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Transport in plants AS Biology [jm]


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Part I

Explain the need for transport systems in multicellular plants
Describe the distribution of xylem and phloem tissue in roots, stems and leaves
Explain the absorption process in roots
Describe transport mechanisms

Part II

List factors that affects rate transpiration
Describe xerophyte properties
List the series of events that leads to translocation

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Transport in plants AS Biology [jm]

  1. 1. Moving water, minerals and sugars Jorge Melo
  2. 2. Introduction Plants use photosynthesis to convert light energy to chemical energy Simple organic substances, such as CO2, H2O and ions are used in their raw form to produce glucose and other carbohydrates.
  3. 3. Introduction How does the plants obtain H2O and CO2? Does the plants have a circulatory system like us?
  4. 4. Introduction American sequoias (giant redwood) Height of 60 meters How these trees lift water?
  5. 5. Objectives Explain the need for transport systems in multicellular plants Describe the distribution of xylem and phloem tissue in roots, stems and leaves Explain the absorption process in roots Describe transport mechanisms
  6. 6. Plants have two separate transport tissues  Xylem tissue: Water and ions travel upwards Roots Stems Leaves Flowers Fruits  Phloem tissue: Sucrose and other assimilates travel upwards and downwards  Movement of water in the xylem and phloem is by mass flow. Everything travels in the same direction within each of column of xylem or phloem  Note that neither plant transport system carries O2 or CO2
  7. 7. Xylem and Phloem 2 distinct transport systems In both the walls of the tubes are further thickened by the addition of:  Cellulose (organic compound –polysaccharide)  Lignin (woody material)
  8. 8. Water transport in 3 parts  Transpiration (or evapo-transpiration) is the transport of water and minerals from roots to leaves. It involves three basic steps: 1- Absorption at the roots. 2 - Capillary action in the xylem vessels. 3 - Evaporation at the leaf.
  9. 9. Roots Root hair Single-celled extensions of some cells Very thin (200-250 µm) A single root can have thousands Increases the surface area Absorbs water by osmosis
  10. 10. Roots Osmosis Movement of H2O molecules from an area of high concentration to an area of lower concentration lower solute concentration in the soil Higher solute concentration in the root High water potential in soil Low water potential in roots
  11. 11. Two different routs  Apoplast pathway:  When H2O soaks through the cell walls and then seeps across the root from cell wall to cell wall and through the spaces between cells Symplast pathway:  When H2O enters the cell walls and moves from cell to cell by osmosis  Or through strands of cytoplasm that makes direct connection between adjacent cells- plasmodesmata
  12. 12.  When water reaches the stele the apoplast pathway is blocked. Endodermis cells (stele) have suberin (waterproof)
  13. 13.  Casparian strip: belt of waxy material, allows only minerals in the symplast to pass into the vascular cylinder through the plasma membrane of endodermal cells. Cells in the vascular cylinder transport water and minerals throughout the plant.
  14. 14. Task 1
  15. 15. Xylem Long narrow cells Xylem elements  Start as living cells (nucleus, cell wall)  Then differentiated into specialised structures and died  No living material  Just empty shells
  16. 16. Protoxylem: The first one toXylem be developed behind root and shoot tips. Lignin added Primary xylem in rings and spirals to form annular vessels (rings). Metaxylem: more mature and walls are fully lignin. Secondary thickening Secondary Xylem The seasonal growth of the xylem shows up as annual rings. The ring from the previous year transports little water but is useful for support.
  17. 17. Primary xylem Protoxylem Metaxylem
  18. 18. Root cross section
  19. 19. Stem
  20. 20. Leaf
  21. 21. Leaf cross section
  22. 22. Task 2
  23. 23. Palisade and spongy mesophyll cells have verylarge internal surface for gas exchange.As the carbon dioxide concentration in the air isso low (0.04%), the surfaces are large so thatenough can be absorbed for photosynthesis.
  24. 24. The air inside leaves is always fully saturated withwater vapour.Usually, the air outside is less saturated than thisand so a concentration gradient for water vapourexists between the air spaces and the outside.Water vapour therefore diffuses down thishumidity gradient.The pathway with the least resistance isthrough the stomata. It is open during theday to allow CO2 in and water out. In mostplants it is closed at night.
  25. 25. How does the water goes up?
  26. 26. Transpiration drives themovement of water in plants • The loss of water from leaves by transpiration causes water to travel upwards through the plant by mass flow. • The mechanism is called ‘cohesion-tension’ and it works as follows:
  27. 27. Cohesion-tension theory Water loss caused by transpiration Causes a pulling force Negative pressure produced Transpiration pull
  28. 28. Cohesion-tension theory2 important factors of the water: Cohesion: H2O molecules tend to stick together by hydrogen bonding Adhesion: H2O molecules tend to stick to the inside of the xylem
  29. 29. Cohesion-tension theory Root Absorption through osmosis Endodermal cells actively secrete mineral salts Why? To keep the water potential in the xylem lower Causing water to be drawn through the endodermis “pulling” of water caused by cortex cells produce positive hydrostatic pressure inside the xylem , forcing water upwards Root pressure
  30. 30. Cohesion-tension theory Capillarity Third force Water tends to rise inside narrow tubes by capillary action Capillarity relies upon the tendency of water molecules to stick to walls of xylem vessels by adhesion. This force may be important in the upward movement of water in small plants but no relevance in large trees
  31. 31. How does the water goes up? Transpiration pull (negative pressure) Root pressure (positive pressure) Capillarity (small plants)2 important factors of the water: Cohesion: H2O molecules tend to stick together Adhesion: H2O molecules tend to stick to the inside of the xylem
  32. 32. Transpiration Spongy mesophyll cells are not tightly packed Air spaces are direct contact with the air outside the leaf, through small pores called stomata If air outside the leaf contains less H2O vapour then inside There is a H2O potential gradient from the air spaces inside the leaf to the outside
  33. 33. Task 3
  34. 34. Moving water, minerals and sugars Jorge Melo
  35. 35. How does the water goes up? Transpiration pull (negative pressure) Root pressure (positive pressure) Capillarity (small plants)2 important factors of the water: Cohesion: H2O molecules tend to stick together Adhesion: H2O molecules tend to stick to the inside of the xylem
  36. 36. Objectives List factors that affects rate transpiration Describe xerophyte properties List the series of events that leads to translocation
  37. 37. Potometer Measures the water absorption Estimate the rate of transpiration Air/water tight Water transpired Water entering to xylem
  38. 38. Factors affecting rate of transpirationLight intensity:  Affects the opening and closing of the stomata  ROT  Indirect effect
  39. 39. Factors affecting rate of transpirationHumidity: Humid atmosphere Contains a lot of H2O molecules Reduction of the water potential gradient between the air spaces and atmosphere ROT decreases Low humidity increases ROT
  40. 40. Factors affecting rate of transpirationTemperature: Temperature kinetic energy Rate of diffusion through the stomata pores Air is able to hold more water molecules at higher temperatures ROT
  41. 41. Factors affecting rate of transpirationWind speed: Still air makes the H2O molecules to accumulate around the stomata pores (leaves) Reduces the H2O potential gradient and slows the ROT Wind disperse H2O molecules gradient in H2O potential ROT
  42. 42. Xerophytes Vs Mesophytes
  43. 43. xerophytes A plant adapted to live in dry conditions They have a range of adaptations to reduce the loss of water vapour by transpiration.
  44. 44. xerophytes Leaves  Small to reduce the surface area  Thick to reduce surface area: volumes ratio
  45. 45. xerophytes Sunken Stomata
  46. 46. xerophytes Stomata  Set deep inside the leaf so that they are at the base of a depression full of water vapour  Some plants open their stomata at night to store and absorb CO2
  47. 47. xerophyte Thick waxy cuticles reduce water loss through the epidermis
  48. 48. Xerophytes Rolling up of leaves Lower surface faces inside and traps humid air next to the stomata Varies with conditions
  49. 49. Xerophytes Leaf hairs Trap damp air Reduces air movement Cut down transpiration
  50. 50. Task 1
  51. 51. Transport in the Phloem• Most photosynthesis occurs in the leaves.• The reactions take place in the chloroplasts.• The compounds that the plant makes are called assimilates.• Many of these are exported form the leaves to the rest of the plant in the phloem.
  52. 52. Sources and Sinks• The transport of these assimilates is called translocation.• This literally means ‘from place to place’. • Assimilates are loaded in the phloem in the leaves, they are often called sources.• They are transported to other parts of the plant, such as roots, stems, flowers, fruits and seeds. These are called sinks.
  53. 53. Movement in the Phloem in an activetransport• The transport of these assimilates is called translocation• Sucrose and other assimilates travel throughout a plant in phloem sieve tubes.• These are made from cells called sieve elements.
  54. 54. Sieve tube• Made of sieve elements• Living cells • No nucleous • Ribosomes or tonoplast• Diameter 10 15 um• End walls: sieve plates• Large pores
  55. 55. Alongside sieve tubesare companion cells.Mesophyll cells in theleaf are close to veinscontaining sieve tubes.Sucrose travels tothe phloemcompanion cells intwo ways.
  56. 56. From cell to cell throughthe plasmodesmata.Along cell walls in themesophyll.Carrier proteins in thecell surface membranesof companion cellsactively pump sucroseinto the cytoplasm.From here it passesthrough plasmodesmatainto a sieve element.
  57. 57. The accumulation of sucroseand other solutes, such asamino acids, in sieveelements lowers the waterpotential so that waterdiffuses in by osmosis fromadjacent cells and form thexylem.This creates pressure inthe sieve elementscausing the liquid(phloem sap) to flow outof the leaf.
  58. 58. Phloem sieve elements areadapted for transport as ithas:• End walls that have sieve pores allowing sap to flow freely.• Little cytoplasm to impede the flow of sap.• Plasmodesmata to allow assimilates to flow in from companion cells.
  59. 59. Sieve elements differform xylem vesselsbecause they are alive.They have somecytoplasm withorganelles.They are notlignified, as they donot need towithstand the sameforces as exist in thexylem.
  60. 60. Sucrose is unloaded atsinks.This is taken up by thecells and is respired orstored s starch.This reduces theconcentration ofphloem sap and lowersthe pressure, so helpingto maintain a pressuregradient form source tosink so the sap keepsflowing in the phloem.
  61. 61. Task