Cell Walls And Their Role In Regulating Plant Cell Shape

Question 1

A plant cell =

Answer 1

Cell wall + protoplastr

Question 2

Cellulose in the cell wall key points

Answer 2

• the most abundant organic macromolecule on earth
• glucose polymer
• highly ordered
• long, ribbon-like structures

Question 3

Cellulose forms

Answer 3

Microfibrils

Question 4

Microfibrils - what are they / what do they do

Answer 4

Highly ordered structures that are strong and form a major component of primary of both primary and secondary cell walls

Question 5

Two phases of cell wall structure

Answer 5

Phase 1: Crystalline Microbibrillar phase
- cellulose

Phase 2: Noncrystalline Matrix
- Pectin polysacchaarides
- Hemicellulose polysaccharides

Plus a network of Extensin (a protein)

Question 6

Hemicellulose

Answer 6

A heterogenous group of polysaccarides.
- Long chain of one type of sugar and short side chains form rigid structure.

Question 7

Pectin

Answer 7

Branches, negatively charged polysaccharides
- bind water and have gel like properties
- not as strong as hemicellulose - more like jam
- negatively charged so can bind water

Question 8

The potein extension

Answer 8

• the extensibility (expansion) of cells can be controlled by extensin cross linking
• extension cross-linking of pectin and cellulose dehydrates the cell wall, reduces extensibility and increases strength

Question 9

The protein extensin while growing / not growing

Answer 9

• when cell still growing, extensions are random, cell takes up water and expands
• when stopped growing, the extension cross-linking dehydrates and strengthens, much more rigid. Cant get bigger but much stronger

Question 10

How the primary wall is synthesised

Answer 10

Co-ordinated synthesis and delivery of:

• cellulose microfibrils are synthesised at plasma membrane
• polysaccharides (pectin and hemicellulose) in the Golgi apparatus are transported to the wall in the vesicles
• cell wall proteins (extensions) from the rER

The vesicles fuse to the plasma membrane

Question 11

What is rosette?

Answer 11

A big protein complex (enzyme) that spans the plasma membrane and miles through it leaving begins a cellulose trail that is excluded to the outside of the membrane as a cellulose
- it makes cellulose by joining together glucose molecules, creating layers of microfibrils.

Question 12

Exocytosis - what is it and what type of exocytosis is needed for the making of the cell wall

Answer 12

• Transports material out of the cell or delivers it to the cell surface
• Constitutive exocytosis releases extracellular matrix proteins (brings pectin and stuff to the cell surface to make cell wall)

Question 13

Different parts of the cytoskeleton are responsible for making different parts of the wall

Question 14

The cellulose-producing rosettes moves ———- to the cortical Microtubules

Answer 14

Parallel

Question 15

Where are cortical tubules located and how do they interact with rosettes to determine shape

Answer 15

• they are right up against plasma membrane
• as the rosette moves through and makes cellulose, it runs around the Microtubules
• position of Microtubules therefore determine where fibril is put down thus determining shape

Question 16

Middle lemalla

Answer 16

• found at the other side of the primary wall to the plasma membrane
• it is a sticky substance that sticks cells together
• made of mainly pectin

Question 17

3 key functions of the cell wall:

Answer 17

• influences cell morphology: ( cell shape - different shapes of cell performing different functions thus must be different structures)
• provides structural support: (support comes from primary but mainly secondary)
• prevents excessive water uptake

Question 18

How the two different orientations of the cellulose microfibrils influence cell morphology

Answer 18

Randomly orientated:
- the cell will expand equally in all directions (sphare)

Right angles to the ultimate long axis of the cell
- the cell will expand longitudinally along that axis

Question 19

How the cell wall provides structual support:

Answer 19

• protoplast expands and pushes against cell wall, as water enters the cell by osmosis.
• cells become rigid and this maintains the plant structure
• wilting occurs when the protoplast is not pushing against cell wall
• water loss from cells reduces protoplast volume and the protoplast does not press on the cell wall.

Question 20

What limits the volume of water that can be taken up

Answer 20

Pressure from the cell wall

(Vacuoles are important in this process because they contain water and make up such a large portion of the protoplast)

Question 21

Vacuole definition

Answer 21

An organelle surrounded by a single membrane
- highly selective, controlling much of what enters and leaves the vacuole
- water moves in the vacuoles by osmosis (passive transport)

Question 22

How many vacuoles does a typical mature plant have

Answer 22

One

Question 23

Osmosis

Answer 23

The diffusion of water across a selectively permeable membrane

Question 24

Movement in osmosis occurs from:

Answer 24

• a high water (low solute) concentration
  To
• a low water (high solute) concentration

Question 25

Vacuole function in regulating cell shape

Answer 25

• high conc of solutes in the vacuole
• this results in water uptake into the vacuole by osmosis
• the plant cell wall limits water uptake and prevents the cell bursting
• plant cells build up large internal pressure that contributes to plant structural suppport

Question 26

The secondary cell wall

Answer 26

• produced only after growth has stopped
• thicker and stringer then primary walls
• provides more structural support then primary cell wall

(Not all plant cells have a secondary cell wall)

Question 27

Secondary cell wall: structure

Answer 27

• made up of Multiple layers (3)
• microfibrils in each layer have different orientations
• this strengthens the secondary wall

Question 28

Chemical characteristics of secondary cell wall

Answer 28

• more cellulose
• less pectin (hense why its stronger)
• Lignin

Question 29

Lignin definition and function

Answer 29

• a complex polymer and is the second most abundant organic macromolecule
• confers strength, rigidity to the secondary cell wall and acts to exclude water

(Provides structural support for specific cell types such as transporting water cells, and for the whole plant)

Question 30

Plasmodesmata definition

Answer 30

• intercellular connections, that enable cell to cell communication

Question 31

How the plasmodesmata work:

Answer 31

• the plasma membrane is continuous
• small enough to prevent organelle movements - although endoplasmic reticulum is connected through plasmadesmata
• ALLOWS THE FREE EXCHANGE OF FREE MOLECULES