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Chapter 36

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  1. Chapter 36 Resource Acquisition and Transport in Vascular Plants

  2. Overview: Underground Plants • The success of plants depends on their ability to gather and conserve resources from their environment • The transport of materials is central to the integrated functioning of the whole plant • Diffusion, active transport, and bulk flow work together to transfer water, minerals, and sugars

  3. Fig. 36-1

  4. Fig. 36-2-1 H2O H2Oand minerals

  5. Fig. 36-2-2 CO2 O2 H2O O2 H2Oand minerals CO2

  6. Fig. 36-2-3 CO2 O2 Light Sugar H2O O2 H2Oand minerals CO2

  7. Concept 36.1: Land plants acquire resources both above and below ground • The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water • The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis • Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss

  8. Shoot Architecture and Light Capture • Stems serve as conduits for water and nutrients, and as supporting structures for leaves • Phyllotaxy,the arrangement of leaves on a stem, is specific to each species

  9. Fig. 36-3 32 24 42 40 29 16 19 11 21 27 3 8 34 6 14 13 Shootapicalmeristem 26 1 22 5 9 18 Buds 4 10 2 17 31 7 12 23 15 25 20 28 1 mm

  10. Light absorption is affected by the leaf area index, the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows • Leaf orientation affects light absorption

  11. Fig. 36-4 Ground areacovered by plant Plant ALeaf area = 40%of ground area(leaf area index = 0.4) Plant BLeaf area = 80%of ground area(leaf area index = 0.8)

  12. Root Architecture and Acquisition of Water and Minerals • Soil is a resource mined by the root system • Taproot systems anchor plants and are characteristic of most trees • Roots and the hyphae of soil fungi form symbiotic associations called mycorrhizae • Mutualisms with fungi helped plants colonize land

  13. Fig. 36-5 2.5 mm

  14. Concept 36.2: Transport occurs by short-distance diffusion or active transport and by long-distance bulk flow • Transport begins with the absorption of resources by plant cells • The movement of substances into and out of cells is regulated by selective permeability

  15. Diffusion and Active Transport of Solutes • Diffusion across a membrane is passive, while the pumping of solutes across a membrane is active and requires energy • Most solutes pass through transport proteins embedded in the cell membrane

  16. The most important transport protein for active transport is the proton pump • Proton pumps in plant cells create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work • They contribute to a voltage known as a membrane potential

  17. Fig. 36-6 EXTRACELLULAR FLUID CYTOPLASM _ + _ Proton pumpgenerates mem-brane potentialand gradient. H+ + ATP H+ _ + H+ H+ H+ H+ _ H+ + H+ H+ _ +

  18. Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes

  19. Fig. 36-7 _ + EXTRACELLULAR FLUID CYTOPLASM _ + K+ _ + K+ K+ K+ K+ K+ _ + K+ Transport protein _ + (a) Membrane potential and cation uptake _ + H+ NO3− H+ _ NO3− + _ H+ + H+ H+ H+ H+ H+ NO3− _ NO3− + NO3− _ H+ + NO3− H+ H+ _ H+ + (b) Cotransport of an anion with H+ _ + H+ H+ H+ H+ _ S + _ H+ + H+ H+ S H+ H+ H+ S _ S S + H+ _ + S H+ _ H+ + (c) Cotransport of a neutral solute with H+

  20. Fig. 36-7a _ + CYTOPLASM EXTRACELLULAR FLUID _ + K+ _ + K+ K+ K+ K+ K+ _ + K+ Transport protein _ + (a) Membrane potential and cation uptake

  21. In the mechanism called cotransport, a transport protein couples the diffusion of one solute to the active transport of another

  22. Fig. 36-7b _ + H+ NO3− H+ _ NO3− + _ H+ + H+ H+ H+ H+ H+ NO3− _ NO3− + NO3− _ NO3− H+ + H+ H+ _ H+ + (b) Cotransport of an anion with H+

  23. The “coattail” effect of cotransport is also responsible for the uptake of the sugar sucrose by plant cells

  24. Fig. 36-7c _ + H+ H+ S H+ _ _ H+ + H+ H+ S H+ H+ H+ S _ S S + H+ _ + H+ S _ H+ + (c) Cotransport of a neutral solute with H+

  25. Diffusion of Water (Osmosis) • To survive, plants must balance water uptake and loss • Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure

  26. Water potential is a measurement that combines the effects of solute concentration and pressure • Water potential determines the direction of movement of water • Water flows from regions of higher water potential to regions of lower water potential

  27. Water potential is abbreviated as Ψand measured in units of pressure called megapascals (MPa) • Ψ = 0 MPa for pure water at sea level and room temperature

  28. How Solutes and Pressure Affect Water Potential • Both pressure and solute concentration affect water potential • The solute potential (ΨS) of a solution is proportional to the number of dissolved molecules • Solute potential is also called osmotic potential

  29. Pressure potential (ΨP) is the physical pressure on a solution • Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast

  30. Measuring Water Potential • Consider a U-shaped tube where the two arms are separated by a membrane permeable only to water • Water moves in the direction from higher water potential to lower water potential

  31. Fig. 36-8 (a) (d) (b) (c) Positivepressure Increasedpositivepressure Negativepressure(tension) 0.1 Msolution Purewater H2O H2O H2O H2O ψP = 0ψS= 0 ψP = 0ψS= −0.23 ψP = 0ψS= 0 ψP = 0.23ψS= −0.23 ψP= 0ψS = 0 ψP = 0.30ψS= −0.23 ψP = −0.30ψS= 0 ψP = 0ψS= −0.23 ψ ψ= 0 MPa ψ= 0 MPa ψ= 0.07 MPa ψ= −0.30 MPa ψ= −0.23 MPa ψ= 0 MPa = −0.23 MPa ψ= 0 MPa

  32. Fig. 36-8a (a) 0.1 Msolution Purewater H2O ψP= 0ψS = −0.23 ψP= 0ψS= 0 ψ= −0.23 MPa ψ= 0 MPa

  33. The addition of solutes reduces water potential

  34. Fig. 36-8b (b) Positivepressure H2O ψP= 0.23ψS = −0.23 ψP= 0ψS= 0 ψ= 0 MPa ψ= 0 MPa

  35. Physical pressure increases water potential

  36. Fig. 36-8c (c) Increasedpositivepressure H2O ψP =  ψS = −0.23 ψP= 0ψS= 0 0.30 ψ= 0.07 MPa ψ= 0 MPa

  37. Negative pressure decreases water potential

  38. Fig. 36-8d (d) Negativepressure(tension) H2O ψP =ψS = −0.23 ψP = −0.30ψS = 0 0 ψ= −0.30 MPa ψ= −0.23 MPa

  39. Water potential affects uptake and loss of water by plant cells • If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis Video: Plasmolysis

  40. Fig. 36-9 Initial flaccid cell: ψP = 0ψS = −0.7 0.4 M sucrose solution: Pure water: ψ= −0.7 MPa ψP = 0ψS = 0 ψP = 0ψS = −0.9 ψ= 0 MPa ψ = −0.9 MPa Plasmolyzed cell Turgid cell ψP = 0ψS = −0.7 ψP = 0ψS = −0.9 ψ = −0.9 MPa ψ= 0 MPa (b) Initial conditions: cellular ψ < environmental ψ (a) Initial conditions: cellular ψ > environmental ψ

  41. Fig. 36-9a Initial flaccid cell: ψP= 0ψS = −0.7 0.4 M sucrose solution: ψ= −0.7 MPa ψP= 0 ψS = −0.9 ψ= −0.9 MPa Plasmolyzed cell ψP= 0 ψS = −0.9 ψ= −0.9 MPa (a) Initial conditions: cellular ψ > environmental ψ

  42. Fig. 36-9b Initial flaccid cell: ψP = 0ψS = −0.7 Pure water: ψ= −0.7 MPa ψP = 0ψS = 0 ψ= 0 MPa Turgid cell ψP = 0.7ψS = −0.7 ψ = 0 MPa (b) Initial conditions: cellular ψ < environmental ψ

  43. If the same flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid Video: Turgid Elodea

  44. Turgor loss in plants causes wilting, which can be reversed when the plant is watered

  45. Fig. 36-10

  46. Aquaporins: Facilitating Diffusion of Water • Aquaporins are transport proteins in the cell membrane that allow the passage of water • The rate of water movement is likely regulated by phosphorylation of the aquaporin proteins

  47. Three Major Pathways of Transport • Transport is also regulated by the compartmental structure of plant cells • The plasma membrane directly controls the traffic of molecules into and out of the protoplast • The plasma membrane is a barrier between two major compartments, the cell wall and the cytosol

  48. The third major compartment in most mature plant cells is the vacuole, a large organelle that occupies as much as 90% or more of the protoplast’s volume • The vacuolar membrane regulates transport between the cytosol and the vacuole

  49. In most plant tissues, the cell wall and cytosol are continuous from cell to cell • The cytoplasmic continuum is called the symplast • The cytoplasm of neighboring cells is connected by channels called plasmodesmata • The apoplast is the continuum of cell walls and extracellular spaces

  50. Fig. 36-11 Cell wall Cytosol Vacuole Plasmodesma Vacuolar membrane Plasma membrane (a) Cell compartments Key Apoplast Apoplast Transmembrane route Symplast Apoplast Symplast Symplastic route Apoplastic route (b) Transport routes between cells