4.4 - Cell communities/connections

Question 1

role of extracellular matrix on shaping cells (2)

Answer 1

1. cell-cell and cell-extracellular matrix interactions control cell shape/function and determine developmental fate of cells/tissues
2. cell-adhesion molecules mediate cell-cell adhesion, receptors mediate cell-matrix adhesions

Question 2

what does the extracellular matrix consist of?

Answer 2

complex meshwork of proteins and polysaccharides that contribute to tissue structure and function

Question 3

2 main polysaccharide components of plant cell wall (2)

Answer 3

1. cellulose (predominant)
2. pectin

Question 4

plant cell wall structure (2)

Answer 4

1. arranged in layers on extracellular side of plasma membrane
2. cellulose forms matrix with other polysaccharides and structural proteins to form structure that resists compression and tension

Question 5

cellulose (2)

Answer 5

1. carbohydrate polymer of glucose
2. very tough and water impermeable

Question 6

whats unusual about cellulose synthesis in plant cells?

Answer 6

synthesised on surface of cell (by cellulose synthase embedded in the membrane)

Question 7

microtubule role in cellulose synthesis

Answer 7

microtubules serve as guides to orient movement of cellulose synthase complex

Question 8

what influences the direction in which a plant cell elongates under turgor pressure (internal water pressure)

Answer 8

orientation of cellulose microfibrils within the plant cell wall
(cell tends to elongate in direction perpendicular to orientation of microfibrils, great tensile strength)

Question 9

plasmodesmata (3)

Answer 9

1. intercytoplasmic channels of plasmodesmata pierce plant cell walls, connecting cytoplasms of all cells in plant
2. each plasmodesmata lined with plasma membrane common to the 2 connected cells (usually contains fine tubular structure - desmotubule)
3. derived from smooth ER

Question 10

what forms bulk of connective tissues in animal cells

Answer 10

extracellular matrix (no cell wall)

Question 11

what are the names of cells that produce extra cellular matrix in connective tissues? (2)

Answer 11

1. in most tissues - fibroblast
2. in bones - osteoblasts

Question 12

key components of of extra cellular matrix in animals (2)

Answer 12

1. collagen
2. proteoglycans

Question 13

collagen (2)

Answer 13

1. animal version of cellulose
2. composed of peptides (not polysaccharides)

Question 14

proteoglycans (2)

Answer 14

1. carbohydrates with protein component that have charged residues which attract water
2. allows ECM to function bit like gel

Question 15

where is collagen found? (2)

Answer 15

• most abundant protein in animals
  1. skin
  2. connective tissues etc

Question 16

collagen structure (3)

Answer 16

1. 3 intertwined helical polypeptide chains
2. a-helices stabilised by steric repulsion of proline rings (not H-bonds)
3. striated appearance - only glycine can fit in interior, spaced every 3 residues

Question 17

collagen fibrils (3)

Answer 17

1. many rodlike collagen molecules covalently cross-linked in intracellular space
2. 3 intertwined chains also interact via H-bonds
3. gives collagen fibrils tensile strength greater than steel

Question 18

collagen organisation in skin

Answer 18

collagen fibrils organised into sheets oriented in different directions to resist tensile strength in different directions

Question 19

collagen organisation in cells (principally fibroblasts)

Answer 19

achieve organisation by moving around intercellular spaces laying down collagen in oriented fashion then reshaping it

Question 20

how can collagen be post-translationally modified and what is the enzyme/cofactor? (2)

Answer 20

1. hydroxylation of proline and lysine
2. catalysed by prolyl hydroxylase, requires ascorbate (vitamin C) as cofactor

Question 21

integrins (adaptor proteins linking cytoskeleton to ECM)

Answer 21

transmembrane proteins bind to actin cytoskeleton in cytoplasm and to extra cellular matrix proteins via fibronectin

Question 22

fibronectin

Answer 22

binds to actin cytoskeleton and collagen making link between cytoskeleton and extra cellular matrix

Question 23

epithelia

Answer 23

majority of cell types organised into multicellular sheets of cells joined together side by side to form a barrier

Question 24

where do epithelia cover?

Answer 24

external surface of a body and line internal cavities

Question 25

how does epithelia contrast connective tissue?

Answer 25

primarily carries mechanical load through cell-cell connections and cytoskeleton (connective tissues use extra cellular matrix)

Question 26

2 faces of epithelial cells (2)

Answer 26

1. apical face - free and exposed to outside 2. basal face - rests on sheet of connective tissue

Question 27

basal lamina

Answer 27

lies between basal face and underlying connective tissue, specialised ECM layer to which the cells are attached

Question 28

tight junctions (2)

Answer 28

1. seel neighbouring cells together in epithelial sheet (prevents leakage of molecules between them) 2. formed from proteins (claudins and occludins) arranged in strands along lines of junction

Question 29

desmosomes (3)

Answer 29

1. join intermediate filaments (keratin) between cells (prevents cells ripping apart under mechanical stress) 2. built around proteins of cadherin family (nucleate a cytoplasmic plaque anchoring intermediate filaments) 3. leads to great tensile strength in sheet (skin epidermis) - hemidesmosomes = similar but connect cells to basement membrane

Question 30

gap junctions (3)

Answer 30

1. provide neighbouring cells with direct channel of intercytosolic communication (allows passage of small water soluble ions/molecules) 2. 2 opposing plasma membranes linked by protein assemblies (connexons) 3. 2 connexons join across intercellular gap - forms aqueous channel connecting cytoplasms of 2 cells

Question 31

adherens junctions (2)

Answer 31

1. join actin bundle in one cell to actin bundle in another (allowing contractile movements) 2. contraction of actin filament causes layer to roll - leads to tube-like structures even spheres (crucial during embryo development)

Question 32

effect of contraction of apical bundles of actin filaments linked from cell to cell via adherens (2)

Answer 32

1. causes epithelial cells to contract at their apex 2. depending on orientation causes epithelial sheet to invaginate or roll up into tube

Question 33

animal cell junctions (4)

Answer 33

1. tight junctions 2. desmosomes 3. gap junctions 4. adherens junctions

Question 34

main function of tight junctions

Answer 34

prevent leakage of cells in epithelial sheets

Question 35

main function of desmosomes

Answer 35

provides tensile strength and prevents cells from ripping under mechanical stress

Question 36

main function of gap junctions

Answer 36

allows passage of small water soluble ions and molecules between cells

Question 37

main function of adherens junctions

Answer 37

allows contractile movement of cells