Water In Plants

Outline Molecular Movement  Diffusion  Osmosis Water Movement  Cohesion-Tension Theory  Regulation of Transpiration  Transport of Organic Solutes  Pressure-Flow Hypothesis Mineral Requirements for Growth

Molecular Movement Diffusion  Movement of molecules from a region of higher concentration to a region of lower concentration. - Move along a concentration gradient. - Move until equilibrium reached.

Diffusion

Osmosis Osmosis is diffusion of water through a differentially permeable membrane from a region where the water is more concentrated to a region where it is less concentrated.  Water enters a cell by osmosis until the osmotic potential is balanced by the resistance to expansion of the cell wall. - Turgor Pressure  Pressure Potential

Osmosis Water Potential of a plant is essentially its osmotic potential and pressure potential combined. Water flows from the xylem to the leaves, evaporates within the leaf air spaces, and transpires through the stomata into the atmosphere.

Osmosis

Molecular Movement Plasmolysis  Loss of water through osmosis is accompanied by shrinkage of protoplasm away from the cell wall. Imbition  Colloidal material and large molecules usually develop electrical charges when they are wet, and thus attract water molecules.

Molecular Movement Active Transport  Plants absorb and retain solutes against a diffusion, or electrical, gradient through the expenditure of energy. - Involves proton pump.

Water and Its Movement Through The Plant More than 90% of the water entering a plant passes into leaf air spaces and then evaporates through the stomata into the atmosphere (Transpiration).  Usually less than 5% of water escapes through the cuticle. Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display

Cohesion - Tension Theory When the negatively charged end of one water molecule comes close to the positively charged end of another water molecule, weak hydrogen bonds hold the molecules together.  Water molecules adhering to capillary walls, and each other, create a certain amount of tension.

Cohesion - Tension Theory When water transpires, the cells involved develop a lower water potential than the adjacent cells.  Creates tension on water columns, drawing water from one molecule to another, throughout the entire span of xylem cells.

Regulation of Transpiration Changes in turgor pressure occur when osmosis and active transport between the guard cells and other epidermal cells cause shifts in solute concentrations.  When photosynthesis is not occurring in the guard cells, potassium ions leave, and the stomata close. - An increase in potassium ions causes a lowering of the water potential and osmosis leading to turgid guard cells.

Regulation of Transpiration Stomata of most plants are open during the day and closed at night.  Stomata of many desert plants open only at night. - Conserves water, but makes carbon dioxide inaccessible during the day. Humidity plays an inverse role in transpiration rates.  High humidity reduces transpiration, while low humidity accelerates it.

Regulation of Transpiration If a cool night follows a warm, humid day, water droplets may be produced through hydathodes at the tips of veins of some plants (Guttation). Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display

Transport of Organic Solutes in Solution One of most important functions of water in the plant involves the translocation of food substances in solution by the phloem.  Most of our knowledge on this subject came from studying aphids feeding on phloem.

Pressure-Flow Hypothesis Organic solutes flow from a source where water enters by osmosis.  Organic solutes are moved along concentration gradients between sources and sinks.

Mineral Requirements for Growth Essential Elements

Macronutrients and Micronutrients Macronutrients are used by plants in greater amounts.  Nitrogen, potassium, calcium, phosphorus, magnesium, and sulfur. Micronutrients are needed by the plants in very small amounts.

Review Molecular Movement  Diffusion  Osmosis Water Movement  Cohesion-Tension Theory  Regulation of Transpiration  Transport of Organic Solutes  Pressure-Flow Hypothesis Mineral Requirements for Growth

Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display