The plant cell wall

Structure of plant cells. The cell wall. Plasmodesmata. Apoplast and symplast. Cytoskeleton in plant cells. Endomembranes in plant cells, plastids. Cell types and organization levelsThe plant cell wall. Plasmodesmata. Apoplast and symplast.

Index

References Taiz & Zeiger. Plant Physiology. 4th Edition Buchanan, Gruissem, Jones. Biochemistry and molecular biology of plants

How doplants manage to stand without an skeleton? Common structural features solar collectors non motile: adaptations structural strength against gravity avoid desiccation vascular system

THE CELL WALL

PRIMARY WALLS: extensible walls formed by growing cells Unspecialized and similar in all cells Variations possible (thickness) SECONDARY WALLS: formed after enlargement has finished Highly specializedin structure and composition Water-proof (ligning and suberin) MIDDLE LAMELLA: Glue cells together PITS PLASMODESMATA

Plants: Plasmodesmata
• • Plasmodesmata are channels that perforate plant cell walls Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell
Cell walls

Interior of cell

Interior of cell

0.5 µm

PlasmodesmataPlasma membranes

CELL WALL
CHARACTERISTICS Permeable to water Rigid (thinest 100 nm from parenchyma) Resistant Staining preferences FUNCTIONS Structural: allow great heights. Middle lamella “glues” cells Exoskeleton Controls cell growth; need to control expansion of cells Fruit maduration Abscission Storage structure-(constituting monomers) Interaction with microorganisms: structural barrierStructural cell wall components
Polysaccharides, polymers of sugar, are the principal components of the cell wall and form its main structural framework

1. Fibrous 2. Matrix

Cellulose Hemicelluloses Pectins

Long chains of sugar molecules covalently linked at various positions, some being decorated with side chains of various lengths

Proteins: structural or enzimatic Coatingsubstances: incrustation: lignin & suberin aposition: waxes and cutin Mineral salts

Polymers of specific sugars are further defined by their linkage type and the configuration of the anomeric carbon. The anomeric carbon can show an alfa (down) or beta (up) substitution in solution. Sugars in polymers are always in the ring form. During polymerization, the anomeric carbon of one sugar molecule isjoined to the hydroxyl group of another sugar or othe compound (phenylpropanoid) by glicosidic linkage. Because the aldehyde of this sugar is able to reduce copper under alkaline conditions, it classically is described as a reducing sugar, and hence, it is called the reducing end.. Branched polysaccharides will have a nonreducing sugar at the end of each side chain and at the terminus of the backbonebut only a single reducing end. Cellulose has a reducing end and nonreducing end. For the 1-4 β linkage to occur, sugars need to be inverted 180º, and therefore, this rends a linear polymer. In contrast, other linkages yield in a helical polymer.

Further reading: Buchanan. Biochemistry and molecular biology of plants. PPp 52-58;61;64-65. figures 2.24 and 2.25, 2.26

LE 5-8

Cellulosemicrofibrils in a plant cell wall Cell walls Microfibril

0.5 µm Plant cells

Cellulose molecules

β Glucose monomer

ORGANIZATION OF CELLULOSE IN THE CELL WALL

Level 1. CELLULOSE
Cellulose chains: 2000-6000 glu residues primary wall, over 10000 in secondary walls

Level 4: MACROFIBRIL PACKS 40 µ

Level 1. Cellulose fibre

Level 2. Microfibril 36 units of cellulose INERT INSOLUBLEHIGHLY STABLE

Level 3: MACROFIBRILS (secondary walls) 15-20 µ

Crystalline cellulose

ORGANIZATION CELLULOSE (1,4 ß-D-Glu linear polymer) (36 molecules of cellulose)

MICROFIBRIL

MACROFIBRIL (15-20 m) aprox. 200 cellulose chains PACKS of MACROFIBRILS ( over 250 microfibrils 40 m)

* Biosynthesis

CELLULOSE SYNTHASE
Catalyzes the beta 1,4 glycosidic bond formation of the...