Chapter 7: Transport in plants

Plants require a transport system to ensure that all the cells of a plant receive a sufficient amount of nutrients.  Plants need transport systems to meet their metabolic demands (glucose, hormones, mineral ions are required for various processes within plants), to efficiently move substances up and down and to compensate for their relatively small SA:V ratio (generally plants cannot rely on diffusion alone).

Vascular System

Plants have a vascular system which involves a network of vessels (vascular tissue) running through the leaves, stem and roots. These three parts are the main organs involved in transport. The vascular system is comprised of two distinct types:

    • 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))

Xylem

  • The functions of xylem tissue in a plant are:
    • Vascular tissue that carries dissolved minerals and water up the plant
    • Structural support
    • Food storage

They are long cylinders made of dead tissue with open ends, therefore they can form continuous column. The xylem is oriented toward the adaxial surface of the leaf (usually the upper side). Xylem is a complex tissue. Xylem is composed of 4 elements: tracheids, vessels, and xylem parenchyma and xylem fibers.

 

Phloem

  • The function of phloem tissue in a plant is to:

Transport organic compounds, particularly sucrose, from the source (eg. leaf) to the sink (eg. roots). The transport of these compounds can occur up and down the plant

  • Phloem is a complex tissue also made up of various cell types; its bulk is made up of sieve tube elements which are the main conducting cells and the companion cells.

The phloem is also a pathway to signaling molecules and has a structural function in the plant body. It is typically composed of three cell types: sieve elements, parenchyma, and sclerenchyma. While the main role of the phloem tissue is to transport carbohydrates from sources to sinks through the sieve elements, phloem is also composed of parenchyma cells, which play a key role in the storage of water, non-structural carbohydrates and storage proteins.

  • In the roots the vascular bundle is found in the Centre and on the edges of the center core is the phloem tissue.
  • In the stems, the vascular bundles are located around the outside and the phloem tissue is found on the outside (closest to the epidermis).
  • In the leaves, the vascular bundles form the midrib and veins and therefore spread from the center of the leaf in a parallel line. The phloem tissue is found on the lower side of the bundles (closest to the lower epidermis).

Transport of Water

  • Water is transported in the plant through the combined efforts of individual cells and the conductive tissues of the vascular system. Water from the soil enters the root hairs by moving along a water potential gradient and into the xylem through either the apoplast or symplast pathway. It is carried upward through the xylem by transpiration, and then passed into the leaves along another water potential gradient. Within a plant mineral ions and organic compounds (eg. sucrose) are transported by being dissolved in water. The dissolved mineral ions are transported in the xylem tissue and the dissolved organic compounds are transported in the phloem tissue.
  •  In the leaf, some water is lost through evaporation from the stomata and the remaining fluid moves along a water potential gradient from the xylem into the phloem, where it is distributed along with the organic nutrients produced by photosynthesis throughout the plant.

Transpiration

The movement of water through a plants xylem is largely due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules. Transpiration refers to the loss of water vapour from a plant to its environment by diffusion and the transpiration stream refers to the movement of water from the roots to the leaves.

The transpiration stream is the movement of water up the stem enables processes such as photosynthesis, growth and elongation as it supplies the plant with water which is necessary for all these processes.

Transpiration involves osmosis, evaporation from the surface of mesophyll cells into intercellular spaces and diffusion of water vapour.

The importance of transpiration is given below:

    • 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.

Movement of water through Roots

Water enters through root hair cells and moves into the xylem tissue in the center of the roots. The root hair cells provide large surface area for the movement of water to occur. Minerals are also absorbed through the root hair cells by active transport.

There are pathways from root hair to xylem inside the roots:

  1. Apoplast Pathway:
    Here water passes from root hair to xylem through the walls of intervening cells without crossing any membrane or cytoplasm. This pathway provides the least resistance to movement of water. However, it is interrupted by the presence of impermeable lignosuberin casparian strips in the walls of endodermal cells.
  2. Symplast pathway
    A smaller amount of water travels through the symplastic pathway, which is the cytoplasm and plasmodesmata or vacuole of the cells. The water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata. The movement of water in the symplastic pathway is slower than the apoplastic pathway.

 

Water movement in Xylem Up to stem

  • Movement in the xylem is by mass flow of the whole solution, and the force is either the tension pull of transpiration or root pressure, or both.
  • In general, however, water movement in the xylem is by transpiration pull.

Water is passively transported into the roots and then into the xylem.

  • The pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits(non-lignified areas), which then causes water to move up the xylem vessels (due to the cohesive and adhesive properties of the water). This movement is called transpiration stream.
  • The forces of cohesion and adhesion cause the water molecules to form a column in the xylem. Water moves from the xylem into the mesophyll cells, evaporates from their surfaces and leaves the plant by diffusion through the stomata.

Roles of Stomata

  • Transpiration is mainly controlled by the pairs of guard cells that surround stomata (plural, stoma is singular)
  • Guard cells open the stomata when they are turgid and close the stomata when they lose water
  • When the stomata are open there is a greater rate of transpiration and of gaseous exchange
  • When the stomata close transpiration and gaseous exchange decrease
  • As stomata allow gaseous exchange (CO2 in and O2 out) they are generally open during the day

Translocation

Translocation is the movement of materials in plants from the leaves to other parts of the plant. Nutrients, mainly sugars, are created in the leaves during photosynthesis. These are then transported throughout the plant through phloem, which are a long series of connected cells.

While in translocation, Sucrose enters the phloem in a process known as active loading. The proton pumps of companion cell uses ATP to transport hydrogen ions into the surrounding tissue, thus creating diffusion gradient, which causes the hydrogen ion to diffuse back into the companion cell. As a result the sucrose diffuses out of the companion cells down the concentration gradient into sieve tube through plasmodesmata.

 

 

This sucrose then actively transported into the sieve tubes of phloem as a result a concentrated medium produced inside the sieve tube
Then water molecules from xylem enters into the sieve tubes which helps the transportation of food from leaves to the storage part
In storage part sucrose actively transports from sieve tubes to storage cells
Transportation through phloem is bidirectional because during spring season sucrose present in the storage part is transported to the young leaves.