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Water in Plants. Chapter 9. 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. Molecular Movement.
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Water in Plants Chapter 9
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
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
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
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
Molecular MovementOsmosis • Osmosis main way H2Oenters plants from environment
Molecular Movement • Plasmolysis - loss of H2Othrough osmosis • Accompanied by shrinkage of protoplasm away from cell wall Normal cells versus plasmolyzed cells
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
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
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
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
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
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
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
Regulation of Transpiration • When photosynthesis does not occur, stomata close • K+ leave guard cells • H2O follows • Guard cells become less turgid and stomata close
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
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
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
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
Mineral Requirements for Growth • Essential Elements - essential building blocks for compounds synthesized by plants
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
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