Class 12 Biology Chapter 6 Plant Water Relation textbook solutions

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Class 12 Biology Chapter 6 Plant Water Relation Textbook Solutions

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Q. 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

2. The most widely accepted theory for ascent of sap is …………..
 a. capillarity theory
 b. root pressure theory
 c. diffusion
 d. transpiration pull theory

3. Water movement between the cells is due to ………….
a. T. P.
b. W. P.
c. DPD
d. incipient plasmolysis

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

5. Surface tension is due to …………..
a. diffusion
b. osmosis
c. gravitational force
d. cohesion

6. Which of the following type of solution has lower level of solutes than the solution?
 a. Isotonic
b. Hypotonic
c. Hypertonic
d. Anisotonic

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

8. Water absorption takes place through …………. 
a. lateral roots
b. root cap
c. root hair
d. primary root

9. Due to low atmospheric pressure the rate of transpiration will ………….
 a. increase
 b. decrease rapidly
 c. decrease slowly
 d. remain unaffected

10. Osmosis is a property of ………………
a. solute
b. solvent
c. solution
d. membrane

Q. 2 Very Short Answer Questions.

1. What is osmotic pressure?
Answer : Osmosis is defined as the flow of water/solvent molecules through a semipermeable membrane from a region of low to high solute concentration.

2. Name the condition in which protoplast of the plant cell shrinks.
Answer : The phenomenon by which protoplast of a cell shrinks from the wall is plasmolysis. It is the process where the shrinkage and contraction of plant cells take place. This happens because of loss of water.

3. What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
Answer : If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. … Pressure can build up in a plant system when water enters a plant cell due to diffusion causing a pressure built up against the cell wall, it makes the cell turgid.

4. Which type of solution will bring about deplasmolysis?
Answer : Plasmolysis is the process in which cells lose water in a hypertonic solution. The reverse process, deplasmolysis or cytolysis, can occur if the cell is in a hypotonic solution resulting in a lower external osmotic pressure and a net flow of water into the cell.

5. Which type of plants have negative root pressure?
Answer : Clark (1874) tested over 60 species of woody plants in Massachusetts and found exudation from only a few species, including maple, birch, walnut, hop hornbeam, and grape. Sap flow ceases as leaves develop and increasing transpiration produces negative pressure or tension in the xylem sap.

6. In which conditions transpiration pull will be affected?
Answer : Transpiration pull will be maximum under open stomata, dry atmosphere and moist soil. Answer: Transpiration pull will be maximum under open stomata, dry atmosphere and moist soil.

7. Mention the shape of guard cells in Cyperus.
Answer : As mentioned, guard cells are bean/kidney-shaped cells located on plant epidermis. As such, they, like trichomes and pavement cells, are also epidermal cells.

8. Why do diurnal changes occur in osmotic potential of guard cells?
Answer : It is generally believed that, by lowering the osmotic potential, higher solute concen- trations can contribute to a greater capacity for turgor maintenance.

9. What is symplast pathway?
Answer : It provides the movement of water from one cell to another cell by plasmodesmata. Symplast, which is also called as cytoplasmic is an inner side of the plasma membrane.

Q. 3 Answer the Following Questions

1. Describe mechanism for absorption of water. 
Answer : In higher plants water is absorbed through root hairs which are in contact with soil water. Mainly, there are two ways/ modes of absorption of water viz, passive absorption and active absorption.

a. Passive absorption :
It is the main way of absorbing water through the roots and not by the roots from soil into the plant. The driving force is transpiration pull and it thus proceeds through DPD gradient. There is no expenditure of energy (ATP) as water moves in accordance to the concentration gradient. Hence, it is passive absorption. About 98% of the total water absorbed in plants, occur passively. Passive absorption occurs during day time when transpiration is in progress. It stops at night when transpiration stops.

b. Active absorption :
Here, water is absorbed due to activity of roots. Root cells play active role in the absorption of water. The driving force is the root pressure developed, in the living cells of root. Active absorption occurs usually at night when transpiration stops due to closure of stomata. As water absorption is against the DPD gradient, there is expenditure of ATP (energy) generated through the respiratory activity of cells. Active absorption may be of two kinds viz, osmotic and non-osmotic.

2. Discuss theories of water translocation. 
Answer : The transport of water with dissolved minerals from root to other aerial parts like stem and leaves, against the gravity, is called translocation or ascent of sap. Several mechanisms/ theories have been put forth to explain the mechanism of translocation of water. The theories include-vital force theory, relay pump theory, physical force theory, root pressure theory, etc. We shall consider following three theories :

Root Pressure Theory: Root pressure is a positive pressure that develops in the xylem sap of the root of some plants. It is a manifestation of active water absorption. Root pressure is observed in certain seasons which favour optimum metabolic activity and reduce transpiration.

Capillarity theory (physical force theory): 
According to this theory, physical forces and dead cells are responsible for ascent of sap. This theory was put forth by Bohem in (1863). Wick dipped in an oil lamp, shows capillarity due to which oil is raised upwards. The conduction of water in a straw dipped in water, is raised to a certain height because of capillarity.

Transpiration pull theory states that. Water is pulled from above and not pushed from below through roots. As there is gravitational force downside but still two forces are there which helps to pull the water upward I.e. Cohesive and adhesive forces.

3. What is transpiration? Describe mechanism of opening and closing of stomata.
Answer : The loss of water in the form of vapour is called transpiration that occurs through leaves, stem, flowers and fruits. Most of the transpiration occurs through the leaves (called foliar transpiration). The actual water loss during transpiration occurs through three main sites – cuticle, stomata and lenticels. Accordingly, three types of transpiration are recognized viz.
cuticular, stomatal and lenticular.

Machanism of opening and closing of stomata :

• The stomata are very minute apertures, usually found on the epidermis of the leaves. Each stoma is surrounded by two kidney-shaped special epidermal cells, known as guard cells.
• Opening and closing of stoma is controlled by turgor of guard cells. During day time, guard cells become turgid due to endosmosis. Thus turgor pressure is exerted on the thin walls of guard cells. Being elastic and thin, lateral walls are stretched out.
• Due to kidney or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma. During night time, guard cells become flaccid due to exosmosis.
• Flaccidity closes the stoma almost completely. Endosmosis and exosmosis occur due to diurnal changes in osmotic potential of guard cells. Different theories are proposed to explain diurnal changes in osmotic potential.

• According to theory of proton transport (Levitt-1974), stomatal movement occurs due to transport of protons H+ and K+ ions. During daytime, starch is converted into malic acid.
• Malic acid dissociates to form Malate and protons. Protons are transported to subsidiary cells and K+ ions are imported from them.
• Potassium malate is formed that increases osmolarity and causes endosmosis. Uptake of K+ ions is always accompanied with Cl¯ ions.
• At night, uptake of K+ and Cl- ions is prevented by abscissic acid, changing the permeability of guard cells. Due to this guard cells become hypotonic and thereby become flaccid.

• According to starch-sugar inter-conversion theory (Steward 1964), during day time, enzyme phosphorylase converts startch to sugar, thus increasing osmotic potential of guard cells cosing entry of water there by gaurd cells are stretched and stoma widens. The reverse reaction occures at night brining about the closure of stoma.

4. What is transpiration? Explain role of transpiration.
Answer :

The loss of water in the form of vapour is called transpiration that occurs through leaves, stem, flowers and fruits. Most of the transpiration occurs through the leaves.

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

Root pressure theory :
The root-pressure hypothesis of sap rise holds that pressures in trees are several times atmospheric pressure, not nearly enough to transport water to the top of the tallest trees. Furthermore, root pressures tend to be lowest when water loss from leaves (transpiration) is highest, exactly when trees most need water.

Objections/ limitations of root pressure theory:
Although, ascent of sap takes place due to root pressure, there are certain objections raised, such as

• i. It is not applicable to plants taller than 20 meters.
• ii. Ascent of sap can also occur even in the absence of root system.
• iii. Root pressure value is almost nearly zero in taller gymnosperm trees.
• iv. In actively transpiring plants, no root pressure is developed.
• v. Xylem sap under normal condition is under tension i.e. it shows negative hydrostatic pressure or high osmotic pressure.

6. Explain capillarity theory of water translocation.
Answer : Capillarity theory (physical force theory): According to this theory, physical forcesand dead cells are responsible for ascent of sap. This theory was put forth by Bohem in(1863). Wick dipped in an oil lamp, shows capillarity due to which oil is raised upwards. The conduction of water in a straw dipped in water, is raised to a certain height because of capillarity.

The height to which water is raised depends on the diameter of the straw. Capilarity is because of surface tension, and forces of cohesion (attraction between like molecules) and adhesion (attraction between unlike molecules). Xylem vessel/ tracheid with its lumen is comparable with straw. Water column exist because of combined cohesive and adhesive forces of water and xylem wall, due to capillarity. It is because of capillarity water is raised or conducted upwards against the gravity, to few centimeters only.

7. Why is transpiration called ‘a necessary evil’?
Answer : transpiration is a necessary evil. Stomata allows gaseous exchange. Carbon dioxide is taken in for its fixation by Calvin cycle and oxygen produced during the photolysis is given out. This gaseous exchange takes place by the process of diffusion.

8. Explain movement of water in the root. 
Answer : Water can now move from the root hair cells and across the parenchyma cells of the cortex in two major ways. Some water passes through the cells by osmosis. Most water travels either in, or between the cell walls (of the parenchyma cells) by simple diffusion.

9. Define and or explain the terms: 
1. Osmosis : semi-permeable membrane from a region of high concentration to a region of low concentration, tending to equalise the concentrations of the water.

2. diffusion : Diffusion is the net passive movement of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration.

3. plasmolysis : Plasmolysis is the process where a cell’s contents shrink away from the cell wall when placed in a hypertonic solution.

4. imbibition :  imbibition The uptake of water by substances that do not dissolve in water, so that the process results in swelling of the substance.

5. guttation : The loss of water in the form of liquid is called guttation. It occurs through special structures called water stomata or hydathodes.

6. transpiration : The loss of water in the form of vapour is called transpiration that occurs through leaves, stem, flowers and fruits.

7. ascent of sap :  The transport of water with dissolved minerals from root to other aerial parts like stem and leaves, against the gravity, is called translocation or ascent of sap.

8. active absorption : Absorption of water against the concentration of gradient by the use of metabolic energy or ATP

9. DPD : The difference in the diffusion
pressures of pure solvent and the solvent in a solution is called Diffusion Pressure Deficit (DPD) or Suction Pressure (SP).

10. turgor pressure : Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall.

11. water potential : Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. DPD refer as water potential.

12. wall pressure : the pressure exerted on the contents of a plant cell by the cell wall that is equal in force and opposite in direction to the turgor pressure.

13. root pressure : Root pressure is a force or the hydrostatic pressure generated in the roots that help in driving the fluids and other ions from the soil in upwards directions into the plant’s

10. Distinguish between a) Osmotic pressure and turgor pressure 
Answer : Osmotic pressure is the minimum pressure that needs to be applied on the solution to prevent the inward flow of its pure solvent across a semipermeable membrane and turgor pressure is the pressure inside the cell pushes plasma membrane against the cell wall of the plant cell.

b) Diffusion and osmosis.
Answer : Osmosis: Osmosis is the movement of solvent particles across a semipermeable membrane from a dilute solution into a concentrated solution. … Diffusion: Diffusion is the movement of particles from an area of higher concentration to lower concentration. The overall effect is to equalize concentration throughout the medium.

11. Enlist macronutrients and micronutrients required for plant growth.
Answer :

1. Macronutrients include carbon, hydrogen, nitrogen, oxygen, phosphorous, potassium, calcium, sulfur, and magnesium. 
2. Micronutrients are boron, chlorine, manganese, iron, zinc, copper, and molybdenum. A plant uses these nutrients to support its growth, life cycle, and biological functions.

12. How are the minerals absorbed by the plants?
Answer : Soil serves as main source for minerals. Minerals constitute most commonly occuring solid, crystalline inorganic materials obtained from earth’s crust. Minerals play an important role in the day to day life of plant. Minerals are absorbed by plants in the ionic (disolved) form, mainly through roots and then transported.

The analysis of plant ash demonstrates that minerals are absorbed by plants from soil and surroundings. Absorption of minerals is independent of that of water. Absorbed mineral ions are pulled in upward direction along with xylem sap because of transpiration pull. This could be understood when the ascending sap is analysed. Mineral ions are needed in the areas of the plant viz. apical, lateral, young leaves, developing flowers, fruits, seeds and storage organs. Hence, from the source (root), these are pulled and transported ascendingly through the sap.

Q. 4 Long Answer Questions.

1. Describe structure of root hair.
Answer :

Root hair is cytoplasmic extension (prolongation) of epiblema cell. Each root hair may be approximately 1 to 10mm long and tube like structure. It is colourless, unbranched, short-lived (ephemeral) and very delicate. It has a large central vacuole surrounded by thin film of cytoplasm, plasma membrane and thin cell wall, which is two layered. Outer layer is composed of pectin and inner layer is made up of cellulose. Cell wall is freely permeable but plasma membrane is selectively permeable.

2. Write on journey of water from soil to xylem in roots.
Answer : Water from the soil enters the root hairs by moving along a water potential gradient and into the xylem through either the apoplast or symplast pathway. It is carried upward through the xylem by transpiration, and then passed into the leaves along another water potential gradient. In the leaf, some water is lost through evaporation from the stomata and the remaining fluid moves along a water potential gradient from the xylem into the phloem, where it is distributed along with the organic nutrients produced by photosynthesis throughout the plant.

3. Explain cohesion theory for translocation of water.
Answer : Cohesion- tension theory (Transpiration pull theory) :This is presently widely accepted theory explaining ascent of sap in plants. It was put forth by Dixon and Joly (1894). This theory is based on two principles.Cohesion and adhesion, and transpiration pull :A strong force of attraction between water molecules, is called cohesive force. While a strong force of attraction between water molecules and lignified wall of lumen of xylem vessel, is called adhesive force. Due to combined cohesive and adhesive forces a continous water column is developed (formed) in the xylem right from root upto the tip of the topmost leaf in the plant.

Transpiration pull : The transpiration pull developed in the leaf vessel is transmitted down to root and thus accounts for the ascent of sap. Excess water is lost in the form of vapour, mainly through the stomata found on leaf. This water loss increases D. P. D. of mesophyll cells. These cells withdraw water ultimately from xylem in the leaf. In otherwords, due to continous transpiration, a gradient of suction pressure (i.e. D. P. D.) is developed right from guard cells up to the xylem in the leaf. This will create a tension (called negative pull or transpiration pull) in the xylem. Consequently, water column is pulled out of xylem. Thus, water is pulled upwards passively against the gravity leading to the ascent of sap.

4. Write on the mechanism of opening and closing of stoma.
Answer :

Stomata are minute, elliptical pores bounded by two kidney/ dumbbell shaped guard cells. Guard cell is a type of epidermal tissue which may be called as modified,

Opening and closing of stoma is controlled by turgor of guard cells. During day time, guard cells become turgid due to endosmosis. Thus turgor pressure is exerted on the thin walls of guard cells. Being elastic and thin, lateral walls are stretched out. Due to kidney or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma. During night time, guard cells become flaccid due to exosmosis. Flaccidity closes the stoma almost completely. Endosmosis and exosmosis occur due to diurnal changes in osmotic potential of guard cells. Different theories are proposed to explain diurnal changes in osmotic potential.

According to starch-sugar inter- conversion theory (Steward 1964), during day time, enzyme phosphorylase converts startch to sugar, thus increasing osmotic potential of guard cells cosing entry of water there by gaurd cells are stretched and stoma widens. The reverse reaction occures at night brining about the closure of stoma.

According to theory of proton transport (Levitt-1974), stomatal movement occurs due to transport of protons H+ and K+ ions. During daytime, starch is converted into malic acid. Malic acid dissociates to form Malate and protons. Protons are transported to subsidiary cells and K+ ions are imported from them. Potassium malate is formed that increases osmolarity and causes endosmosis. Uptake of K+ ions is always accompanied with Cl¯ ions. At night, uptake of K+ and Cl- ions is prevented by abscissic acid, changing the permeability of guard cells. Due to this guard cells become hypotonic and thereby become flaccid.

5. What is hydroponics? How is it useful in identifying the role of nutrients?
Answer : It is a subcategory of hydroculture and is a technique of growing flora using mineral nutrient solutions in the liquid medium, without soil.

The main advantage of using hydroponics method is:

1. Conservation of water and nutrients.
2. No more use of fertilizers, pesticides, and other chemicals.
3. It can be grown anywhere as it requires very less space for growing and involves a soil-free condition.
4. It minimizes the loss of nutrients and has a lot more accurate control over the nutrients required for the plants.
5. Plant growth is completely dependent on the nutrient solution provided. Thus, there is a controlled plant growth.

6. Explain the active absorption of minerals.
Answer : Active mineral absorption is an active process and thus it requires the expenditure of metabolic energy (ATP). Active mineral absorption can occur both along and against the concentration gradient by osmosis or through special carrier proteins in the plasma membrane.

7. Write on macro- and micro nutrients required for plant growth.
Answer : Plant nutrition is the study of the chemical elements and compounds necessary for plant growth, plant metabolism and their external supply. In its absence the plant is unable to complete a normal life cycle, or that the element is part of some essential plant constituent or metabolite.

Minerals that play important role in the day to day life, are called essential elements. About 36 to 40 elements are incorporated in the plant’s life.

Some minerals like C, H, O, P, N, S, Mg required in large quantity, are called macro elements. While minerals like Cu, Co, Mn, B, Zn required in small quantity, are called micro elements.

Maharashtra State Board Class 12 Biology Solutions

• Chapter 1: Reproduction in Lower and Higher Plants 
• Chapter 2: Reproduction in Lower and Higher Animals
• Chapter 3: Inheritance and Variation
• Chapter 4: Molecular Basis of Inheritance
• Chapter 5: Origin and Evolution of Life
• Chapter 7: Plant Growth and Mineral Nutrition
• Chapter 8: Respiration and Circulation
• Chapter 9: Control and Co-ordination
• Chapter 10: Human Health and Diseases
• Chapter 11: Enhancement of Food Production
• Chapter 12: Biotechnology
• Chapter 13: Organisms and Populations
• Chapter 14: Ecosystems and Energy Flow
• Chapter 15: Biodiversity, Conservation, and Environmental Issues