2.3b

• Xylem


  • Responsible for the transport of water and mineral ions
  • Vascular tissue in plants
• Phloem


  • Responsible for translocation of organic solutes, eg. sucrose, and amino acids
  • Vascular tissue in plants
• Xylem structure


  • Water is conducted through vessels and tracheids, which are dead cells due to lignin deposition in the walls
  • Fibres provide support
  • Xylem parenchyma acts as packing tissue
  • Tracheids are present in flowering plants, ferns and conifers, but vessels are only present in flowering plants
• Water uptake by the roots involves absorption of water by osmosis as the soil solution has a higher water potential than the vacuole of the root hair cell
• Pathways for water movement across the root cortex

  • Apoplast pathway
  • Symplast pathway
  • Vacuolar pathway
• Endodermis


  • Forms a ring surrounding the vascular tissue in the centre of the root
  • Cell walls impregnated with suberin, forming an impermeable Casparian strip that drives water from the apoplast pathway into the cytoplasm
  • Helps to regulate the movement of water, ions and hormones into and out of the xylem
• Root pressure is created by the water potential of endodermal cells being raised by water being forced into them by the Casparian strip and the active transport of sodium ions into the xylem, lowering the water potential of fluid in the xylem and forcing water into the xylem by osmosis
• Mineral uptake by roots


  • Minerals are actively transported into the root hair cells against their concentration gradient
  • They can also pass along the apoplast pathway in solution and then enter the cytoplasm via active transport and pass into the xylem
• Transpiration


  The evaporation of water vapour from the leaves or other above-ground parts of the plant, out through stomata into the atmosphere
• Adhesive forces


  Created between the charges on the water molecules and their attraction with the hydrophilic lining of the xylem vessels
• Cohesive forces


  Created by the attractive forces between water molecules due to their dipolar charges forming hydrogen bonds
• Capillarity


  The movement of water up narrow tubes by capillary action
• Factors affecting the rate of transpiration


  • Temperature
  • Humidity
  • Air movement
  • Light intensity
• A potometer measures the rate of water uptake, which approximates the transpiration rate if cells are turgid
• Mesophytes


  • Live in temperate regions with adequate water supply but must survive times of water scarcity
  • Adaptations include closing stomata, shedding leaves, becoming dormant, producing overwintering seeds
• Xerophytes


  • Adapted to living in dry environments by reducing water loss
  • Adaptations include sunken stomata, hairs around stomata, rolled leaves, thick cuticle
• Hydrophytes


  • Grow partially or fully submerged in water
  • Adaptations include stomata on upper leaf surface, large air spaces in stems and leaves, poorly developed xylem, little or no cuticle, lack of support tissue
• Translocation


  The transport of the products of photosynthesis (sucrose) and amino acids from where they are produced (source) to where they are used or stored (sink)
• Phloem structure


  • Consists of sieve tubes (perforated walls forming longitudinal tubes), companion cells (dense cytoplasm, nucleus, mitochondria), and phloem parenchyma (packaging tissue)
• Experiments using radioactive labelling, ringing, and aphid feeding have provided evidence that phloem is the vessel involved in translocation
• Mass flow theory of translocation


  1. Photosynthesising cells produce glucose, which is converted to sucrose lowering the water potential, causing water to enter by osmosis and forcing sucrose into the phloem

  2. Increased solutes in phloem lower water potential, causing water to move in by osmosis, raising hydrostatic pressure

  3. Sucrose and solutes move by mass flow from high to low hydrostatic pressure

  4. At sink cells, sucrose diffuses out, lowering phloem pressure and causing water to move from phloem to xylem, then up by transpiration