The cell wall

Question 1

What is the primary marcomolecule in the cell wall?

Answer 1

Cellulose. It is a glucose polymer and is highly ordered with a long ribbon-like structure.

Question 2

What does cellulose form?

Answer 2

Microfibrils which are highly organised structures and are strong. They are a major component of both the primary and the secondary cell wall. The fibres run in different ways to each other to resist force.

Question 3

What is phase 1 of the primary cell wall?

Answer 3

The crystalline microfibillar phase (cellulose)

Question 4

What is phase 2 of the primary cell wall?

Answer 4

The noncrystalline matrix (not as ordered)

• Pectic polysaccharides
• Hemicellulose polysaccharides
• Extensin

Question 5

What is hemicellulose?

Answer 5

Hemicellulose is a heterogenous group of polysaccharides. The long chains of one type of sugar and short side chains form a rigid structure.

Question 6

What is pectin?

Answer 6

Pectin is a negatively branched group of polysaccharides. They bind water and have gel like properties. Not structural support, more like a matrix. Acts as a glue.

Question 7

What is extensin?

Answer 7

Extensin controls rigidity of the primary cell wall. Extensin crosslinking with pectin and cellulose dehydrates the wall and reduces extensibility and increases the strength of the wall.

Question 8

What is the extensibility (expansion) of the cell controlled by?

Answer 8

Extensin cross linking. When crosslinked there is a very ordered structure. Crosslinking excludes some of the water from the pectin and changes the property of the cell wall. Cell will become more rigid and strong because of this.

Question 9

What happens in a young cell, to do with expansion of the protoplast?

Answer 9

A young cell is always growing. When growing the protoplast will expand and push against the cell wall. The cell wall will expand as it pushes against it. Cell won’t expand in extensin crosslinking, but in a young cell it is still disordered so it can expand.

Question 10

Where is cellulose made?

Answer 10

At the plasma membrane. Made from rosettes. Rosettes take cellulose momomers and make them into polymers, excretes them into the cell. Long chains of glucose form the microfibrils.

Question 11

Where are the polysaccharides in the cell wall transported from?

Answer 11

From the golgi in vesticles to the cell wall

Question 12

Where are cell wall proteins made (extensin)

Answer 12

At the rough eR

Question 13

How is the primary cell wall layed down?

Answer 13

The microfibrils are layed down first. The vesicle fuse to the plasma membrane and this releases the material out of the cell.

Question 14

How is the cytoskeleton important in the formation of the cell wall?

Answer 14

Microtubules line the plasma membrane and controls how the microfibils are laid down. The cellulose-producing rosettes move parallel to the cortical microtubules which is important for cell shape.

Question 15

What are the cell walls functions in regulating cell shape?

Answer 15

• Influences cell morphology
• Provides structural support (individual cells and plants as a whole)
• Prevents excessive water uptake

Question 16

What does the orientation of cellulose microfibrils influence and why?

Answer 16

Influences cell morphology.
A) randomly orientated and the cell will expand in all directions

B) Right angles to the ultimate long axis of the cell and the cell will expand longitudinally.

Plant cells come in many shapes and this will be influenced by the laydown of microfibrils.

Question 17

How does the protoplast have an effect on structural support for the cell?

Answer 17

The protoplast pushes against the cell wall. The cells become rigid and this maintains the plant structure. Wilting will occur when the protoplast is not pushing. Water loss from cells reduce the protoplast volume and the protoplast does not press on the cell wall. As water enters the cell the protoplast expands and pushes against the cell wall. Pressure from the can wall limits the volumes of water that can be taken up.

Question 18

Why are vacuoles important in cell structure?

Answer 18

They contain and make up such a large portion of the protoplast.

Question 19

What is a vacuole?

Answer 19

An organelle surrounded by a single membrane. Highly selective and controls much of what enters and leaves the vacuole. This water moves by osmosis.

Question 20

What are vacuoles functions in regulation?

Answer 20

There is a high conc of solutes in the vacuole which results in high water uptake. The plant cell wall limits water uptake and prevents the cell from bursting which means they build up a large internal pressure that contributes to plant structural support. e.g. If wilting can’t get as much light for photosynthesis.

Question 21

What is the secondary cell wall?

Answer 21

Not all plants have it, and it is thicker and stronger then the primary cell wall, provides more structural support. It is produced only after cell growth has stopped.

Question 22

What are the chemical characteristics of the secondary cell wall?

Answer 22

More cellulose, less pectin and lignin

Question 23

What is lignin?

Answer 23

The second most abundant organic marcomolecule and the reason we cannot breakdown cellulose. They are complex polymers. Confers strength, rigidity to the secondary cell wall and acts to exclude water.

Question 24

How do cells with a cell wall communicate?

Answer 24

Through plasmodesmata which are intercellular connections that enable cell to cell communication.

• The plasma membrane is continuous
• Small enough to prevent organelle movements
• Allows the free exchange of small molecules.