1. 9.2 Transport in the Phloem
Plant Science
9.2
Transport in the Phloem

2. Review
Active Transport
Passive Transport
Photosynthesis
Osmosis

3. Transport in the phloem
Where does photosynthesis happen in the plant?
The phloem moves sugar

4. Transport in the Phloem
Structure-Function relationship
Sieve tubes
Made of sieve tube cells
Sieve plate
Reduced cytoplasm
No nucleus
Ridged walls
Companion cells
Genetic and metabolic functions
Mitochondria
Folding of membrane
Transport proteins
Plasmodesmata

6. Transport in the Phloem
Identification of Phloem in cross sections of stem and root

7. Transport in the phloem
Source to sink
Translocation 
When organic compounds are produced in leafs in excess, the leaf is the source.
Sink can be flowers, fruits, roots
When organic compounds are released from storage, the storage structure is the source
Sink can be fruits, flowers

8. Transport in the Phloem
Phloem loading
Apoplast
Through cell walls
Active Transport
Symplast
Between cells
plasmodesmata

9. Transport in the phloem
Apoplast
A. . proton pumps move H+s into extracellular environment (hydrolysis of ATP)
B. Creation of proton gradient outside the cell
C. H+ moves passively into the cell via a co- transport protein that takes sucrose with it
D. Sucrose builds up in sieve tube

10. Transport in the Phloem
Increase in solute (sucrose) Water moves from xylem to phloem
Water uptake in phloem
Phloem sap movement
Pressure gradient is maintained by unloading of sucrose at sink

11. Transport in Phloem The solutes are unloaded
Sap solution becomes hypotonic 
Water is drawn out of the phloem and back into the xylem
This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source

12. Transport in the Phloem
Incompressibility of water allows transport along hydrostatic pressure gradients.
at normal temperature, the compressibility of water is so low that it can be considered incompressible
therefore, any change in pressure at one end of the transport vessel will result in an equal change in pressure at the other end of the transport vessel
this allows phloem sap to move up or down, with or against gravity, as it moves from source to sink

13. Translocation Rate
Aphids are a group of insects that feed on sap extracted from phloem

14. Translocation Rate
Aphids can be used to collect sap at various sites along a plant's length and thus provide a measure of phloem transport rates

15. Factors Affecting Translocation Rate
The rate of photosynthesis
The rate of cellular respiration
The rate of transpiration
The diameter of the sieve tubes