w10 txtbk Flashcards
tissues
cooperative assembly of cells and matrix woven together to form a distinctive multicellular unit with a specific function
extracellular matrix
complex network of polysaccharides and proteins secreted by cells
- a structural component of tissues that also influences their development and physiology
what causes a leaf to wilt?
osmotic swelling of the cell, limited by the resistance of the cell wall, can keep the chamber bloated
mass of swollen chambers forms a semirigid tissue
if water is lacking, the cells shrink and the leaf wilts
turgor pressure
a swelling pressure that develops as the result of an osmotic imbalance b/w the interior of the plant cell and its surroundings
how is a 2nd cell wall produced
once cell growth stops and the wall no longer needs to expand, it’s produced either by thickening of the primary wall or by deposition of new layers with a different composition underneath old ones
cellulose microfibils
long thin polysaccharide fiber that helps strengthen plant cell walls
resists compression and tension
how do cellulose microfibrils aid the cell in changing structure
for plant cell to grow/change, cell wall has to stretch or deform
-because the cellulose microfibrils resist stretching, their orientation governs the direction in which the growing cell enlarges
by controlling the way it lays down its wall, the plant cell controls its own shape and direction of growth of the tissue
what guides enzyme complexes?
microtubules serve as tracks that help direct movement of enzyme complexes
-cytoskeleton controls shape of plant cell and modeling of plant tissues
connective tissue
bulk of tissue occupied by extracellular matrix, and the cells that produce the matrix are scattered within it
the tensile strength is provided by fibrous proteins
collagens
triple-stranded, fibrous protein thats a major component of the extracellular matrix and connective tissues; its an abundant main protein in animal tissues
osteoblasts
the cells that manufacture collagen and live within connective tissues’ extracellular matrix and are named according to the tissue type in which they reside
fibroblasts
cell type that produces the collagen-rich extracellular matrix in connective tissues such as skin and tendon
also produce other macromolecules of the matrix
consequence of assembly occurring prematurely?
before secretion, the cell would become choked with its own products
in the case of collagen, the cells avoid this by secreting collagen in a precursor form called procollagen, which has additional peptide extensions at each end (collagen fibrils)
integrins
family of transmembrane proteins present on cell surfaces that allow cells to make and break attachments to the extracellular matrix, helping them to crawl through tissue
don’t interact directly with collagen fibers
undergo conformational changes
fibronectin
extracellular matrix protein that helps cells attach to the matric by acting as a “linker” that binds to a cell-surface integrin molecule on one end and to a matrix component (e.g. collagen) on other
glycosaminoglycans (GAGs)
polysacc. chain that can form a gel that acts as a “space filler” in the extracellular matrix of connective tissues; helps animal tissues resist compression
-negatively charged
proteoglycans
molecule consisting of one or more glycosaminoglycan chains attached to a core protein
these aggregates can form gels that regulate the passage of molecules through the extracellular medium and guide cell migration
how do GAGs cause swelling pressure?
even at low conc, GAGs form hydrophilic gels
-their negative charges attract a cloud of positive cation that draw large amounts of water into matrix
this hydration gives rise to a swelling pressure, whichs balanced by tension in collagen fibers interwoven with the GAGs
when the matrix is rich in collagen and large qty’s of GAGs are trapped in the mesh, both the swelling pressure and counterbalancing tension are big = resistant to compression
Which of the following determines the direction in which cellulose microfibrils are laid down in the extracellular space of a plant cell?
1. amount of turgor pressure within cell
2. availability of sugar monomers for cellulose microfibril elongation
3. orientation of microtubules in cell wall
4. orientation of microtubules on cytoplasmic side of plasma membrane
- orientation of microtubules on cytoplasmic side of plasma membrane
Microtubules beneath the cell wall serve as tracks that guide the movement of the enzyme complexes that synthesize the cellulose microfibrils.
Fibroblasts are the major cellular component in which tissues?
connective
Fibroblasts are the major cellular component in connective tissues. Fibroblasts manufacture and inhabit the extracellular matrix in skin, tendon, and many other connective tissues
epithelium
sheet of cells covering an external surface or lining an internal body cavity
-some mainly act as a protective barrier(+takes up nutrients) and others have biochemical functions
-cover the external surface of the body and line all internal cavities
structure of epithelial sheet
2 faces:
Apical surface
-free and exposed to air or to bodily fluid
Basal surface
-attached to thin, tough sheet of extracellular matrix called basal lamina
-composed of specialized collagen and protein called laminin (provides adhesive sites for integrin molecules)
Differ bc each contain a diff. set of molecules that reflect the polarized organization of individual epithelial cells
-this polarity is crucial for function
-imports food from gut lumen to apical surface and exports food from basal surface into tissue
what does polarity depend on in epithelial cells?
depends on the junctions that the cells form with one another and with the basal lamina
-these cell junctions control the arrangement of system of membrane-associated intracellular proteins
tight junction
cell-cell junction that seals adjacent epithelial cells together, preventing the passage of most dissolved molecules from one side of epithelial sheet to the other
-seal neighboring cells tg so water-soluble molecules cant easily leak b/w them
w/o tight junctions to prevent leakage, the pumping activities of absorptive cells like those in the gut would be pointless
+ the composition of extracellular fluid would become the same on BOTH sides of epithelium
how do tight junctions play a key role in maintaining polarity of individual epithelial cells
1) tight junctions around apical region of each cell prevent diffusion of proteins in plasma membrane and keep the contents of apical domain of plasma membrane separate
2) in many epithelia, tight junctions are sites of assembly for the complexes of intracellular proteins that govern the apical-based polarity of cell interior
types of epithelium
Adherens junctions, desmosomes
-bind one epithelial cell to another
hemidesmosome
-bind epithelial cells to basal lamina
all provide mechanical strength to the epithelium by the same strategy
-the proteins that form the junctions span PM and linked inside cell to cytoskeletal filaments
Adherens junctions and desmosomes
both built around transmembrane proteins that belong to the cadherin family
-a cadherin molecule in PM of one cell binds directly to identical caherin molecule in PM of its neighbour
called homophilic binding
-this binding requires the Ca2+ be present in extracellular medium
structure of adherens junction
each cadherin molecule is tethered inside its cell, via linker proteins to actin filaments
often the adherens junctions form a continuous adhesion belt around each of the interacting epithelial cells
network of actin filaments also contains myosin filaments and can CONTRACT, giving the epithelial sheet ability to develop tension and change its shape
these movements are crucial during embryonic development, when they create structures such as the neural tube
-give rise to brain and spinal cord, and lens vesicle, which develops into the lens of the eye
desmosome
connects to keratin filaments
-has great tensile strength and is characteristic of tough, exposed epithelia (skin)
hemidesmosome
structure that anchors epithelial cells to basal lamina beneath them
(look like half a desmosome = name)
gap junctions
in animal tissues, specialized connection b/w juxtaposed cells thru which ions and small molecules can pass from one cell to other
the channel allows inorganic ions and small, water-soluble molecules to move directly from the cytosol of one cell to another
-this flow create electrical and metabolic coupling b/w cells
in many tissues can be opened or closed in response to extracellular or intracellular signals
tight junction function
seals neighboring cells tg in epithelial sheet to prevent leakage of extracellular molecules b/w them; helps polarize cells
adherens junction
joins actin bundle in one cell to a similar bundle in neighboring cell
desmosome function
joins intermediate filaments in one cell to those in a neighbour
gap junction function
forms channels that allow small, intracellular, water-soluble molecules, including inorganic ions and metabolites, to pass from cell to cell
hemidesmosome function
anchors intermediate filaments in a cell to basal lamina
plasmodesmata
cell-cell junction that connects one plant cell to next; consists of a channel of cytoplasm lined by membrane
cytoplasmic channels lined with PM, allowing passage of both small and large molecules
