transport in plants

• The outer layer of the root (as is in stems and leaves) is the epidermis.
• • Vascular tissue is concentrated in a central stele (vascular cylinder).
  • The stele is mainly composed of xylem tissue with a smaller amount of phloem tissue.
  • Immediately outside the stele is a single layer of cells called the endodermis.
• A layer of undifferentiated cells is found between the epidermis and the endodermis called the cortex. Cells in the cortex have small air spaces between them and the cells may be rich in starch grains.
• Plant root

  A) Epidermis

  B) Cortex

  C) Endodermis

  D) xylem

  E) phloem

  F) stele

  
• Xylem vessels are made up of cells whose end walls have broken down, resulting in long hollow tubes ideal for carrying water and mineral ions.  In these cells, a secondary wall, impregnated with lignin, is formed inside the primary cellulose wall. Lignin provides great strength and prevents the vessels from collapsing when under pressure exerted by the transpiration stream "sucking" water up the plant.
• In protoxylem (first xylem vessels to develop in a plant) the lignin produces an annular or spiral pattern.  These patterns allow the vessels to elongate along with other tissues in the growth regions (root and shoot tips).

  
• In metaxylem (xylem vessels that are more mature) there is more extensive lignification arranged in patterns known as reticulate and pitted preventing further growth.  Reticulated vessels are thickened by interconnecting bars of lignin.  Pitted vessels are uniformly thickened, except at pores seen as pits that allow rapid movement of water and ions out of the vessels to the surrounding cells.

  
• Protoxylem and metaxylem often occur together. New xylem cells are produced in a meristematic region (the cambium) between the xylem and phloem. In roots the protoxylem is pushed to the outer edge of the stele as the metaxylem forms behinds it. Protoxylem cells are generally smaller and have less thickened walls than the metaxylem.
• Phloem is the tissue responsible for the transport of sucrose (the transport carbohydrate in plants) though amino acids and other organic solutes are also present. Phloem tissue is living and consists of sieve tube elements and companion cells.
• The transporting cells are the sieve tube elements which lie end to end and form a continuous stack – the sieve tube. They have a small amount of cytoplasm containing endoplasmic reticulum and mitochondria lining the inside of the cellulose wall. The end walls, where two sieve tube elements meet together form the sieve plate. Here the end walls have numerous pores (perforations) in them like a sieve through which sugars pass.
• Sieve elements have microtubules that extend between sieve elements and pass through sieve pores. They are thought to be involved in translocation of solutes.
• Next to each sieve tube element are companion cells. The cytoplasm of the companion cell is linked via plasmodesmata to that of the sieve tube element. They have a dense cytoplasm with many mitochondria and have high levels of metabolic activity. They act as supporting cells, carrying out many metabolic activities for the highly specialised sieve tube elements.
• Translocation of organic solutes in phloem:

  • Movement of substances like sucrose through the phloem.
  • Essential for distributing nutrients within plants.
  • Moves from leaves to growing parts and roots for storage.
  • Requires energy expenditure.

  Mechanism of translocation:

  • Evidence suggests an active process.
  • Companion cells, rich in mitochondria, play a key role.
  • ATP is used to move sucrose into companion cells.
  • Sucrose is then loaded into sieve tube elements for transport.
  • Metabolic poisons like potassium cyanide can halt translocation.
• The distribution of xylem and phloem tissue is different in plant stems. The vascular tissue is arranged as vascular bundles around the outside of the stem – the advantage being the provision of greater support necessary in stems to support branches and leaves.
• The protoxylem is usually in the section of xylem closer to the centre of the stem, with the metaxylem in the section of xylem closer to the outer edge of the stem.
• A vascular bundle continues into the leaf as the midrib, which branches to form smaller veins that are distributed throughout the leaf. The leaf veins are typically found in the spongy mesophyll just below the palisade layer.
• cross section diagrams

  A) endodermis

  B) xylem

  C) phloem

  D) cortex

  E) epidermis

  F) vascular bundle

  G) phloem

  H) xylem

  I) vascular bundles

  J) phloem

  K) xylem

  L) upper epidermis

  M) lower epidermis

  N) root

  O) stem

  P) leaf

  
• Comparing xylem and phloem (xylem)

  • Transports water and inorganic ions
  • Lignin - to waterproof and strengthen the cell wall
  • Cells are dead - no contents present
  • No companion cells
  • No sieve tube elements
  • Movement one way (up only)
  • Consists of protoxylem (spiral and annular) and metaxylem (reticulate and pitted)
  • No metabolic activity, movement of water by transpiration stream.
• Comparing xylem and phloem (phloem)

  • Transports sucrose (the transport carbohydrate in plants), amino acids, and other organic solutes
  • No lignin
  • Living cells
  • Companion cells that aid transport of solutes
  • Sieve tube elements with microtubules to help with translocation
  • Movement two way (up and down)
  • Only comes in one form (phloem)
  • High metabolic activity to aid in translocation of organic substances, e.g., move sucrose from palisade cells to phloem. Energy provided by respiration.
• Transpiration can be defined as: the evaporation of water from the mesophyll surface and the subsequent diffusion of water vapour through the stomata and into the atmosphere.
• Water is absorbed into the root hair cells by osmosis, ions by active transport (or facilitated diffusion if diffusion gradient is favourable).
• apoplast pathway: This pathway involves water moving (by capillarity) along the cellulose microfibrils of the cell walls. Most water moves via this pathway due to the limited resistance to water movement. As water moves through the wall, the cohesive properties of water (aided by the hydrogen bonding) helps pull the water column along.
• Symplast pathway: This pathway involves water moving by osmosis through the cytoplasm of the cortex cells.  As a root hair cell takes in water, its water potential becomes less negative and is higher than the adjacent cell in the root cortex, creating a water potential gradient.  Water moves from the root hair cell into the cortex cell by osmosis and so on across the cells of the cortex.

  Between adjacent cells there are strands of cytoplasm called plasmodesmata.  These connections allow water molecules to pass between the cytoplasm of one cell and the next.