9. The Plant Cell Wall I

Question 1

Overview: Give an overview of the cell wall!

Answer 1

Middle lamella, pirmary walls, seccondary wall

Question 2

Why is the cell wall important for the cell?

Answer 2

1. Establishment of a counterpressure to the water potential of the protoplasts -> “exoskeleton”
2. determines the mechanical stability of tissues
3. • Conductive elements such as e.g. the xylem, which is subjected to a strong transpiration suction
4. holds cells together in a tissue context
5. determines and limits the elongation growth of plant cell
6. contributes to the water balance, because the cell wall determines the relationship between turgor pressure and cell volume
7. Diffusion barrier, limiting in size to macromolecules that can reach the plasma membrane
8. structural barrier to pathogen infection

Question 3

Can the cell react on higher pressure?

Answer 3

Osmotic water influx into vacuole and cytosol leads to turgor pressure on the cell wall, which the cell wall counteracts with the wall pressure

Question 4

Which chemical-molecular compounds are in the cell wall?

Answer 4

Carbohydrates: Pectins, Hemicelluloses, Celluloses
Proteins: Structural proteins, Modifying enzymes
Hydrophobic polymers: Lignins, cutins, suberines

Question 5

Cell wall compound pectine: they are a mixture of what? Where are they abundant?

Answer 5

Groups of Carbohydrates in Cell Walls:

Abundant in middle lamella and primary wall
Protopectin is a mixture of: Galacturonans Rhamnogalacturonans

Question 6

Protopectin is made of… G

Answer 6

Homogalacturonan Molecules
Monomeres: α-D-Galacturonic acid (dissociated anion) and α-D-Galacturonic acid methyl ester
Highly methylated galacturonan = pectin
Polymer with Ca2+ or Mg2+ bridges

Question 7

What kind of pectin can you find on the flank of the pollen tube?

Answer 7

Pectin Methyl Esterase (PME) Catalyzes the Transformation

to Non-Esterified Pectin e.g. on the Flank of the Pollen Tube

Question 8

Protopectin is made of … (R)

Answer 8

Rhamnogalacturonan Molecules of Protopectin
Monomers: α-D-Galacturonic acid (dissociated anion) and α-L-Rhamnose
in the polymer: Arabinan-, Galactan- or Arabinogalactan Side Chains

Question 9

Hemicellulloses: they are a mixture of what? Where can you find it?

Answer 9

Groups of Carbohydrates in Cell Walls: Hemicellulloses
Main components of the primary
wall: Hemicelluloses contain Pentoses
e.g. D-xylose, L-arabinose, Hexoses such as D-glucose, Dgalactose and D-mannose

Question 10

One example of the hemicellulose - monomers and polymers

Answer 10

Monomeres: β-D-Glucose and α-D-Xylose

with alpha 1-6 and beta 1-4 linkage

Question 11

Cell wall compound Cellulose: they are a mixture of what? Where are they abundant?

Answer 11

Groups of Carbohydrates: Cellulose

Found in primary and secondary wall
about 10% of the primary wall
up to 94% of the secondary wall
Linear [β1-> 4] glucan from β-D-glucose
(or its dimer)
very long-chain (up to 15,000 glucose subunits) and straightened out

Question 12

Formation of the Middle Lamella and the Primary Wall

at the End of Cytokinesis - how does that take place?

Answer 12

1. Cell plate fusion with parental plasma membrane
2. Formation of middle lamella
3. Formation of primary wall

Question 13

How are Cellulose synthase (CESA) complexes transported?

Answer 13

Note: Cellulose synthase (CESA) complexes are transported via the TGN to the phragmoplast and cellulose is already synthesized during cell plate formation

Question 14

Vectorial Cellulose Synthesis is Determined by what?

Answer 14

Cortical Microtubules

Question 15

Which size have macro- and microfibrils?

Answer 15

Macrofibrils:
Bundles of microfibrils, diameter very variable, 10 to 60 nm.
Lengths also variable, 1 to 13 µm (equivalent to 2,000 - 25,000 glucose units)

Microfibrils:
Directly from rosette complex: 18 cellulose molecules in cross section, about 3 nm diameter

Question 16

What do you know about the rosette compelxes?

Answer 16

Rosette complexes - number of subunits not finally clarified
Cellulose synthase complex (CSC) = rosette complex
- hexameric, 6 units (visible electron microscopically)
- each unit probably composed of 3 cellulose synthase (CESA) subunits
- therefore microfibrils in cross section of 18 cellulose molecules but also deviations: e.g. so far no CSCs observed in cotton

Question 17

Initiation of Cell Wall Precursor Delivery During Cell Plate Formation

Answer 17

The phragmoplast (a cylinder of microtubles) appears after chromosomes have separated.
The cell plate forms a vesicle fuse at the middle line of the phragmoplast.
The cell plate grows radially outward toward future side of division.

Question 18

Cell Wall Precursor Delivery During Cell Plate Formation

Answer 18

The expanding cell plate nears the edges of the cell.
The membrane of the cell plate begins to fuse with the plasma membrane.
Once fusion is finished cellulose is synthetisized to complete the

Question 19

Plasmodesmata serve

Answer 19

symplastic transport and can also be (ab)used by phytopathogenic viruses