Cells walls and role of regulating plant cell shape

Question 1

what is cellulose

Answer 1

Cellulose is a highly ordered glucose polymer structures that are strong and form the major component of both primary and secondary walls.

Question 2

what are the 2 phases of cell walls

Answer 2

1. microfibrils (crystalline phase)
   - cellulose
2. matrix (non crystalline phase)
   - pectin polysaccharides
   - hemicellulose polysaccharides
   and there is a network of Extensin, a protein (if more structure is needed - more extensin is produced)

Question 3

what is hemicellulose

Answer 3

a heterogeneous group of polysaccharides, there is a long chain of one type of sugar and short side chains form a rigid structure

Question 4

what is pectin

Answer 4

a branched, negatively charged polysaccharide that can bind water and have gel-like properties

Question 5

how is the expansion of cells controlled

Answer 5

through extensin cross-linking of pectin and cellulose which dehydrates the cell wall, reduces extensibility and increase strength

Question 6

where are cellulose microfibrils made

Answer 6

synthesised at the plasma membrane

Question 7

where are pectin and hemicellulose made

Answer 7

synthesised in the golgi complex and are transported to the plasma membrane in vesicles

Question 8

where is extensin (cell wall proteins) made

Answer 8

synthesised in the rER and transported via golgi to plasma membrane in vesicles, which fuse with the plasma membrane

Question 9

what are cellulose-producing rosettes

Answer 9

are protein complexes (enzymes) that span the plasma membrane and they more parallel to the cortical microtubules

Question 10

what is the middle lamella in plant cells

Answer 10

a thin, pectin-rich layer that acts as a “glue” or cementing substance between the primary cell walls of adjacent plant cells

Question 11

3 functions of cell walls in regulating cell shape

Answer 11

1. influences cell morphology
   - when cellulose microfibrils are randomly oriented the cell will expand in all directions equally
   - if they are right angles to the ultimate long axis of the cell, the cell will expand longitudinally along the axis
2. provides structural support
   - the protoplast pushes against the cell wall, the cell becomes rigid and maintains the plant structure
   - water loss reduces protoplast volume
3. prevents excessive water uptake
   - water enters the cell via osmosis, the protoplast expands and pushes against cell wall
   - the pressure from cell wall limits water intake

Question 12

3 functions of vacuole in regulating cell shape

Answer 12

1. high concentration of solutes in vacuole results in water uptake in vacuole via osmosis
2. plant cell wall limits the water uptake and prevents cell from bursting
3. the build up of pressure (turgor pressure) contributes to plant structural support

Question 13

the structure of secondary cell wall

Answer 13

• made up of multiple layers, the microfibrils in each layer have different orientations which strengthens the secondary cell wall.
• it contains more cellulose, less pectin and lignin

Question 14

what is lignin

Answer 14

is a complex polymer that confers strength and rigidity to secondary wall and acts to exclude water

Question 15

function of secondary cell wall

Answer 15

1. they only form after cell growth has stopped s they are thicker and stronger than primary cell wall, therefore provide more structural support
2. provides structural support for specific cell types such as water transporting cells

Question 16

what are plasmodesmata

Answer 16

cytoplasmic connections with a desmotubule (from ER) in the center, that enable cell communication by allowing the free exchange of small molecules