Water in Plants

1. Water in Plants Chapter 9

2. Outline • Molecular Movement • Water and Its Movement Through the Plant • Regulation of Transpiration • Transport of Food Substances (Organic Solutes) in Solution • Mineral Requirements for Growth

3. Molecular Movement • Diffusion - movement of molecules from region of higher concentration to region of lower concentration • Molecules move along concentration gradient • State of Equilibrium - molecules distributed throughout available space • Rate of diffusion depends on pressure, temperature and density of medium

4. Molecular Movement • Solvent - liquid in which substances dissolve • Semipermeable Membranes - membranes in which different substances diffuse at different rates • All plant cell membranes • Osmosis - diffusion of H2O through semipermeable membrane from region where H2O more concentrated to region where less concentrated

5. Molecular MovementOsmosis • Osmotic Pressure - pressure required to prevent osmosis • Osmotic potential balanced by resistance of cell wall • Pressure Potential (Turgor Pressure) - pressure that develops against walls as result of H2Oentering cell • Turgid Cell - firm cell due to H2Ogained by osmosis • H2Opotential of cell = osmotic pressure + pressure potential Turgid cell • H2Omoves from cell with higher H2Opotential to cell with lower H2O potential

6. Molecular MovementOsmosis • Osmosis main way H2Oenters plants from environment

7. Molecular Movement • Plasmolysis - loss of H2Othrough osmosis • Accompanied by shrinkage of protoplasm away from cell wall Normal cells versus plasmolyzed cells

8. Molecular Movement • Imbibition • Large molecules (i.e., cellulose and starch) develop electrical charges when wet, and attract H2O molecules • H2Omolecules adhere to large molecules • Results in swelling of tissues • Imbibition 1ststep in germination of seed Seeds before and after imbibition

9. Molecular Movement • Active Transport - process used to absorb and retain solutes against diffusion, or electrical, gradient by expenditure of energy • Involves proton pump (enzyme complex in plasma membrane energized by ATP molecules) • Transport proteins - facilitate transfer of solutes to outside and to inside of cell

10. Water and Its Movement Through the Plant • Transpiration - H2Ovapor loss from internal leaf atmosphere • >90% of H2O entering plant transpired • H2Oneeded for: • Cell activities • Cell turgor • Evaporation for cooling • If more H2O lost then taken in, stomata close

11. Water and Its Movement Through the Plant • Cohesion-Tension Theory - transpiration generates tension to pull H2Ocolumns through plants from roots to leaves • H2Ocolumns created when H2Omolecules adhere to tracheids and vessels of xylem and cohere to each other

12. Water and Its Movement Through the Plant • Cohesion-Tension Theory • When H2Oevaporates from mesophyll cells, they develop lower H2Opotential than adjacent cells • H2Omoves into mesophyll cells from adjacent cells with higher H2O potential • Process continued until veins reached • Creates tension on H2Ocolumns, drawing H2Oall way through entire span of xylem cells • H2Ocontinues to enter root by osmosis

13. Regulation of Transpiration • Stomatal apparatus regulates transpiration and gas exchange • StomatalApparatus - 2 guard cells + stoma (opening) • Transpiration rates influenced by humidity, light, temperature, and CO2 concentration

14. Regulation of Transpiration • When photosynthesis occurs, stomata open • Guard cells use energy to acquire K+ from adjacent epidermal cells • Causes lower H2Opotential in guard cells • H2Oenters guard cells via osmosis • Guard cells become turgid and stomata opens

15. Regulation of Transpiration • When photosynthesis does not occur, stomata close • K+ leave guard cells • H2O follows • Guard cells become less turgid and stomata close

16. Regulation of Transpiration • Stomata of most plants open during day/closed at night • H2Oconservation in some plants: • Stomata open only at night - Desert plants • Conserves H2O, but makes CO2inaccessible during day • Undergo CAM Photosynthesis • CO2converted to organic acids and stored in vacuoles at night • Organic acids converted to CO2during day • Stomata recessed below surface of leaf or in chambers • Desert plants, pines

17. Regulation of Transpiration • Guttation - loss of liquid H2O • If cool night follows warm, humid day, H2Odroplets produced through hydathodes at tips of veins • In absence of transpiration at night, pressure in xylem elements forces H2Oout of hydathodes Guttation in barley plants

18. Transport of Organic Solutes in Solution • Important function of H2Ois translocation of food substances in solution by phloem • Pressure-Flow Hypothesis - organic solutes flow from source, where H2Oenters by osmosis, to sinks, where food utilized and H2O exits • Organic solutes move along concentration gradients between sources and sinks

19. Transport of Organic Solutes in Solution • Specifics of Pressure-Flow Hypothesis: • Phloem Loading - sugar enters by active transport into sieve tubes • H2Opotential of sieve tubes decreases and H2Oenters by osmosis • Turgor pressure develops and drives fluid through sieve tubes toward sinks • Food substances actively removed at sink and H2Oexits sieve tubes, lowering pressure in sieve tubes • Mass flow occurs from higher pressure at source to lower pressure at sink • H2Odiffuses back into xylem

20. Mineral Requirements for Growth • Essential Elements - essential building blocks for compounds synthesized by plants

21. Mineral Requirements for Growth • Macronutrients - used by plants in greater amounts • N, K, Ca, P, Mg, and S • Micronutrients - needed by plants in very small amounts • Fe, Na, Cl, Cu, Mn, Co, Zn, Mo, and B • When any required element deficient in soil, plants exhibit characteristic symptoms

22. Review • Molecular Movement • Water and Its Movement Through the Plant • Regulation of Transpiration • Transport of Food Substances (Organic Solutes) in Solution • Mineral Requirements for Growth