules (collagen, heparin, fibrin) other domains recognize and bind cell surface receptors via the RGD (arg-gly-asp) sequence

Question 47

effects of fibronectin on cell movement

Answer 47

fibronectin acts as a bridging molecule btw cells and the ecm cell migration requires fibronectin many kinds of cancer cells over or underproduce fibronectin - uncontrolled migration

Question 48

effects of fibronectin on blood clotting

Answer 48

the soluble form of fibronectin in blood is called plasma fibronectin it promotes blood clotting bc it has domains for binding fibrin (blood clotting protein) can attach blood platelets to fibrin as the clot forms

Question 49

laminins

Answer 49

adhesive glycoprotein of the ecm localized predominantly in the basal lamina of epithelial cells consist of three long polypeptides (alpha, beta, gamma) disulfide bonds hold the polypeptides together have several domains

Question 50

basal lamina

Answer 50

thin sheets of specialized ecm material that separates epithelial cells from underlying connective tissues structural support and a permeability barrier contain type IV collagen, proteoglycans, laminins, and another glycoprotein entactin or nidogen

Question 51

integrins

Answer 51

family of cell surface receptors that bind to fibronectins or laminins; integrate the cytoskeleton with the ecm

Question 52

structure of integrins

Answer 52

consist of two large transmembrane polypeptides (alpha and beta) extracellular parts of the subunits form the binding sites and specificity is mainly dependent on the alpha unit

Question 53

focal adhesion

Answer 53

localized points of attachment btw cell surface integrin molecules and the extracellular matrix contain clustered integrin molecules that interact with bindles of cytoskeletal actin microfilaments via several linker proteins

Question 54

hemidesmosomes

Answer 54

found in epithelial cells point of attachment btw cell surface integrin molecules of epithelial cells and the basal lamina contains integrin molecules that are anchored via linker proteins to intermediate filaments of the cytoskeleton

Question 55

plaque

Answer 55

dense layer of fibrous material located on the cytoplasmic side of adhesive junction such as desmosomes, hemidesmosomes, and adherens junction composed of intracellular attachment proteins that link the junction to the appropriate type of cytoskeletal filament

Question 56

plakin

Answer 56

family of proteins involved in linking the integrin molecules of a hemidesmosome to intermediate filaments of the cytoskeleton

Question 57

anchorage-dependent growth

Answer 57

requirement that cells be attached to a solid surface such as the ecm before they can grow and divide

Question 58

costamere

Answer 58

an attachment structure at the surface of striated muscle align in register with Z discs around the surface of muscle cells and physically attach muscle fibers at the surface of muscle cells to the plasma membranes, which in turn are anchored to the ECM surrounding the muscle cells

Question 59

3 types of ECM attachments

Answer 59

1 2 3 costamere - important in human disease

Question 60

cell wall

Answer 60

rigid, nonliving structure exterior to the plasma membrane of bacterial, algal, fungal, and plant cells

Question 61

protoplast

Answer 61

cell with no cell wall

Question 62

what does the cell wall consists of

Answer 62

consist predominantly of long fibers embedded in a network of branched molecules contain cellulose microfibrils enmeshed in a complex network of branched polysaccharides (hemicellulose and pectins) and glycoproteins called extensins

Question 63

cellulose

Answer 63

predominant polysaccharide in plant cell walls consisting of repeating glucose units linked by beta 1->4 bonds

Question 64

microfibrils

Answer 64

aggregate of several dozen cellulose molecules laterally crosslinked by hydrogen bonds serves as a structural component of plant and fungal cell walls

Question 65

hemicelluloses

Answer 65

heterogeneous group of polysaccharides deposited along with cellulose in the cell walls of plants and fungi to provide added strength each consist of a long, linear chain of a single kind of sugar (glucose or xylose) with short side chains

Question 66

pectins

Answer 66

branched polysaccharides, rich in galacturonic acid and rhamnose found in plant cell walls, where they form a matrix in which cellulose microfibrils are embedded

Question 67

extensins

Answer 67

only protein found in cell walls group of related glycoproteins that form rigid, rodlike molecules tightly woven into the cell walls of plants and fungi covalently crosslinked to one another and cellulose

Question 68

ligins

Answer 68

insoluble polymers of aromatic alcohols that occur mainly in woody plant tissues, where they contribute to the hardening of the cell wall and the structural strength we associate with wood

Question 69

middle lamella

Answer 69

first layer of the plant cell wall to be synthesized ends up farthest away from the plasma membrane, where it functions to hold adjacent cells together

Question 70

primary cell wall

Answer 70

flexible portion of the plant cell wall that develops beneath the middle lamella while cell growth is still occurring contains a loosely organized network of cellulose microfibrils

Question 71

expansins

Answer 71

a protein that allows local remodeling of plant cell walls role in plant cell growth, fruit softening, abscission may act by weakening hydrogen bonds within the cell wall and are activated at low pH

Question 72

secondary cell wall

Answer 72

rigid portion of the plant cell wall that develops beneath the primary cell wall after cell growth has ceased contains densely packed, highly organized bundles of cellulose microfibrils

Question 73

plasmodesmata, desmotubule, annulus

Answer 73

cytoplasmic channels through relatively large opening in the cell wall, allowing continuity of the plasma membranes from two adjacent cells - function like gap junctions (allow ions and larger molecules to pass) each is lined with plasma membrane common to the two cells, with the desmotubule in the central channel annulus lies btw the desmotubule and membrane lining the plasmodesma, provides continuity btw adjacent cells