Solutions-Class-12-Biology-Chapter-6-Plant Water Relations-Maharashtra Board

Plant Water Relations

Maharashtra Board-Class-12th-Biology-Chapter-6


Question 1. Multiple Choice Questions

(1) In soil, water available for absorption by root is ...................

(a) gravitaional water

(b) capillary water

(c) hygroscopic water

(D) combined water

Answer :

(b) capillary water

(2) The most widely accepted theory for ascent of sap is ..............

(a) capillarity theory

(b) root pressure theory

(c) diffusion

(d) transpiration pull theory

Answer :

(d) transpiration pull theory

(3) Water movement between the cells is due to .............

(a) T. P.

(b) W. P.

(c) DPD

(d) incipient plasmolysis

Answer :

(c) DPD

(4) In guard cells, when sugar is converted into starch, the stomatal pore .............

(a) closes almost completely

(b) opens partially

(c) opens fully

(d) remains unchanged

Answer :

(a) closes almost completely

(5) Surface tension is due to ..............

(a) diffusion

(b) osmosis

(c) gravitational force

(d) cohesion

Answer :

(d) cohesion

(6) Which of the following type of solution has lower level of solutes than the


(a) Isotonic

(b) Hypotonic

(c) Hypertonic

(d) Anisotonic

Answer :

(b) Hypotonic

(7) During rainy season wooden doors warp and become difficult to open or to close

because of ...............

(a) plasmolysis

(b) imbibition

(c) osmosis

(d) diffusion

Answer :

(b) imbibition

(8) Water absorption takes place through .............

(a) lateral roots

(b) root cap

(c) root hair

(d) primary root

Answer :

(c) root hair

(9) Due to low atmospheric pressure the rate of transpiration will .............

(a) increase

(b) decrease rapidly

(c) decrease slowly

(d) remain unaffected

Answer :

(a) increase

(10) Osmosis is a property of ..................

(a) solute

(b) solvent

(c) solution

(d) membrane

Answer :

(c) solution


Question 2 Very short answer questions.

(1) What is osmotic pressure?

Answer :

The pressure exerted due to osmosis is osmotic pressure.

(2) Name the condition in which protoplast of the plant cell shrinks.

Answer :

During plasmolysis, protoplast of cell shrinks and recedes from cell wall.

(3) What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?

Answer :

The water potential of pure water or a solution increases when a pressure larger than atmospheric pressure is applied to it.

(4) Which type of solution will bring about deplasmolysis?

Answer :

Placing a plasmolysed cell in hypotonic solution will bring about deplasmolysis.

(5) Which type of plants have negative root pressure?

Answer :

Plants showing excessive transpiration have negative root pressure.

(6) In which conditions transpiration pull will be affected?

Answer :

Temperature variations during the day and night might cause gas bubbles to develop, affecting transpiration pull.

(7) Mention the shape of guard cells in Cyperus.

Answer :

Kidney shaped and dumbbell shaped guard cells are seen.

(8) Why do diurnal changes occur in osmotic potential of guard cells?

Answer :

During the day, phosphorylase enzyme activity transforms starch to sugar, and at night, sugar is converted back to starch. This alters the osmotic potential of guard cells.

(9) What is symplast pathway?

Answer :

When water is absorbed by root hair, it travels from one living cell to another via plasmodesmatal connections, which is referred to as the symplast pathway across the root.

Question 3 Answer the following questions.

(1) Describe mechanism for absorption of water.

Answer :

  • Water absorption occurs in two modes: active absorption and passive absorption.
  • Passive absorption accounts for 98% of all absorption.
  • Passive absorption requires no energy cost.
  • The driving force is transpiration pull, and water travels according to the concentration gradient. Water is drawn upwards.
  • It happens during the day when there is active transpiration.
  • Active absorption occurs most often at night, when transpiration ceases owing to stomatal closure.
  • Water absorption occurs against the D.P.D. gradient, requiring A.T.P. energy, which is obtained by respiration.
  • Active absorption may be osmotic or non-osmotic type.
  • Root pressure is important for osmotic absorption.

(2) Discuss theories of water translocation.

Answer :

  • Water translocation is the movement of water and dissolved minerals from roots to aerial regions.
  • The movement is against gravity and is referred to as sap ascension.
  • The translocation takes place through the lumen of water carrying tissue xylem, namely vessels and tracheids.
  • Different ideas for translocation mechanisms have been considered, such as vital force theory (Root pressure), relay pump, physical force (capillary), and so on.
  • The most commonly recognised explanation is the cohesion tension theory, often known as the transpiration pull theory.

(3) What is transpiration? Describe mechanism of opening and closing of stomata.

Answer :

  • The loss of water as vapour is referred to as transpiration.
  • Stomatal transpiration is a major kind of transpiration that involves minute pores.
  • Stomata are surrounded by accessory cells and separated by two guard cells.
  • The turgidity of guard cells controls the opening and shutting of stomata.
  • Endosmosis causes guard cells to become turgid, and their lateral thin and elastic wall bulges or stretches out.
  • The inner thick and inelastic wall is pushed apart, allowing the stoma to open during the day.
  • When guard cells become flaccid owing to exosmosis at night, the wall relaxes and the stoma closes.
  • Endosmosis and exosmosis occur as a result of changes in the osmotic potential of guard cells.

(4) What is transpiration? Explain role of transpiration.

Answer :

Transpiration : The loss of water from plant body in the form of vapour is called transpiration.

Role of transpiration :

  • It removes excess of water.
  • It helps in the passive absorption of water and minerals from soil.
  • It helps in the ascent of sap.
  • As stomata are open, gaseous exchange required for photosynthesis and respiration, is facilitated.
  • It maintains turgor of the cells.
  • Transpiration helps in reducing the temperature of leaf and in imparting cooling effect.

(5) What is significance of transpiration? Explain root pressure theory and its limitations.

Answer :

Significance of transpiration :

Advantages :

  • It removes excess of water.
  • It helps in the passive absorption of water and minerals from soil.
  • It helps in the ascent of sap.
  • As stomata are open, gaseous exchange required for photosynthesis and respiration, is facilitated.
  • It maintains turgor of the cells.
  • Transpiration helps in reducing the temperature of leaf and in imparting cooling effect.


  • Excessive transpiration leads to wilting and injury in the plant. It may also lead to the death of the plant.

Root pressure theory :

  • Root pressure theory is proposed by J. Pristley.
  • For translocation of water, activity of living cells of root is responsible.
  • Absorption of water by root hair is a constant and continuous process and due to this a hydrostatic pressure is developed in cortical cells.
  • Owing to this hydrostatic pressure i.e. root pressure, water is forced into xylem and further conducted upwards.
  • Root pressure is an osmotic phenomenon.

Limitations of root pressure theory :

  • Not applicable to tall plants above 20 metres.
  • Even in absence of root pressure ascent of sap is noticed.
  • In actively transpiring plants, root pressure is not developed.
  • In taller gymnosperms, root pressure is zero.
  • Xylem sap is under tension and shows negative hydrostatic pressure.

(6) Explain capillarity theory of water translocation.

Answer :

  • Capillarity theory of water translocation is proposed by Bohem.
  • According to this theory, physical forces and dead cells are responsible for ascent of sap.
  • Capillarity is because of surface tension and cohesive forces and adhesive forces of water molecules.
  • Xylem vessels and tracheids are tubular elements having their lumen.
  • In these elements water column exists due to combined action of cohesive and adhesive forces of water and lignified wall.
  • As a result of this capillarity water is raised upwards.

(7) Why is transpiration called ‘a necessary evil’?

Answer :

  • The loss of water in the form of water vapour is called transpiration.
  • About 90 — 93% of transpiration occurs through stomata, small apertures located in the epidermis of leaves.
  • For stomatal transpiration to occur, stoma must remain open, during day time.
  • When stomata are open then only the gaseous exchange needed for respiration and photosynthesis, will take place.
  • If stomatal transpiration stops, it will directly affect productivity of plant through the loss of photosynthetic and respiratory activity.
  • The process is necessary evil because water which is important for plant is lost in the process.
  • At the same time, it helps in absorption of water and its translocation. Hence it cannot be avoided.
  • Hence for productivity, stomata must remain open. Consequently, transpiration cannot be avoided.
  • Hence, Curtis regarded transpiration as ‘a necessary evil’.

(8) Explain movement of water in the root.

Answer :

  • Root hairs absorb water by imbibition then diffusion which is followed by osmosis.
  • As water is taken inside the root hair cell it becomes turgid i.e. increase in turgor pressure [T.P.]
  • Root hair cell has less D.P.D. but adjacent cortical cell has more D.P.D.
  • The inner cortical cell has more osmotic potential so it will suck water from root hair cell.
  • Root hair cell becomes flaccid and ready to absorb soil water.
  • Water is passed on similarly in inner cortical cells.
  • Water moves rapidly through loose cortical cells up to endodermis and through passage cells in pericycle.
  • From pericycle due to hydrostatic pressure developed it is forced into protoxylem.

(9) Define and or explain the terms:

(i) Osmosis,

Answer :

Osmosis : It is a special type of diffusion of solvent through a semipermeable membrane.

(ii) Diffusion

Answer :

Diffusion : It is the movement of ions/atoms/molecules of a substance from the

region of higher concentration to the region of their lower concentration. -

(iii) Plasmolysis

Answer :

Plasmolysis : Exo-osmosis in a living cell when placed in hypertonic solution is called plasmolysis.

(iv) Imbibition

Answer :

Imbibition : It is swelling up of hydrophilic colloids due to adsorption of water.

(v) Guttation

Answer :

Guttation : The loss of water in the form of liquid is called guttation.

(vi) Transpiration

Answer :

Transpiration : The loss of water from plant body in the form of vapour is called transpiration.

(vii) ascent of sap

Answer :

Ascent of sap : The transport of water with dissolved minerals in it from root to other aerial parts of plant against the gravity is called ascent of sap.

(viii) active absorption.

Answer :

Active absorption : Water absorption by activity of root which is against the D.P D. gradient along with expenditure of A.T.P. energy generated by respiration is the

process of active absorption.

(ix) DPD.

Answer :

Diffusion Pressure Deficit [D.R D.) : The difference in the diffusion pressures of pure solvent and the solvent in a solution is called diffusion pressure deficit.

(x) Turgor pressure

Answer :

Turgor pressure : It is the pressure exerted by turgid cell sap on to the cell membrane and cell wall.

(xi) Water potential.

Answer :

Water potential : Chemical potential of Water is called water potential.

(xii) Wall pressure.

Answer :

Wall pressure : Thick and rigid cell wall exerts a counter pressure to turgor pressure developed on the cell sap is called wall pressure that operates in opposite


(xiii) Root pressure.

Answer :

Root pressure : As absorption of water by root hair being a continuous process, a sort of hydrostatic pressure is developed in living cells of root, this is called root


(10) Distinguish between

(a) Osmotic pressure and turgor pressure

Answer :

Osmotic pressure Turgor pressure
The pressure exerted due to osmosis is called osmotic pressure. The pressure exerted by turgid cell sap on cell membrane and cell wall, is called turgor pressure.
It is pressure caused by water when it moves by osmosis. It is pressure caused by content of cell (cell sap).
It is generated by the osmotic flow of water through a semipermeable membrane. It is maintained by osmosis.

(b) Diffusion and osmosis.

Answer :

Diffusion osmosis
Diffusion is the movement of ions, atoms or molecules of solute, liquids or gases. Osmosis is the special type of diffusion of water or solvent.
Diffusion involves the flow of diffusing particles in both the directions. Osmosis involves the unidirectional Ilow of solvent molecules.
Diffusion does not occur through a semi-

permeable membrance.

Osmosis occurs through a semi-permeable membrance.
Diffusion occurs from a place of higher

concentration of diffusing particles to a place of lower concentration.

Osmosis occurs from a solution of lower

concentration to a solution of higher


(11) Enlist macronutrients and micronutrients required for plant growth.

Answer :

  • Some minerals which are required in large amounts for plant metabolism are macronutrients. E.g. C, H, O, P, N, S, Mg, Ca, K. etc.
  • Some minerals which are required in small amounts for plant metabolism are micronutrients. E.g. Cu, Co, Mn, B, Zn, Cl, etc.

(12) How are the minerals absorbed by the plants?

Answer :

  • Soil is the chief source of minerals for the plants.
  • Minerals get dissolved in the soil water.
  • Minerals are absorbed by the plants in the ionic form mainly through roots.
  • Absorption of minerals is independent of water.

Question 4 Long answer questions.

(1) Describe structure of root hair.

Answer :

  • Root hairs helps plants in absorbing water from the soil.
  • In the absorption zone, root hairs are present.
  • Ephemeral (short-lived) structures called unicellular extensions are created by epidermal cells. e.g., root hairs.

  • Root hairs are nothing but cytoplasmic extensions of epiblema cell.
  • Root hairs are long tube like structures of about 1 to 10 mm.
  • They are colourless, unbranched and very delicate structures.
  • A large central vacuole is surrounded by thin layer of cytoplasm, plasma membrane and outer cell wall.
  • The cell wall of root hair is thin and double layered with outer layer of pectin and inner layer of cellulose which is freely permeable.

(2) Write on journey of water from soil to xylem in roots.

Answer :

  • The readily accessible capillary water from the soil is absorbed by unicellular root hairs, which are tubular extensions of epiblema cells.
  • Imbibition, diffusion, and osmosis are the three physical processes that deal with the absorption of water.
  • Imbibition is the process through which water molecules adsorb on root hair cell walls.
  • They diffuse through the cell wall, which is freely permeable, and enter the root hair cell.
  • They penetrate through the semipermeable plasma membrane through the mechanism of osmosis.
  • The root hair cell becomes turgid, which causes an increase in turgor pressure and a drop in D.P.D. value.
  • Since the next cortical cell has a higher osmotic potential, it has a higher D.P.D. value.
  • Thus, the cortical cell absorbs water from the turgid epidermal cell. This process continues as a result of the gradient of suction pressure that forms from cell to cell until the endodermis' thin-walled passage cells.
  • It will exit the endodermis and enter the pericycle before being driven into the protoxylem cell by hydrostatic pressure.
  • The pathway of water is by apoplast and symplast.
  • When water passes through cell wall and intercellular spaces of cortex it is apoplast pathway.
  • When water passes across living cells through their plasmodesmatal connections it is symplast pathway.

(3) Explain cohesion theory for translocation of water.

Answer :

  • This is very widely accepted theory of ascent of sap proposed by Dixon and Joly.
  • It is based on the concepts of water molecule adhesion and cohesion as well as plant transpiration.
  • Cohesion, a strong force of attraction between water molecules, and adhesion, a strong force of attraction between water molecules and the lignified walls of xylem components, are two more strong forces of attraction.
  • Sap rises via the Xylem components' lumen.
  • A continuous water column is maintained in the xylem from the root to the aerial portions, or leaves, due to cohesive and adhesive forces.
  • Stomata allow for transpiration, and leaf vessels acquire a transpiration pull as a result.
  • This tension or pull is transmitted downwards through vein to roots which triggers ascent of sap.
  • In transpiration, water is lost in vapour form and this increases D.P.D. of mesophyll cells that are near guard cells.
  • Mesophyll cells absorb water from xylem in leaf and a gradient of D.P.D. or suction pressure (S.P.) is set.
  • Owing to this gradient from guard cell to xylem in leaf, a transpiration pull or tension is created in xylem.
  • Hence water column is pulled upward passively against gravity.

(4) Write on the mechanism of opening and closing of stoma.

Answer :

  • Transpiration takes place through stomata.
  • Turgidity of guard cells controls opening and closing of stomata.
  • The movement is caused by turgor pressure acting on the guard cell's unevenly thickened wall.
  • Stomata open as a result of the outer, thin, elastic wall stretching out and pulling the inner, thick, inelastic wall.
  • Stomata close when the guard cells become floppy.
  • The enzyme phosphorylase turns starch into sugar during the day, in accordance with the starch-sugar interconversion theory.
  • Because sugar is osmotically active, guard cells' O.P. is raised. The water is taken up by auxiliary cells. Turgidity causes the stoma to open and the walls to stretch.
  • During night-time sugar is converted to starch and hence guard cells loose water and become flaccid. Hence there is closure of stomata.
  • According to proton transport theory, the movement is due to transport of H+ and K+
  • Subsidiary cells are reservoirs of K+
  • Starch is converted to malic acid which dissociate into malate and proton (H+) during day.
  • Proton transported to subsidiary cells and K+ ions are taken from it. This forms potassium malate in guard cells.
  • Potassium malate increases osmotic potential and endo osmosis occurs hence turgidity of guard cells. —> stomata opens.
  • The uptake of K+ and CI- ions is stopped by abscissic acid formed during night. This changes permeability. Guard cells become hypotonic and loose water as they become flaccid stomata close.

Note : The following questions (5), (6) & (7) are not based on textual matter of Chapter 6. These topics are covered in Chapter 7, but answers are given here.

(5) What is hydroponics? How is it useful in identifying the role of nutrients?

Answer :

  • Growing plants in aqueous (soilless) medium is known as hydroponics. [Greek
  • word hudor = water and ponos = work]
  • It is a method for growing plants that involves giving them all the nutrients they need in the water they are supplied.
  • Plant roots are suspended in this liquid with the proper support to create a nutritious medium. This medium is made by dissolving the necessary salts of macronutrients and micronutrients in the right proportion.
  • The symptoms of various mineral nutrient deficiencies may be studied in great detail using hydroponics.
  • With the aid of their root hairs, plants may absorb provided nutrient solutions or dissolved ions of mineral nutrients from their environment.
  • While preparing the required nutrient medium particular nutrient can be totally avoided and then the effect of lack of that nutrient can be studied in variation of plant growth.
  • Any visible change noticed from normal structure and function of the plant is the symptom or hunger sign considered.
  • For e.g. Yellowing of leaf is observed due to loss of chlorophyll pigments or Chlorosis is noticed if Magnesium is lacking as it is a structural component of chlorophyll pigment.

(6) Explain the active absorption of minerals.

Answer :

  • Plants absorb minerals from the soil with their root system.
  • Minerals are absorbed from the soil in the form of charged particles, positively charged cations and negatively charged anions.
  • The absorption of minerals against the concentration gradient which requires expenditure of metabolic energy is called active absorption.
  • The ATP energy derived from respiration in root cells is utilized for active absorption.
  • Ions get accumulated in the root hair against the concentration gradient.
  • These ions pass into cortical cells and finally reach xylem of roots.
  • Along with the water these minerals are carried to other parts of plant.

(7) Write on macro- and micro nutrients required for plant growth.

Answer :

  • Plants absorb mineral nutrients from their surroundings.
  • For a proper growth of plants about 35 to 40 different elements are required.
  • Plants absorb these nutrients in ionic or dissolved form from soil with their root system e.g. Phosphorus as PO4, Sulphur as SO42
  • Based on their requirement in quantity, they are classified as major nutrients or macronutrients and those needed in small amounts are minor or micronutrients.
  • Macroelements are required in large amounts, as they play nutritive and structural roles e.g. C, H, O, P, Mg, N, K, S and Ca. — Ca pectate cell wall component, Mg component of chlorophyll.
  • C, H, O are non-mineral major elements obtained from air and Water e.g. CO2 is source of carbon, Hydrogen from water.
  • Microelements are required in traces as they mainly have catalytic role as co-factors or activators of enzymes.
  • Microelements may be needed for certain activity in life cycle of plant e.g. B for pollen germination, Si has protective role during stress conditions and fungal attacks, Al enhances availability of phosphorus.
  • The important micronutrients for plant growth are Mn, B, Cu, Zn, Cl.

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