Chapter 2- The cell wall

What are the three main parts of a plant cell wall?

Middle lamella, primary wall and secondary wall.

What is the middle lamella's main function?

It acts as a glue binding adjacent plant cells together.

Which polysaccharides are characteristic of the middle lamella?

Pectic polysaccharides.

Where is the secondary wall deposited?

Inside the primary wall, after the cell begins to differentiate.

What are the two main functions or components of the secondary wall?

Main function: support. Main components: cellulose and lignin.

What are the three layers of the secondary wall called?

S1, S2 and S3.

How do the S1, S2 and S3 layers differ?

The orientation and direction of their cellulose microfibrils differ.

How many glucose residues can a cellulose chain contain?

As many as 25,000 glucose residues.

What is cellobiose, and how does it relate to cellulose?

A glucose-glucose disaccharide; cellulose consists of glucose residues arranged mainly as cellobiose units.

How do cellulose chains form microfibrils, and what do microfibrils provide?

Hydrogen bonds form within and between cellulose chains. Microfibrils provide tensile strength to the wall.

Two types of hemicellulose are ____ and ____

xyloglucans gluconarabixyclans

Whats the main feature of hemicellulose

They don't link to form microfibrils; they however form hydrogen bonds with cellulose

How are pectic polysaccharides extracted from the cell wall

Using hot water, dilute acid or calcium chelators

Whats the main type of sugar that makes up pectic polysaccharides

Galacturonic acid; Pectic polysaccharides are polymers of galacturonic acid.

How do pectic polysaccharide polymers form a gel?

Adjacent polymers are crosslinked by divalent cations like calcium to form a gel.

Where are pectic polysaccharides initially assembled

In the golgi apparatus where they are initially methylated.

How do pectic polysaccharide polymers form a gel

They are joined together by calcium cations after the removal of their methy groups by the enzyme pectin methylesterase.

Name another type of pectic polysaccharide polymer

Rhamnogalacturonan II

Rhamnogalcturonan II is a polymer of ___ and ___. These polymers are crosslinked by ___

rhamnose galacturonic acid boron atoms

Name the main functions of pectic polysaccharides

Determine wall porosity

Provide charged wall surface for cell to cell adhesion

Cell to cell recognition

Pathogen recognition

What are the amino acids usually found in wall glycoproteins

hydroxyproline proline and glysine

Extensin is a ____ which are also called ____

Wall glycoprotein Hydroxyproline rich glycoprotein

What is the purpose of wall proteins in the cell wall

They act as scaffolding to construct other wall components.

They provide structural support

The two main groups of wall proteins are ___ and ___ proteins

Hydroxyproline Rich glycoproteins(HRGP) and arabinogalactan

What's the function of lignin in the cell wall

It acts as a structural agent and helps resist pathogen and fungal attacks.

Whats the function of suberin, waxes and cutin in the plant cell wall

They provide wall strength and water proofing

Water is responsible for the ___ and ___ of the hydrated cell wall.

flexibility extensibility

How does the cell wall mantain the cell shape

Its analogous to the exoskeleton for every cell.

Protoplasts are round

Name two functions of the cell wall

mantaining cell shape

carbohydrate storage

How does the cell wall mantain the cell shape?

Acts as an exoskeleton with protoplasts giving the cells their round shape

Prevents the cell from bursting in hypotonic medium (resists water pressure)

Controls the rate and direction of cell growth / controls plant morphogenesis ( plant shape development)

How does the cell wall contribute to cell metabolism

Some proteins in the wall are enzymes for transport and secretion

How does the cell wall protect the cell from pathogens?

Suberized cells form a physical barrier that prevents the entry of pathogens.

Does the cell wall serve any storage function?

Yes, cell walls act as carbohydrate repositories esp in seeds