1. Unit 9.2 Transport in the phloem
IB DP Biology
Unit 9.2
Transport in the phloem

2. 9.2.u1
Plants transport organic compounds from sources to sinks.

3. Transport in Angiosperms
Phloem translocation
Translocation
The movement of substances from one part of a plant to another in the phloem
From source (sucrose made in leaves) to the sink (sucrose stored in roots)
Sieve-tube members
The cells that make up the phloem

4. Transport in Angiosperms
Phloem translocation
Symplastic route
Sucrose manufactured in mesophyll cells travels intracellularly to phloem sieve-tube members (STMs)
Apoplastic route
Sucrose manufactured in mesophyll cells travels extracellularly to companion cells and STMs

5. Transport in Angiosperms

6. 9.2.u3
Active transport is used to load organic compounds into phloem sieve tubes at the source.
9.2.u4
High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.

7. 9.2.u5
Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.
9.2.u2
Incompressibility of water allows transport along hydrostatic pressure gradients.

8. Transport in Angiosperms
Phloem translocation
Proton pumps in companion cells and STMs
Driven by ATP
Pump H+ into extracellular environment
Sucrose enters companion cells and STMs by co-transport
H+ moves down its concentration gradient back into companion cells and STMs

9. Transport in Angiosperms
Pressure flow in phloem
Loading of sucrose into STMs at source causes water to enter by osmosis
By reducing water potential inside STMs

10. Transport in Angiosperms
Pressure flow in phloem
Absorption of water generates hydrostatic pressure
Forces the sap to flow along the tube
Gradient of pressure is reinforced by the unloading of sucrose and the loss of water at the sink (root)

11. Transport in Angiosperms
Phloem transport
Translocation moves organic molecules (sugars, amino acids) from their source through the tube system of the phloem to the sink
Phloem vessels still have cross walls called sieve plates that contain pores

12. Transport in Angiosperms
Phloem transport
Companion cells actively load sucrose into the phloem
Water follows the high solute in the phloem by osmosis
A positive pressure potential develops moving the mass of phloem sap forward

13. Transport in Angiosperms
Phloem transport
The sap must cross the sieve plate
Phloem still contains a small amount of cytoplasm along the walls
The organelle content is greatly reduced

14. Transport in Angiosperms
Phloem transport
Companion cells actively unload the organic molecules at sinks
Using proton pumps
Organic molecules are stored (sucrose as starch, insoluble) at the sink
Water is released and recycled in xylem

15. Transport in Angiosperms

16. Transport in Angiosperms
Phloem transport
Plants will not transport glucose
It is used directly in respiration and is metabolically active
Sucrose is soluble and transportable
Not metabolically active in respiration
At the sink it is necessary to have the transported molecule insoluble (no osmotic effect) and inactive (no respiration effect)

17. Transport in Angiosperms
Phloem transport
Sources:
Photosynthesising leaves
Storage region at the beginning of a growing season (tuber)
Sinks
Meristems (regions of plant cell growth)
Storage area at the end of a season (tuber)

18. Transport in Angiosperms
Phloem transport problems!
Does not account for:
How the direction of sap travel could be reversed
How the resistance of the sieve plate is overcome

19. Transport in Angiosperms
Source produces organic molecules
Glucose from photosynthesis produced
Glucose converted to sucrose for transport
Companion cell actively loads the sucrose in phloem
Water follows from xylem by osmosis
Sap volume and pressure increased to give Mass flow
Unload the molecules at sink by the companion cell
Sucrose stored as the insoluble and unreactive starch
Water that is released is picked up by the xylem
Water recycles as part of transpiration to re supply the sucrose loading

20. Transport in Angiosperms

21. Transport in Angiosperms

22. 9.2.a1
Structure–function relationships of phloem sieve tubes.

23. Transport in Angiosperms

24. 9.2.s2
Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.

25. Transport in Angiosperms
Measuring phloem transport rates
Phloem is thin and moves slowly
Measuring rate was difficult for long time

26. Transport in Angiosperms
Measuring phloem transport rates
Aphids (insects) drink phloem sap
Use mouth-parts called stylets

27. Transport in Angiosperms
Measuring phloem transport rates
If the stylet is cut off after insertion, phloem continues to flow
Can now measure what is in the phloem

28. Transport in Angiosperms
Measuring phloem transport rates

29. Transport in Angiosperms
Measuring phloem transport rates
Plants use CO2 from the air to make sugar in the leaves
Using radioactive CO2 makes it easy to test how long it takes sugar to move from the leaves down the phloem

30. MAJOR SOURCES
Thank you to my favorite sources of information when making these lectures!
John Burrell (Bangkok, TH)
Dave Ferguson (Kobe, JA)
Stephen Taylor (Bandung, IN)
Andrew Allott – Biology for the IB Diploma
C. J.Clegg – Biology for the IB Diploma
Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate
Howard Hugh’s Medical Institute –
Mr. Hoye’s TOK Website –
And all the contributors at