3.4.2 Mass transport in plants

What is the function of the xylem tissue?

Transports water and minerals through the stem, up the plant to the leaves of plants

How is xylem tissue adapted for its function?

• Cells joined with no end walls, forming a continuous tube → water flows as a continuous column

• Cells contains no cytoplasm/nucleus → easier water flow as no obstructions

• Thick cell walls with lignin → provides support/withstands tension/prevents water loss

• Pits in side walls → allow lateral water movements

Explain the cohesion - tension theory of water transport in the xylem

1. Water lost from leaf by transpiration (water evaporates from mesophyll cells into air spaces and water vapour diffuses through open stomata)

2. Reduced water potential of mesophyll cells

3. Water is drawn out of the xylem, down a water potential gradient

4. Creating tension in xylem

5. Hydrogen bonds result in cohesion between water molecules so water is pulled up as a continuous column

6. Water also adheres (sticks) to walls of xylem

7. Water enters roots via osmosis

What four factors affect the rate of transpiration?

Light intensity

Temperature

Wind intensity

Humidity

What is the function of phloem tissue?

Transports organic substances (eg sucrose) in plants

What is translocation?

The movement of assimilates such as sucrose from source cells (where made, eg leaves) to sink cells (where used/stored, eg roots) by mass flow

Explain the mass flow hypothesis for translocation in plants

At source, sucrose is actively transported into phloem sieve tubes/cells by companion cells

This lowers water potential in sieve tubes so water enters from xylem by osmosis

This increases hydrostatic pressure in sieve tubes at source and creates a hydrostatic pressure gradient

So mass flow occurs → movement from source to sink

At sink, sucrose is removed by active transport to be used by respiring cells or storage organs