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NCERT Class 11 Biology Chapter 11 Transport in Plants
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Transport in Plants
Chapter: 11
BIOLOGY
TEXTUAL QUESTIONS ANSWERS
Q.1. What are the factors affecting the rate of diffusion?
Ans. Factors Influencing Diffusion-
(i) Density – Rate of diffusion of a substance is inversely proportional to square root of its relative density. [Graham Law]
(ii) Permeability of Medium Rate of diffusion decreases with density of the medium.
(iii) Temperature – rise in temperature increases the rate of diffusion.
(iv) Diffusion Pressure Gradient – Rate of diffusion is directly proportional to difference of diffusion pressure at the two ends of a system and inversely proportional to the distance between the two.
Q.2. What are porins? What role do they play in diffusion?
Ans. The diffusion of substances along the concentration gradient through fixed membrane transport protein like porins or aquaporins or water channels without involving energy expenditure is called facilitated diffusion.
Q.3. Describe the role played by protein pumps during active transport in plants.
Ans. Active transport occurs against the concentration gradient. It is carried out by membrane carrier proteins called pumps. They use ATP energy for transport across the membrane. The rate of active transport reaches the maximum when all protein pumps are used in transport.
Q.4. Explain why pure water has the maximum water potential.
Ans. Kinetic energy is possessed by water molecules and they are in random motion. Pure water has highest concentration of water molecules. Therefore, pure water has the maximum water potential.
Q.5. Differentiate between the following:
(a) Diffusion and Osmosis.
Ans. Diffusion and osmosis: Diffusion occurs along the concentration gradient i.e. from region of higher concentration to region of lower concentration. Movement by diffusion is passive and slow. Osmosis is the movement of solvent or water from its higher chemical potential or higher diffusion pressure (dilute solution) to its lower chemical potential or lower diffusion pressure (conc. solution) without allowing the diffusion of solute across a semipermeable membrane.
(b) Transpiration and Evaporation.
Ans. Transpiration and evaporation Transpiration:
(i) The loss of water in the vapour form from the exposed parts of a plant is called transpiration.
(i) Stomata are the main source of transpiration.
Evaporation:
(i) It is a physical loss of water which occurs from any free living or non living surface.
(ii) It is entirely driven byenvironmental factors.
(c) Osmotic Pressure and Osmotic Potential.
Ans. Osmotic pressure and osmotic potential Osmotic pressure-
(i) It is maximum pressure which develops in an osmotically active solution when it is separated from its pure solvent by a semipermeable membrane.
(ii) Osmotic pressure is numerically equal to osmotic potential but osmotic potential has a negative value & osmotic pressure has a positive value.
Osmotic potential-
(i) It is decrease in the chemical potential of pure water due to presence of solute particles in it.
(ii) It dependents on number of solute particles.
(d) Imbibition and Diffusion.
Ans. Imbibition and diffusion Imbibition: The absorption of water by solid particles of an adsorbent causing it to enormously increase in volume without forming a solution is called imbibition. Amount of imbibition depends on–
(i) Water potential gradient between adsorbent and water
(ii) Affinity of adsorbent for water.
Diffusion occurs along the concentration gradient i.e. from region of higher concentration to region of lower concentration. Movement by diffusion is passive and slow.
(e) Apoplast and Symplast pathways of movement of water in plants.
Ans. Apoplast and symplast pathways of movement of water in plants
Apoplast Pathway-
(i) Water passes from root hair to xylem through walls of intervening cells without crossing any membrane or cytoplasm.
(ii) The pathway provides the least resistance to movement of water.
(iii) It is interrupted by the presence of impermeable casparian strips in the walls of endodermal cells.
Symplast Pathway-
(i) Water passes from cell to cell through their protoplasm.
(ii) The cytoplasm of the adjacent cells is connected through bridges called plasmodesmata.
(iii) Water has to pass through plasmalemma for entering into symplast.
(iv) It is called transmembrane pathway.
(iv) Symplastic movement is aided by cytoplasmic streaming of individual cells.
(v) Slower than apoplastic movement.
(f) Guttation and Transpiration.
Ans. Guttation and transpiration Transpiration:
(i) The loss of water in the vapour form from the exposed parts of a plant is called transpiration.
(ii) Stomata are the main source of transpiration.
Guttation-
(i) The loss of water in the form of liquid droplets from the margins and tips of leaves and other parts of an uninjured or intact plant is called guttation.
(ii) Guttation takes place through special structures called hydathodes. They are usually found on the margins and tips of the leaves.
Q.6. Briefly describe water potential. What are the factors affecting it?
Ans. Water Potential –
(i) It is the difference in the free energy or chemical potential of water in a system and that of pure water at the same temperature and pressure.
(ii) Chemical potential of pure water at normal temperature and pressure in zero.
(iii) Water moves from high water potential to low water potential.
(iv) Water potential or ψw is the sum total of solute potential [ψs] and pressure potential [ψₚ].
ψw = ψₛ + ψₚ
where ψₛ has a negative value while ψₚ has a positive value.
(v) Water Potential has a negative value eg -5 atm
Q.7. What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
Ans. The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. Water diffuses into cell and build up pressure against the cell wall. This makes the cell wall turgid.
Q.8. (a) With the help of well-labelled diagrams, describe the process of plasmolysis in plants, giving appropriate examples.
Ans. Plasmolysis-
(i) Shrinkage of protoplast of a cell from its cell wall under the influence of hypertonic solution is called plasmolysis.
(ii) Pressure potential (ψₚ) is zero and the osmotic concentration of cell interior is just equivalent to that of external solution. The cell is called flaccid.
(iii) Plasmolysis is a result of exosmosis.
(b) Explain what will happen to a plant cell if it is kept in a solution having higher water potential.
Ans. Cell placed in hypertonic solution will loose water and becomes flaccid. Turgor potential decreases and coresponds wall pressure. It finally becomes zero. Water potential is equal to solute potential ψw = ψₛ + ψₚ = ψₛ + 0 = ψₛ.
Q.9. How is the mycorrhizal association helpful in absorption of water and minerals in plants?
Ans. Mycorrhizal Water Absorption-
(i) In mycorrhiza a large number of fungal hyphae are associated with the young roots.
(ii) The fungal hyphae have a large surface area to absorb water and minerals.
(iii) Mycorrhizal association between fungus and root is obligate.
(iv) Eg – Pinus and Orchid Seeds do not germinate without mycorrhizal association.
Q.10. What role does root pressure play in water movement in plants?
Ans. Root Pressure-
(a) It is a positive pressure that develops in the xylem sap of roots.
(b) It is caused by active water absorption due to osmosis.
(c) It is responsible for pushing up water to small heights in the stem
Q.11. Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Ans. Cohesion – Tension – Transpiration pull – given by Dixon and Joly.
(a) Driving force is transpirational pull from the leaves.
(b) There is continuous water column from root, stem and leaves which is maintained by three components.
1. Cohesion – Mutual attraction between water molecules.
2. Adhesion – Attraction of water molecules to polar surface of tracheids and vessels elements.
3. Surface tension – Water molecules are attracted to each other in liquid state more than to water in gaseous state.
(c) These properties give water high tensile strength (i.e. ability to resist a pulling force) and high capillarity (i.e. ability to rise in thin tracheids and vessel elements)
(d) It can pull the water to 130 m in height.
(e) External Factors Affecting Transpiration-
(i) Reality Humidity – The rate of transpiration is inversely proportional to the relative humidity.
(ii) Atmospheric Temperature – rise in temp increases transpiration.
(iii) Light – Stoma open in presence of light and close in darkness.
(iv) Air Movements – movement of air increases transpiration.
(v) Availability of Water – increased availability of water increases transpiration.
(f) Internal Factors Affecting Transpiration-
(i) Number and Position of Stomata.
(ii) Root/Shoot Ratio – low ratio decrease transpiration.
(iii) Mucilage and Solutes – hold water and decrease transpiration.
(g) Significance of Transpiration-
(i) Ascent of Sap.
(ii) Cooling Effect by evaporating water.
(iii) Turgidity – Keeps cell turgid.
(iv) Photosynthesis – Transpiration supplies water for Photosynthesis.
Q.12. Discuss the factors responsible for ascent of xylem sap in plants.
Ans. Forces responsible for ascent of sap —
1. Root Pressure – It is a positive pressure that develops in the xylem sap of roots. It is caused by active water absorption due to osmosis. It is responsible for pushing up water to small heights in the stem.
2. Cohesion – Tension – Transpiration pull — Driving force is transpirational pull from the leaves.
There is continuous water column from root, stem and leaves which is maintained by three components: Cohesion – Mutual attraction between water molecules, Adhesion – Attraction of water molecules to polar surface of tracheids and vessel elements, Surface tension-Water molecules are attracted to each other in liquid state more than to water in gaseous state.
(i) These properties give water high tensile strength (i.e. ability to resist a pulling force) and high capillarity (i.e. ability to rise in thin tracheids and vessel elements).
(ii) It can pull the water to 130 m in height.
Q.13. What essential role does the root endodermis play during mineral absorption in plants?
Ans. Mineral Uptake by Roots-
(i) The movement of ions from soil to interior of root is against concentration gradient and requires an active transport.
(ii) Specific ion pumps occur in the membrane of root hairs which pump ions from soil to root hairs.
(iii) Ions are transported inwardly to xylem by transport proteins in endodermis cells along the concentration gradient.
Q.14. Explain why xylem transport is unidirectional and phloem transport bi-directional.
Ans. Since the source sink relationship is variable depending on season or plants need, the direction of movement of organic solutes in phloem can be upwards or downwards i.e. bidirectional.
The transport of water in the xylem takes place only from the roots to the leaves. Therefore, the movement of water and nutrients in the xylem is unidirectional.
Q.15. Explain pressure flow hypothesis of translocation of sugars in plants.
Ans. Mechanism of Phloem Translocation is Mass flow or Pressure Flow Hypothesis. It was given by Munich: Organic substances move from the region of high osmotic pressure [source] to the region of low osmotic pressure [sink] in a mass flow due to the development of a gradient of turgor pressure.
Q.16. What causes the opening and closing of guard cells of stomata during transpiration?
Ans. Stomata are the main source of transpiration. Each stoma is surrounded by specialized green epidermal cells called guard cell. Guard cells contain chloroplasts. Opening of stomata depends on turgor pressure of guard cells and radial orientation of cellulose microfibrils in cell walls of guard cells. Guard cells swell up due to increased turgor and stomata open.
