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Absorption and Movement of Water

​Water uptake into the plant is by osmosis through root hair cells. The presence of these cells massively increases the surface area of the root for absorption to occur. Mineral elements exist in the form of ions. Uptake is greatest in regions of the root hairs. Uptake occurs mainly by active transport into the root hair cells. 


This diagram shows the movement of water across the root. There is a water potential gradient across the root, which means the water is actually transported by osmosis. The two main pathways are the apoplast and the symplast. Basically, the apoplast is through the cell walls - these are freely permeable to all small molecules and the plant has no control over the movement of substances through them (they can also be seen as non-living). The symplast can also take the vacuolar pathway. It is important to note that the Casparian strip effectively blocks the apoplast pathway at the endodermis. Therefore, all water enters the stele through the symplast. This prevents leakage of water from xylem vessels and aids the development of root pressure (an upward force, pushing water through the stem).


Although each plant cell is encased in a box-like cell wall, it turns out that communication between cells is just as easy, if not easier, than between animal cells. Fine strands of cytoplasm, called plasmodesmata, extend through pores in the cell wall connecting the cytoplasm of each cell with that of its neighbours.

Plasmodesmata provide an easy route for the movement of ions, small molecules like sugars and amino acids and even macromolecules like RNA between cells.


The extent to which a membrane permits or restricts the movement of a substance is called membrane permeability. It depends on the membrane composition, as well as the chemical nature of the solute. Permeability can be measured readily by determining the rate at which a solute passes through a membrane under a specific set of conditions.


Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration.

Osmosis is the process by which a liquid substance passes spontaneously through a semi-permeable (or selectively permeable) membrane. The net movement of water from a hypotonic solution into a hypertonic plant cell is an example of osmosis. Osmosis is a special case of diffusion.

Hypertonic Solutions 
These contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel.

Hypotonic Solutions
These contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode.

Isotonic Solutions
These contain the same concentration of solute as another solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic.

Plants, because of their cell walls and large central vacuoles can take advantage of steep water gradients to maintain the shape and rigidity of leaves and stems. The central vacuole is hypertonic to ground water. The subsequent intake of water by a plant cell creates great hydrostatic pressure (turgor or water pressure) against their cell walls.

Water is able to "climb" up the stems of plants by capillary action because of its adhesion to the cellulose. This effectively removes some water, reducing the water gradient and thus allowing more water to take its place higher up the stem.

Types of Osmosis

Depending upon the movement of water into or out of the cell, osmosis is of two types.

The osmotic inflow of water into a cell, when it is placed in a solution, whose solute concentration is less than the cell sap, is called endosmosis. e.g. swelling of raisins, when they are placed in water:

The osmotic outflow of water from a cell, when it is placed in a solution, whose solute concentration is more than the cell sap, is called exosmosis. e.g. shrinkage of grapes, when they are placed in strong sugar solution.

The tendency on the part of molecules, atoms or ions of gases, liquids and solids to get evenly distributed throughout the available space, on account of their kinetic motion is called diffusion. The molecules move from a region of their higher concentration to a lower one.

The phenomenon of diffusion is commonly observed in our daily life. For example, if a bottle of volatile substance such as perfume is opened in one corner of the room, the fragrance of the perfume will spread to the other corners. Similarly, if a small crystal of copper sulphate is put in a beaker containing water, an intense blue colour will appear around the crystal. It decreases with the increase of distance from the crystal. It shows that the molecules are diffusing into water. The molecules of water also move towards the copper sulphate to occupy the place left by the molecules of copper sulphate. This movement finally results into an equal and uniform distribution of copper sulphate and water molecules throughout the solution.

Importance of Diffusion in Plants

  1. The exchange of gases like carbon dioxide and oxygen through stomata takes place by diffusion.
  2. Transpiration or loss of water vapour from the aerial parts of the plants involves the process of diffusion.
  3. Diffusion is involved in passive uptake of mineral salts.
  4. Fragrance of flowers spreads in the air by diffusion.
  5. Diffusion plays an important role in imbibition and osmosis.


The "living" content of a cell, the protoplasm, is surrounded by a membrane called plasma membrane or plasmalemma. The protoplasm is usually next to the cell wall, so that the Plasmalemma can hardly be seen. To display it, the cells are transferred into a high salt or sugar solution. As a result, the protoplasm shrinks and detaches itself from the wall. The process is reversible and is called plasmolysis.

Significance of Plasmolysis
Plasmolysis is a vital phenomenon and has the following importance:
  1. The OP (Osmotic Pressure) of a cell can be measured by plasmolysis. The OP of a cell is roughly equal to the OP of a solution that causes plasmolysis in the cell.
  2. Salting of pickles, meat, fish, etc. and addition of sugar to jams, jellies, cut fruits, etc., prevent their decay by microbes, as the latter get killed due to plasmolysis or due to high concentration of salt or sugar.
  3. By salting, weeds can be prevented from growing on tennis courts and the growth of plants can be prevented in the cracks of walls.
  4. Plasmolysis is helpful in determining whether a particular cell is living or dead as plasmolysis does not occur in a dead or non-living cell.


This is a surface or adsorption phenomenon where solid matter adsorbs or imbibes solvent molecules. For example, hydrophilic or water-loving substances or imbibants like gums, pectins, polysacchrides imbibe or adsorb water. The adsorption or surface uptake of water by hydrophilic substances is followed by the absorption of water, where water is taken up into the inner parts. During imbibition, there is an increase in volume and also energy is released in the form of heat. When seeds are kept in water, the seeds swell due to the imbibition of water by the hydrophilic chemicals present in the seed coat and the water also becomes slightly warmer due to the release of heat energy during imbibition.

Significance of Imbibition
  1. In ancient times, boulders were split by inserting wooden stakes into crevices of the boulders and pouring water on to the stakes. The increase in volume caused by the wooden stake, which imbibed water caused the total splitting of the boulder.
  2. Hydrophilic chemicals in the seed coat and cotyledons imbibe water and swell during the process of seed germination.
  3. During the early stages of soil water absorption, hydrophilic chemicals found in the root hair cell wall imbibe water.
  4. Imbibition plays a role in the ascent of sap.

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