1. Transport in phloem
Topic 9.2

2. U: Plants transport organic compounds from sources to sinks
Phloem found in stems, roots, leaves
Composed of sieve tubes made up of living cells
Columns of sieve tube cells, separated by sieve plates (perforated walls)
Sieve tube cells associated with companion cells
Phloem transports organic compounds (sugar, a.a.)
Translocation = transport of org comp
From source to sink
Source = where org comp loaded into phloem
Sink = where org comp unloaded, used

3. From source to sink!

4. Comparision of phloem & blood vessels
Transports substances in either direction
No valves or central pump
Fluid flows inside tubes because of pressure gradients
Energy needed to generate pressure, so transport of sap is active
Transports blood in one direction
Valves (in veins) and central pump (heart)
Fluid flows inside tubes because of pressure gradients
Energy needed to generate pressure, so transport of blood is active

5. Sources vs Sinks sources sinks Photosynthetic tissues: Storage organs:
Mature green leaves
Green stems
Storage organs:
Germinating seeds
Tap roots or tubers at start of growth season
Roots that are growing or absorbing mineral ions using energy from cell respiration
Parts of plant growing or developing food stores:
Developing fruits
Developing seeds
Developing tab roots or tubers
Growing leaves

6. U: Active transport is used to load organic compounds into phloem sieve tubes at the source
Sap = sugars & minerals dissolved in H2O, transported by phloem
Sucrose is most prevalent solute in sap (good form of carb to transport since it’s not in a form plants use for respiration)
Phloem loading = bringing sugars INTO phloem

7. U: Incompressibility of water allows transport by hydrostatic pressure gradients
Sucrose build-up in companion cell draws water in
Build-up of pressure caused by:
Rigid cell walls
Incompressibility of water
Water flows from high to low pressure
At sink, sucrose withdrawn from phloem & either used as energy for growth or converted into starch
Loss of sucrose causes less pressure, so water that carried that solute to the sink is drawn back in to the transpiration stream in xylem

8. U: High concentrations of solutes in phloem at source lead to water uptake by osmosis

9. U: Raised hydrostatic pressure causes contents of phloem to flow toward sink

10. App: Structure-function relationship in phloem sieve tubes

11. Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets & radioactively labelled CO2.

12. NOS: Developments in scientific research follow improvements in apparatus: experimental methods for measuring phloem transport rates using aphid stylets & radioactively-labelled CO2 were only possible when radioisotopes became available
14C = radioactive isotope of C
Unstable; emits radiation as it decays
Radioactively-labeled C in CO2 can be fixed during photosynthesis, releasing radiation
Both formation and movement of radioactive C can be traced
Geiger counters measure radiation levels in a crop

13. Skill: Identification of xylem & phloem in microscope images of stem & root