Lecture 7 - Cell Walls And Their Role In Regulating Plant Cell Shape

Question 1

What is a protoplast?

Answer 1

The plasma membrane and everything inside the membrane.

Question 2

What are plant cell components?

Answer 2

The cell wall and the protoplast.

Question 3

What is cellulose?

Answer 3

A highly ordered glucose polymer that forms long ribbon-like structures. It is the most abundant organic macromolecule on earth.

Question 4

What is the major component of the cell wall?

Answer 4

Cellulose. It’s properties are good for structure of the cell wall

Question 5

What does cellulose form?

Answer 5

Cellulose forms microfibrils. The ribbons of cellulose line up next to each other and cross link, forming the microfibres.

Question 6

What are the two phases of cell wall structure?

Answer 6

Phase 1: crystalline microfibrillar phase - cellulose

Phase 2: Noncrystalline matrix - pectin polysaccharides and hemicellulose polysaccharides

+ network of extensin (a protein)

Question 7

What is hemicellulose?

Answer 7

A heterogeneous group of polysaccharides. Long chain of one type of sugar and short side chains form a rigid structure

(Not as strong as cellulose)

Question 8

What is pectin?

Answer 8

Branched negatively charged polysaccharides.

Question 9

What are the properties of pectin?

Answer 9

Pectin binds water and is gel like. It sticks on top of other polysaccharides.

Question 10

What does extensin cross linking with pectin and cellulose do?

Answer 10

Dehydrates the cell wall, reduces extensibility and increases strength

Question 11

What is extensibility of cell controlled by?

Answer 11

Extensin cross linking

Question 12

What is the steps of synthesis of the primary cell wall?

Answer 12

1) cellulose microfibrils are laid down at the plasma membrane
2) The polysaccharides (pectin and hemicellulose) are transported from the golgi complex to the plasma wall in vesicles.
3) cell wall proteins (extensins) are released from the rough ER and are released out of the cell

Question 13

What is a rosette?

Answer 13

An enzyme complex (cellulose synthase) that pushes through the entire cell from inside to outside.

Question 14

Why are microtubules the most important in context to making the primary cell wall?

Answer 14

Because the microtubules close to the plasma membrane will help lay down the cell wall. It is important for knowing where to put the cellulose, thus controls shape of cell.

Question 15

Why is it important to have cytoplasmic streaming?

Answer 15

So the chloroplast move around the cell to maximise photosynthesis/get an even amount of light

Question 16

Describe the relationship between rosettes and microtubules

Answer 16

The rosette associates with the microtubule. Because the plasma membrane is dynamic, the rosette can move along the microtubule and produces cellulose microfibril.

Question 17

What is the middle lamella made out of and what does it do?

Answer 17

The middle lamella is made of pectin so it is gel like and it sticks stuff together

Question 18

What is in the primary cell wall?

Answer 18

Pectin
cellulose
hemicellulose

Question 19

What does the cell wall do?

Answer 19

Influence cell morphology
Provide structural support
Prevent excessive water uptake

Question 20

What influences cell morphology

Answer 20

The orientation of the cellulose microfibrils influences cell morphology

Question 21

Describe the two different orientations of cellulose microfibrils

Answer 21

a) randomly orientated - when randomly orientated the cell will expand equally in all directions meaning that the cellulose will have equal pressure
b) right angles to the ultimate long axis of the cell - The cell will expand longitudinally along that axis. (When expanding it is hard to push it in other ways)

Question 22

How does the cell wall provide structural support?

Answer 22

When the protoplast pushes against the cell wall. This makes the cell become rigid and makes it maintain its structure.

Question 23

What happens when there is water loss from the cell?

Answer 23

When water is lost from the cell, the protoplast will lose volume causing it to be unable to push on cell wall.

Question 24

How does the cell wall prevent excessive water uptake?

Answer 24

When water enters the cell by osmosis the protoplast expands and pushes against the cell wall. The pressure created from the cell wall limits the volume of water that can be taken up.

Question 25

Why are vacuoles important in preventing excessive water uptake?

Answer 25

Because the vacuole contains water and makes up a large portion of the protoplast.

Question 26

What is the structure of a vacuole?

Answer 26

The vacuole is an organelle surrounded by a single membrane.

Question 27

Describe the single membrane of the vacuole

Answer 27

The membrane is highly selective and controls much of what enters and leaves the vacuole.

Question 28

Describe the vacuoles function in regulation of cell shape

Answer 28

Due to the high concentration of solutes in the vacuole, there is water uptake by osmosis. The plant cell wall limits the amount of water uptake to prevent the cell from bursting which builds up a large internal pressure that contributes to plant structural support.

Question 29

What is the secondary cell wall?

Answer 29

The secondary cell wall is produced after cell growth has stopped. It provides more structural support than the primary cell wall because it is thicker and stronger.
Not all plant cells have a secondary cell wall.

Question 30

Describe the physical structure of the secondary cell wall.

Answer 30

The secondary cell wall is made up of multiple layers of cellulose with the microfibrils in each layer having different orientations which strengthens the secondary cell wall.

Question 31

Describe the chemical characteristics of the secondary cell wall structure

Answer 31

The secondary cell wall has more cellulose, less pectin and has lignin.

Question 32

What is lignin and what is its function?

Answer 32

Lignin is a complex polymer that confers strength and rigidity to the secondary cell wall and acts to exclude water.

Question 33

What does the secondary cell wall provide structural support for?

Answer 33

It provides support for specific cell types such as water transporting cells and for the whole plant.

Question 34

How do you cells with a cell wall communicate?

Answer 34

Through the plasmodesmata.

Question 35

What is the plasmodesmata?

Answer 35

The plasmodesmata are intercellular connections that enable cell to cell communication.

Question 36

How does the plasmodesmata work?

Answer 36

The plasma membrane is continuous so through the use of the plasmodesmata, free exchange of small molecules is allowed but because the plasmodesmata is small it prevents organelles to move besides parts of the ER that can poke through.