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9.2 transport in the phloem of plants

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IB Biology 2015

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9.2 transport in the phloem of plants

  1. 1. 9.2 Transport in the phloem of plants Essential idea: Structure and function are correlated in the phloem of plants http://commons.wikimedia.org/wiki/File:Taraxacum_offici nale,_central_leaf_vein,_Etzold_green_2.JPG
  2. 2. Understandings Statement Guidance 9.2 U.1 Plants transport organic compounds from sources to sinks. 9.2 U.2 Incompressibility of water allows transport along hydrostatic pressure gradients. 9.2 U.3 Active transport is used to load organic compounds into phloem sieve tubes at the source 9.2 U.4 High concentrations of solutes in the phloem at the source lead to water uptake by osmosis. 9.2 U.5 Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.
  3. 3. Applications and Skills Statement Guidance 9.2 A.1 Structure–function relationships of phloem sieve tubes 9.2 S.1 Identification of xylem and phloem in microscope images of stem and root 9.2 S.2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively- labelled carbon dioxide
  4. 4. https://classconnection.s3.amazonaws.c om/558/flashcards/183558/png/picture91 335365188079.png 9.2 A.1 Structure–function relationships of phloem sieve tubes
  5. 5. Lumen of the sieve tube with no organelles that would obstruct the flow of sap Cell Wall resists high pressures inside the sieve tube Sieve Plate cross walls strengthens the sieve tube with pores that allow sap to pass through in either direction https://classconnection.s3.amazonaws.c om/558/flashcards/183558/png/picture91 335365188079.png Cell Organelles necessary for carrying out metabolic activities The cytoplasm of Sieve-tube Membrane and Companion cells Is connected by plasmodesmata Phloem sap (mostly Sugar) Passes vertically Through pores in The wall between Sieve-tube members 9.2 A.1 Structure–function relationships of phloem sieve tubes
  6. 6. 9.2 S.1 Identification of xylem and phloem in microscope images of stem and roots
  7. 7. http://cnx.org/contents/addad899-ddc7-489f-918a-30ff0b88911b@4/Roots 9.2 S.1 Identification of xylem and phloem in microscope images of stem and roots
  8. 8. • The transport of materials in plants is called translocation. The organic compounds synthesized during photosynthesis are carried in the phloem tube. • Plants will not transport glucose as it is used directly it must be converted into sucrose. • Sucrose move through the plant:  With increased water pressure  The use of ATP (active transport) Sink is any tissue that is accepting sugar Source is any tissue that makes sugar available to the plant 9.2 U.1 Plants transport organic compounds from sources to sinks.
  9. 9. • During the growing season plant leaves produce glucose. • Glucose is used to produce ATP for metabolic activities. • Excess glucose is converted into sucrose and stored in the roots of the plant (called the sink). • At the end of a growing season plant tubers becoming the source of glucose, as the leaves glucose production ceases. During non growing seasons plants convert sucrose back into glucose to carry out metabolic activities (sink becomes the source). Potatoes http://media-2.web.britannica.com/eb- media/42/82542-004-4A7EA186.jpg 9.2 U.1 Plants transport organic compounds from sources to sinks.
  10. 10. Summer Source for carbohydrates Summer Sink for carbohydrates Winter/Spring Source for Carbohydrates
  11. 11. Pressure-Flow Hypothesis •Hydrostatic pressure is pressure in a liquid. •The term water potential is used to describe the tendency for water molecules to move within and between cells. •A net movement of water will occur from one region to another as a result of a difference in water potential. •Water moves by osmosis from an area of high water concentration to an area of low water concentration. •In the diagram water flow out of the xylem and into the phloem due to this gradient. •This change in water pressure in the phloem helps push sucrose to the sink cell (storage site) in the plant. http://cnx.org/resources/cd8839949717d6b04f4f ade2e6b0f7cf/Figure_30_05_07.jpg 9.2 U.2 Incompressibility of water allows transport along hydrostatic pressure gradients.
  12. 12. http://cnx.org/resources/cd8839949717d6b04f4f ade2e6b0f7cf/Figure_30_05_07.jpg High hydrostatic water pressure moves water into the Phloem High hydrostatic water pressure moves water into the Xylem Low water potential gradient Low water potential gradient 9.2 U.2 Incompressibility of water allows transport along hydrostatic pressure gradients.
  13. 13. • Phloem tissue is found in plants (stems, roots & leaves). • Phloem is composed of sieve tubes. Sieve tubes along with companion cells composes a column of specialized cells making up most of the phloem. • The sieve tubes that make up phloem tissue are composed of living cells. These cells have mitochondria which generate ATP needed for active transport • Sieve plates are found at either end of a sieve tube. They contain by perforated walls called sieve plate pores 9.2 U.3 Active transport is used to load organic compounds into phloem sieve tubes at the source.
  14. 14. http://image.slidesharecdn.com/transportinanimalsandplants4-150218054406- conversion-gate02/95/transportation-structure-of-plants-10-638.jpg?cb=1424320835 • Sucrose (green triangle in the diagram) is actively transported into the phloem form leaf cells indirectly.  Step one: ATP is used to pump H+ across the leaf cell into the phloem cells, creating a concentration gradient of H+ ions.  Step two: Co- transport proteins use the concentration gradient to move sucrose and H+ simultaneously, allowing protons down the concentration gradient 9.2 U.3 Active transport is used to load organic compounds into phloem sieve tubes at the source.
  15. 15. 1. The source during the growing season is the leaf which produces organic molecules. 2. Photosynthesis produced glucose in leaf cells. 3. Glucose is converted to sucrose for transport. 4. Companion cell actively loads the sucrose increasing the concentration in the phloem. 5. Water flows from xylem by osmosis into phloem (water from the xylem helps transport sucrose from the phloem). This one reason why close proximity is import between the two. Combined Transpiration/ Translocation 9.2 U.4 High concentrations of solutes in the phloem at the source lead to water uptake by osmosis
  16. 16. 6. SAP volume and pressure increased leading to mass flow (material moving together with water). 7. Unload the organic molecules by the companion cell occurs at the sink (during the growing season this is the roots). 8. Sucrose is converted and stored as insoluble starch. 9. Water mixed with sucrose is released and flows back into the xylem by osmosis. 10. Water recycles as part of transpiration to resupply sucrose loading. * Remember transpiration is the movement of water from the roots to the leaves of the plant. Combined Transpiration/ Translocation 9.2 U.4 High concentrations of solutes in the phloem at the source lead to water uptake by osmosis
  17. 17. 1.Translocation moves the 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. 2. Companion cells actively load sucrose (soluble, not metabolically active) into the phloem. 3. Water follows the high solute in the phloem by osmosis. A pressure develops moving the mass of phloem sap forward. Translocation theory Click4biology.com 9.2 U.5 Raised hydrostatic pressure causes the contents of the phloem to flow towards sink.
  18. 18. 4. The sap must cross the sieve plate. 5. The phloem still contains a small amount of cytoplasm along the walls but the organelle content is greatly reduced. 6. Companion cells actively unload (ATP used) the organic molecules 7. Organic molecules are stored (sucrose as starch, insoluble) at the sink. Water is released and recycled in xylem.Click4biology.com Translocation theory 9.2 U.5 Raised hydrostatic pressure causes the contents of the phloem to flow towards sink.
  19. 19. • An understanding of sap flow rates through phloem has come from the use of an insect called an aphid and its feeding technique • Aphids have a long piercing mouthpart called a stylets, which they insert into the plants sieve tube and draw out sap. http://upload.wikimedia.org/wikipedia/commons/1/11/Aphid-giving-birth.jpg 9.2 S.2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively- labelled carbon dioxide
  20. 20. http://plantsinaction.science.uq.edu.au/book/export/html/23 9.2 S.2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively- labelled carbon dioxide 1. Plants grown in a lab with leaves covered with a clear bag and then exposed to CO2 containing the radioactive isotope C14 . 2. The radioactive CO2 is taken and incorporated into glucose by the process of photosynthesis. The glucose is then converted into sucrose and move through the phloem of the plant. 3. In the experiment colonies of Aphids feed on the sucrose in the phloem at different locations of the plant stem. 4. When feeding sucrose drips down into container below the colonies. The distant between the colonies is used to calculate sucrose flow rates in the phloem.
  21. 21. Experiment numbers 1 2 3 Distance between aphid colonies (mm) 650 340 630 Time for radioactivity to travel between colonies (hours) 2.00 1.25 2.50 Rate of movement (mm hours -1 ) 32.5 27.2 25.4 Example with two colonies 9.2 S.2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively- labelled carbon dioxide

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