1. 9.2 Transport in the phloem of plants
Sponge: Set up Cornell Notes on pg. 31
Topic: 9.2 Transport in the phloem of plants
Essential Question: If water travels from roots to leaves how can plants move sugars in the opposite direction?
Objective: Students will understand structure-function relationships of phloem sieve tubes
If water travels from roots to leaves how can plants move sugars in the opposite direction?
Key Vocabulary:
Phloem
Sugar source
Sugar sink
Companion cells
Translocation- Pressure flow hypothesis
BIOZONE:
Text: 629,
Digital Text: 157

2. Plant organic molecule movement
Phloem: made up of living cells that transport their contents in various directions
Links parts of the plants that need a supply of sugar, to parts that have a surplus
Composed of sieve (siv) tubes
Connected to one another by sieve plates
Have pores which allow the movement of water and dissolved sugar throughout the plant
pore

3. Understandings:
Plants transport organic compounds from sources to sinks

4. Sources and Sinks Movement is from a source to a sink
Source: plant organ that is a net producer of sugar through photosynthesis
Leaves are primary sugar sources
Contain all the major organs for photosynthesis
Sink: plant organ that uses or stores sugar
Roots, buds, stems, seeds and fruits are all sugar sinks

5. Sources and Sinks

6. Sources and Sinks
It is possible for some structures to be both a source and a sink
Sinks can turn into sources and vice versa
Bulb
Tuber

7. Plant organic molecule movement
Companion cells are connected to sieve tube members by plasmodesmata
Microscopic channels which enable transport and communication
Perform many of the genetic and metabolic functions of the sieve tube cells
Supplies the ATP needed for active transport

8. Translocation
Translocation: The movement of organic molecules (sugars) in plants
The sugars are dissolved in H2O and the solution is referred to as phloem sap
Include:
Mostly sugars (sucrose)
Amino acids
Plant hormones
Small RNA molecules

9. Understandings:
High concentrations of solutes in the phloem at the source lead to water uptake by osmosis

10. Translocation: Pressure Flow Hypothesis
1. Phloem loading at source by active transport
ATP active transport of sugar into phloem
Creates high solute concentration in sieve tube (reducing H₂O)
Causes osmosis from surrounding cells

11. Translocation: Pressure Flow Hypothesis
Apoplast route: travels through cell walls
H+ transported out of companion cells using ATP
H+ build up
Flow through co-transport protein
Energy released carries sucrose into phloem
Symplast route: travels between cells

12. Understandings:
Incompressibility of water allows transport along hydrostatic pressure gradients
Raised hydrostatic pressure causes the contents of the phloem to flow toward the sink

13. Translocation: Pressure Flow Hypothesis
2. H²O uptake causes a hydrostatic pressure gradient and a flow of phloem sap
Adding solutes to a limited space filled with water increases pressure (+)
Phloem sap will move from higher to lower pressure areas by passive transport
HIGH PRESSURE
LOW PRESSURE

14. Translocation: Pressure Flow Hypothesis
3. Removal of sugar at the sink
Need ATP 
Pressure is diminished by the removal of the sugar into the sink
Sugar used as a energy source for growth or converted to starch
Starch is insoluble and exerts no osmotic effect

15. Translocation: Pressure Flow Hypothesis
4. Xylem recycles H₂O
The relatively pure H²O is carried by xylem from the sink back to the source

16. Understandings:
Active transport is used to load organic compounds into phloem sieve tubes at the source

17. Translocation: Pressure Flow Hypothesis
The loading of sugar at the sieve tube at the source and the removal of the sugar at the sink is accomplished by active transport
Chemiosmotic process involving protein pumps
Only the loading and unloading of the sugar requires energy
The transport in the tube is passive

18. Data Based Questions: Explaining H2O movement
Explain movement of water from
A to C
C to D
D to B
B to A

19. Crash Course: Vascular Plants (10m06-11m11s)

20. Application:
Structure-function of phloem sieve tubes

21. Structure-function relationship of phloem sieve tubes
Sieve tube- companion cell association- abundant mitochondria in companion cells
Supports active transport of sucrose
Rigid cell walls
Allow for establishment of pressure necessary to achieve the flow of phloem
Infolding of plasma membrane
Increases the phloem loading capacity
Plassmodesmata connect cytoplasm of companion cell with sieve tube
Accommodate the movement of sugars between the two cells
Sieve plates with pores
Perforated walls means that the resistance to the flow of phloem sap will be lower

22. Electron micrograph of phloem

23. Electron micrograph of phloem
vii ii v i vi
iii iv

24. Nature of Science:
Developments in scientific research follow improvements in apparatus– experimental methods for measuring phloem transport rates using aphid stylets

25. NOS: Translocation and aphids
The pressure that occurs within the phloem, as well as the composition of phloem sap, has been demonstrated using the stylets of aphids
Phloem sap is nutrient rich
But the only animals to consume it as the main part of their diet are insects belonging to a group called the Hemiptera
Aphids
Whitefly
Mealybugs
Psyllids

26. NOS: Translocation and aphids
Aphids penetrate plant tissues to reach the phloem (p) using mouth parts called stylets (st)
If the aphid is anaesthetized and the stylet is severed, phloem will continue to flow out of the stylet
Both the rate of flow and the composition of the sap can be analyzed
The closer the stylet is to the sink, the slower the rate at which the phloem sap will come out

27. Understandings:
Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide

28. Analysis: Experiments using aphid stylets

29. Analysis: Experiments using aphid stylets
a) (i) active transport of sugar (ii) create high solute concentration; water drawn in by osmosis; b) (i) no oligosaccharides at sucrose concentration below 0.25 mol dm-3; oligosaccharides concentration rises between 0.25 and 0.50 mol dm-3; no further increase above 0.50 mol dm-3; (ii) to reduce water loss from aphid/gut cells by osmosis; c) (i) poor source of amino acids, with many (especially essential amino acids) at a lower percentage in phloem sap that aphid proteins; (ii) plants synthesize amino acids for making plant proteins; plant and aphid proteins have different amino acid composition; d) (i) feed aphids on phloem sap containing antibiotics; test aphid growth rates/protein synthesis rates/amino acid contents; (ii) physiological problems have to be overcome; problem of lack of essential amino acids;

30. Nature of Science:
Developments in scientific research follow improvements in apparatus– experimental methods for measuring phloem transport rates using radioactively-labelled carbon dioxide were only possible when radioisotopes became available

31. NOS: Radioisotopes can be used to study translocation
Carbon-14 is an isotope of carbon that is radioactive
Radioactively –labelled carbon within CO2 can be fixed by plants in photosynthesis
It will release radiation that can be detected using radiation detectors
Formation and movement of radioactive molecules can be traced

32. NOS: Radioisotopes can be used to study translocation
A Geiger counter can be used to measure radiation levels in a crop of sunflowers
The sunflowers in this picture are being used for bioremediation of soil contaminated with radiation

33. Animation: Transport of water and sugar in plants Activity
Use the animation to fill answer the questions on the animation analysis activity