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

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

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

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

Transport in Angiosperms

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.

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.

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

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

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)

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

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

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

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

Transport in Angiosperms

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)

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)

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

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

Transport in Angiosperms

Transport in Angiosperms

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

Transport in Angiosperms

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

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

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

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

Transport in Angiosperms Measuring phloem transport rates

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

MAJOR SOURCES Thank you to my favorite sources of information when making these lectures! John Burrell (Bangkok, TH) www.click4biology.info Dave Ferguson (Kobe, JA) http://canada.canacad.ac.jp/High/49 Stephen Taylor (Bandung, IN) www.i-biology.net 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 – www.hhmi.org/biointeractive Mr. Hoye’s TOK Website – http://mrhoyestokwebsite.com And all the contributors at www.YouTube.com