transport in plants

• Plants require a transport system to ensure all cells receive sufficient nutrients through the combined action of xylem tissue for water and dissolved minerals in transpiration, and phloem tissue for sugar translocation
• In the roots, xylem and phloem are components of the vascular bundle, with xylem vessels arranged in an X shape surrounded by endodermis, and phloem found on the outside
• In non-wooded plants, xylem provides support and flexibility to the stem, while phloem is located on the outside of the vascular bundle
• Xylem vessels transport water and minerals, provide structural support, have open ends for a continuous column, contain pits for water movement, and are thickened with lignin deposited in spiral patterns for flexibility
• Phloem vessels are tubes made of living cells, involved in translocation of nutrients, consist of sieve tube elements and companion cells, and have cytoplasm linked through plasmodesmata for communication and substance flow
• Transpiration is the process where plants absorb water through roots, move it up the plant, release it as water vapor through leaves, enabling processes like photosynthesis, growth, and temperature control
• Transpiration involves osmosis from xylem to mesophyll cells, evaporation from mesophyll cells' surface, and diffusion of water vapor out of stomata, with factors affecting transpiration rate including light, temperature, humidity, and water availability
• Xerophytes are plants adapted to dry conditions with features like small leaves, densely packed mesophyll, thick waxy cuticle, closed stomata, hairs and pits to trap moist air, rolled leaves to reduce exposure, and leaf loss to decrease water loss
• Plants respond to low water availability by:

  • Closing the stomata to prevent water loss
  • Containing hairs and pits to trap moist air, reducing water vapor potential
  • Rolling their leaves to reduce exposure of lower epidermis to the atmosphere, trapping air
  • Shedding leaves to decrease the number of leaves and reduce water loss via transpiration
  • Storing water in parenchyma tissue
  • Having large extensive roots to increase water uptake
• In the movement of water in the root:

  • Water enters through root hair cells and moves into the xylem tissue located in the center of the root
  • Root hair cells provide a large surface area for water movement
  • Minerals are absorbed through root hair cells by active transport
  • Water can move across the cortex of the root into xylem via the symplast pathway or the apoplast pathway
  • When water reaches the endodermis, it encounters a layer of suberin known as the Casparian strip, which cannot be penetrated by water
• Water moving in the xylem up the stem:

  • Water is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient
  • Root pressure aids in pushing water upwards by osmosis
  • Cohesion-tension theory explains the flow of water in the xylem, supported by capillary action
• Translocation is an energy-requiring process that transports assimilates dissolved in water in the phloem between sources and sinks

  • Sucrose enters the phloem through active loading
  • Water enters the sieve tube elements via osmosis, increasing hydrostatic pressure
  • Mass flow of water from source to sink down the hydrostatic pressure gradient supplies assimilates where needed