Transport in plants

• Dicotyledonous (dicots) plants

  • Seeds contain two cotyledons (seed leaves)
  • Network of veins
  • Leaves have broad blades (leaf surface) and petioles (stalks)
  • Tap root with lateral branches
  • Non-woody tissue
• Herbaceous dicots

  Have a relatively short life cycle (one growing season)
• Plants need transport systems to meet their metabolic demands, to efficiently move substances up and down and to compensate for their relatively small SA:V ratio
• Vascular system


  Network of vessels (vascular tissue) running through the leaves, stem and roots
• Xylem

  Transports water and mineral ions from the roots to the rest of the plant
• Phloem

  Transports substances from the source (eg. leaf) to the sink (eg.root)
• The xylem and phloem are arranged together in vascular bundles
• The bundles are laid out differently in the leaves, stem and roots
• Functions of xylem tissue

  • Vascular tissue that carries dissolved minerals and water up the plant
  • Structural support
  • Food storage (parenchyma)
• Xylem tissue is found, along with phloem tissue and other tissues, in vascular bundles
• In the roots the vascular bundle is found in the centre and the centre core of this is xylem tissue
• This helps the roots withstand the pulling strains they are subjected to as the plant transports water upwards and grows
• Vascular bundles in stems

  • Located around the outside
  • Xylem tissue found on the inside (closest to the centre of the stem)
  • Helps support the plant (against lateral forces; flexibility)
• Vascular bundles in leaves

  • Form the midrib and veins
  • Spread from the centre of the leaf in a parallel line
  • Phloem tissue found on the upper side of the bundles (closest to the upper epidermis)
• Phloem tissue

  • Transports organic compounds, particularly sucrose from the source (e.g. leaf) to the sink (e.g. roots)
  • Transport can occur up and down the plant
• Phloem tissue

  • Complex tissue made up of various cell types
  • Bulk is made up of sieve tube elements (main conducting cells)
  • Also contains companion cells
• Other phloem cell types

  • Parenchyma for storage
  • Strengthening fibres
• Location of vascular bundles

  • In roots, vascular bundle is in the centre with phloem on the edges
  • In stems, vascular bundles are around the outside with phloem on the outside
  • In leaves, vascular bundles form midrib and veins spreading from the centre
• Xylem tissue

  • Vascular tissue that transports dissolved minerals and water around the plant
  • Provides structural support
  • Stores food
• Xylem cell types

  • Tracheids
  • Vessel elements
  • Xylem parenchyma
  • Sclerenchyma cells (fibres and sclereids)
• Most xylem tissue is made up of tracheids and vessel elements, which are both types of water-conducting cells
• Difference between xylem and phloem

  • Xylem is dead, phloem is living
  • Xylem has lignified cell walls, phloem does not
• Phloem sieve tube elements and companion cells

  • Sieve tube elements are the main conducting cells
  • Companion cells control the metabolism of their associated sieve tube members
  • Companion cells play a role in loading and unloading of sugars into the phloem
• Water and mineral transport

  • Dissolved in water
  • Mineral ions transported in xylem
  • Organic compounds transported in phloem
• Plant root functions

  • Uptake of water and mineral ions
  • Can have root hairs to increase surface area for absorption
• Water uptake

  Passive process by osmosis (from higher to lower water potential)
• Mineral uptake

  Can be passive by diffusion or active by transport
• Plants need constant supply of water and minerals to compensate for water loss by transpiration and support photosynthesis and protein production
• Apoplast pathway

  • Water moves through cell walls, dead cells, and xylem
  • Faster movement than symplast pathway
  • Blocked by Casparian strip in endodermis
• Symplast pathway

  • Water moves through cytoplasm and plasmodesmata
  • Slower movement than apoplast pathway
  • Water enters cells by osmosis
• Water does not move by osmosis in the apoplast pathway as the molecules are in the cell wall which is freely permeable
• Forces aiding water movement

  • Root pressure
  • Transpiration pull
  • Capillarity
• Transpiration

  • Loss of water vapour from leaves
  • Driving force is the water potential gradient from soil to atmosphere
  • Provides cooling and aids mineral uptake
• Around 99% of water absorbed is lost through transpiration
• Absorption of water and mineral salts

  1. Continuous absorption of water

  2. Continuous absorption of mineral salts
• Around 99% of the water absorbed is lost through evaporation
• Transpiration

  Water moves from the plant's stem and its leaves
• Transpiration

  The loss of water vapour from a plant to its environment by diffusion
• Transpiration stream

  The movement of water from the roots to the leaves
• Advantages of transpiration

  • It provides a means of cooling the plant via evaporative cooling
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants