phloem

function of phloem

transports organic molecules like sugars in the leaves (translocation)

structure of the phloem

1. living cells, including sieve tube elements and companion cells

2. STE - have perforated end walls or “sieve plates”, joined end to end to form sieve tubes.

3. STE - few organelles, as the companion cells have lots of them

4. each companion cell is linked to sieve tube elements by many plasmidesmata

Describe the mass flow hypothesis for phloem loading

1. Companion cell – contains H⁺ ions, large/many mitochondria, which enables the cell to actively transport H⁺ into the source cell.

2. H⁺ and sucrose are transported from the Source cell into the Companion cell via facilitated diffusion through a co-transporter protein, down the concentration gradient.

3. Sucrose then diffuses down the concentration gradient, via the plasmodesmata, from the Companion cell into the sieve tube element (phloem).

4. Sucrose in the sieve tube element lowers the water potential.

5. Water moves down the water potential gradient from the xylem into the sieve tube element by osmosis.

6. This causes an increase in hydrostatic pressure in the sieve tube element.

7. Water (and sucrose) move to an area of lower hydrostatic pressure. N.B. This can be up or down the phloem.

Describe the mass flow hypothesis for phloem unloading.

1. Sucrose in the sieve tube element (down a concentration gradient) diffuses via the plasmodesmata into the Companion cell at the base of the plant.

2. In the sink cell, the sucrose is converted into glucose and fructose (hydrolysis). The glucose is then converted to starch (condensation). Maintaining the concentration gradient.

3. In the Sink cell, sucrose is converted into insoluble starch for storage.

4. Movement of sucrose into the Companion and Sink cell increases the water potential of the sieve tube element.

5. Water moves into the xylem (low water potential due to active uptake of mineral ions) via osmosis down a water potential gradient.

what is a source and sink?

source = where products are formed e.g leaf

sink = where it’s needed e.g root