Plant Water Relation EXERCISES part-1

 

XII CH-6

 Plant Water Relation-EXERCISES part-1

1) Multiple Choice Question:

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

• 1. gravitational water
• 2. capillary water
• 3. hygroscopic water
• 4. combined water

Solution: In soil, water available for absorption by root is capillary water.

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

• 1. capillarity theory
• 2. root pressure theory
• 3. diffusion
• 4. transpiration pull theory

Solution: The most widely accepted theory for ascent of sap is transpiration pull theory.

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

• 1. T.P.
• 2. W.P.
• 3. DPD
• 4. incipient plasmolysiS

Solution: Water movement between the cells is due to DPD

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

• 1. closes almost completely
• 2. opens partially
• 3. opens fully
• 4. remains unchanged 

Solution: In guard cells, when sugar is converted into starch, the stomatal pore closes almost completely.

5)Surface tension is due to ____________.

• 1. diffusion
• 2. osmosis
• 3. gravitational force
• 4. cohesion

Solution: Surface tension is due to cohesion.

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

• 1. Isotonic
• 2. Hypotonic
• 3. Hypertonic
• 4. Anisotonic

Solution: Hypotonic

7)During rainy season wooden doors warp and become difficult to open or to close because of ______

• 1. plasmolysis
• 2. imbibition
• 3. osmosis
• 4. diffusion

Solution: During rainy season wooden doors warp and become difficult to open or to close because of imbibition.

8)Water absorption takes place through ______.

• 1. lateral roots
• 2. root cap
• 3. root hair
• 4. primary root

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

• 1. increase
• 2. decrease rapidly
• 3. decrease slowly
• 4. remain unaffected 

Solution: Due to low atmospheric pressure, the rate of transpiration will increase.

10) Osmosis is a property of ______.

• 1. solute
• 2. solvent
• 3. solution
• 4. membrane

2) Very short answer question.

1) What is osmotic pressure?

Ans- Osmosis-

i. The pressure exerted due to osmosis is called osmotic pressure.

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

Ans-Plasmolysis

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

Ans-

1. The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. 
2. For example: when water diffuses into a plant cell, it causes pressure to build up against the cell wall.
3.  This makes the cell wall turgid. This pressure is termed as pressure potential and has a positive value.
4.  If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. 
5. It is equivalent to pumping water from one place to another. 
6. 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?

Ans Hypotonic solution can bring about deplasmolysis.

5) Which type of plants have negative root pressure?

ans

The plants in which transpiration occurs rapidly especially during midsummer shows negative root pressure.

6)In which conditions transpiration pull will be affected?

ans:

For transpiration pull to operate, the water column should be unbroken and continuous. However, due to temperature fluctuations during day and night, gas bubbles may enter in water column breaking the continuity.

7)Mention the shape of guard cells in Cyperus.

ans:In Cyperus, both kidney-shaped and dumbbell-shaped guard cells are present.

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

ans:

• 1. According to Steward, diurnal changes occur in the osmotic potential of guard cells due to starch-sugar inter-conversion.
• 2. Whereas according to Levitt active transport of potassium ions into the guard cells and out of them causes diurnal changes in the osmotic potential of guard cells.
• 3. Endo-osmosis and exo-osmosis occur due to diurnal changes in osmotic potential of guard cells.  

9)What is the symplast pathway?

ans:

When water passes across from one living cell to another living cell through plasmodesmata, then it is called the symplast pathway. It is also called the trans-membrane pathway.

3)Answer the following question.

1)Describe the mechanism for absorption of water.

ans:A mechanism for absorption of water:

1. In plants, water is absorbed mainly by two processes: Passive absorption and Active absorption

2. Passive absorption:

• a. About 98% of the total water absorbed in plants occurs passively.
• b. In passive absorption, living cells of the root do not play an important role in water absorption.
• c. The driving force is transpiration pull and it thus proceeds through the DPD gradient.
• d. There is no expenditure of energy (ATP) as water moves in accordance with the concentration gradient. Hence, it is passive absorption.
• e. Passive absorption occurs during day time when transpiration is in progress. It stops at night when transpiration stops.
• f. Rapid transpiration creates tension in the xylem vessel due to negative water potential. This tension is transmitted to xylem in the roots. Consequently, water is pulled upwards passively.
• g. During passive absorption, no ATP is utilized. Thus, the rate of respiration is not affected.

3. Active absorption:

• a. In this water is absorbed due to the activity of roots.
• b. Root cells play an active role in the absorption of water.
• c. The driving force is the root pressure developed, in the living cells of the root.
• d. Active absorption occurs usually at night when transpiration stops due to closure of stomata.
• e. As water absorption is against the DPD gradient, there is an expenditure of ATP (energy) generated through the respiratory activity of cells.

2)Discuss theories of water translocation.

ans:Theories of water translocation:

• i. Various theories have been put forth to explain the mechanism of translocation of water.          These theories include Vital force theory, Relay pump theory, Physical force theory, Root pressure theory, etc.
• ii. Root Pressure Theory (Vital Theory): This theory was proposed by J. Priestley. According to this theory, the activity of living cells of the root is responsible for the translocation of water.
• iii. Capillarity theory (physical force theory): This theory was put forth by Boehm in (1863). According to this theory, physical forces and dead cells are responsible for the ascent of sap.
• iv. Cohesion- tension theory (Transpiration pull theory): This theory was put forth by Dixon and Jolly (1894). This is presently a widely accepted theory explaining the ascent of sap in plants. This theory is based on two principles i.e. Cohesion and adhesion, and transpiration pull.

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

ans:

Transpiration:

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

Mechanism of opening and closing of stomata:

• 1. The opening and closing of stoma is controlled by turgor of guard cells.
• 2. During day time, guard cells become turgid due to endo-osmosis.
• 3. Thus turgor pressure is exerted on the thin walls of guard cells.
• 4. Being elastic and thin, lateral walls are stretched out.
• 5. Due to kidney or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma.
• 6. During night time, guard cells become flaccid due to exo-osmosis.
• 7. Flaccidity closes the stoma almost completely.
• 8. Endo-osmosis and exo-osmosis occur due to diurnal changes in osmotic potential of guard cells.
• 9. According to starch-sugar inter-conversion theory (Steward 1964), during day time, enzyme phosphorylase converts starch to sugar, thus increasing the osmotic potential of guard cells causing entry of water, thereby guard cells are stretched and stoma widens. The reverse reaction occurs at night bringing about the closure of the stoma.

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

4)What is transpiration? Explain role of transpiration.

ans:Transpiration:

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

Role of transpiration:

• i. It removes excess of water.
• ii. It helps in the passive absorption of water and minerals from the 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 the turgor of the cells.
• vi. Transpiration helps in reducing the temperature of leaf and in imparting a cooling effect.  

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

ans:Significance of transpiration:

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

Root pressure theory (Vital theory):

• 1. This theory was proposed by J. Priestley.
• 2. According to this theory, the activity of living cells of root is responsible for the translocation of water.
• 3. When a stem of a potted plant is cut few inches above the soil by a sharp knife, xylem sap is seen flowing out/ oozing out through the cut end.
• 4. This exudation at the cut end of the stem is a good proof for the existence of root pressure.
• 5. As water absorption by roots is a constant and continuous process, hydrostatic pressure is developed in the living cells of cortex of the root. This is termed as root pressure (coined by S. Hales).
• 6. It is due to root pressure water along with dissolved minerals is not only forced into xylem but it is also conducted upwards against the gravity.
• 7. Root pressure seems to be largely an osmotic phenomenon and its development is an active process.
• 8. The value of root pressure is +1 to +2 bars which is enough to pump water to a height of 10 to 20 meters.
• 9. The factors like oxygen, moisture, the temperature of the soil, salt contents, etc. influence the root pressure.

Limitations of root pressure theory:

Although, ascent of sap takes place due to root pressure, there are certain objections raised, such as;

• 1. It is not applicable to plants taller than 20 meters.
• 2. Ascent of sap can also occur even in the absence of root system.
• 3. Root pressure value is almost nearly zero in taller gymnosperm trees.
• 4. In actively transpiring plants, no root pressure is developed.
• 5. Xylem sap under normal condition is under tension i.e. it shows negative hydrostatic pressure or high osmotic pressure.
• Thus, root pressure is not the sole mechanism explaining the ascent of sap in all plants of varying heights.

6)Explain capillarity theory of water translocation.

ans:

Capillarity theory of water translocation:

• 1. This theory was put forth by Boehm in (1863).
• 2. According to this theory, physical forces and dead cells are responsible for the ascent of sap. For e.g. 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.
• 3. Capillarity is because of surface tension, and forces of cohesion (attraction between like molecules) and adhesion (attraction between unlike molecules).
• 4. Xylem vessel/ tracheid with its lumen can be compared with straw.
• 5. Water column exists because of combined cohesive and adhesive forces of water and xylem wall, due to capillarity.
• 6. Due to capillarity, water is raised or conducted upwards against gravity, to few centimeters only.

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

ans:

Curtis (1926) regarded transpiration as ‘a necessary evil’, because;

• 1. For stomatal transpiration to occur, stoma must remain open, during day time.
• 2. When stomata are open then only the gaseous exchange needed for respiration and photosynthesis will take place.
• 3. If stomatal transpiration stops, it will directly affect the productivity of the plant through the loss of photosynthetic and respiratory activity.
• 4. Hence for productivity, stomata must remain open.
• 5. Consequently transpiration cannot be avoided.

8) Explain the movement of water in the root.

ans:

• Journey of water from soil to xylem in roots (from epiblema upto xylem in the stelar region):
• 1. Water is absorbed by root hair cells through processes like imbibition, diffusion, osmosis which occur sequentially.
• 2. Water passes through the epidermal cell (epiblema), cortex, endodermis, Casparian strip, pericycle, and then to protoxylem.
• 3. When the root hair cell absorbs water it becomes turgid. Its turgor pressure increases, but its DPD value decreases.
• 4. However, the immediately adjacent cortical cell inner to it, has more DPD value, because its O. P. is more.
• 5. Therefore, cortical cells will absorb water from the turgid root hair cell. It then becomes turgid.
• 6. The flaccid root hair cell now absorbs water from the soil.
• 7. Water from the turgid cortical cell is absorbed by the inner cortical cell and the process goes on.
• 8. Thus, a gradient of suction pressure (DPD) is developed from cells of epiblema to the cortex of the root.
• 9. Consequently water moves rapidly across the root through loosely arranged living cells of cortex, followed by passage cells of endodermis and finally into the cell of pericycle.
• 10. Protoxylem is in close proximity to the pericycle.
• 11. It is due to root pressure, water from pericycle is forced into the xylem.
• 12. Pathway of water across the root occurs in two types: Apoplast pathway and Symplast pathway
• 13. Apoplast pathway: When some amount of water passes across the root through the cell wall and the intercellular spaces of cortical cells of the root, it is then called the apoplast pathway. This pathway occurs up to endodermis.
• 14. Symplast pathway: When water passes across from one living cell to another living cell through plasmodesmata, then it is called the symplast pathway. It is also called the trans-membrane pathway. 

9) Define and or explain the term:Plasmolysis

• Solution1:
• Plasmolysis - The shrinkage of cytoplasm of a living cell as a result of exosmosis is known as plasmolysis.
• Solution2:
• 1. When a living cell is placed in a hypertonic solution, exo-osmosis occurs. This is called plasmolysis.
• 2. During plasmolysis, the protoplast of the cell shrinks and recedes from the cell wall due to which cell becomes flaccid. Such a cell is called a plasmolysed cell.
• 3. In a plasmolyzed cell, a gap is developed between the cell wall and the protoplast. This gap is filled up by the outer solution

10) What is osmotic pressure?

Explain the term osmosis.

• Solution:
• i. The pressure exerted due to osmosis is called osmotic pressure.
• ii. Osmotic pressure is a pressure of the solution, which is required in opposite direction, so as to stop the entry of solvent molecules into the cell.
• OR
• Osmotic pressure of a solution is equivalent to the pressure which must be exerted upon it to prevent flow of solvent across a semipermeable membrane.

11) Define and or explain the term:

Diffusion

Solution:

• 1. Diffusion means to disperse.
• 2. Diffusion can be defined as the movement of ions/ atoms/ molecules of a substance from the region of their higher concentration to the region of their lower concentration till equilibrium is reached.
• 3. The movement is due to the kinetic energy of the molecules.
• 4. Water passes into the cell by diffusion through a freely permeable cell wall.

 

12) Define and or explain the term:imbibition

ans :

• 1. Imbibition is swelling up of hydrophilic colloids due to the adsorption of water.
• OR
• The adsorption of water by hydrophilic compounds is called imbibition.
• 2. Substance that adsorbs water/liquid is called imbibant and water/ liquid that gets imbibed is called imbibate.
• 3. The root hair cell wall is made up of pectic compounds and cellulose which are hydrophilic colloids.
• 4. During imbibition, water molecules get tightly adsorbed without the formation of a solution.
• 5. Imbibition continues until the equilibrium is reached. In other words, water moves along the concentration gradient.
• 6. Imbibition is significant in soaking of seeds, swelling up of dried raisins, kneading of flour etc.

13) Define and or explain the term:Guttation

• ans:1. The loss of water in the form of liquid is called guttation.
• 2. It occurs through special structures called water stomata or hydathodes.

14) Define and or explain the term:Transpiration

• ans:
• i. The loss of water in the form of vapour is called transpiration that occurs through leaves, stem, flowers and fruits.
• ii. Transpiration occurs through three main sites - cuticle, stomata, and lenticels.

15) Define and or explain the term:Ascent of sap

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

16) Define and or explain the term:Active absorption

• ans:1. In this water is absorbed due to the activity of roots.
• 2. Root cells play an active role in the absorption of water.
• 3. The driving force is the root pressure developed, in the living cells of the root.
• 4. Active absorption occurs usually at night when transpiration stops due to the closure of stomata.
• 5. As water absorption is against the DPD gradient, there is an expenditure of ATP (energy) generated through the respiratory activity of cells.