2.2.5 carbohydrates 3: polysaccharides as structural units Flashcards
1
Q
describe cellulose
A
- found in plants
- forms cell walls
- tough, insoluble & fibrous
- homopolysaccharide made of long chains (up to 15000) beta glucose molecules bonded together via condensation reactions forming glycosidic bonds
2
Q
what are cellulose chains like
A
straight & lay side by side
3
Q
describe structure of cellulose –> esp. bonding
A
- hydrogen & hydroxyl groups on carbon 1 are inverted in beta glucose = every other beta glucose molecule is rotated 180 degrees
^^this & the beta 1-4 glycosidic bond prevent chain spiralling - hydrogen bonding between rotated alpha glucose molecules in each chain = strength
- hydrogen bonding between rotated beta glucose molecules in diff. chains = additional strength –> hydroxyl group on C 2 sticks out
4
Q
when are microfibrils formed
A
when 60 to 70 cellulose chains are bound together
- 10-30nm in diameter
5
Q
what are macrofibrils
A
microfibrils bundled together
–> contains up to 400 microfibrils embedded in pectins to form plant cell walls
6
Q
why is cellulose an excellent material for plant cell walls
A
- macrofibrils/microfibrils have high tensile strength (glycosidic bonds & hydrogen bonds between chains)
- macrofibrils run in all directions & criss-cross = strength
- difficult to digest cellulose due to glycosidic bonds between glucose molecules (less easy to break)
7
Q
why do the features of cellulose help a plant cell wall to do it’s job
A
- plants do not have rigid skeleton & each cell needs strength to support plant
- space between macrofibrils for water/mineral ions to pass on way in/out of cell = cell wall fully permeable
- wall has high tensile strength = prevents bursting if turgid & supports plant –> turgid cells press against each other & supports cell –> wall protects cell membrane
- macrofibril structure reinforced by other substances for extra support or to make walls waterproof eg. cutin & suberin (waxes)
8
Q
name 2 other structural polysaccharides
A
- bacterial cell walls
- exoskeletons
9
Q
describe bacterial cell walls
A
- whole structure surrounding cell = peptidoglycan
–> long polysaccharide chains lying in parallel, cross-linked by short peptide chains (made of amino acids)
10
Q
describe exoskeletons
A
- insect/crustacean exoskeletons made of chitin
–> has an acetylamino group (not hydroxyl group) on carbon 2
–> forms cross-links between long parallel chains of acetylglucosamine
