Discuss the factors responsible for ascent of xylem sap in plants.
Xylem sap ascends mainly due to forces generating in the foliage of plants as a result of active transpiration. Thus, the factors which enhance the rate of transpiration are also the factors responsible for ascent of xylem sap in plants.
Various factors responsible for ascent of xylem sap in plants are as follows:
(i) Capillarity: There is limited rise of water in narrow tubes or capillaries due to forces of cohesion amongst molecules of water and their property of adhesion to other substance.
(ii)Root pressure: It is positive pressure that pushes sap from below due to active absorption by root.
(iii)Transpiration pull: Transpiration in aerial parts brings the xylem sap under negative pressure or tension due to continuous withdrawal of water by them. Water column does not break due to its high tensile strength related to high force of cohesion and adhesion.
Given below is a table. Fill in the gaps.
| Property |
Simple diffusion |
Facilitated transport |
Active transport |
|
| i. | Highly selective | Yes | ||
| ii. | Uphill transport | Yes | ||
| iii. | Requires ATP | |||
Movement of substances in xylem is unidirectional while in phloem it is bidirectional. Explain.
Water moves up against gravity and even for a tree of 20 m height, the tip receives water within two hours. The most important physiological phenomenon which is responsible for the upward movement of water is _________
Match the following and choose the correct option.
| A. | Leaves | (i) | Anti-transpirant |
| B. | Seed | (ii) | Transpiration |
| C. | Roots | (iii) | Negative osmotic potential |
| D. | Aspirin | (iv) | Imbibition ‘ |
| E. | Plasmolyzed cell | (v) | Absorption |
Options:
(a) A—(ii), B—(iv), C—(v), D—(i), E—(iii)
(b) A—(iii), B—(ii), C—(iv), D—(i), E—(v)
(c) A—(i), B—(ii), C—(iii), D—(iv), E—(v)
(d) A—(v), B—(iv), C—(iii), D—(ii), E—(i)
Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
A flowering plant is planted in an earthen pot and irrigated. Urea is added to make the plant grow faster, but after some time the plant dies. This may be due to ________.
What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
How is the mycorrhizal association helpful in absorption of water and minerals in plants?
Osmosis is a special kind of’diffusion, in which water diffuses across the cell membrane. The rate and direction of osmosis depends upon both ________
Smaller, lipid soluble molecules diffuse faster through cell membrane, but the movement of hydrophilic substances are facilitated by certain transporters which are chemically ________.
Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
In a passive transport across a membrane, when two protein molecules move in opposite direction and independent of each other, it is called as ________
The plant cell cytoplasm is surrounded by both cell wall and cell membrane. The specificity of transport of substances are mostly across the cell membrane, because _________ .
Absorption of water from soil by dry seeds increases the ________ thus helping seedlings to come out of soil.
The C4 plants are twice as efficient as C3 plants in terms of fixing C02 but lose only _________ as much water as C3 plants for the same amount of C02 fixed.
(a) With fhe help of well-labelled diagrams, describe the process of plasmolysis in plants, giving appropriate examples.
(b) Explain what will happen to a plant cell if it is kept in a solution having higher water potential.
Differentiate between the following:
(a) Diffusion and Osmosis
(b) Transpiration and Evaporation
(c) Osmotic Pressure and Osmotic Potential
(d) Imbibition and Diffusion
(e) Apoplast and Symplast pathway of movement of water in plants
(f) Gutta’tion and Transpiration
