1. 9.2 Transport in the phloem of plants
Sponge: Set up Cornell Notes on pg. 39
Topic: 9.2 Transport in the phloem of plants
Essential Question: What is the major difference between xylem and phloem?
What is the major difference between xylem and phloem?
Key Vocabulary:
Translocation- Pressure flow hypothesis
BIOZONE:
Text: 629,

2. Applicaiton:
Structure-function relationships of phloem tubes

3. Plant organic molecule movement
Phloem: made up of living cells (unlike xylem) that transport their contents in various directions
Occurs in ALL parts of the plants (not just the stems)
Mostly sieve (siv) tube members
Connected to one another by sieve plates to form sieve tubes
Sieve plates have pores which allow the movement of water and dissolved organic molecules throughout the plant
pore

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

5. Plant organic molecule movement
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

6. Plant organic molecule movement
It is possible for some structures to be both a source and a sink
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 in plants (products of photosynthesis)
The organic molecules 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
Sugar loading at source
Creates high solute concentration in sieve tube (reducing H₂O)
Causes osmosis from surrounding cells

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

12. Translocation: Pressure Flow Hypothesis
2. H²O uptake causes hydrostatic pressure
The positive pressure in the sieve tube results in a flow (bulk flow) of the phloem sap
Adding solutes to a limited space filled with water increases pressure
Two areas with different hydrostatic pressure produce a hydrostatic pressure gradient
H₂O with its dissolved solutes will move from higher pressure area to lower pressure area

13. Translocation: Pressure Flow Hypothesis
3. Removal of sugar at the sink
Pressure is diminished by the removal of the sugar into the sink
Sugars are changed at the sink to starch
Starch is insoluble and exerts no osmotic effect

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

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

16. 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

17. Crash Course: Vascular Plants (4m40s-11m53s)

18. Skill:
Identification of xylem and phloem in microscope images of stem root

19. ID xylem and phloem in light micrographs
Xylem cells are generally larger than phloem cells
Phloem cells tend to be closer to the outside of the plant in stems and roots

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

21. Translocation
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

22. Translocation
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

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

24. Analysis: Experiments using aphid stylets

25. 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