4.4 - Cell communities/connections Flashcards
role of extracellular matrix on shaping cells (2)
- cell-cell and cell-extracellular matrix interactions control cell shape/function and determine developmental fate of cells/tissues
- cell-adhesion molecules mediate cell-cell adhesion, receptors mediate cell-matrix adhesions
what does the extracellular matrix consist of?
complex meshwork of proteins and polysaccharides that contribute to tissue structure and function
2 main polysaccharide components of plant cell wall (2)
- cellulose (predominant)
- pectin
plant cell wall structure (2)
- arranged in layers on extracellular side of plasma membrane
- cellulose forms matrix with other polysaccharides and structural proteins to form structure that resists compression and tension
cellulose (2)
- carbohydrate polymer of glucose
- very tough and water impermeable
whats unusual about cellulose synthesis in plant cells?
synthesised on surface of cell (by cellulose synthase embedded in the membrane)
microtubule role in cellulose synthesis
microtubules serve as guides to orient movement of cellulose synthase complex
what influences the direction in which a plant cell elongates under turgor pressure (internal water pressure)
orientation of cellulose microfibrils within the plant cell wall
(cell tends to elongate in direction perpendicular to orientation of microfibrils, great tensile strength)
plasmodesmata (3)
- intercytoplasmic channels of plasmodesmata pierce plant cell walls, connecting cytoplasms of all cells in plant
- each plasmodesmata lined with plasma membrane common to the 2 connected cells (usually contains fine tubular structure - desmotubule)
- derived from smooth ER
what forms bulk of connective tissues in animal cells
extracellular matrix (no cell wall)
what are the names of cells that produce extra cellular matrix in connective tissues? (2)
- in most tissues - fibroblast
- in bones - osteoblasts
key components of of extra cellular matrix in animals (2)
- collagen
- proteoglycans
collagen (2)
- animal version of cellulose
- composed of peptides (not polysaccharides)
proteoglycans (2)
- carbohydrates with protein component that have charged residues which attract water
- allows ECM to function bit like gel
where is collagen found? (2)
- most abundant protein in animals
1. skin
2. connective tissues etc
collagen structure (3)
- 3 intertwined helical polypeptide chains
- a-helices stabilised by steric repulsion of proline rings (not H-bonds)
- striated appearance - only glycine can fit in interior, spaced every 3 residues
collagen fibrils (3)
- many rodlike collagen molecules covalently cross-linked in intracellular space
- 3 intertwined chains also interact via H-bonds
- gives collagen fibrils tensile strength greater than steel
collagen organisation in skin
collagen fibrils organised into sheets oriented in different directions to resist tensile strength in different directions
collagen organisation in cells (principally fibroblasts)
achieve organisation by moving around intercellular spaces laying down collagen in oriented fashion then reshaping it
how can collagen be post-translationally modified and what is the enzyme/cofactor? (2)
- hydroxylation of proline and lysine
- catalysed by prolyl hydroxylase, requires ascorbate (vitamin C) as cofactor
integrins (adaptor proteins linking cytoskeleton to ECM)
transmembrane proteins bind to actin cytoskeleton in cytoplasm and to extra cellular matrix proteins via fibronectin
fibronectin
binds to actin cytoskeleton and collagen making link between cytoskeleton and extra cellular matrix
epithelia
majority of cell types organised into multicellular sheets of cells joined together side by side to form a barrier
where do epithelia cover?
external surface of a body and line internal cavities
how does epithelia contrast connective tissue?
primarily carries mechanical load through cell-cell connections and cytoskeleton (connective tissues use extra cellular matrix)
2 faces of epithelial cells (2)
- apical face - free and exposed to outside
- basal face - rests on sheet of connective tissue
basal lamina
lies between basal face and underlying connective tissue, specialised ECM layer to which the cells are attached
tight junctions (2)
- seel neighbouring cells together in epithelial sheet (prevents leakage of molecules between them)
- formed from proteins (claudins and occludins) arranged in strands along lines of junction
desmosomes (3)
- join intermediate filaments (keratin) between cells (prevents cells ripping apart under mechanical stress)
- built around proteins of cadherin family (nucleate a cytoplasmic plaque anchoring intermediate filaments)
- leads to great tensile strength in sheet (skin epidermis)
- hemidesmosomes = similar but connect cells to basement membrane
gap junctions (3)
- provide neighbouring cells with direct channel of intercytosolic communication (allows passage of small water soluble ions/molecules)
- 2 opposing plasma membranes linked by protein assemblies (connexons)
- 2 connexons join across intercellular gap - forms aqueous channel connecting cytoplasms of 2 cells
adherens junctions (2)
- join actin bundle in one cell to actin bundle in another (allowing contractile movements)
- contraction of actin filament causes layer to roll - leads to tube-like structures even spheres (crucial during embryo development)
effect of contraction of apical bundles of actin filaments linked from cell to cell via adherens (2)
- causes epithelial cells to contract at their apex
- depending on orientation causes epithelial sheet to invaginate or roll up into tube
animal cell junctions (4)
- tight junctions
- desmosomes
- gap junctions
- adherens junctions
main function of tight junctions
prevent leakage of cells in epithelial sheets
main function of desmosomes
provides tensile strength and prevents cells from ripping under mechanical stress
main function of gap junctions
allows passage of small water soluble ions and molecules between cells
main function of adherens junctions
allows contractile movement of cells
