The Plant Cell Surface

Question 1

the strength of plant cells is provided by?

Answer 1

cell walls

Question 2

what do cell walls allow plants to do? (2)

Answer 2

1. withstand the high turgor pressure exerted by the uptake of water
2. the cell wall is also a permeability barrier to large molecules, though not an obstacle for gases, ions, or other small water soluble molecules

Question 3

what do plant cells walls consists of?

Answer 3

mainly long cellulose microfibrils enmeshed in a network of branched polysaccharides and glycoproteins called extensins

Question 4

Cellulose

• how abundant?
• structure?
• composition?
• what do they generate?

Answer 4

• predominant polysaccharide of the cell wall, most abundant organic macromolecule on earth
• long, ribbon-like, stabilized by intramolecular hydrogen bonds
• 50-60 cellulose associated laterally to form microfibrils found in cell walls
• cellulose microfibrils twisted together (rope-like fashion) generate even larger structures called macrofibrils

Question 5

the two main types of polysaccharides in a plant cell are?

Answer 5

hemicellulose and pectins

Question 6

Hemicelluloses
what are they?
what do they consist of?

Answer 6

• heterogenous group of polysaccharides
• each consisting of a long linear chain of a single kind of sugar (glucose or xylose) with short side chains (which contain many kinds of sugars) bonded into a rigid network

Question 7

Pectins 
what are they?
side chains contain?
what do they form?
what else do they do?

Answer 7

they are branched polysaccharides with backbones called rhamnogalacturonans

• the side chains have the same monosaccharides found in hemicellulose
• pectin molecules form the matrix in which cellulose microfibrils are embedded
• also bind adjacent cell walls together

Question 8

Extensins

• what are they?
• structure?
• what happens in the cell wall?
• least abundant?
• most abundant?

Answer 8

• group of related glycoproteins, which are deposited in soluble form
• rigid, rod-like molecules that are tightly woven into the complex polysaccharide network of the cell wall
• in the cell wall, extensins become covalently crosslinked to one another and cellulose
• least abundant in cell walls of actively growing tissue
• most abundant in cell walls of tissues that provide mechanical support to the plant

Question 9

Lignins
what are they?
where are they?

Answer 9

• insoluble polymers of aromatic alcohols found mainly in woody tissue
• localized between cellulose fibrils

Question 10

The Cell Walls Are Synthesized in Several Discrete Stages:

How is the cell wall secreted? creating what?

Answer 10

the cell wall is secreted in steps, creating a series of layers

Question 11

The Cell Walls Are Synthesized in Several Discrete Stages:

what is the first structure of the cell wall?

Answer 11

-middle lamella

Question 12

The Cell Walls Are Synthesized in Several Discrete Stages:
what is the second structure of the cell wall?
what is its composition?
when is it formed?

Answer 12

• Primary cell wall
• loosely organized network of cellulose, hemicellulose, pectins, and glycoproteins
• formed while cells are still growing, relatively thin, flexible structure

Question 13

The Cell Walls Are Synthesized in Several Discrete Stages:

while shoots and roots are still growing, what helps cell walls retain pliability?

Answer 13

expansins

Question 14

The Cell Walls Are Synthesized in Several Discrete Stages:
what is the last structure of the cell wall?
when is it formed?
what is the composition?(2)
what does it contribute to the cell wall?

Answer 14

• secondary cell wall
• formed when calls have stopped growing
• cellulose and lignins are the primary constituents of the secondary cell wall
• densely packed bundles of cellulose microfibrils in parallel and oriented at an angle to adjacent layers
• great strength and rigidity of the wall

Question 15

Plasmodesmata

• what are they?
• what do they do?
• what are they lined with?
• where is the annulus located
• what does the annulus do?
• when are plasmodesmata formed?
• plasmodesmata functions like?

Answer 15

• cytoplasmic channels through openings in the cell wall, allowing cytoplasmic continuity between two adjacent cells
• permit direct cell-cell communication through the cell wall
• lined with plasma membrane common to the two cells with the desmotubule in the central channel
• the annulus lies between the desmotubule and the membrane lining the plasmodesmata
• annulus is thought to provide the cytoplasmic continuity between adjacent cells, allowing molecules to pass freely from one cell to the next
• plasmodesmata are formed at the time of cell division while the new wall is being formed
• plasmodesmata functions like gap junctions