lecture 7 - plant cell wall

Question 1

What does the plant cell wall enclose?

Answer 1

The protoplast.

Question 2

What is the major component of the plant cell wall?

Answer 2

Cellulose

Question 3

What is the structure of cellulose?

Answer 3

Chains of glucose (a polymer), forming ribbons

Question 4

What are cellulose micro fibrils?

Answer 4

Layers of cellulose ribbons bonded together into strong chains

Question 5

What are the 2 phases of the cell wall?

Answer 5

Crystalline microfibrillar phase and Non-crystalline matrix

Question 6

What is the chemical component of the Cell Wall Phase 1?

Answer 6

Cellulose molecules (which form cellulose/fibre microfibrils)

Question 7

What are the three components of Phase 2 of the plant cell wall?

Answer 7

Pectin and hemicellulose polysaccharides, extensin (a protein)

Question 8

What is the structure of hemicellulose?

Answer 8

Long chains of 1 type of sugar, with oligosaccharide side chains.

Question 9

What are the properties of hemicellulose?

Answer 9

Rigid structure, but not as strong as cellulose.

Question 10

What is the structure and properties of pectin?

Answer 10

Branched negatively charged polysaccharides, which bind water, giving it gel-like properties.

Question 11

What element of plant cell structure does extensin determine?

Answer 11

Extensibility (expansion)

Question 12

How does extensin limit extensibility?

Answer 12

When extension fibres partake in cross-linking, the cell wall become dehydrated, and the cell cannot expand as the wall becomes more rigid.

Question 13

Where are cellulose microfibrils synthesised?

Answer 13

At rosettes on the cell membrane

Question 14

Where are cell wall polysaccharides synthesised?

Answer 14

Pectin and hemicellulose are made in the golgi, and transported via vesicles.

Question 15

Where are cell wall proteins (extensin) synthesised?

Answer 15

In the Rough Endoplasmic Reticulum, then transported by vesicles to the cell wall.

Question 16

What is the process of cellulose microfibril production?

Answer 16

Rosettes at the plasma membrane move parallel to the underlying cortical microtubules, forming parallel microfibrils on the other side of the plasma membrane.

Question 17

How are the cell wall components structured in the wall?

Answer 17

Microfibrils of cellulose form layers, each with microfibrils running in different directions. Hemicellulose molecules sit around the larger microfibrils. Pectin sits within the wall as well as forming a gel-like layer between the cell walls of neighbouring cells.

Question 18

What are the functions of the plant cell wall? (3)

Answer 18

Influence cell morphology, structural support, prevent excessive water uptake

Question 19

How does the cell wall influence cell morphology?

Answer 19

The direction of microfibrils in the cell wall determines the direction of expansion when the protoplast pushes against it. Randomly/evenly oriented will allow the cell to expand in all directions into a spherical shape. If the microfibrils are at right angles to the long axis of the cell, it will expand longitudonally.

Question 20

How does the cell wall provide structural support to a plant cell?

Answer 20

The protoplast (filled with the water filled central vacuole) pushes against the cell wall until it becomes rigid.

Question 21

What is wiliting, and how does it affect the cell wall?

Answer 21

When water flows out of the protoplast/vacuole, there is less pressure on the cell wall, so it becomes less rigid

Question 22

How does the cell wall prevent excessive water uptake by the cell?

Answer 22

When the vacuole absorbs large amounts of water via osmosis, the cell wall exerts pressure on the protoplast, preventing it from expanding more.

Question 23

What is the central vacuole in plant cells?

Answer 23

A membrane bound organelle, which takes up lots of water and molecules, taking up a large amount of the protoplast.

Question 24

What is osmosis, in terms of the vacuole?

Answer 24

The process in which water flows from a low solute concentration to a high solute concentration, allow it to flow in and out of the vacuole, expanding or diminishing it.

Question 25

What are the 3 states of the cell wall, which are dependent on the water levels within the cell?

Answer 25

Turgid, flaccid, plasmolyzed

Question 26

What does turgid mean in terms of a plant cell?

Answer 26

The vacuole is filled with water, so the protoplast pushes out firmly onto the cell way, making it stiff

Question 27

What does flaccid mean in terms of the plant cell?

Answer 27

There is less water in the vacuole, so the protoplast exerts less force on the cell wall, and it does become as rigid and is flaccid

Question 28

What is a plasmolyzed plant cell?

Answer 28

A cell which has lost a lot of water, so the cell wall loses a lot of strength.

Question 29

What is the purpose of secondary cell walls?

Answer 29

To provide thicker, stronger structural support to specfic types of cells.

Question 30

Do all plant cells have a secondary cell wall?

Answer 30

No

Question 31

What is the structure of the secondary cell wall?

Answer 31

Three layers of cellulose microfibrils (more than in the primary cell wall), with hemicellulose, but less pectin. Walls contain lignin

Question 32

What is lignin?

Answer 32

A complex hydrocarbon polymer that excludes water, making the secondary wall strong, rigid and difficult to break down.

Question 33

What are plasmodesmata?

Answer 33

Intercellular connections - gaps in the cell wall that allow molecules to pass to neighbouring cells so they can communicate.

Question 34

Are the plasmodesmata small enough to prevent organelles from getting through?

Answer 34

Yes, except some pieces of ER slip through in parts.

Question 35

How is the plasma membrane continuous at the plasmodesmata?

Answer 35

It lines the cell wall on either side and coats the sides of the plasmodesmata gaps.

Question 36

What cell wall components does extensin cross link?

Answer 36

cellulose and pectin