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NCERT Solutions for Class 11 Chapter Transport in plants -
Question 1- What are the factors affecting the rate of diffusion?
Answer: Factors affecting diffusion are - Concentration gradient, Permeability of membrane; separating the substance, Temperature and Pressure.
Question 2 - What are porins? What role do they play in diffusion?
Answer: Special type of protein called Carrier proteins form porin channels in the outer membranes of plastids, mitochondria and some bacteria. The porin channels allow the molecules to pass through membrane. Molecules stick to the transport protein and are transported by facilitated diffusion.
Question 3 - Describe the role played by protein pumps during active transport in plants.
Answer: Protein pumps are used to transport substances against a concentrations gradient. This is a type of active transport. Active transport is carried out by membrane proteins. Pumps are proteins which use energy to carry substances across the cell membrane.
Question 4 - Explain why pure water has the maximum water potential.
Answer: The Water potential decreases as we start adding solute to it. Pure water has the highest concentration of water molecules. Hence, pure water has the highest water potential.
Question 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
(f) Guttation and Transpiration
Answer :
(a)Diffusion and Osmosis - When diffusion happens across a semi-permeable membrane, it is called osmosis. Semi-permeable membrane is not necessary in all cases of diffusion.
(b) Transpiration and Evaporation - Loss of water from plant leaves due to evaporation is called transpiration, while conversion of water into water vapour at any temperature is called evaporation.
(c) Osmotic Pressure and Osmotic Potential - The pressure which needs to be applied to stop osmosis and prevent the flow of water across a semi-permeable membrane is called Osmotic pressure. The tendency of water to move across a semi-permeable membrane is called osmotic potential.
(d) Imbibition and Diffusion - diffusion is moment of molecules from high concentration to low concentration. Diffusion is slow and happens over short distances only. Diffusion of water into solid materials is called imbibition.
(e)Apoplast and Symplast pathways - the apoplast pathway is made up of cell wall and intercellular spaces. It is not found at Casparian strips. Symplast pathway is made of interconnected protoplast. Sensitive cytoplas to transport substances.
(f)Guttation and Transpiration - Release of water in liquid phase from plants, is called guttation. Evaporative loss of water from plants is called transpiration. In guttation, water comes out in liquid form, while in transportation, water comes out in gaseous form.
Question 6 - Briefly describe water potential. What are the factors affecting it?
Answer: The tendency of water to move from one area to another is called as water potential. The water potential decreases on addition of solute. So pure water has highest water potential. Water potential depends upon solute potential and pressure potential from the following relation - Ww = Ws + Wp .
Question 7 - What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
Answer: If a pressure more than atmospheric pressure is applied to pure water or a solution, the water potential of pure water or a solution increases. This happens because of pressure potential which develops due to increased pressure.
Question 8 - With the help of well-labelled diagrams, describe the process of plasmolysis in plants, giving appropriate examples.
Answer: When a plant cell is placed in hypertonic solution, the plant cell loses water and hence it result in loss of the turgor pressure. This makes the cell flaccid. Further water loss will lead to plasmolysis, in which the pressure decreases to an extent where the protoplasm of the cell sperates from the cell wall. This leaves gaps between the cell wall and the membrane. This can also lead to cytorrhysis or complete collapse of the cell wall.
Question 9 - Explain what will happened to a plant cell if it is kept in a solution having higher water potential.
Answer: When a plant cell is placed in solution having higher water potential, that is hypotonic solution, the cell gains water and thus gains turgor pressure. This leads to the cell getting swollen. But the rigidity of the cell wall, prevents the cell from bursting.
Question 10 - How is the mycorrhizal association helpful in absorption of water and minerals in plants?
Answer: Mycorrhiza is a symbiotic association of a fungus with roots of higher plant like pinus. The hyphae of fungus form a network around young roots and thus increase the surface area. This helps in getting access to more water and minerals for the plants.
Question 11 - What role does root pressure play in water movement in plants?
Answer: Root pressure is not very strong pressure. It does not play any major role in water movement in tall plants. Root pressure contributes towards reestablishment of continuous chains of water molecules in the xylem; which often break under enormous tensions created by transpiration pull.
Question 12 - Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Answer: Transpiration creates a suction force inside the xylem. This suction force is called transpiration pull. This is powerful enough to pull the water column from beneath. Adhesion, cohesion and surface tension are the important physical properties of water which help in the upward movement of water through xylem. Temperature, light, humidity, wind speed, number and distribution of stomata, etc. are factors that affect transpiration.
Transpiration helps the plants in :
• Transport of water that is required for photosynthesis.
• cooling the leaves due to evaporation of water.
• Maintaining the turgidity and shape of plant parts.
Question 13 - Discuss the factors responsible for ascent of xylem sap in plants.
Answer:
Cohesion: Mutual attraction between water molecules is called cohesion.
Adhesion: Attraction of water molecules to polar surfaces is called adhesion.
Surface Tension: Any liquid has a tendency to occupy the least possible surface area. This property is called surface tension.
These three together give high tensile strength to water. The high tensile strength imparts an ability to resist a pulling force and high capillarity.
Question 14 - What essential role does the root endodermis play during mineral absorption in plants?
Answer: Minerals need to be actively absorbed by the epidermal cells. Specific proteins in the membranes of root hairs actively pump ions from the soil to the epidermal cells.
Question 15 - Explain why xylem transport is unidirectional and phloem transport bi-directional.
Answer: Water transported through xylem is utilised in photosynthesis and most of the water is lost through transpiration. Renewed demand for water is once again supplied through the same channel. Hence, transport through xylem is unidirectional. In case of phloem transport, food is transported from source to sink. Leaves are the usual source and storage organs are the usual sink. But the storage organs become source when new buds emerge during early spring. In that case, a reverse flow of food is required. Hence, movement through phloem is bi-directional.
Question 16 - Explain pressure flow hypothesis of translocation of sugars in plants.
Answer: The pressure flow or mass flow hypothesis is -
When glucose is prepared at the source, it is converted to sucrose. The sucrose moves into the companion cells and then into the living phloem sieve tube cells; through active transport. This process of loading at the source produces a hypertonic condition in the phloem. Water from the adjacent xylem moves into the phloem by osmosis. This results in an increase of osmotic pressure. It forces the phloem sap to areas of lower pressure, i.e. towards the sink . The osmotic pressure must be reduced at the sink. Active transport moves the sucrose out of the phloem sap into the cells in the sink. Once the sugar is removed, the osmotic pressure decreases and water moves out of the phloem.
Question 17 - What causes the opening and closing of guard cells of stomata during transpiration?
Answer: A change in the turgidity of guard cells results in closing or opening of stomata. The inner wall of the guard cell; towards the stomatal aperture; is thick and elastic. An increase in turgidity results in the thin outer walls to bulge out. This forces the inner wall into a crescent shape and results in opening of stoma. The orientation of the microfibrils in the cell walls of the guard cells also helps in opening of stomata. These meicrobifibrils are radially oriented and thus make it easy for the stoma to open up. Loss in turgidity of the guard cells, leads to resumption of shape of the elastic inner wall of the guard cell and the stoma close
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