Cell Walls Flashcards
What is the role of xyloglucans in plant cell walls?
act as tethers between microfibrils via hydrogen bonds
when cells become rigid they expand out, stretching out the xyloglucan which makes it taut
to expand the cell even more the chains can be cut or hydrogen bonds removed
What is the structure of xyloglucan?
exists as a hemicellulose
glucose backbone connected via β-1-4 glycosidic bonds
most glucose have xylose attached, some xylose have galactose and some galactose have fucose
side chains linked via β-1-6 bonds
What can xyloglucans be split by?
endo-β-glucanase (cellulase)
xyloglucan endotransglucosylase (XET)
hydroxyl radicals (*OH)
How does endo-β-glucanase (cellulase) split xyloglucan?
hydrolysis
How does xyloglucan endotransglucosylase (XET) split xyloglucan?
uses two chains
xyloglucans (and other polysaccharides) have a non reducing end (depicted on left) and reducing end (depicted on right)
XET attaches non reducing end of acceptor onto reducing end of donor which splits donor chain and leaves
XET may reversibly untether microfibrils
allows for expansion while retaining strength
What is the source of hydroxyl radicals in the cell wall?
product of fenton reaction
most likely thing the radical will react w is a polysaccharide
called non-enzymic scission
What are expansins?
cause cell wall creep by mediating cell wall loosening
work optimally at pH4 and break xyloglucan-cellulose hydrogen bonds
refer to ‘velcro model’
What is the role of phenols in the cell wall?
attached to polysaccharides, can be oxidised which crosslinks polysaccharides together which stops cell growth
What enzyme oxidises phenols in the cell wall?
peroxidase
activity correlates with growth cessation
How are new wall polysaccharides synthesised?
UDP-glucose is a high energy molecule that donates glucose on to other molecules in a rosette (bunch of cellulose synthases)
ATP is required to add another glucose onto UDP
What does the rate of cell expansion depend on?
increased rate of wall building
loosening of pre-existing cell wall material
absence of tightening of pre-existing cell wall material
What is the perforation plate?
in healthy living plant
when xylem differentiates, normal cells degenerate into drain pipe like cells which have holes
side walls intact
allows transport of sap
involves breaking down of cell wall
What is the abscission zone?
zone where dying leaves are cut off from plant
involves breaking down of cell wall
What needs to happen for a fruit to ripen?
degradation of cell walls to allow softening
How do pathogenic fungi penetrate plant through the cuticle?
by use of cutinases
What are examples of biologially important situations where the cell wall is broken down post mortem?
- rotting eg on forest floor digested by fungi
- digestion in animal’s guts
What group of enzymes digest polysaccharides by splitting them in the middle? Give some examples.
glycanases (endohydrolases)
eg -endo-β-glucanase (cellulase)
- endo-pectinase (endopolygalacturonase)
What group of enzymes digest polysaccharides from the end? Give some examples.
glycosidases (exohydrolases)
split 1 off from non-reducing end
nature has evolved a different glycosidase for each polysaccharide
eg - β-glucosidase
- β-xylosidase etc
Most glycosidases (exohydrolases) digest from the end of a polysaccharide 1 sugar residue at a time. What is an exception to this?
β-glucosidase which releases cellulose two at a time
known as a cellobiose
the resulting disaccharide can then be hydrolysed into 2 monosaccharides of glucose
What would be an example of glycanases and glycosidases acting synergistically on microfibrils?
digestion of leaf litter by fungi
What are some examples of enzyme-proofing agents?
lignin - complex polymer made up of random organisation of phenolic isomers
silica - chemically similar to glass
extensin - a hydroxyproline-rich glycoprotein
cutin and suberin - aliphatic polyesters, form cuticle on epidermis of leaf
Which primary metabolite is used as the building block of lignin?
phenylalanine
How many different isomers is lignin built up of and what do they exist as?
4 isomers
exist as free radicals
each isomer can transform into any other isomer via non-enzymic reactions as well as coupling with each other
any isomer can bond with any isomer
How does a lignified cell wall cause expansion of cell wall to be diminished?
undergoes stress-transfer
