Journey of water from plant root hair cell till a leaf then to the atmosphere:

This journey takes place in three steps:

Entry of water from soil into root: (osmosis)

    • Absorption of water from soil takes place mainly in the zone of root hairs.
    • As the root grows, new root hairs are produced. Old root hairs degenerate and die. Absorption of water and mineral salts occur in new areas of the root.
    • Roots hair is a tiny tubular outgrowth of an epidermal cell.
    • Root hair extends and penetrate into the space between soil particles, close to the water present in soil.
    • The cell sap present in the root hair cell is highly concentrated because of sugars and mineral salts as compared to soil water so water potential is low in the cell.
    • These two solutions of different concentration are separated by a semi-permeable membrane present on the surface of root hair surface.
    • In this situation, water moves into root hairs from soil (where water potential is high)by the process of osmosis.

Entry of water with ions and mineral salts from soil into root: (active transport)

    • Water also enters into root hairs through active transport.
    • In this situation concentration of sugars and mineral salts in the cell sap is higher than that in the soil.
    • In this stage energy is required to carry out the process of active transport because the root hairs have to absorb the ions against the concentration gradient.
    • The energy is required.
    • This energy comes from cellular respiration in the root hair cell.

What happens when once the water enters into the root hair cell?

    • Once the water enters into the root hair cell, dilutes the cell sap.
    • Now this cell sap is more dilute as compared to the next cell.
    • Hence water passes from root hair to the inner cell by the process of osmosis.
    • This process continues until the water enters the xylem vessels and move up the plant.

Entry of water from roots into the stem:

    • Water moves from roots into the stem by three ways:
      • Root pressure
      • Capillary action
      • Transpiration pull
    • Root pressure

      • is the pressure generated by osmosis and active transport of water, which helps to force the water to move from the root hairs into the xylem vessels and moves up the stem along the xylem vessels.
    • Capillary action :

      • The force of attraction between water molecules is called cohesion.
      •  The force of attraction between water molecules and inner surface of xylem vessels is called adhesion.

      •   Both cohesion and adhesion help the water molecules to move upward in xylem vessel and this effect is called capillary action.

      • However, it must be said that capillary action is only possible in small plants and is not responsible for water rising up a tall tree.
    • Transpiration pull:

      • is a strong suction force produced in xylem vessels to draw the water in leaves from stem.
      • This transpiration pull is  produced or generated because of the loss of water in atmosphere from  aerial parts of the plant in form of water vapours.

Water from leaf to the atmosphere:

The movement of water inside a leaf occurs in the following way:

    • Water from stem reaches the leaf (xylem) and continuously moves out of the mesophyll cells by osmosis.
    • This continues movement form a thin film of moisture over the surface of mesoplyll cell.
    • This water moves into intercellular air spaces and eventually accumulates near large air spaces near stomata.
    • From stomata, water diffuses out to the drier air outside the leaf(transpiration).

What happens when water from mesophyll cells evaporates into the inter-cellular spaces of the leaf?

    • As water evaporates from mesophyll cells, the water potential of the cell sap decreases.
    • The mesophyll cells starts to absorb the water from deeper cell inside the leaf by osmosis.
    • These deeper cells remove the water from xylem vessels.
    • This result in a suction force(transpiration pull) which drag/pull the whole column of water up the xylem vessel.

Test yourself:

    • Name the process through which water enters into the root hair cells.
    • Why the process of active transport occurs during the absorption of ions and mineral salt through root hair cell?
    • What is a transpiration pull?
    • Why does transpiration pull important for the absorption of water through root hair cells?
    • What is capillary action?
    • Name two process that are important component for capillary action.
    • What is a root pressure?
    • Why root pressure is important for the absorption of water through root hair cells?
    • What happens when water enters into the leaf from stem(xylem vessels)?
    • Name the process through which water leaves the leaf into the atmosphere.
    • What is transpiration stream?