Objectives 9.2.10 & 9.2.11

1. Objectives 9.2.10 & 9.2.11 Asia Bowman Shantel Butler William Middleton Destinee Miguest Shanese Stuckey IB Biochemical Biology P.2

2. 9.2.10 • Outline four adaptations of xerophytes that help to reduce transpiration.

3. What are Xerophytes?! • Plants that are adapted to arid climates. • Xerophytes have evolutionary adaptations that reduce transpiration • These plants have various leaf modifications that reduce the rate of transpiration

4. Adaptations: • Xerophytes have small, thick leaves • This limits water loss by reducing surface area relative to leaf volume. • Stomata are recessed in “crypts” • protects the stomata from hot, dry wind • Trichomes • Breaks up the flow of air, allowing the chamber of the crypt to have a higher humidity than the surrounding atmosphere.

5. Adaptations: 4. One of the most elegant adaptations to arid habitats is found in ice plants and other succulent plants of the family Crassulaceae and in representatives of many other plant families. • CO2 is assimilated by an alternative photosynthetic pathway known as CAM, for crassulacean acid metabolism. • Because the leaf takes in its CO2 at night, the stomata can close during the day, when transpiration is most severe.

7. 9.2.11 • Outline the roles of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots).

8. Translocation of Phloem Sap • Xylem sap flows in a direction that generally does not allow it to function in exporting sugar from leaves to other parts of the plant… on the other hand • The prevalent solute in phloem sap is sugar, primarily the disaccharide sucrose in most plant species. • The phloem transports the organic products of photosynthesis throughout the plant. • Phloem sap may also contain minerals, amino acids, and hormones in transit from one part of the plant to another.

9. Phloem • Translocation: the transport via phloem of food in a plant. • sugar source: a plant organ in which sugar is being produced by either photosynthesis or the break down of starch. • sugar sink: an organ that is a net consumer or storer of sugar. • Examples: • Mature leaves = primary sugar sources • Sugar sink supplied by phloem = growing roots, shoot tips, stems, and fruit

10. Phloem • Storage organ depends on the season. • Example: • Sugar sink during the summer • In the spring the storage organ becomes a source as its starch is broken down to sugar which is carried away in the phloem to the growing buds of the shoot system. • Other solutes may be transported to sinks along with sugar. • Example: Minerals that reach leaves in xylem may later be transferred in the phloem to developing fruit.

11. Loading of sucrose into phloem

12. Loading • Sugar from the mesophyll cells of a leaf and other sources must be loaded into sieve-tube members before it can be exported to sugar sinks. • From mesophyll cells to sieve-tube members through symplast, passing from cell to cell through plasmodesmata • Sucrose reaches sieve-tube members through a mixture of symplastic and apoplastic paths • Sample path: • Sucrose diffuses through cytoplasm (symplast) from mesophyll cells into small veins • Much of the sugar moves from the cells into the walls (apoplast) of the companion cells near sieve-tube members • Sieve-tube member- a chain of living cells that form sieve-tubes in phloem

13. Companion cells pass the sugar they accumulate into the sieve-tube member through plasmodesmata linking the cells • Plasmodesmata- an open channel in the cell wall of plants through which cytosol connect adjacent cells • Transfer cells- companion cells that have been modified with numerous ingrowths on their walls to increase the cells’ surface area and enhance the transfer of solutes between apoplast and symplast • Sieve-tube members accumulate sucrose to concentrations two to three times higher than concentrations in mesophyll • Phloem loading requires active transport

14. Unloading • At the sink end of a sieve tube, phloem unloads the sucrose • The concentration of free sugar in the sink is lower than that in the sieve tube because the unloaded sugar is consumed during growth of the sink cells or is converted to starch

15. TRIVIA!!! 1. Imagine cutting a live twig from a tree and examining the cut surface of the twig with magnifying glass. You locate the vascular tissue and observe a growing droplet of fluid exuding from the cut surface. The fluid is most likely: a. Phloem sap b. Xylem sap c. Guttation fluid d. Fluid of the transportation stream

16. Answer: A . Phloem sap

17. TRIVIA!!! 2. The movement of sap from a sugar source to a sugar sink a. occurs through the apoplast of sieve-tube members. b. May translocate sugars from the breakdown of stored starch in a root up to developing shoots c. Is similar to the flow of xylem sap in depending on tension, or negative pressure. d. Depends on the active pumping of water into sieve tubes at the source end

18. Answer B: May translocate sugars from the breakdown of stored starch in a root up to developing shoots

19. TRIVIA!!! 3. What cells mediate the photosynthesis-transpiration compromise? • Answer: guard cells

20. TRIVIA!!! 4. ______________ have evolutionary adaptations that reduce transpiration. • Answer: xerophytes

21. TRIVIA!!! • The prevalent solute in phloem sap is _______, primarily the disaccharide sucrose in most plant species. • Answer: sugar