Transport in plants (xylem, phloem, transpiration) Knowledge-Based Study

Study notes

1. The Need for Specialized Transport Systems

   Unlike small unicellular or non-vascular plants like mosses, large vascular plants have a relatively small surface area-to-volume ratio. Consequently, simple diffusion over long distances is far too slow to supply essential nutrients and water to all cells.

   To overcome this limitation, vascular plants have evolved two specialized long-distance transport tissues collectively known as vascular tissue:

   • Xylem: Responsible for conducting water and dissolved inorganic mineral ions from roots upward to stems and leaves.
   • Phloem: Responsible for transporting organic nutrients, particularly sucrose and amino acids, from sites of synthesis or storage to sites of growth or utilization.

   Together, xylem and phloem form discrete vascular bundles arranged strategically throughout roots, stems, and leaves.

   Structure of Xylem Tissue

   Mature xylem is primarily a non-living conducting tissue. Its primary functional conducting cells are vessel elements (predominant in angiosperms) and tracheids (present in all vascular plants).

   • Hollow, Non-Living Tubes: At functional maturity, these cells lose their cytoplasm, nucleus, and end walls, forming an uninterrupted continuous column through which water can flow with minimal resistance.
   • Lignified Cell Walls: The cell walls are heavily impregnated with lignin, a tough polymer that provides immense tensile strength and prevents vessels from collapsing inward under the negative pressure generated by transpiration.
   • Pits: Unlignified thin areas in lateral walls called pits allow lateral water movement between adjacent vessels.

   Transport in the xylem is strictly unidirectional, moving exclusively upward from roots to shoots.

   Mechanism of Water Ascent

   Water movement in the xylem is driven primarily by transpiration, which is the loss of water vapor from plant leaves via evaporation through stomata.

   The ascent of sap occurs through the cohesion-tension theory:

   1. Evaporation at Stomata: Water evaporates from the moist mesophyll cell surfaces into substomatal air spaces and diffuses out through open stomata into the atmosphere.
   2. Tension Generation: Loss of water lowers the water potential of mesophyll cells, drawing water out of leaf xylem vessels and creating negative hydrostatic pressure (tension).
   3. Cohesion of Water Molecules: Hydrogen bonding between individual water molecules produces strong cohesion, holding the water column together as an unbroken continuum from roots to leaves.
   4. Adhesion to Cell Walls: Hydrogen bonding between water molecules and hydrophilic xylem wall components (adhesion) prevents the column from breaking under gravity.

   Regulating Transpiration Rates

   Because stomatal aperture governs gas exchange for photosynthesis, plants continually balance carbon dioxide intake with water conservation. Several environmental variables significantly impact transpiration rates:

   • Light Intensity: Higher light stimulates stomatal opening to allow carbon dioxide intake for photosynthesis, significantly increasing the transpiration rate.
   • Temperature: Increased temperature elevates the kinetic energy of water molecules, accelerating evaporation and increasing the capacity of air to hold water vapor.
   • Humidity: Higher atmospheric humidity flattens the water potential gradient between internal leaf air spaces and the surrounding atmosphere, reducing the rate of transpiration.
   • Wind / Air Movement: Moving air sweeps away the humid boundary layer of water vapor surrounding the leaf surface, steepening the diffusion gradient and elevating transpiration.

   Structure and Function of Phloem

   Phloem is a living tissue responsible for translocation, the distribution of soluble organic products of photosynthesis (assimilates), primarily sucrose.

   • Sieve Tube Elements: Elongated living cells aligned end-to-end. Their end walls are perforated into sieve plates. Although alive, mature sieve tube elements lack a nucleus, vacuoles, and ribosomes to minimize resistance to sap flow.
   • Companion Cells: Metabolically active cells connected to sieve tube elements via numerous plasmodesmata. They contain dense cytoplasm, numerous mitochondria, and carry out vital metabolic functions to maintain sieve elements and actively load sucrose.

   The Source-to-Sink Concept

   Unlike xylem transport, phloem transport is bidirectional, moving from a source (site of sugar production or release, such as mature leaves or germinating tubers) to a sink (site of sugar utilization or storage, such as growing roots, shoot tips, developing fruits, and seeds).