Cell Wall

1
Cell Wallstructure and function

• The three layers
• 1). The middle lamella (M)
• 2). The primary wall (1)
• 3) The secondary wall (2)
• (some cells may not have 2nd wall)

2
(No Transcript)
3

• The composition
• M 1 2
• Cellulose
• Hemicellulose
• Pectin
• Protein ?
• Lignin - -

4

• 3. Molecular structure
• cellulose at least 500 glucose molecules
  1,4-beta-glycoside cellulose fibrils intra- and
  inter-molecular H-bonds make the cellulose
  molecules bundle together into a fibril.
• Hemicellulose mixture of cellulose backbones
  with side chains. Can interact with fibrils via
  backbones.
• Pectins poly-galacturonic acid (from galactose)
  with many COO- negatively charged easily bind
  metals especially calcium forming a geljello.
  Middle lamella to cement cell wall of linked
  cells. Some cells such as mesophyll cells that
  need air/CO2 for photosynthesis may have cracked
  middle lamella (rich in air space)
• Proteins cell wall proteins are very important
  for the function and dynamics of the wall. Often
  have a lot of prolines and other hydrophilin
  amino acids for interaction with cellulose and
  other carbohydrates.
• A typical primary wall is combination of the
  above molecules fibrils embedded in a
  mixture/matrix of polysaccharides and proteins.
  Such interaction involve both covalent and
  non-covalent (H-bond).

5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10

• 4. Biosynthesis
• complexity involve a number of organelles
  (ER/Golgi, PM) more than 100 enzymes and a dozen
  of monosaccharides (precursor sugars). Both
  linear and branched connections.
• Model systems to study wall synthesis protoplast
  and cell division.
• Wall-less cellhow to make it by enzyme
  digestion. If protoplasts intend to survive, the
  first thing is to generate new wall.
• During cell division the preprophase band
  (arrow)--microtubule bundle that marks the plane
  for cell division. At telophase, the polar
  spindles and golgi vesicles form structure called
  phragmoplast (arrow)

11
Cell platethe phragmoplast recruit more vesicles
that fuse into a membrane-like structure made of
polysaccrides. The cell plate extend to connect
with the cell wall of the mother cellthe new
wall is born, so is the new cell.
Cell plate

• 3). Biosynthesis of cellulose
• Monomer glucose (photosynthesis or starch during
  germination)
• Activation step P-glucoseUTP UDPGPpi
• UDPG transport to the cell wall site (not sure
  how)
• UDPG polymerization addition of G to the chain
  by cellulose synthase
• very important enzyme (genes cloned from
  plantsa big family of genes)
• one of the genes was identified by screening for
  weak cell wall mutant.
• e. Deposition of cellulose and orientation of the
  fibrils cellulose is rigid because
• It is bundled together with the rest of the
  fibril as soon as made. Membrane-enzyme move
  during the synthesis. The orientation of the
  fibrils are controlled by microtubule and actin
  filament

12
(No Transcript)
13
5. Wall composition and cell type

• The primary cell wall of all cells are rather
  similar in composition, but the secondary wall is
  very different that marks the specific features
  of the cell type. For example, lignin is rich in
  supporting tissues that need a lot of strength
  (lignin is a polymer formed by cinnamic
  acidsaromatic ring structuretough). Cutin and
  wax (polyester of long chain fatty acid
  hydrocarbon/fatty alcohol) are found in
  epidermal layer (surface) for prevention of water
  loss because the materials are hydrophobicthink
  about a leaf without wax/cutin layer (cellulose
  is hydrophilic and loss water easily).
• 2) Cell wall dynamics and plant development Cell
  wall is not fixed after its synthesis, it is
  constantly changing during development. The
  formation of the xylem vessel and sieve
  tubesliving cells go through cell death and wall
  thickening ripening of fruits involves cell wall
  degradation falling leaves is due to formation
  of abscission zone in the petiole (degrading cell
  wall) maturation of cotton fiber

14
(No Transcript)
15
(No Transcript)
16
6. Cell wall function

• Cell support from giant redwood trees to small
  seedlings
• of Arabidopsis. What support them? Mechanism
  different!
• 2) Cell shape 90 waterno shape but shape of
  water container.
• Wall is also referred to as little wooden box
  but the box is elastic. What control the shape of
  the cells? The orientation of the fibrils.

Fat seedling
Thin seedling
3) Cell expansion cell wall loosening is a
critical step of cell growth. How to loosen up
the wall? Enzymes and expansinsenzymes that
cleave the covalent bonds expansins are family
of wall proteins that break H-bonds in the
fibrils. After wall loosening, enzymes re-build
larger wall by adding new materials. Discovery of
expansinsa case study.
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20

1. Measuring wall-loosening activity by assaying
   cellulose paper strength
2. Extract proteins from cell wall---bioassay
3. Fractionate proteins and ---bioassay
4. Purify proteinsbioassay
5. Sequence proteinsdeduce DNA sequence and clone
   gene (library screening or directly get the
   sequence from the genome database!)
6. Express recombinant protein and confirm activity
   by bioassay.

4) Cell fate some work indicates that wall
determines fate of the cell during development.
The fucus experiment
A
A
B
Laser surgery
A
B
A
B
B
5) Cell adhesion and cell-cell recognition
pectin cement, proteins may serve as adhesion
molecules (some evidence). In the pollen tube
extension inside the pistil tissues during
pollination and fertilization processes, some
evidence on cell wall proteins as recognition
signal. In the pollen germination and
compatibility between the pollen and stigma,
small proteins may serve as signals for
receptor-like protein kinasesa signaling pathway
(details later).
21
6). Cell wall and defense the great walla
physical barrier for invaders. Chemical
enforcement during infection/attack by wall
thickening and lignifying wall fragments as
signaling molecules to trigger a systematic
defense process involving gene expression, cell
death, or wall thickening etc 7). Cell-cell
communication and development many cells
communicate by talking to each other during
development/ Small molecules such as hormones and
peptides all need to go through the wall to
target cells. Some of the receptor-like protein
kinases are connecting cytoplasm, plasma
membrane, and cell wall and play a key role in
cell growth. This is WAKnext guest lecture.