TRANSPORT IN PHLOEM TOPIC 9 2 U Plants

TRANSPORT IN PHLOEM TOPIC 9. 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

FROM SOURCE TO SINK!

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

Sources vs Sinks SOURCES • Photosynthetic tissues: – Mature green leaves – Green stems • Storage organs: – Germinating seeds – Tap roots or tubers at start of growth season SINKS • 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

U: Active transport is used to load organic compounds into phloem sieve tubes at the • source Sap = sugars & minerals dissolved in H 2 O, 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

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caused by: – Rigid cell walls – Incompressibility of water • Water flows from high to low pressure • At sink, sucrose withdrawn from phloem & either used as energy for growth or converted into starch • Loss of sucrose causes less pressure, so water that carried that solute to the sink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

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

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

App: Structure-function relationship in phloem sieve tubes

Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets & radioactively labelled CO 2.

NOS: Developments in scientific research follow improvements in apparatus: experimental methods for measuring phloem transport rates using aphid stylets & radioactively-labelled CO 2 were only possible when radioisotopes became available • 14 C = radioactive isotope of C – Unstable; emits radiation as it decays • Radioactively-labeled C in CO 2 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

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