1. The leaf is the main photosynthetic factory (Fig. 36.1, p. 702)

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1 TRANSPORT IN PLANTS A. Introduction 1. The leaf is the main photosynthetic factory (Fig. 36.1, p. 702) a. This requires a transport system to move water and minerals from the roots to the leaf. This is accomplished by the xylem. b. Carbohydrates are produced in the leaf and must be moved to the rest of the plant. This is accomplished by the phloem. 1) In general then carbohydrates, in the form of sucrose, move downward in the phloem. 2) Can you think of an exception to this? Yes, when moves in the of the year. c. Carbon dioxide enters the leaf by small pores, stomates. 2. Symplast is a continuum of cytoplasm from cell to cell by cytoplasmic strands called plasmodesmata. (fig. 36.4, p. 705) 3. Apoplast is a continuum of matrix, cell wall (fig.36.4, p. 705). a. Solutions move by crossing plasma membranes b. Solutions move via symplast. c. Solution move via apoplast 4. ATP (adenosine triphosphate) is the energy source for pumping inorganic ions and sugar during active transport. 5. H 2 O always moves by passive transport. B. Vascular tissue 1. Both xylem and phloem are complex tissues. This means that they are composed of more than one type of cell. 2. Xylem (fig , p. 689 and fig. 35.9, p. 688) is composed of: a. xylem vessels b. tracheids c. parenchyma cells d. fibers

2 3. Phloem (fig , p. 689 and fig , p. 690) is composed of a. sieve tube cells b. companion cells c. parenchyma cells C. Movement of water and minerals 1. Pathway is from soil into root hairs, which are extensions of epidermal cells, through root tissues into the xylem (figure 1 from Raven and Johnson; fig 36.4, p. 705 and fig.; 36.8, p. 707). Figure 1. Pathway of minerals from soil to xylem ( Fig. 39.9, p. 784 from Raven and Johnson, Biology, 6 th. ed.) 2. Minerals are actively absorbed; water follows passively. 3. Water and minerals can move through both symplast and apoplast. 4. Route root hair, cortex, endodermis, apoplast transport is stopped by Casparian strip (waxy) and material must be transferred to symplast. (figs 36.4 and 36.5, p. 705) a. This assures that any mineral entering the stele must pass through selectively permeable membranes. b. Once in the endodermal cells or parenchyma cells within the stele, the solution is transferred to the walls of these cells, apoplast, which are in direct connection with the xylem vessels. 4. Root pressure a. It is due to accumulation of solutes by active transport that lowers the water pressure. b. Water flows in and forces fluid up the xylem. 6. Transpiration a. It depends upon evaporation and cohesion. b. The route is the same as through the root, then xylem of stem and leaf, mesophyll cells, intercellular space, stomate, and atmosphere. c. Figure 36.8, p. 707 illustrates the generation of transpirational pull

3 in a leaf. d. Figure 2 shows an overview of transport in plants; the water pressure is given at different locations in the plant. 1) Pure water has a high water pressure of 0 MPa (megapascal), one MPa is equal to 10 bars, a bar is about the same as one atm. of pressure. 2) Water pressure gets lower from soil to air. Figure 2. An overview of transport in plants with water pressure indicated at various locations along the water route. D. Transport of Sugar Translocation 1. This occurs in the phloem. 2.Phloem loading is the pumping of sucrose from mesophyll cells into companion cells and the sieve tubes. (fig , p. 712) a. ATP energy is used for the transport into companion cells. b. The sucrose is concentrate in the phloem. c. Usually passes by way of the symplast. 3. Movement of sucrose and water in sieve tubes. a. As sucrose is pumped into sieve tubes, water follows which creates a hydrostatic pressure which forces solution, sap, to flow along the sieve tube. b. Sucrose and water flow to lower concentration in the root. c. Sucrose is actively unloaded which reduces the concentration of sucrose in lower tissues. Sucrose is used, stored as sucrose or converted to starch and stored. (fig.36.14, p. 712) D Opening and closing of stomates (fig , p. 709) 1. Guard cells surrounding the stomate regulate the opening and closing of the stomate. 2. Potassium ions play an important role. 3. ATP powered ion transport channels causes potassium ions to enter the guard cells from surrounding epidermal cells

4 ` 4. Water follows the ions by osmosis and results in the guard cells becoming turgid. a. The thick walls on the inside result in the cell bulging outward causing the thick inner wall to bulge inward. b. This results in the stomate opening. 5. When the potassium ions leave the guard cells, the stomate closes. 6. The guard cells possessing chloroplasts provides them with an alternate source of ATP to power the pump. DISCUSSION QUESTIONS OVER TRANSPORT IN PLANTS 1. Is movement of solutions faster or slower in the xylem than in the phloem? Explain. 2. What does it mean when it is said that both xylem and phloem are complex tissues? 3. How is the absorptive area of the root increased? 4. Trace a molecule of water from the soil to the leaf. 5. What are the two forces that contribute to transpiration? 6. In general what happens to water pressure as it, the water, passes from the soil to the leaf of the plant? Explain the significance of this change. 7. Explain the mechanism by which the stomates are opened and closed. 8. Compare transport of carbohydrate molecules in animals and plants. 9. What is the role of companion cells in the transport of carbohydrate? CRITICAL THINKING QUESTIONS 1. It is said that water is not moved by active transport. Yet, water enters the root hairs. How is this accomplished? 2. Wilting is referred to as a protective mechanism for plants. Explain how this could be.

5 OBJECTIVE QUESTIONS 1. Water moving across root can move in the (A) symplast (B) apoplast (C) both A and B (D) neither A or B. 2. In general water and minerals move (A) upward in the xylem (B) upward in the phloem (C) downward in the xylem (D) downward in the phloem. 3. Carbohydrates are moved in plants in the form of a (A) monosaccharide (B) disaccharide (C) polysaccharide. 4. Movement of water is always by (A) active transport (B) passive transport. 5. In deciduous plants, one always finds (A) xylem vessels (B) Tracheids (C) both A and B. 6. In the epidermal cells of the roots, ATP is used to absorb (A) minerals (B) water (C) both A and B (D) neither A or B. 7. The Casparian strip is found in the walls of the (A) epidermal cells (B) cortex cells (C) endodermis cells. 8. Root pressure is the result of (A) evaporation (B) active absorption of minerals (C) cohesive force of water. 9. Transpiration is largely the result of (A) evaporation (B) cohesive attraction of water molecules (C) both A and B (D) neither A or B. 10. (A) Sucrose loading (B) Sucrose unloading (C) both A and B (D) neither A or B requires the expenditure of energy, ATP.