NCERT Solutions for Class 11 Biology Chapter 11: Transport in Plants
1. What are the factors affecting the rate of diffusion?
Ans. The rate of diffusion can be affected by a number of factors. The rate of diffusion can be affected by a number of factors.
||The greater the concentration gradient, higher the driving force and so higher the rate of diffusion.|
||With increase in temperature of the medium, the kinetic energy of the particles will increase. This helps the molecules to move faster and diffusion rate becomes faster.|
||Diffusion rate increases as the membrane permeability increases.|
||Pressure plays an important role in the diffusion of gases as gases diffuse from a region of higher partial pressure to a region of lower partial pressure.|
||The more the size of the molecule the higher the inertia it has so the less the rate of diffusion.|
||The more the viscosity of the medium the more difficult it is for the molecule to move, the less the diffusion rate.|
2. What are porins? What role do they play in diffusion?
Ans. Porins are proteins lined hydrophilic channels present in the outer membranes of the plastids such as chloroplast, mitochondria and the membranes of the bacteria.
Roles of porins in diffusion:
1. Porins are beta barrel proteins that cross cellular membranes and act as pores through which molecules can diffuse.
2. Porins act as a channel for selective diffusion of different molecules.
3. Depending on the size of the porin, the interior of the protein may either be filled with water or have a stopper segment.
3. Describe the role played by protein pumps during active transport in plants.
Ans. In plants active transport occurs against the concentration gradient i.e., from lower concentration to higher concentration point. Each protein pump is specific about what substance it should carry through the membrane. The protein pumps are made up of specific proteins called trans-membrane proteins. These specific
proteins make a complex with the substance to be transported across the membrane, using the energy derived from ATP. The substance finally gets liberated into the cytoplasm as a result of the dissociation of the protein–substance complex.
4. Explain why pure water has the maximum water potential.
Ans. For pure water, water potential is always taken as zero at standard temperature and pressure. For normal water, water potential is expressed as the sum of solute potential (ψs) and pressure potential(ψp).
ψw = ψp + ψs
When some solute dissolved in water, the water potential of pure water decreases and termed as solute potential, this is always negative and is maximum when water is pure and at atmospheric pressure.
5. 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 pathways of movement of water in plants.
(f) Guttation and Transpiration.
Ans. (a) Differences between diffusion and osmosis are as follows :
||It operates in liquid medium through semi- permeable membrane.|
||Its applicable only to the solvent part of the solution.|
||Here solvent [ liquid or water] moves from the area of higher free energy or chemical potential to the area of its lower free energy or chemical potential.|
||It depends on the degree of reduction of free energy of one solvent over the another.|
||It is depended upon solute potential.|
||This process does not equalise the concentration of solvent on the two side of the semi-permeable membrane.|
(b) Differences between transpiration and evaporation are as follows:
||It is a physical process that occurs on any free surfaces either living or non-living.|
||No much force is involved.|
||Evaporation stops when air got saturates near the surface.|
||Its rate increases at high wind flow condition and continues as long as water is available.|
(c) Differences between osmotic pressure and osmotic potential are as follows:
|Osmotic pressure||Osmotic potential|
||It is lower of free energy of water in a confined or open system due to the presence of solute particles.|
||It develops in a closed or open system.|
||It develops in a closed or open system.|
(d) Differences between Imbibition and diffusion are as follows:
||It is the movement of all types of substances from the higher free energy to lower free energy point.|
||It doesn’t produce heat but can’t develop high pressure.|
(e) Differences between apoplast pathway and symplast pathway are as follows:
||It consists of living parts of plant body e.g. ,plasmodesmata.|
||It is comparatively slower pathway to absorb as resistance is slightly more.|
||Metabolic state of roots doesn’t affect the pathway.|
(f) Differences between Guttation and Transpiration are as follows:
||It is the process of loosing excess water by the plant in vapour form.|
||The transpired water is pure water.|
||Mostly occurs during hotter period of the day and very negligible amount during night.|
||It occurs through the lamina of the leaves.|
||It occurs through the stomata, lenticels and epidermal cell.|
||Stomata can be open or close.|
||Even under water stress condition this process continues.|
||Excessive transpiration cause wilting.|
6. Briefly describe water potential. What are the factors affecting it?
Ans. Water potential (ψ) quantifies the tendency of water to move from one part to the other during various cellular processes such as diffusion, osmosis etc. Its unit is Pascal (Pa). For pure water, water potential is always taken as zero at standard temperature and pressure. For normal water, water potential is expressed as the sum of solute potential (ψs) and pressure potential (ψp).
ψw = ψp + ψs
Pressure potential is the pressure which develops in an osmotic system due to osmotic entry or exit of water from it. It has a positive value except in Xylem. This pressure potential plays a major role in the ascent of water through the stem.
When some solute is dissolved in water, the water potential of pure water decreases. This is termed as solute potential (ψs), which is always negative. For a solution at atmospheric pressure, ψw = ψs.
The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. It is termed as pressure potential. It is denoted by ψp and has a positive value, although a negative pressure potential is present in the xylem. This pressure potential plays a major role in the ascent of water through the stem.
Water potential is affected by the following factors:
1. Solute potential.
2. Pressure potential.
5. Water gain or loss.
7. What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
Ans. If pressure is greater than the atmospheric pressure then the water potential of pure water or a solution will increases. For example, when water diffuses into a plant cell, pressure is built up against the cell wall and makes the cell wall turgid. This pressure is termed as pressure potential and has a positive value and plays a major role in the ascent of water through the stem.
8. (a) With the 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.
Ans. (a) Under the influence of hypertonic solution, the protoplast of the cell shrinkages from its cell wall and this phenomenon is called plasmolysis. First water is moves out from the cytoplasm and then from the vacuole. When water drawn out of the cell through diffusion into the extracellular (outside cell) fluid, causes the protoplast to shrink away from the walls.
Various stages in plasmolysis: A. Normal cell B. Incipient plasmolysic C. Plasmolysed cell
Examples of plasmolysis:
- Preservation of meat, fishes with salt.
- Preservation of jams and jellies by sweetening with sugar.
- Salt kills the weeds of lawn by means of plasmolysis in their cells.
- To determine the osmotic pressure of a cell plasmolysis method is used.
(b) When the cells are placed in a higher water potential solution [hypotonic solution] or dilute solution as compared to the cytoplasm, water diffuses into the cell causing the cytoplasm to build up a pressure against the wall, that is called Turgor pressure and this turgor pressure is ultimately responsible for enlargement and extension growth of cells.
9. How is the mycorrhizal association helpful in absorption of water and minerals in plants?
Ans. A mycorrhiza is a symbiotic association of a fungus with a root system. The fungal filaments form a network around the young root or they penetrate the root cells. The hyphae have a very large surface area that absorb mineral ions and water from the soil from a much larger volume of soil compare to root. The fungus provides minerals and water to the roots, in turn the roots provide sugars and nitrogen containing compounds to the mycorrhizae. For example, Pinus seeds cannot germinate and establish without the presence of mycorrhizae.
10. What role does root pressure play in water movement in plants?
Ans. As various ions from the soil are actively transported into the vascular tissues of the roots, water follows (its potential gradient) and increases the root pressure inside the xylem. This is responsible for pushing up water to small heights in the stem. During water transport, root pressure provides only a modest push. They obviously do not play a major role in water movement up in the tall trees. The greatest contribution of root pressure may be to re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration.
11. Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Ans. In 1894 transpiration pull or cohesion-tension theory was originally proposed by Dixon and Joly. This was further improved by Dixon in 1914. According to this theory, a continuous column of water is present in the xylem channels of the plant due to cohesive force of water molecules. There is another force of adhesion which holds water at the walls of xylem vessels. During transpiration in plants, water is lost, in form of water vapour, from the mesophyll cells to exterior, through stomata. This results decrease in turgor pressure of these cells and leads to increase in diffusion pressure deficit (DPD). Now these cells take water from adjoining cells and the turgor pressure of those adjoining cells decreases. This process is repeated and ultimately water is absorbed from the nearest xylem vessels of leaf. As there is a continuous water column inside the xylem elements, a tension or pull is transmitted down and finally transmitted to root, resulting in the upward movement of water.
Factors affecting transpiration include both environmental and internal factors.
(a) Relative humidity: The rate of transpiration is inversely proportional to the relative humidity.
(b) Atmospheric temperature: At high temperature stomata even open in darkness. Besides producing a heating effect, it lowers the relative humidity of the air and increases vapour pressure inside transpiring organ. Consequently, rate of transpiration increases.
(c) Light: Most of the transpiration occurs through stomata so the rate of transpiration is quite high in light. It falls down appreciably in the darkness.
(d) Air movements: Transpiration is lower in the still air because water vapours accumulate around the transpiring organs and reduce the DPD of the air. The movement of the air increases the rate of transpiration by removing the saturated air around the leaves.
(e) Atmospheric pressure: Low atmospheric pressure enhances evaporation rate, produces air currents and increases the rate of transpiration.
(f) Availability of water: The rate of transpiration depends upon the rate of absorption of soil water by roots. This is further influenced by a number of soil factors like soil water, soil particles, soil temperature, soil air, etc.
(a) Leaf area (transpiring area): A plant with large leaf area will shows more transpiration than another plant with less leaf area.
(b)Leaf structure: Leaf structure affects transpiration in following ways:
- Cuticular transpiration decreases with the thickness of cuticle and cutinisation of epidermal walls.
- Most of the transpiration takes place through the stomata so their number and position influences the rate of transpiration.
- The sunken stomata are device to reduce the rate of transpiration by providing an area where little air movement occurs.
(c) Root/shoot ratio: A low root/shoot ratio decreases the rate of transpiration while a high ratio increases the rate of transpiration.
(d) Mucilage and solutes: They decrease the rate of transpiration by holding water tenaciously.
Transpiration is useful to plants in the following ways:
(a) This process helps in removal of excess water.
(b) The ash and sugar content of the fruit improve quality of fruits.
(c) Transpiration prevents overheating of leaves.
(d) Pole in ascent of sap and improve turgidity.
(e) Mineral salts could distribute uniformly due to transpiration.
(f) Photosynthesis– Transpiration supplies water for photosynthesis.
12. Discuss the factors responsible for ascent of xylem sap in plants.
Ans. 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:
(a) 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. It happens in tracheids and vessel elements.
(b) Root pressure: As various ions from the soil are actively transported into the vascular tissues of the roots, water follows (its potential gradient) and increases the pressure inside the xylem. This positive pressure is called root pressure, and can be responsible for pushing up water to small heights in the stem. It is positive pressure that pushes sap from below due to active absorption by root.
(c) Transpiration pull: As water evaporates through the stomata, since the thin film of water over the cells is continuous, it results in pulling of water, molecule by molecule, into the leaf from the xylem. Also, because of lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air. 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.
13. What essential role does the root endodermis play during mineral absorption in plants?
Ans. Like all cells, the endodermal cells have many transport proteins embedded in their plasma membrane; they let some selective solutes to cross the membrane. Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem. Because of the layer of suberin, the root endodermis has the ability to actively transport ions in one direction only.
14. Explain. Why xylem transport is unidirectional and phloem transport bi-directional?
Ans. Transport over longer distances proceeds through the vascular system (the xylem and the phloem) is called translocation. In rooted plants, transport in xylem (to water and minerals) is essentially unidirectional, from roots to the stems. Organic and mineral nutrients however, undergo multi-directional transport. Food, primarily sucrose, is transported by the vascular tissue, phloem, from a source to a sink. Usually the source is part of the plant which synthesises the food, i.e., the leaf, and sink, the part that needs or stores the food. But, the source and sink may be reversed depending on the season, or the plant’s needs. Since the source-sink relationship is variable, the direction of movement in the phloem can be upward or downward, i.e., bi-directional. Hence, unlike one-way flow of water in xylem, food in phloem tissues can be transported in any required direction.
15. Explain pressure flow hypothesis of translocation of sugars in plants.
Ans. The pressure flow hypothesis is the accepted mechanism used for the translocation of sugars from source to sink. Glucose prepared by photosynthesis in leaves is converted to sucrose (a disaccharide). The sugar is then moved in the form of sucrose into adjacent companion cells and then into the living phloem by active transport. This process of loading at the source produces a hypertonic conditions in the phloem.
Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds up, the phloem sap will move to areas of lower pressure. At the sink, osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar converting it into energy, starch, or cellulose. As sugars are removed, the osmotic pressure of the phloem decreases and water moves out of the phloem.
16. What causes the opening and closing of guard cells of stomata during transpiration?
Ans. Transpiration is the evaporative loss of water mainly through the stomata of leaves by plants. The immediate cause of the opening or closing of the stomata is change in the turgidity of the guard cells. The inner wall of each of the guard cells, towards the pore or stomatal aperture, is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape and thus the stomata opens. The opening of the stomata is also aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radically rather than longitudinally making it easier for the stomata to open. When the guard cells lose turgor, due to water loss (or water stress) the elastic inner walls regain their original shape, the guard cells become flaccid and the stomata closes.