Absorption of water

Absorption of water, Mechanism of water absorption, Active osmotic absorption of water, Non-osmotic absorption of water, passive absorption of water

1. ABSORPTION OF WATER R. NITHYA M. Sc., M. Phil., (Ph. D) ASSISTANT PROFESSOR IN BIOTECHNOLOGY SRI ADI CHUNCAHNAGIRI WOMEN’S COLLEGE, CUMBUM, THENI DT, TAMIL NADU.

2. Absorption of water Uptake of water by plants Plants absorb water from soil through root hairs Absorbed water reaches the leaves through cortex endoderm pericycle & xylem tubes Plants absorb water by physical forces imbibition Diffusion Osmosis & by some biological mechanisms

3. IMBIBITION The absorption of water by a solid substance for its swelling is called imbibitions. The substances absorbing water are called imbibiants . The imbibiants do not dissolve in water. Cell wall, proteins, polysaccharides, gum, dry seeds, etc. are good imbibiants . As a result of imbibitions a pressure is developed in the imbibiant . It is called imbibition pressure . Example: the swelling of dry seeds soaked in water The breaking of seed coat and germination of seeds is due to imbibition . Imbibition of water by the cell walls of the root hairs results in absorption of water.

4. DIFFUSION The movement of particles or molecules from a region of higher concentration to a region of lower concentration is called diffusion. The diffusing particles have certain pressure called the diffusion pressure . It is directly proportional to the number or concentration of the diffusing particles. Therefore the diffusion takes place always from a region of higher diffusion pressure to a region of lower diffusion pressure, i.e. along the concentration gradient. CO2 and O2 of atmosphere enters the leaf cells through diffusion. The ions of salt are absorbed by diffusion

5. OSMOSIS The movement of water from the solution of lower concentration to the solution of higher concentration through a semi-permeable membrane is called osmosis. The semi-permeable membrane allows only solvent and not the solute to pass through it. During osmosis, solvent molecules move from the hypotonic (less concentrated) solution to hypertonic (highly concentrated ) solution. Osmotic diffusion of solvent molecules will not take place, if two solutions are isotonic(neutral). Ex. If a cell is placed in water, the cell sap is hypertonic and water is hypotonic . So water moves into cell by osmosis. When the cell is placed in a salt solution, the cell sap is hypotonic and the salt solution is hypertonic . So water moves out of the cell by osmosis.

6. DIFFUSION PRESSURE DEFICIT (DPD) The difference in the diffusion pressure between a solution and its solvent is called diffusion pressure deficit (DPD). The DPD is important for the ability of cells to absorb water. Hence, it is called suction pressure (SP). The DPD of a solution is always lesser than its solvent. It increases with increase in the concentration of solution and decreases with the dilution factor in the solution.

7. TURGOR PRESSURE The pressure developed within the protoplasm to stretch the cell wall for increasing the cell volume due to intake of water is called turgor pressure (T.P ). This hydrostatic pressure develops in cells due to endosmosis . As a result of entry of water into the cell sap, a pressure is developed ; It presses the protoplasm against the cell wall and the cell becomes turgid . This distended condition of the cell is called turgidity or turgescence . The pressure exerted by the protoplasm against the cell wall is called turgor pressure.

8. SITE OF WATER ABSORPTION Water is mainly absorbed through root hairs Area rich in root hair – Root hair zone Billions of root hairs present in root system ROOT HAIRS Tubular hair like projections of the epidermal cells Epidermis bearing root hair is known as piliferous layer Each root hair is single cell 0.5 – 1.0 cm in length 10u in diameter Wall of root hair is permeable to water Made up of cellulose & pectic substances are strong hydrophilic in nature During root growth older root hairs die off and new root hairs appear from the younger portion of the root.

9. PATH OF ABSORBED WATER Soil water Root hair cells Cortical cells Endodermis(passage cells) Pericycle cell Xylem cells Leaves

10. MECHANISM OF WATER ABSORPTION Karmer (1949) proposed that water is absorbed by two mechanisms they are Active absorption Passive absorption Active absorption Absorption of water by plants with the use of energy is known as active absorption. Metabolic energy is consumed Types of active absorption Two types 1. Active osmotic absorption 2.Active non-osmotic absorption

11. ACTIVE OSMOTIC ABSORPTION Absorption of water by root hairs by the operation of osmotic forces is called active osmotic absorption Water moves from – medium of lower concentration – medium of higher concentration Water moves from – hypertonic solution – hypotonic solution This mechanism was proposed by Atkins (1916) Priestly and Arnold (1952) Anderson and House (1967) Levitt (1969) and others .

12. MECHANISM INVOLVES THE FOLLOWING STEPS Soil water absorbed into cell wall of root hairs cell by imbibition Osmotic pressure of the cell sap is higher than – OP of the soil water OP of cell sap is usually between 3 to 8 atmosphere OP of soil water is less than 1 atm. As the root hair cell contains less water – It has less turgor pressure (TP) High OP and less TP of root hair cell results in increased diffusion pressure deficit (DPD). This leads to increase in suction pressure of root hairs.

13. Hence soil water enters the root hair cell through aquaporins present in semi-permeable plasma membrane by endosmosis. As a result OP, suction pressure(SP) & DPD of root hair become lower & its turgor pressure is increased. Now the cortical cell next to root hair cells have higher OP, DPD and Lower TP in comparison to root hair cells. Therefore, water is drown into the adjacent cortical cells from the root hair cells by endoosmosis . In this way water move gradually by cell to cell osmotic diffusion and reaches the inner most cortical cells. This process uses some amount of metabolic energy .

14. Then water moves to endodermis and passage cells From passage cells it enters the pericycle by osmotic diffusion From pericycle water moves to xylem of roots A pressure is developed inside the xylem it is called root pressure. This pressure lift the water to certain height in xylem.

15. ACTIVE NON-OSMOTIC ABSORPTION Absorption of water by plant roots against the osmotic gradient with expenditure of metabolic energy is called active non-osmotic absorption. This concept was proposed by Thiman (1951)and karmer (1959). Water absorption is takes place when OP of soil water is higher than the OP of cell sap . Water molecules moves through the aquaporins present in PM . Suction force is not at all provided by osmotic pressure of cell sap. Aquaporin is phosphorylated by ATP to facilitate movement of water across the membrane. This type of water absorption takes place all the time.

16. Following are the supporting point for this theory Factors which inhibit respiration also decrease water absorption Poisons retarding the metabolic activities of root cells also retard water absorption . Auxins which stimulate the metabolic activities of the cells, increase the absorption of water by plant roots.

17. PASSIVE ABSORPTION Absorption of water by plant roots due to transpiration pull from the top of the plants and without using energy is called passive absorption. Root hair cells do not play any active role. Transpiration causes – absorption of water Water is pulled up to the leaves not pumped up by the roots. Purely physical process Passive absorption is high when the rate of transpiration is high .

18. MECHANISM OF PASSIVE ABSORPTION Forces for absorption of water arise from leaves of plants Forces arise due to transpiration Transpiration increases the concentration of cell sap in the leaves. Increased concentration of cell sap increases the DPD . As a result water from the xylem vessels move into the mesophyll cells of leaves. This results in a DPD in the xylem vessels . Water forms a continuous column from leaves to roots through xylem vessels, the DPD is transmitted to the root . This create tension in the root hair cells This tension sucks the soil water into the root hair cells.

19. Water moves across the root cortex and xylem as a mass flow column. Water moves through Apoplastic pathway – through intercellular pathway Symplast pathway – through plasmodesmata or Transmembrane pathway – through aquaporins It is believed that greatest amount of water is pulled up by passive absorption. Water is absorbed through roots and not by roots .

20. PASSIVE ABSORPTION

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