Physicsl Defenses

Question 1

Physical defences:

Answer 1

• Cellulose cell wall – (passive) physical barrier (also contains chemical defences).
• Active: thickened and strengthened with additional cellulose
• Lignin – further thickening and waterproofing of xylem of cell walls (after infection)
• Waxy cuticle – (passive) physical barrier for water (containing pathogens) from entering cells inn
  upper epidermis. Also stops water collecting on leaf, so pathogens can’t survive on leaf surface.
• Casparian strip – (passive) physical barrier forces water entering roots to pass through plasma
  membrane – prevents entry of toxins/pathogen in water.
• Stomatal closure – when pathogens are detected, guard cells close the stomata in that part of the
  plant, to prevent them from entering.
• Tylose formation – balloon-like swelling that blocks xylem, preventing pathogen spread.
• Necrosis – deliberate cell suicide. Killing cells surrounding infection, limits pathogen’s access to water
  and nutrients and stops it spreading. Necrosis is brought about by intracellular enzymes activated by injury that destroy damaged cells – brown spots on leaves.

Callose:
Active: During times of stress (pathogen invasion) it gets deposited between plant cell walls and plasma membranes, stopping pathogens entering cells. Callose deposition in the plasmodesmata limits virus spread between cells. Also deposited in cell wall to make them thicker/stronger
Passive: Callose is a large polysaccharide that is deposited around the sieve plates in the sieve tubes in the phloem at the end of a growing season – blocking flow & preventing a pathogen spreading around the plant.

Question 2

Chemical defences (all active):

Answer 2

• Saponins – destroy cell membranes of fungi (chitin) and other pathogens
• Phytoalexins – inhibit growth, metabolism and reproduction of pathogens.
• Stimulates secretion of chitinases that break down chitin cell wall of fungi
• Chemicals that are toxic to insects (insecticides/insect repellents) – this reduces the amount of insect
  feeding in plants, reducing the risk of plant infection by plant viruses carried by insect vectors.
• Antibacterial compounds/antibiotics – break down peptidoglycan cell wall.
• General toxins, chemicals that are broken down to toxic cyanide ions.

Question 3

Physical defences: cellulose cell wall

Answer 3

• Cellulose cell wall – (passive) physical barrier (also contains chemical defences).
• Active: thickened and strengthened with additional cellulose

Question 4

Physical defences: lignin

Answer 4

• Lignin – further thickening and waterproofing of xylem of cell walls (after infection)

Question 5

Physical defences: waxy cuticle

Answer 5

• Waxy cuticle – (passive) physical barrier for water (containing pathogens) from entering cells inn
  upper epidermis. Also stops water collecting on leaf, so pathogens can’t survive on leaf surface.

Question 6

Physical defences: casparian strip

Answer 6

• Casparian strip – (passive) physical barrier forces water entering roots to pass through plasma
  membrane – prevents entry of toxins/pathogen in water.

Question 7

Physical defences: stomatal closure

Answer 7

• Stomatal closure – when pathogens are detected, guard cells close the stomata in that part of the
  plant, to prevent them from entering.

Question 8

Physical defences: tylose formation

Answer 8

• Tylose formation – balloon-like swelling that blocks xylem, preventing pathogen spread.

Question 9

Physical defences: necrosis

Answer 9

• Necrosis – deliberate cell suicide. Killing cells surrounding infection, limits pathogen’s access to water
  and nutrients and stops it spreading. Necrosis is brought about by intracellular enzymes activated by injury that destroy damaged cells – brown spots on leaves.

Question 10

Physical defences: callose

Answer 10

Callose:
Active: During times of stress (pathogen invasion) it gets deposited between plant cell walls and plasma membranes, stopping pathogens entering cells. Callose deposition in the plasmodesmata limits virus spread between cells. Also deposited in cell wall to make them thicker/stronger
Passive: Callose is a large polysaccharide that is deposited around the sieve plates in the sieve tubes in the phloem at the end of a growing season – blocking flow & preventing a pathogen spreading around the plant.