Plant Cell Walls - 16
311 Syllabus

Wall Growth

Diffuse vs Tip Growth

Tip Growth occurs at the tip of an elongating structure (i.e. Root Hairs & Pollen Tubes).

Tip Growth occurs with Root Hairs and Pollen Tubes.

Diffuse Growth is More Common and cell growth is evenly distributed over the Cell's Surface.

Microfibril Orientation provides Polarity in Diffuse Growth

Turgor Pressure in Cell -> Same in all Directions

Meristematic Cells = Isodiametric

When Cellulose = Isotropic (random arrangement) -> Equal growth occurs in all directions -> Sphere

When Cellulose = Anisotropic (uneven arrangement) -> Directional Growth occurs -> Many possible Shapes.

Most Plant Cells -> Cylindrical Growth

Cellulose Microfibrils -> Mainly deposited on Long Walls of Cylindrical Cells

Transverse Orientation to Long Axis of Growth

Resemble Barrel Hoops

Resist Lateral Expansion -> Columnar Cell Shape

Note the Cylindrical Shape of this Cell. If we could see the Microtubules in the Cytoplasm of this Cell it would look like the Image to the right.	This image shows the Microtubules in the outer cytoplasm of a cell like that on the left. These direct the orientation of the Cellulose fibrils in the Cell Wall. This orientation resists lateral expansion but permits vertical enlargement.

Older Microfibrils (Outermost Wall Layers)

Shifted Parallel to the axis of Elongation

Thinned by elongation

More recent Microfibrils -> 

Transverse orientation

Greater influence on the Direction of Cell Growth

If you look carefully at the image on the right you can see that the innermost Cellulose Microfibrils have a more transverse
(Side <-> Side) orientation that those on the surface facing you. The latter have a more random appearance. This is due to 2 factors.

1] Their original Orientation
2] Changes in orientation that occur during cell growth

Based on the orientation of the outermost Cellulose Microfibrils, in which direction do you think the cell above has enlarged? (Vertical or Horizontal?)

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