1. Transport Mechanisms in Plants

2. Transport in vascular plants occurs on three scales
Physical forces drive the transport of materials in plants over a range of distances
Transport in vascular plants occurs on three scales
Transport of water and solutes by individual cells, such as root hairs
Short-distance transport of substances from cell to cell at the levels of tissues and organs

3. Vascular tissue transports nutrients throughout a plant; such transport may occur over long distances

4. Roots take in water and minerals where they are transported through the stem to the leaves where photosynthesis occurs

5. During primary growth three zones develop in the growing root
Root Structure and
During primary growth three zones develop in the growing root
Zone of cell division - contains apical meristem, forms new undifferentiated cells (M)
Zone of elongation - cells enlarging in size (G1), becoming specialized

6. Organization of tissues in young roots occurs in the zone of maturation

7. Differentiation results in:
Epidermis which contains root hairs
Cortex
Endodermis separates the cortex and vascular cylinder, contains casparian strip
Pericyle first layer within vascular cylinder
Xylem and phloem

8. Lateral roots
Arise from within the pericycle, the outermost cell layer in the vascular cylinder

9. Water-conducting cells
Have rigid, lignin-containing secondary cell walls
Dead at maturity, only cell walls remain
Form system of tubes that convey water from roots to stem or leaves
Tracheids are long with tapered ends
Vessel elements are wider, shorter, less tapered
Make up xylem tissue

10. Food-conducting cells
Sieve tube members
Arranged end to end forming tubes
Have thin primary walls and no secondary walls
Alive at maturity
Ends form perforated sieve plates that allow plasmodesmata to pass sugars and ions between cells
Flanked by companion cells that make proteins for sieve tube members (lose nuclei, ribosomes, other organelles during development

11. Water-conducting cells of the xylem and sugar-conducting cells of the phloem

12. Water and minerals transported from soil into water-conducting xylem tissue must pass through epidermis and cortex. They must pass at some time through the plasma membranes of some root cells. Because plasma membranes are selectively permeable, only certain solutes can enter the xylem

13. The Roles of Root Hairs and Cortical Cells
Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where specialized epidermal structures called root hairs are located
Root hairs account for much of the surface area of roots and allow for absorption of enough water and inorganic ions to survive and grow
Water enters a plant through root hairs by osmosis because they have a higher solute concentration and lower water concentration than the surrounding soil

14. Substances must be in solution to enter roots
Plants absorb minerals in ionic form in solution (nitrogen = N03-, NH4+; phosphorus = HPO4-, H2PO4-; sulfur = SO42-; magnesium = Mg2+; potassium = K+; calcium = Ca2+; iron = Fe2+, Fe3+)

15. Most plants form mutually beneficial relationships with fungi, which facilitate the absorption of water and minerals from the soil

16. Mycorrhizae and Plant Nutrition
The fungus benefits from a steady supply of sugar donated by the host plant, and may help protect against certain pathogens in the soil
In return, the fungus increases the surface area of water uptake and mineral absorption and supplies water and minerals (especially phosphorus) to the host plant

17. Active transport of mineral ions
Occurs if ions are at higher concentrations inside the root, or if they are unable to cross the lipid bilayer of the membrane
Requires specific transport proteins
Proton pumps use ATP to pump hydrogen ions out of the cell and create a positive charge outside the cell and a negative charge inside the cell (membrane potential), as well as a strong concentration gradient of hydrogen ions from outside to inside the cell

18. There are two pathways through which water can move into the xylem tissue of roots
1. SYMPLASTIC Pathway - water and selected solutes cross cell wall and plasma membrane of an epidermal cell. Cells of root are interconnected by plasmodesmata, so cytoplasm is continuous through root cells
Solution moves from cell to cell through plasmodesmata without further crossing cell membranes of other cells, until it reaches the endodermis

19. 2. APOPLASTIC Pathway - solution moves inward along cell walls and intracellular spaces in cortex layer
Solution does not enter cytoplasm of epidermis or cortex cells
The solution does not cross the plasma membrane, so there is no selection of solutes until they reach endodermis

20. Water traveling by apoplastic route enters xylem only by crossing plasma membrane of endodermal cells, this means that selection of ions occurs at the endodermal cell membrane
Water and ions traveling by symplastic route are selected at epidermal cells, and travel through endodermal cells
Endodermal cells then discharge solution into xylem

21. The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder

22. The Endodermis: A Selective Sentry
Surrounds the vascular cylinder (stele) and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue

23. Plants lose an enormous amount of water (more than 90%) through transpiration, the loss of water vapor from leaves and other aerial parts of the plant
The transpired water must be replaced by water transported up from the roots

24. Factors Affecting the Ascent of Xylem Sap
Xylem sap is the solution of inorganic ions that flows from plant’s roots to tips of leaves through xylem

25. Pushing Xylem Sap: Root Pressure
At night, when transpiration is very low, root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential
Water flows in from the root cortex generating root pressure

26. Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous dicots

27. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism
Transpiration, the loss of water from the leaves and aerial parts of plants, supplies the pulling force

28. Water vapor in the airspaces of a leaf
Transpirational Pull
Water vapor in the airspaces of a leaf
Diffuses down its water potential gradient and exits the leaf via stomata

29. Transpiration produces negative pressure (tension) in the leaf

30. Cohesion and Adhesion in the Ascent of Xylem Sap
The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution
Transpiration is facilitated by cohesion and adhesion
Cohesion - the sticking together of water molecules due to hydrogen bonding between water molecules. This allows water to form continuous strings or columns of water molecules from roots to leaves

31. As transpiration occurs in leaves, water molecules break off the ends of the vertical string of water molecules. Cohesion puts tension in string of water molecules, and as one molecule exits the leaf, another is tugged into place right behind it
This is called the transpiration-cohesion-tension mechanism

32. Xylem Sap Ascent by Bulk Flow: A Review
The movement of xylem sap against gravity
Is maintained by the transpiration-cohesion-tension mechanism

33. Environmental factors affecting transpiration
Humidity - decreased humidity increases transpiration
Wind increases transpiration by carrying water away
Temperature - increases transpiration by causing more water to evaporate
Soil water - dry soil decreases transpiration because stomata close as a result of loss in turgor
Carbon dioxide - high levels cause stomata to close decreasing transpiration

34. Stomata help regulate the rate of transpiration
Leaves generally have broad surface areas

35. Both of these characteristics
Increase photosynthesis
Increase water loss through stomata

36. Effects of Transpiration on Wilting and Leaf Temperature
Plants lose a large amount of water by transpiration
If the lost water is not replaced by absorption through the roots

37. Stomata: Major Pathways for Water Loss
This means that 90% of the water taken in by a plant is lost to transpiration rather than being used in photosynthesis or growth
Even though plants lose large amounts of water through transpiration, this is necessary to move xylem sap upward through plant

38. Each stoma is flanked by guard cells
Which control the diameter of the stoma by changing shape

39. Changes in turgor pressure that open and close stomata

40. Blue light triggers proton pumps which causes active transport of potassium ions into the cell
Water moves in, but because of uneven thickening in the cell walls of the guard cells, the outer edge (thinner) elongates more than the inner edge (thicker), causing the cell to bend which opens the stoma

41. The hormone abscisic acid causes the rapid diffusion of potassium ions out of the cell, and water follows passively causing the stoma to close

42. Xerophyte Adaptations That Reduce Transpiration
Xerophytes
Are plants adapted to arid climates
Have various leaf modifications that reduce the rate of transpiration:
Small thick leaves that decrease surface area to reduce water loss
Reduced number of stomata

43. Xerophyte Adaptations That Reduce Transpiration
The stomata of xerophytes are concentrated on the lower leaf surface and are often located in depressions that shelter the pores from the dry wind and increase humidity near stomata

44. Xerophyte Adaptations That Reduce Transpiration
Hair-like trichomes on surface trap layer of water vapor

45. Xerophyte Adaptations That Reduce Transpiration
They may drop leaves during driest months and become dormant
Use C4 or CAM metabolism

46. Organic nutrients are translocated through the phloem
The main function of phloem is to transport food (sugar) molecules made during photosynthesis
Phloem sap moves freely from one cell to the next through perforations in the sieve plates of sieve-tube members

47. Sugar Transport in Phloem
Phloem sap may contain amino acids, hormones, and mRNA but mostly the disaccharide sucrose

48. Movement from Sugar Sources to Sugar Sinks
Phloem sap travels from a sugar source to a sugar sink, regardless of direction

49. A sugar source A sugar sink
Is a plant organ that is a net producer of sugar, such as mature leaves
A sugar sink

50. Sugar must be loaded into sieve-tube members at sources before being exposed to sinks

51. Proton pumping and cotransport of sucrose and H+
In many plants
Phloem loading requires active transport
Proton pumping and cotransport of sucrose and H+

52. Pressure Flow: The Mechanism of Translocation in Angiosperms
In studying angiosperms researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure

53. Pressure-Flow Mechanism
Sugar is loaded into phloem at source by active transport. This raises solute concentration inside phloem tube
High solute concentration causes water to move in by osmosis. Inward flow of water raises water pressure at source end of tube
At sink, sugar is actively transported out of the phloem tube

54. The building of water pressure at source end, and reduction of pressure at the sink end cause water to flow from source to sink, or from high pressure to low pressure

55. The pressure flow hypothesis explains why phloem sap always flows from source to sink
Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms

56. Xylem sap according to transpiration-cohesion-tension mechanism is pulled upward through xylem
Phloem sap according to pressure-flow mechanism is pushed from source to sink through phloem