9. The Plant Cell Wall I Flashcards
Overview: Give an overview of the cell wall!
Middle lamella, pirmary walls, seccondary wall
Why is the cell wall important for the cell?
- Establishment of a counterpressure to the water potential of the protoplasts -> “exoskeleton”
- determines the mechanical stability of tissues
- Conductive elements such as e.g. the xylem, which is subjected to a strong transpiration suction
- holds cells together in a tissue context
- determines and limits the elongation growth of plant cell
- contributes to the water balance, because the cell wall determines the relationship between turgor pressure and cell volume
- Diffusion barrier, limiting in size to macromolecules that can reach the plasma membrane
- structural barrier to pathogen infection
Can the cell react on higher pressure?
Osmotic water influx into vacuole and cytosol leads to turgor pressure on the cell wall, which the cell wall counteracts with the wall pressure
Which chemical-molecular compounds are in the cell wall?
Carbohydrates: Pectins, Hemicelluloses, Celluloses
Proteins: Structural proteins, Modifying enzymes
Hydrophobic polymers: Lignins, cutins, suberines
Cell wall compound pectine: they are a mixture of what? Where are they abundant?
Groups of Carbohydrates in Cell Walls:
Abundant in middle lamella and primary wall
Protopectin is a mixture of: Galacturonans Rhamnogalacturonans
Protopectin is made of… G
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
What kind of pectin can you find on the flank of the pollen tube?
Pectin Methyl Esterase (PME) Catalyzes the Transformation
to Non-Esterified Pectin e.g. on the Flank of the Pollen Tube
Protopectin is made of … (R)
Rhamnogalacturonan Molecules of Protopectin
Monomers: α-D-Galacturonic acid (dissociated anion) and α-L-Rhamnose
in the polymer: Arabinan-, Galactan- or Arabinogalactan Side Chains
Hemicellulloses: they are a mixture of what? Where can you find it?
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
One example of the hemicellulose - monomers and polymers
Monomeres: β-D-Glucose and α-D-Xylose
with alpha 1-6 and beta 1-4 linkage
Cell wall compound Cellulose: they are a mixture of what? Where are they abundant?
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
Formation of the Middle Lamella and the Primary Wall
at the End of Cytokinesis - how does that take place?
- Cell plate fusion with parental plasma membrane
- Formation of middle lamella
- Formation of primary wall
How are Cellulose synthase (CESA) complexes transported?
Note: Cellulose synthase (CESA) complexes are transported via the TGN to the phragmoplast and cellulose is already synthesized during cell plate formation
Vectorial Cellulose Synthesis is Determined by what?
Cortical Microtubules
Which size have macro- and microfibrils?
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