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BIOSCI 340 > Plant cell walls > Flashcards

Plant cell walls Flashcards

1
Q

what are the components of a plant cell wall?

A 
polysaccharides
proteins and glycoproteins
lignin and other phenolic compounds
inorganic compounds 
water
other polymers
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2
Q

what is cutin

A

a polymer found in cell wall

3D polyester of fatty acids and hydroxy fatty acids

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3
Q

what is suberin

A

a polymer found in cell walls

has an aromatic and polyester domain

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4
Q

what is the most common component of cell wall waxes?

A

hydrocarbons (mostly odd chained alkanes)

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5
Q

name a very resistant polymer that occurs in the outer wall

A

sporopollenin

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6
Q

describe the common features of monosaccharides in cell wall polysaccharides

A 
all are 5- or 6-carbons
all are aldoses
some D-, some L-isomers
some are rings (5 or 6 membered)
linked by glycosidic bonds
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7
Q

name the 9 monosaccharides present after acid hydrolysis of cell wall polysaccharides

A 
D-glucose
D-galactose
D-mannose
D-xylase
L-arabinose
D-glucronic acid
D-galacturonic acid
L-rhamnose
L-fucose
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8
Q

what do primary cell walls of eudicotyledons commonly contain?

A

cellulose (20-40%)
pectic polysaccharides (pectin)
xyloglucans
extensins, structural proteins and glycoproteins

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9
Q

describe a cellulose microfibril

A

cellulose are laterally packed together
chains are parallel
each contains 18 cellulose molecules

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10
Q

what are pectic polysaccharides

A

pectins
block polymers consisting of blocks or domains
common one is homogalacturonan (ie polygalacturonic acid)

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11
Q

what is the second most abundant type of domain?

A

rhamnogalacturonan 1 (RG-1)

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12
Q

describe some features of RG-1

A

alternating galacturonic acid (GalA) and rhamnose (Rha) residues
pectins can attach to Rha –> “hairs”
various plysaccharides are attached
role is not clear

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13
Q

name the 3 types of polysaccharides attached to RG-1

A

arabinins
galactans
arabinogalactans

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14
Q

name the commonest domain

A

homogalacturonan (HG)

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15
Q

in which molecule are two apiose monosaccharides connected by a borate ester?

A

RG-II

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16
Q

describe xyloglucans

A

family of unrelated, similarly structured polysaccharides

bind to cellulose microfibrils

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17
Q

describe extensins

A

hydroxyproline-rich glycoproteins
consist of a protein backbone with repeats (Ser-Hypx5 is common)
formed as soluble that cross link
thought to play a structural role in the cell

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18
Q

what is the composition of secondary walls of eudicotyledons?

A

cellulose (40-60%)
heteroxylans
lignin (15-35%)

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19
Q

what is the composition of primary walls of Poaceae?

A 
cellulose (20-40%)
heteroxylans
(1-->3),(1-->4)-B-glucans
xyloglucans and pectins
structural proteins and glycoproteins including extensins
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20
Q

describe GAX

A

glucuronoarabinoxylans
major non-cellulostic component
ferulic acid is ester linked to GAX via some Ara residues
ferulic acid on adjacent GAX can form dimers

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21
Q

describe (1–>3),(1–>4)-B-glucans

A

linear polysaccharides with 1–>3 and 1–>4 links

some are waters soluble, others not

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22
Q

what is the composition of secondary walls of Poaceae?

A

cellulose (35-40%)
xylans
lignin (15-25%)

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23
Q

describe the model for the architecture of eudicotyledon primary cell walls

A

cellulose microfibrils crosslinked with xyloglucans

extensin-pectin network

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24
Q

describe the model for the architecture of poaceae primary cell walls

A

cellulose microfibrils crosslinked by GAXs with low degrees of substitution
GAXs with higher degrees of substitution and small proportions of pectins
(1–>3),(1–4)-B-glucan may be H bonded to cellulose microfibrils

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25
Q

describe the cellulose microfibril layers in eudicotyledon and poaceae secondary walls

A

S1, S2, and S3 layers
S1, S3: cross helical organisation
S2: more uniform helical organisation

26
Q

describe the role of heteroxylans in the architecture of secondary cell walls

A

associated with cellulose aggregates and with lignin

27
Q

what does lignin do in 2ndary walls

A

displaces water and enhances H bonding
makes wall more rigid
protects polysaccharides from enzyme degradation
forms the Casparian strip

28
Q

which enzymes are used in the biosynthesis of cell wall polysaccharides?

A

glycosyltransferases
transfer glycosyl from nucleotide sugars to acceptors
eg. UDP-glucose –> cellulose

29
Q

is there a template mechanism in the synthesis of polysaccharides?

A

no

30
Q

what are the steps from glucose to cellulose?

A

glc –> glc-6-P –> Glc-1-P –> UDP-Glc –> cellulose

31
Q

where does the synthesis of cell wall polysaccharides occur?

A

in the endomembrane system

RER–>SER–>Golgi apparatus–>Golgi vesicles–> plasma membrane

32
Q

in embryophytes, which polysaccharides are probably synthesis at the plasma membrane?

A

cellulose, callose and probably (1–>3),(1–>4)-B-glucans

all others in Golgi apparatus

33
Q

in which group was the rosette structure of cellulose seen?

A

embryophytes

34
Q

how many CESAs (cellulose synthase) per lobe in rosetta?

A

3 (so 18 cellulose per microfibril)

35
Q

how many CESA genes are found in Arabidopsis?

A

10

36
Q

what are the CSL (cellulose synthase-like) genes involved in?

A

CSLA: heteromannan synthesis
CSLC: xyloglucan synthesis
CSLF, CSLH: (1–>3),(1–>4)-B-glucan synthesis

(CSLF and CSLH not present in Arabidopsis/eudicotyledons)

37
Q

what nucleotide sugars are required for the synthesis of xyloglucan?

A

UDP-Glc, UDP-Xyl, UDP-Gal, UDP-Fuc

38
Q

in Arabidopsis, how many glycosyltransferases are invovled in the synthesis of xyloglucan?

A 
6
1 x (1--4)-B-glucan synthase
2 x a-xylosyltransferase
2 x B-galactosyltransferase
1 x a-fucosyltransferase
39
Q

describe the two mutants involved in XG synthesis

A

mur2: lacks fucosylated XG (mutation in a-fucosyltransferase) –> normal phenotype
mur3: lacks fucosylated XG (mutation in B-galactotransferase) –> normal phenotype

40
Q

which two closely related genes were shown to encode xylosyltransferase that transfer xyl fro UDP-xyl?

A

XXT1 and XXT2

Arabidopsis with both mutants had no detectable xyloglucan in cell wall. still had normal phenotype

41
Q

what is the second most abundant polymer in the biosphere?

A

lignin (behind cellulose)

42
Q

how is lignin formed?

A

by oxidative polymerisation of hydroxycinnamyl alcohol precursors (monolignols) (form free radicals which polymerise)

coniferyl–>guaiacyl (G subunit)
sinapyl–>syringyl (S subunit)

43
Q

describe lignin in coniferous gymnosperms

A

mostly G subunits

44
Q

describe lignin in angiosperms

A

S and G subunits linked by at least 5 different bonds

45
Q

describe lignin in poaceae

A

S, G and H (p-hydroxyphenyl) subunits

46
Q

how are monolignols formed?

A

by the phenylpropanoid pathway

phenylalanine –> cinnamic acid –> p-coumaric acid

47
Q

why is lignin economically important?

A

pulp and paper industry
forages for ruminant animals
feedstock for the production of biofuels

48
Q

what is the role of lignin in pulp and the paper industry?

A

lignin is removed in producing wood pulp

hardwoods (S, G lignin) is easier to pulp than soft woods (G lignin)

49
Q

why is lignin relevant in foraging for ruminant animals?

A

lignin is linked to cell wall polysaccharides and interferes with their degradation (makes it harder to digest)

50
Q

describe fah mutant in Arabidopsis

A

only contains G lignin

mutation in F5H

51
Q

describe bm1 mutant of Zea mays

A

mutant of CAD

more digestible cell well

52
Q

describe bm3 mutant of Zea mays

A

mutant of COMT

more digestible cell wall

53
Q

describe gene perturbation

A

most common biotechnological approach
e.g. improved forage cell wall digestibility
down regulated p-coumarate 3-hydroxylase (C3H) gene
slighly reduced lignin content
massive increase in H subunit

54
Q

describe turgor pressure

A

gradient in osmotic pressure driver water into cells to develop an internal hydrostatic pressure
makes the cell increase in size and volume

55
Q

how does auxin cause cell loosening

A

acts on WEX

56
Q

describe the two types of cell growth

A

diffuse growth: if irreversible, extension occurs in all walls of the cell

localised growth: if irreversible, growth only occurs in localised area

57
Q

describe the acid growth theory (AGT)

A

when susceptible cells are exposed to auxin, they excrete H+ into the cell wall at an enhanced rate, resulting in a decrease in apoplastic pH –> activates cell loosening

58
Q

describe expansins

A

expressed in expanding cells
no evidence that expansins are polysaccharide hydrolases
may act by disrupting H bonds (induced extension in strips of paper that is predominantly H bonds

59
Q

describe role of XET in cell expansion

A

XET = xyloglucan endotransglycosylase
evidence that is involved in cell expansion in at least some species
some are regulated by auxins

60
Q

describe the role of endo-(1–>4)-B-glucanases in cell expansion

A

EGases
been called cellulases by no evidence that for action on cellulose microfibrils
likely substrates = xyloglucans, (1–>3),(1–4)-B-glucan

61
Q

describe the possible role of OH radicals

A

can cause enzymatic breakage of xyloglucans and cell wall polysaccharides
are formed by Fenton reaction: H202 + Cu+ –> radical OH + OH- + Cu2+

BIOSCI 340 (3 decks)

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