2.2.5 carbohydrates 3: polysaccharides as structural units

Question 1

describe cellulose

Answer 1

• 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

Question 2

what are cellulose chains like

Answer 2

straight & lay side by side

Question 3

describe structure of cellulose –> esp. bonding

Answer 3

• 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

Question 4

when are microfibrils formed

Answer 4

when 60 to 70 cellulose chains are bound together
- 10-30nm in diameter

Question 5

what are macrofibrils

Answer 5

microfibrils bundled together
–> contains up to 400 microfibrils embedded in pectins to form plant cell walls

Question 6

why is cellulose an excellent material for plant cell walls

Answer 6

• 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)

Question 7

why do the features of cellulose help a plant cell wall to do it’s job

Answer 7

• 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)

Question 8

name 2 other structural polysaccharides

Answer 8

• bacterial cell walls
• exoskeletons

Question 9

describe bacterial cell walls

Answer 9

• whole structure surrounding cell = peptidoglycan
  –> long polysaccharide chains lying in parallel, cross-linked by short peptide chains (made of amino acids)

Question 10

describe exoskeletons

Answer 10

• 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