Plant Ch. 15

Front 1

What are the functions of plant cell walls?

Back 1

Maintain and determine cell shape
Resist water pressure so don't lyse in hypotonic medium
Limits cell migration: plants grow by addition
Controls rate and direction of elongation
Physical barrier
Signaling
Recognition responses: self vs. nonself in pollen and style

Front 2

Describe oligosaccharins

Back 2

They are small sugar residues that are important in cell signaling.
1. Stimulate ethylene synthesis: gaseous hormone that causes plants to ripen
2. Induce phytoalexins: defense chemicals produced in response to bacterial/fungal infection
3. Induce chitinase and other enzymes
4. increases Ca concentration in cytosol

Front 3

Describe cellulose.

Back 3

ß 1,4 glucose polymers
Cellobiose: 2 inverted glucose molecules: functional subunit
Polymers contain 1,000-10,000 glucose monomers.
20-40 polymers H-bond to form cellulose microfibril.
H-bonding creates crystalline structure and gives tensile strength = to that of steel. Also gives it resistance to chemical attacks.

Front 4

How is cellulose made?

Back 4

By a particle rosette.
Rosettes are made in the ER and secreted into PM
Each rosette is made up of 5-8 particles that are arranged circularly.
The cellulose microfibrils are embedded in a polysaccharide matrix. These are made in Golgi and exocytosed via vesicles. Proteins also present that were made in ER and exocytosed.

Front 5

Describe hemicellulose.

Back 5

It is bound very tightly to the cell wall.
It is very flexible and highly branched with ß 1-4 linkages: this prevents the cell wall from being too ordered and crystalline.
H-bonded to cellulose microfibrils: spaces them apart.
Polysaccharides are highly variable.
Hydrophilic and hydrated: forms a gel: creating a pillow that resist compression.
Abundant in all cell walls.

Front 6

Describe pectins

Back 6

Alpha 1-4 glucans
Very diverse: can be linear or branched
Often acidic due to COOH group: at pH 7 this is deprotonated and can form salt bridges with Ca or MG that create networks around HC and cellulose. At pH 5 it is protonated and is when cell wall can change shape, grow, and mature.
Secreted with methyl/acetate group on it = less hydrated creating a less rigid gel phase. Can be removed with pectine esterase.
1º function= resist compression forces. Also creates pore size of cell wall.
Abundant in 1º cell wall and middle lamella.

Front 7

Describe structural proteins in cell wall.

Back 7

Classified on aa structure.
Hydrophillic: can form Hbonds and salt bridges with other cell wall components.
Wounding or pathogen attack can induce or increase rate of protein synthesis which leads to increased rigidity of cell wall.
Have high Lysine content (has NH3+ group) which can interact with negative charges on pectin: this creates lots of interactions between aa side chains and carbohydrates.
May contain sites for lignification

Front 8

Give an example of a structural protein in cell wall.

Back 8

Extensin: limits cell expansion and growth
Has evenly spaced Tyrosine : allows for formation of covalent isodityrosine bonds between 2 extensin proteins or in the same protein. H2O2 and peroxidase necessary to form this bond. Creates a more rigid wall and limits growth.
Mostly found in cells that have stopped growing/maturing. Also in cells that have been wounded/ have pathogens. Abundant in cells with lignified walsl

Front 9

Describe functional proteins in cell wall.

Back 9

1.Have oxidative enzymes like peroxidases that catalyze isodityorsine linkages, catalyze lignification, and rigidify/strengthen cell wall.
2. have hydrolytic enzymes that require low pH such as pectinases, cellulase, hemicellulases, and chitinases.
3. Expansins: catalyze cell wall expansion by breaking Hbonds between cellulose microfibril and hemicellulose

Front 10

Describe lignin.

Back 10

Only found in 2º cell walls.
Made of phenol propanoid groups that are linked together covalently in branched polymer.
*No simple repeating pattern
Precursors for lignin synthesis are PP alcohols from phenylalanine.
After they are secreted outside of PM, enzymes link the units together. (This displaces water from the cell= no longer hydrated).
Can be covalently bonded to cellulose, proteins, hemicelluloses.
-Increases rigidity and strength of wall. ALSO creates hydrophobic environment so prevents water movement from outside of cell to inside.
-Seen in fibers (support), vessel members & tracheids (water conduction).
Resist large negative pressures that could cause cell to collapse.
Resist mechanical stress
Provide support against gravity : allows for evolutionary increase in size.
Physical block for pathogens
Chemically resistant to breakdown: indigestible

Front 11

Describe secondary cell walls.

Back 11

Orientation of cellulose microfibrils is in 3 distinct layers that account for all possible angles of mechanical stress = super strong and rigid.

Front 12

What determines the position of a new cell wall before phragmoplast? Fig 1.25

Back 12

A group of MT called pre-prophase band
Microfilaments also play a role in position new cell wall before division.
Interphase: lots of cytosolic MT. Cortical MT occur just underneath PM, wrap around like a band
Pre-prophase: right before cell enters mitosis: MT break down but a band stays associated with PM usually in line with nucleus.
PP band then disintegrates and mitotic spindle forms
The phragmoplast forms where the PP band was.

Front 13

Describe the role of Microfilaments in determining position of new cell wall before phragmoplast

Back 13

Pre-prophase: MF extend through entire cell and band is only at center near nucleus.
During mitosis: MF disappear from where band is: and remains everywhere else, but cortical MF remain. The interaction between MF and MT helps orient/align PP band.