🧖🏼 Movement of Water

Diffution

• The movement of the Molecules of gases, liquids or solutes from the regions of higher concentration to the regions of lower concentration until the molecules are evenly distributed throughout the available space is known as Diffusion.

Osmosis

• Osmosis is essentially a special type of diffusion of liquids.
• The term ‘Osmosis’ was given by Abbe Nollel.
• Osmosis means:

  

• Plasma or cell membrane is a semi-permeable membrane. At higher temperature plasma membrane becomes permeable.

• Here when barrier between two different concentrated solutions is opened and barrier remained only semi-permeable membrane, then the level of higher conc. solution is increased. It is happened only due to flow of water from lower conc. to the higher concentration of solution through semi-permeable membrane.
• Semi-permeable membrane is such type of membrane through which solutes of solution do not pass but solvent passes e.g. plasma-membrane.
• Differentially or Selectively Permeable membrane is such type of membrane through which some particles pass but others do not pass e.g. lipo-protein cell membrane.

Osmotic Pressure or Osmotic Potential (O.P.)

• The applied pressure required to stop osmosis when a solution is separated from pure water by a semi-permeable membrane is called osmotic pressure and usually denoted by ‘ψ’.
• The osmotic pressure of a solution is defined as the excess hydro-static pressure which must be applied to it in order to make its water potential equal to that of pure water.
• One atm. (S.I. unit of pressure) = 1.01 bars
• or one bar = 0.987 atm.
• Due to a number of definitions, the term osmotic pressure is misleading since it denotes a real pressure where as it is only a property of a solution expressed in terms of pressure. Hence the term ‘Osmotic Potential’ is used by modern physiologists. Osmotic potential is that part of water potential which becomes more negative with addition of solutes.
• Osmotic Pressure = Positive
• Osmotic Potential = 🌶 Negative
• It indicates decrease in pressure that occurs due to addition of the solutes.
• In a pure solvent, the value of osmotic potential is Zero which is maximum value.

Diffusion Pressure Deficit/Water Potential (DPD/WP)

• Diffusion pressure deficit (DPD) is also called Suction Pressure (S.P.).
• It is the ability of a cell to draw water. It is the force per unit area (i.e. pressure) by which water enters into a cell.
• A pure solvent is supposed to have maximum diffusion pressure. When certain solute particles are added to the pure solvent, the diffusion pressure of the resulting solution is lowered.
• The amount by which the diffusion pressure of a solution is lowered than that of its solvent at the same temperature and atmospheric pressure, is called DPD. The term DPD was introduced by Meyer in 1938.

• When a cell is fully turgid, suction pressure becomes zero.
• But according to the recent trend, diffusion of water is explained in terms of Water Potential.
• DPD is the positive value whereas water potential is the negative value.
• Water potential concept was formulated by Otto Renner in 1915.
• Turgor Pressure (T.P.): T.P. is the outward pressure exerted by the cell solution on the cell wall which is developed due to osmotic diffusion of water. In an equilibrium inward pressure is also given by the cell wall on the cell solution in an equal amount, which is called Wall Pressure or Hydrostatic Pressure.

• Turgor Pressure is also called Pressure Potential (P.P.) which has positive value.
• Chemical Potential: The free energy per mole of a chemical substance is called its chemical potential. Chemical potential of water in a solution is reduced by the addition of solutes.

Osmotic Relations

• The diffusion of water depends upon its free energy per mole. The wall pressure also increases the free energy of the water contained in a cell. The sum of osmotic potential and wall pressure denotes a net change in the chemical potential of water. This net change is called water potential (ψw).
• Water potential (W.P.) is always negative and the maximum value is zero.
• Water potential is affected by osmotic (solutes) potential, pressure (hydrostatic or turgor) potential and Matric potential (due to water binding matrix).

Ψw = Ψs + Ψp + Ψm

• Ψw = Water Potential (Probably the most useful measurable property in the soil-plant-air system)
• Ψs = Osmotic potential; contribution made by solutes
• Ψp = Pressure potential; contribution made by turgor pressure.
• Ψm = Matric potential; contribution made by matrix. Since Ψm negligible,
• Ψw = Ψs + Ψp
• W.P. = OP + PP

👉🏻 Therefore in osmosis, movement of water takes place from:

• Higher water potential to lower water potential.
• Lower concentration to higher concentration of solution.
• Lower DPD to higher DPD.

Question: A cell (A) having O.P. = 10 atm. and TP = 4 atm. is surrounded by ‘B’ cells of OP = 18 atm. and TP is 6 atm; then movement of water is a) From ‘A’ cell to ‘B’ cell (surrounding cell) b) From ‘B’ cell to ‘A’ cell. c) Both of them. d) No movement of water.

Solution:

• For ‘A’ cell:
  • WP = OP + PP (or TP)
  • = - 10 + 4 = - 6 atm
  • or DPD = OP - TP = 10 – 4 = 6 atm.
• For ‘B’ cell:
• WP = -18 + 6 = -12 atm.
• or DPD = 18 - 6 = 12 atm.

👉🏻 Therefore, movement of water is from ‘A’ cell (i.e. higher W.P. or lower DPD) to ‘B’ cell (i.e. lower WP or higher DPD) means Ans. (a)

• When T.P. increases, the corresponding DPD decreases.
• Endo-osmosis: The diffusion of solvent particles into a living cell or structure is called endo-osmosis.
• Exo-osmosis: The diffusion of solvent out of a living cell or structure is called exo-osmosis.

Role of Osmosis

• Plants absorb large amount of water from soil.
• Movement and distribution of water across cells is due to osmosis.
• Osmosis is responsible for turgidity of plant cells. The leaf, flower & stem tip require turgor for maintaining their form.
• Turgor of the guard cells is absolutely by essential.
• It is essential for the growth of young cells.
• High osmotic conc. increases resistance of the plants to freezing temperature and desiccation.
• It is responsible for turgor pressure in root xylem i.e. root pressure.

Factors affecting Osmotic Pressure

• Concentration of the solute particles: Osmotic pressure depends upon the ratio of solute and the solvent particles. The increase in the solute concentration, increases the osmotic pressure.
• Ionization of the solute molecules: Ionization increases the number of solute particles.
• Hydration of solute molecules: A solution which has hydrated solute molecules is of higher osmotic pressure than otherwise because the water molecules which are associated with the solute molecules are ineffective as a part of the solvent.
• Temperature: Osmotic pressure increases with increase in temperature.

Imbibition

• The term ‘imbibition’ was coined by Sachs.
• The soaking up of water by dry substances due to hydrophilic colloids is called Imbibition.
• It is the first step in the absorption of water.
• The rate of imbibition increases with increase in temperature.
• Imbibition in oily seed is more than in starchy seed.

• Imbibition pressure is also called Metric Potential (Ψm). Metric potential is the component of water potential which is determined by the attraction between water and hydrating colloid or gel-like organic molecules, cell wall etc. (collectively called matrix).
• The metric potential is maximum (most negative) in a dry material. Both living and dead plant cells possess a large amount of carbohydrates, proteins, and polypeptides etc. which are hydrophilic colloids and therefore, have very strong attraction for water.
• Seeds rich in colloidal materials are very good imbibants.

Plasmolysis

• Plasmolysis is the shrinkage of protoplasm due to outward flow of water in a hypertonic solution.
• The point where plasmolysis just starts (not visible) is called incipient plasmolysis.

Cell wall → Either permeable (cellulose cell wall) or impermeable (cork cell, lignified, suberinised)

Cell membrane → Always semi-permeable.

• Space between shrunken protoplasm and the cell wall is occupied by plasmolysing solution i.e. external solution (or salt).
• When a plasmolysed cell is placed in hypotonic solution or pure water, the protoplasm as well as cell as a whole attain their original shape and size respectively due to endo-osmosis. This phenomenon is called De-plasmolysis.
• Examples:
  • Raisins swell in water due to imbibition and endosmosis.
  • Excess of fertilizer in the soil kills the plant due to plasmolysis (exo-osmosis)
  • Salted pickle or meat kills the bacteria due to plasmolysis.

Solution

• Hypotonic solution: it means weak solution. A cell swells in hypotonic solution.
• Hypertonic solution: strong solution. A cell shrinks or become flaccid in such solution due to outward flow of water.
• Isotonic solution: not weak nor strong solution. A cell remains unchanged in it. Eye drops are always isotonic solution.

Absorption of water

• Water is absorbed by the root hairs.
• Root hairs are more developed in xerophytes (arid plants) and absent in hydrophytes.
• First step in absorption of water is imbibition.
• The absorption of water takes place in the terminal portions of roots but the maximum absorption of water takes place in the zone of root hairs i.e. 1 - 10 cm behind the root tip.
• Maximum absorption of inorganic salts is through the zone of cell division.

• Root cap is formed by calyptrogen (= Dermatogen + Periblem).
• Multiple root cap is found in Pandanus (Screw pine).
• Roots of hydrophytes contain root pockets.

• Renner (1912, 1915): First time recognized and classified the water uptake mechanism into “active absorption” and “passive absorption”.

Active Absorption

• Root hairs play an active role.
• Water enters into root hair by osmosis.
• Process occurs against the concentration gradient and
• Metabolic energy is spent. 👉🏻 There are two major theories to explain active absorption:

a) Osmotic Theory

• Atkins (1916) and Priestley (1920-22) were amongst the first to postulate an osmotic theory of active absorption.
• The value of osmotic pressure of the cell sap of the root hairs is generally 2.0 atm, (but varies between 3 to 8 atm) and of soil water is less than 1.0 atm.
• Higher DPD of the cell sap of the root hairs cause endo-osmosis of soil water.
• But a problem is how a sufficient conc. of solutes is maintained in the root xylem to maintain a higher DPD in the xylem sap.

b) Non-osmotic theory

• Water is absorbed against a concentration gradient.
• Absorption require an expenditure of energy released from respiration.
• Renner’s active theory is called ‘osmotic active’.

Passive Absorption

• Process occurs along the concentration gradient.
• And hence energy is not involved.
• Absorption is controlled by transpiration.
• Highest DPD or Lowest Water Potential → in Leaf cells
• Lowest DPD or Highest water potential –> in Root hairs

T/A Experiment

👉🏻 T/A means Transpiration/Absorption ratio

• Let,
  • Weight of bottle with plant = 1000g
  • Initial reading water inside tube is 5 cc. one drop of oil is poured into side tube to check evaporation. After allowing the minimum time i.e. 30 minutes for transpiration and absorption, again reading is noted down.
  • Wt. of bottle with plant = 998g, means water lost = 1000 — 998 = 2g.
  • Final reading of water inside tube = 3 cc. means water absorbed = 5 - 3 = 2 cc.
  • Transpiration /Absorption = 2/2 = 1.
  • Inference: water lost in transpiration is equal to water absorbed.

👉🏻 Factors affecting the rate of water absorption:

• Available Soil Water: The rate of water absorption is uniform between the field capacity and permanent wilting percentage. The decrease in soil water below the permanent wilting point causes considerable decrease in the absorption of water.
• Conc. of the soil solution: The lack of water absorption by plants growing in saline water is an example of physiological dryness.
• Soil aeration: The rate of water absorption is rapid in well aerated soils. Oxygen deficiency and accumulation of CO₂, increases the viscosity of the protoplasm and decreases its permeability, both of which reduce the rate of absorption. During water logging conditions, there is total check on the water absorption which is an example of physiological dryness.

Ascent of Sap

• Ascent of sap is the translocation of water and inorganic solutes.
• Ascent of sap means:
  • Movement of water and inorganic solutes
  • from root to the leaves
  • through xylem vessels
  • against the force of gravity
  • and water Column remains in a state of tension
• Ringing or girdling experiment by Malpighi confirmed the ascent of sap through xylem.

• Mass Flow or Pressure Flow theory for the movement of food was given by Munch.

Mechanism of Ascent of Sap

👉🏻 To explain the mechanism of ascent of sap, various theories are put forwarded which are classified under 3 headings:

Vital Theories

🐠 Relay or Clambering Pump Theory

• This theory was put forwarded by Godlewski (1884) to explain ascent of sap. According to this theory,

“Rhythmic change in the osmotic pressure of the living cells of xylem parenchyma and medullary rays bring about a pumping action of water in an upward direction.”

• Relay pump theory was contradicted by Strasburger (1893). He proved that ascent of sap was independent of living cells because water continued to be transported above even after the killing of living cells by poison (i.e. Picric acid) or high temperature.

🐠 Pulsation Theory

• By Sir J.C. Bose (1923), experiment on Demodium gyrans (Indian telegraph plant, Family: Fabaceae or Leguminosae). According to Sir J.C. Bose,

“Living cells of the inner most layer of the cortex, just outside the endodermis are in a state of rhythmic pulsations which cause the pumping of water for cell to cell in an upward direction.”

• Benedict (1927) found that the actual rate of ascent of sap was 8,000 to 30,000 times as rapid as would be possible according to the Bose theory.

Root Pressure Theory

• ‘Root Pressure’ was coined by Stephan Hales. The hydrostatic pressure developed due to the accumulation of water absorbed by the roots is called root pressure.
• Root pressure is measured by manometer.
• But Root pressure is not sufficient to drive water to a distance of 400 ft. in the trunks of tall trees.

Physical Theories

👉🏻 All these theories consider the dead cells of plant to be responsible for ascent of sap. 🐠 Boehm’s theory (1809)

• According to it, the ascent of sap is partly due to the phenomena of capillarity of the trachea and partly due to the atmospheric pressure.
• The highest column of water attained by capillary forces is 4 ft. and by atmospheric pressure is 34 ft. only.

🐠 Jamin’s chain theory

• Air and water are alternately arranged inside the plant.
• When air expands, it moves up carrying along with it the water column present above it.

🐠 Imbibition Theory

• According to Sachs (1878), imbibition activity of cell walls of xylem is responsible for ascent of sap.

🐠 Cohesion and Adhesion theory or Transpiration Pull theory

• This theory was given by Dixon & Jolly (1894) and Askenasy (1895). This theory is most accepted theory.
• There are three features of this theory:
  • Strong cohesive force or tensile strength of water.
  • Continuity of water column in the plant.
  • Transpiration pull or tension on the unbroken water column.

Loss of water

• The loss of water from the living tissues of plants takes place either in vapour form (i.e. transpiration) or in liquid form (i.e. Guttation, Bleeding).

Transpiration

• The loss of water in the form of vapour from the living aerial parts of the plant is known as transpiration.
• The principal organ of transpiration is leaf.
• Transpiration may be foliar or lenticular.
• Again foliar transpiration is of two types viz. stomata (through stomata) and cuticular (through cuticle).

  

  

Difference between Transpiration & Evaporation

Factors effecting transpiration

• Hot 🥵 dry 💨 windy day causes an increase in transpiration.