Cell Walls

Question 1

What is the major component of the cell wall

Answer 1

Cellulose

Question 2

What is Cellulose

Answer 2

The most abundant organic macromolecule on Earth
Glucose polymer
Highly ordered
Long, ribbon like structures

Question 3

What does cellulose form

Answer 3

Microfibrils

Question 4

Outline the two phases of the cell wall structure

Answer 4

Phase 1: Crystalline microfibrillar phase

Phase 2: Noncreystalline matrix

Question 5

What is the crystalline microfibrillar phase

Answer 5

A highly ordered cellulose microfibrillar structure

Question 6

What is the non crystalline matrix composed of

Answer 6

Pectin polysaccharides
Hemicellulose polysaccharides

Plus a network of extensin

Question 7

What is Hemicellulose

Answer 7

A heterogeneous group of polysaccharides. Long chain of one sugar type and branched chains of another. Form a rigid structure

Question 8

What is pectin

Answer 8

Branched, negatively charged polysaccharides. Bind water and have gel like properties

Question 9

What is extensin

Answer 9

A protein that controls the extensibility of (expansion) of cells

Question 10

How does extensin control extensibility

Answer 10

Extensin cross-linking with pectin and cellulose (in the cell wall) dehydrates the cell wall, reduces extensibility and increases strength

Question 11

How does cellulose get to the cell wall

Answer 11

It is synthesised at the plasma membrane by cellulose synthase rosette that travels along cortical microtubules

Question 12

How do pectin and hemicellulose (polysaccharides) reach the cell wall

Answer 12

They are transported from the Golgi apparatus to the cell wall in vesicles

Question 13

How do cell wall proteins (extensins) get to the cell wall

Answer 13

Made in the rough endoplasmic reticulum and then transported to the cell wall

Question 14

How is cellulose synthesised to make the cell wall

Answer 14

Cellulose synthase rosette move parallel following cortical microtubules (under plasma membrane) and synthesise cellulose on the exterior of the cell thus synthesising the cell wall

Question 15

What is the middle lamella

Answer 15

A layer that cements together the primary cell walls of two adjacent plant cells. It is pectin rich

Question 16

What are the 3 functions of the cell wall in regulating cell shape

Answer 16

Influence cell morphology
Provides structural support
Prevents excess water uptake

Question 17

How does orientation o cellulose microfibrils influence cell morphology

Answer 17

Randomly orientated cellulose microfibrils allow cell to expand equally in all directions (equal cell wall pressure)
Ordered cellulose microfibrils will allow cell to expand perpendicular to the cellulose microfibrils (cell wall pressure prevents expansion along microfibril plane)

Question 18

How does the cell wall provide structural support

Answer 18

Protoplasm pushes against cell wall. Cell becomes rigid and thus maintains plant structure. Many rigid cells result in rigid plant

Question 19

What is wilting

Answer 19

Wilting occurs when the protoplast is not pushing against the cell wall (low turgor pressure) thus plant looses structure

Question 20

How does cell wall prevent excessive water uptake

Answer 20

As water enters cell, protoplast expands and pushes against cell wall.
Pressure from cell wall limits volume of water that can be taken up.

Question 21

Where is water stored in a plant cell

Answer 21

Central vacuole

Question 22

Outline the 3 characteristics of the central vacuole

Answer 22

Organelle surrounded by a single membrane
Is highly selective, controlling much of what enters and leaves
Water crosses membrane via osmosis

Question 23

How does the central vacuole contribute to cell shape/overall plant structure

Answer 23

High concentration of solutes in vacuole
Results in water uptake via osmosis
Plant cell wall limits cell uptake of water (and prevents bursting)
Plant cells build up large internal pressure that contributes to cell shape, and overall plant structure

Question 24

What are the characteristics of the secondary cell wall

Answer 24

Not all plants have a secondary cell wall
Produced only after cell growth has stopped
Thicker and stronger than primary cell wall
Provides more structural support than primary cell wall

Question 25

Outline the structure of the secondary cell wall

Answer 25

Made up of multiple layers
Microfibrils in each layer have different orientations
This strengthens the secondary wall

Question 26

What are the chemical characteristics of the secondary cell wall

Answer 26

More cellulose (stronger)
Less pectin (less gel like substance)
Lignin

Question 27

What is lignin

Answer 27

The second most abundant organic macromolecule
A complex polymer
Confers strength, rigidity to the secondary cell wall and acts to exclude water

Question 28

What does the secondary cell wall provide structural support for

Answer 28

Specific cell types (such as water transporting cells, and for the whole plant)

Question 29

How do cells with a cell wall communicate

Answer 29

Plasmodesmata

Question 30

What are plasmodesmata

Answer 30

Intercellular connections that enable cell to cell communication

Question 31

Outline the characteristics of plasmodesmata

Answer 31

The plasma membrane is continuous from cell to cell
Small enough to prevent organelle movements, although endoplasmic reticulum is connected through plasmodesmata
Allows the free exchange of small molecules