Plant Cell Walls And Defense

Question 1

What are the key roles of plant cell walls?

Answer 1

1. Provide structural support.
2. Protect against pathogens.
3. Regulate cell expansion and water movement.
4. Serve as a barrier to external stressors.

Question 2

How do cell walls differ based on their functions?

Answer 2

1. Meristem: Thin and plastic, allowing growth.
2. Cork: Waterproof and germproof.
3. Xylem: Rigid and waterproof for water transport.
4. Root Cap: Slimy for lubricating soil passage.

Question 3

What is the composition of plant cell walls?

Answer 3

1. 90% polysaccharides (cellulose, hemicellulose, pectin).
2. 10% structural glycoproteins.
3. Some walls contain lignin, cutin, or suberin.

Question 4

What is the middle lamella, and what does it do?

Answer 4

The middle lamella is a pectin-rich layer that acts as an intercellular glue, enabling cell adhesion and expansion.

Question 5

What are the major monosaccharides in cell walls?

Answer 5

• 3-O-methylrhamnose.
• Mannose.
• Xyloglucan.
• Mixed-linkage β-glucans.

Question 6

How does turgor pressure drive cell expansion?

Answer 6

Turgor pressure (up to 5 atm) stretches the cell wall, enabling growth. This is considered irreversible expansion.

Question 7

What governs the direction of cell expansion?

Answer 7

Cellulose microfibrils in the cell wall dictate the direction. Microfibrils separate as the wall expands.

Question 8

Which hormones influence cell growth?

Answer 8

1. Gibberellin: Promotes growth.
2. Auxin: Facilitates elongation by loosening the wall.
3. ABA: Inhibits growth during stress.

Question 9

What are expansins, and how do they affect growth?

Answer 9

Expansins break hydrogen bonds between cellulose and hemicellulose, loosening the wall and allowing expansion.

Question 10

What role do hydroxyl radicals play in cell wall loosening?

Answer 10

Hydroxyl radicals (OH•) cause non-enzymatic scission of polysaccharides, loosening the wall for growth.

Question 11

How does lignin contribute to defence?

Answer 11

Lignin strengthens cell walls, making them resistant to enzymatic degradation by pathogens and waterproof.

Question 12

When is callose deposited in the cell wall?

Answer 12

Callose is deposited in response to:
1. Pathogen attack.
2. Wounding.
3. Plasmodesmata closure.

Question 13

What is XET, and how does it affect the wall?

Answer 13

XET cuts xyloglucan chains and attaches new molecules, allowing microfibril elongation.

Question 14

How do pathogens degrade cell walls?

Answer 14

Pathogens use cell wall-degrading enzymes (CWDEs) like cellulases, pectinases, and hemicellulases to invade cells.

Question 15

What happens during phenolic cross-linking?

Answer 15

Peroxidases mediate the cross-linking of phenolics, stiffening the wall and slowing growth during pathogen attack.

Question 16

Why is cellulose important in biofuel production?

Answer 16

Cellulose is the most abundant organic compound and can be broken down into glucose for biofuels, although lignin hinders digestion.

Question 17

Which enzymes break down cellulose, and how do they act?

Answer 17

1. Endoglucanases: Break cellulose in the middle.
2. Exoglucanases: Remove glucose units from ends.
3. β-glucosidases: Break cellobiose into glucose.

Question 18

What are perforation plates, and where are they found?

Answer 18

Perforation plates are areas where end walls of xylem cells are degraded, allowing sap flow.

Question 19

What is the role of the abscission zone in plants?

Answer 19

The abscission zone forms by cell wall loosening at the stem-petiole interface, allowing leaves to fall.

Question 20

How is lignin produced, and what role does it play in stress transfer?

Answer 20

1. Lignin precursors are polymerized by peroxidases into random structures.
2. Lignin surrounds microfibrils, enabling strong, stress-resistant walls.