Module 7: Transport in Plants

Molecular Movement

  • Diffusion- movement of molecules along a concentration gradient

  • State of equilibrium- when molecules are evenly distributed throughout available space

  • Rate of diffusion depends on pressure, temperature and density of medium

  • Solvent- liquid in which substances are dissolved

  • Solute- what is being dissolved in a solvent

  • Semipermeable membranes- selectively allows molecules to pass through

  • Osmosis- diffusion of water

  • Imbibition- when water molecules adhere to internal surfaces of large molecules resulting in swelling of tissues

    • imbibition is the first step in germination of seed

  • Active Transport- movement of solutes against concentration gradient using energy

    • involves a proton pump- enzyme complex in which plasma membrane energized by ATP molecules

  • Proton sucrose symporter- moves thing in same direction

  • Transport proteins- facilitate transfer of solutes to outside and to inside of cell

Molecular Movement- Osmosis

  • Water potential (Ψw) is responsible for water movement from one region to another

  • Water potential is influenced by osmotic potential and pressure potential

  • Osmotic potential (Ψs)- tendency of water to move in response to differences in solute concentration

  • Osmotic potential determined by solute concentration relative to that of pure water (which is zero)

    • osmotic potential of any solution can only be zero or negative

  • Pressure Potential (Ψp) - tendency of water to move in response to physical pressure

  • Pressure potential is influenced by turgor pressure and wall pressure

    • Turgor pressure- pressure against the cell wall due to water entering the cell

    • Wall pressure- pressure exerted by walls of a cell when it is turgid (firm)

    In summary; Ψw= Ψp + Ψs megapascals (Mpa)

  • Plasmolysis- loss of water through osmosis

    • shrinks protoplasm away from the cell wall

Water and its Movement Through the Plant

  • Osmosis is the primary way water enters plants from environment

  • Leaves have low water potential

  • Soil has high water potential

  • Pathway of water through plant:

    • water enters from soil into cell walls and intercellular spaces of root hairs and roots→ crosses permeable membrane of cortex cells and cytoplasm of endodermis into xylem → flows through xylem to leaves and diffuses out through stomata

    • this is an upward direction, from soil to leaves

  • Transpiration- process in which water vapor is lost from the internal leaf atmosphere

    • more than 90% of the water entering a plant is transpired

  • Why plants need water:

    • physiological activities

    • cell turgor- maintains rigidity of cells

    • cooling- through transpiration

  • 3 hypotheses explain how water moves from root to shoot:

    1. Root pressure = only accounts for short distance water movement

    2. Capillary action = only moves water in short distances

    3. Cohesion-Tension = moves water in long distances

  • Roots have different tissue layers (from outside in):

    • the epidermis and the root hairs

    • the cortex

    • the endodermis

    • the pericycle

    • the vascular tissues

  • Water travels through the root cortex toward the vascular tissues via one of the three pathways:

    1. Transmembrane pathway- water molecules move through water channels aka aquaporins

    2. Apoplastic pathway- water molecules move through porous cell walls

    3. Symplastic pathway- water molecules move through plasmodesmata

Regulation of Transpiration

  • Stomata regulate transpiration and gas exchange

  • Stomata includes two guard cells and stoma (opening)

  • Transpiration rates influenced by humidity, light, temperature, and carbon dioxide concentration

  • Light causes stomata to open:

    1. Protons pumped out of guard cell

    2. K+ and Cl- enter

    3. Water enters

    4. Cells swell, pore opens

  • In absence of photosynthesis or when there is less water for the plant, stomata close.

    • several processes triggered by ABA (abscisic acid) lead to closre of stomata

  • ABA binds to receptors on guard cells

    1. Proton pumping stops

    2. K+ exits

    3. Water exits

    4. Cells shrinks, pore closes

Plant adaptations that Minimizes Water Loss

  • Stomata open only at night - desert plants

    • conserves water, but makes carbon dioxide inaccessible during day

    • these plants undergo CAM photosynthesis

      • carbon dioxide converted to organic acids and stored in vacuoles at night

      • organic acids converted to carbon dioxide during day

  • Stomata located below surface of leaf or in pits

    • e.g., desert plants, pines

Transport of Organic Solutes in Solution

  • occurs in the phloem

  • Pressure-Flow Hypothesis = explains the movement of organic solutes from source to sink

    • a source is where food enters the phloem

    • a sink is where food exits the phloem, or where food is utilized

  • Organic solutes move along concentration gradient from source to sink

  • Model of Pressure-Flow Hypothesis:

    • Phloem loading- sugar enters by active transport into sieve tube

    • water potential of sieve tubes decreased and water enters by osmosis

    • turgor pressure develops and drives fluid through sieve tubes toward sinks

    • bulk flow occurs from higher pressure at source to lower pressure at sink

    • Phloem unloading- sugar is actively removed at sink and water exits sieve tubes, lowering pressure in sieve tubes

    • water diffuses back into xylem