Water in Plants

• Brownian Movement


  The result of the bombardment of the visible particles by invisible water molecules, which are in constant motion themselves
• Diffusion


  Movement of molecules or ions from a region of higher concentration to a region of lower concentration
• Diffusion


  1. Moving along a diffusion gradient - moving from a region of higher concentration to a region of lower concentration

  2. Moving against a diffusion gradient - moving from a region of lower concentration to higher concentration

  3. Equilibrium - distributed throughout the space available
• Factors affecting rate of diffusion

  • Pressure
  • Temperature
  • Density of the medium
• Unaided diffusion requires a great deal of time because molecules and ions are infinitesimally small
• Large molecules move much more slowly than small molecules
• In nature, wind and water currents distribute molecules much faster than they ever could be distributed by diffusion alone
• Osmosis


  Diffusion of water through a semipermeable membrane from a region where the water is more concentrated to a region where it is less concentrated
• Solvents


  Liquids in which substances dissolve
• Water (a solvent) moves freely from cell to cell although the cytoplasm of living cells is bounded by membranes
• Plasma, vacuolar, and other membranes have tiny holes or spaces in them, even though such holes or spaces are invisible to the instruments presently available
• Semipermeable membranes


  Membranes through which different substances diffuse at different rates
• Osmosis ceases if the concentration of water on both sides of the membrane becomes equal
• Osmotic Pressure


  The pressure required to prevent osmosis
• Osmotic Potential (ψs)


  The measure of the potential of water to move from one cell to another as influenced by solute concentration
• Turgor Pressure / Pressure Potential (ψp)


  Develops against the walls as a result of water entering the vacuole of the cell
• Water Potential (ψw)


  The cell's osmotic potential and pressure potential combined (ψw = ψs + ψp)
• Osmosis is the primary means by which water enters plants from their surrounding environment
• How water enters plants

  1. Water from the soil enters the cell walls and intercellular spaces of the epidermis and the root hairs

  2. Travels along the walls until it reaches the endodermis

  3. Crosses the differentially permeable membranes and cytoplasm of the endodermal cells on its way to the xylem

  4. Flows from the xylem to the leaves, evaporates within the leaf air spaces, and diffuses out (transpires) through the stomata into the atmosphere
• The movement of water takes place because there is a water potential gradient from relatively high soil water potential to successively lower water potentials in roots, stems, leaves, and the atmosphere
• Plasmolysis


  Loss of water through osmosis, which is accompanied by the shrinkage of protoplasm away from the cell wall
• Plasmolysis


  • Placing turgid carrot and celery sticks in a 10% solution of salt in water, they soon lose their rigidity and become limp enough to curl around your finger
• If plasmolyzed cells are placed in fresh water before permanent damage is done, water reenters the cell by osmosis, and the cells become turgid once more
• Imbibition


  A type of diffusion where the water is absorbed by the solid particles called colloids, causing an enormous increase in volume
• Imbibition is the initial step in the germination of seeds
• The physical forces developed during germination can be tremendous, even to the point of causing a seed to split a rock weighing several tons
• Imbibition causes seeds to expand, which results in the seed coat or testa being broken
• Imbibition aids in the transport of water into the developing ovules
• Active Transport


  Forcing molecules to go against their concentration gradient
• For plants to take up mineral ions, ions are moved into root hairs, where they are in a higher concentration than in the dilute solutions in the soil. Active transport then occurs across the root so that the plant takes in the ions it needs from the soil around it
• Plant cells generally have a larger number of mineral molecules and ions than exist in the soil immediately next to the root hairs
• Transport Proteins


  Special embedded proteins that facilitate the transport of specific ions through the cell membrane
• Proton Pump


  Helps regulate which ions pass through the cell membrane and this is often accompanied by expenditure of energy-rich ATP molecules
• Adenosine Triphosphate (ATP)


  Energy-carrying molecule found in the cells of all living things. ATP captures chemical energy obtained from the breakdown of food molecules and releases it to fuel other cellular processes
• More than 90% of the water entering a plant passes through and evaporates—primarily into leaf air spaces and then through the stomata into the atmosphere with usually less than 5% of the water escaping through the cuticle
• Mature corn plants each transpire about 15 liters (4 gallons) of water per week, while four-tenths of a hectare (1 acre) of corn may transpire more than 1,325,000 liters (350,000 gallons) in a 100-day growing season
• If humans had requirements similar to those of plants, each adult would have to drink well over 38 liters (10 gallons) of water per day
• Water constitutes about 90% of the weight of young cells
• The thousands of enzyme actions and other chemical activities of cells take place in water, and additional, although relatively negligible, amounts are used in the process of photosynthesis
• The exposed surfaces of the mesophyll cells within the leaf have to be moist at all times, for it is through this film of water that the carbon dioxide molecules needed for the process of photosynthesis enter the cell from the air