3.3.2 The Xylem & Phloem

Question 1

Xylem 1

Answer 1

• Carries dissolved minerals and water up plant
• Structural support
• Food storage

4 Types of tissue :
- Tracheids (long ones with pits)
- Vessel elements (thickened walls and no end plates when mature)
- Xylem parenchyma
- Sclerenchyma cells (fibres and sclerids)

Question 2

Phloem

Answer 2

• Transports sucrose and assimilates from source to sink up and down plant (sap)
• Mainly made of sieve tube elements and companion cells
• Contains living tissue

Question 3

Xylem 2 (adaptations)

Answer 3

Lignified cell walls (in mature cells)
- Adds strength to withstand hydrostatic pressure
- Impermeable to water so reduces water loss
- And cellulose

No end plates
- No resistance to water flow
- Lets mass flow occur, cohesion

No protoplasm (dead cells when mature)
- No resistance to solutes

Pits in wall
- Lateral movement of water
- Incase air bubbles further up or callose closes it because of infection

Small diameter of vessels
- Helps prevent column of water from breaking
- Cohesion

Question 4

Phloem adaptations

Answer 4

Sieve plates with sieve pores
- Continuous movement of organic compounds

Cellulose cell wall
- Strengthens wall to withstand hydrostatic pressure

No nucleus, vacuole or ribosomes (mature)
- maximises space for transportation of assimilates
- Less resistance

Thin cytoplasm
- Less resistance

Companion cell
- Lots of mitochondria
- Transport proteins in plasma membrane
- Plasmodesmata
- Nucleus and RER (coordinates unloading of assimilates)

Question 5

COmparison of xylem and phloem

Answer 5