Long distance transportation of water and minerals , Root pressure , Guttaion

 

In plants, When material is transported long distance like from root to leaves and from leaves to root, In such cases diffusion can not facilitate such transportation.  Merely by the diffusion, materials  move independently depending on their concentration gradients. In long distance transportation, materials are transported in bulk or masses. The movement of material from one place to another place  in plants is termed as Mass flow or Bulk flow.

Mass  flow can be facilitated by  positive hydrostatic pressure from root to leaves  and negative hydrostatic pressure from leaves to roots.Water and minerals, and food are transported  by following the Bulk flow system.

The bulk movement of material  through vascular tissues like xylem and phloem in plants is called translocation.

Absorption of  Water in Plants Epidermal cells of root develops fine and thin protection like hair are called root hairs. These root hairs are derived from the root cap that exist during development of monocot and dicot embryo. These root hairs increase the surface area for absorption of water and minerals. Water is absorbed with minerals  by the root hairs through the process of diffusion. In soil , Water has a higher  potential than cells of root hair. As a result, Water starts to move into the  root hair cells from the soil. After the absorption of water by the root hairs, water is passed into cortex, endodermis, pericycle and finally  into the xylem vessel.

Pathways of water movement into the root 

Water adopts two different pathways when it make entry from the soil into the root.These pathways are - Apoplast and Symplast pathway

Apoplast pathway of water movement

This type of movement takes place through the intercellular spaces and the walls of the cells.This movement does not allow water to cross the casparian strip of endodermis of root due to the deposition of Suberin. The apoplastic movement  does not allow the water to cross the cell membrane. In this kind of movement, water is always transported in mass flow along the concentration gradient.

Symplast pathway of water movement

In  symplastic movement, the water is moved via cytoplasm through the plasmodesmata along the  concentration gradient.This movement of water is slow therefore most of the water is transported  in the roots by  the apoplast  movements.

👌👌Remember - The movement of water is symplastic in the endodermis and  in cortex region whereas In xylem vessels and tracheids, The water movement is apoplastic.

As described earlier, the movement of water  is symplastic in the endodermis Because the endodermis is impervious to water due to the presence of a strip  of suberin  called the casparian strip. Water molecules are not able to cross the  layer of suberin  so they are directed to the next  cells through the cell  membranes Or plasmodesmata by the means of  symplast movement  to reach the cells of the xylem.

Endodermis due to the presence of a layer of suberin has the ability to actively transport ions in one direction only.

Root Pressure:

Different types of  minerals or ions along with the water  are actively transported into the vascular tissues of the roots from the soil. As a result , it increases the pressure inside the xylem. This positive pressure is called root pressure.This root pressure is responsible for  the upward movement of water and pushes up the water up to the little extent of   heights in the stem.

Root pressure does not cause the  upward movement of water in tall trees But root pressure make a   continuous chains of water molecules in the xylem.This chain is affected  due to the phenomenon of transpiration

During night or early morning, When evaporation is low, excess amounts of water are released in the form of small droplets from leaves in plants. This process is termed as Guttation.

Guttation is the result of root pressure. These small droplets of water are released  through openings of veins located in leaves of plants. These openings are termed as  hydathodes or water stomata.

Transport of minerals in plants 

Minerals are present in the soil in the form of ions which cannot transport passively or without energy across cell membranes.

👌👌Remember - The concentration of minerals in the soil is lower than the concentration of minerals in the root.

To facilitate the transport of ions from soils to root, Some carrier proteins are found  in the membranes of root hair cells that actively transport the ions from the soil into the cytoplasm of the epidermal cells.

Carrier proteins of endodermal cells are selective  for the quantity and types of ions or minerals  that reach the xylem. Cell membrane of root hairs allows only some minerals or ions  but not others. 

As the ions have reached xylem through active or passive uptake then their transportation  upto  the stem and to  all parts of the plant takes place  through the transpiration stream.