NCERT Solutions for Class 11 Biology for Chapter 11 Transport in Plants
1. What are the factors affecting the rate of diffusion?
Answer: Diffusion is a process of passive (ATP/energy is not required) movement of solutes from one place (lower concentration) to another (higher concentration). There are several factors that affect rate of diffusion.
(i) Concentration gradient: Diffusion rate depends upon the concentration of solute at the two points. Larger the concentration difference between two point higher the rate of diffusion.
(ii) Permeability of the membrane: More permeable the membrane that is separating the solutes, faster is the process of diffusion.
(iii) Temperature: Increase in temperature increases the rate of diffusion.
(iv) Distance: Diffusion rate is inversely proportional to the distance, solute has to travel.
(v) Size of the molecules: smaller molecules diffuse faster.
2. What are porins? What role do they play in diffusion?
Answer: Porins are the proteins that form huge pores in plasma membrane of bacteria, mitochondria and chloroplast. They act as pores by forming channels in membrane through which a variety of molecules can diffuse across. They are large enough to allow passive diffusion of solutes across the membrane. There are different types of porins for transportation of different types of molecules.
3. Describe the role played by protein pumps during active transport in plants.
Answer: Active transport requires energy to facilitate transport of molecules against their concentration gradient. It is needed where molecules cannot cross plasma membrane by simple diffusion or when transport of molecules against their concentration gradient is required. There are specific protein pumps (protein channels) that facilitate transport of such molecules by using energy of ATPs. Hydrolysis of ATP induces a conformational change in protein pumps that allows movement of molecules through it. When all the protein transporters are in use, the transport reaches its peak level or attains a saturation level. This is a highly selective process.
4. Explain why pure water has the maximum water potenti
Answer: Pure water has maximum water potential since it has maximum kinetic energy.Water potential is defined in terms of tendency of water to flow from one place to another. It can be understood with free energy (kinetic energy in liquid and gaseous state) of water molecules. Presence of any other molecule in water decreases the kinetic energy of its molecules. In the absence of any other molecules, kinetic energy (free energy) of water molecules will be highest so will be the water potential. That’s why pure water has maximum water potential.
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.
Diffusion is the passive movement of molecules along a concentration gradient.
It is the diffusion of solvent (water) across a semi permeable membrane.
It can occur in liquid or gaseous state.
It can occur in liquid medium only.
It does not require presence of water.
Presence of water is a must.
It doesn’t require a semi permeable membrane.
It requires a semi permeable membrane.
It is the loss of water from plants through stomata found on the bottom surface of leaves.
It is the loss of water from any surface (living or non-living).
It is a physiological process.
It is a physical process.
It is influenced by many physiological and environmental factors.
It is driven by only environmental factors.
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semi permeable membrane.
Numerically osmotic pressure is equivalent to the osmotic potential, but the sign is opposite.
It is a positive pressure and its value increases with increase in concentration of solute particles.
It is a negative pressure and its value decreases with increase in concentration of solute.
Imbibition is absorption of water by solids or colloids by simple diffusion.
Diffusion is passive movement of molecules, ions or particles from one area to another along their concentration gradient.
It involves movement of liquid.
It involves movement of liquid, gas and solid particles.
Apoplast pathway of movement involves transport of water through the space found between two adjacent cell walls of epidermis and cortex.
It involves transport of water through protoplast.
It is a faster process and water moves by mass flow.
It is a relatively slower process.
It is the loss of water from leaves of plants in form of water droplets.
It is the loss of water from leaves in form of vapours.
It occurs usually at night or early mornings.
It occurs during day.
It is an uncontrolled process and occurs at the endings of leaf veins.
It is a controlled process and occurs through stomata.
6. Briefly describe water potential. What are the factors affecting it?
Answer: Water potential measures the tendency of water to move from one area to another. Water potential is expressed in potential energy per unit volume and is represented by the Greek letter Ψ. Pure water has maximum water potential and mixing of any solute in it decreases its water potential. The greater the concentration of water in a system, higher is its kinetic energy or water potential. Water potential of pure water at standard temperature and pressure is zero. Solute potential (the magnitude of lowering of water potential due to dissolution of a solute in water) and pressure potential (the magnitude of increase in water potential due to application of pressure greater than atmospheric pressure) are two important components that determine the water potential.
Factors affecting water potential: Water potential is affected by solute concentration and pressure. Increase in solute concentration decreases water potential and is known as solute potential. Application of pressure (greater than atmospheric pressure) increases water potential and is known as pressure potential.
7. What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
Answer: When a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. It helps in transport of water in biological systems like in plants. When water enters the cell it build a pressure against the cell wall and cell wall exert exact opposite pressure to maintain its rigidity.
8. (a) With the help of well-labeled diagrams, describe the process of plasmolysis in plants, giving appropriate example
(b) Explain what will happen to a plant cell if it is kept in a solution having higher water potential.
(a)Plasmolysis is loss of water from cells when they are surrounded by hypertonic (or more concentrated) solution. During plasmolysis, water moved out of the cell, first from the cytoplasm and then vacuoles. This results in detachment of the cell membrane of the plant cell from the cell wall. In plasmolysis, cytoplasm shrinks and gets detached from the cell wall. The cell is said to be plasmolysed. It is a reversible phenomenon. During plasmolysis, water movement occurs across the membrane from an area of high water potential to an area of lower water potential outside the cell.
(b) Water potential is the measurement of tendency of water to move from one place to another. Water flows from the area of high water potential to the area of lower water potential. When a plant cell is placed in a solution of high water potential cell, water moves into the plant cell and build a pressure against the wall called as turgor pressure. Plant cell does not burst because of the presence of cell wall
9. How is the mycorrhizal association helpful in absorption of water and minerals in plants? Answer: Mycorrhizal association is the symbiotic relationship between plants and fung Fungal filaments not only form dense network around the plant roots but also penetrate them and provide large surface area which roots alone cannot provide, for absorption of water and minerals from the soil surrounding it. This strategy helps in providing minerals and water to the plant.
10. What role does root pressure play in water movement in plants?
Answer: When minerals enter the roots of the plant, it decreases the water potential inside the root causing water to move in (water moves from area of high water potential to the area of low water potential). This increases the pressure inside the xylem. This pressure is a positive pressure and called as root pressure.
It helps in pushing water into the roots up to small heights.
It also helps in pushing out excess water in the form of droplets from tip of glass blades or leaves during night or early morning.
11. Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Answer: Transport of water in plants is explained by transpiration pull theory. The theory has two essential features
(i) cohesion between water molecules and adhesion between water and xylem tissues,
(ii) Transpiration pull. Transpiration is the process of loss of water by mesophyll cells of leaves through stomata. During day time stomata are open so water is lost quickly as it reaches from roots to leaves. So a negative hydrostatic pressure is created in mesophyll cells that cause mesophyll cells draw water from nearby veins of leaves. The negative pressure is gradually transmitted downwards from leaves to roots via xylem tissues. This causes continuous upward movement of water column.
Factors affecting transpiration: It is affected by many physiological (number and distribution of stomata) and environmental factors (intensity of light, air, humidity and temperature).
Usefulness to plants:
Transpiration is helpful to plants as it creates negative pressure in xylem that causes continuous upward movement of water.
It helps in absorption of water and minerals from soil.
Water reaching from roots to leaves is used for photosynthesis.
Water also provides cooling effect.
It helps in maintaining turgidity of plant cells.
12. Discuss the factors responsible for ascent of xylem sap in plants.
Answer: Factors responsible for ascent of sap (transport of water and minerals) are-
(i) Transpiration pull: transpiration pull creates negative pressure in xylem that causes continuous upward movement of sap.
(ii) Cohesion (between water molecules) and adhesion (between water molecules and xylem tissues) forces along with transpiration pull create a water column within xylem that helps in continuous upward movement of sap.
13. What essential role does the root endodermis play during mineral absorption in plants?
Answer: Root endodermis carries up the function of uptake of selected mineral ions by active transportation. Minerals present in the soil enter the root epidermis first either by passive or active transportation.
The endodermal cells of the root have transport proteins embedded in their plasma membrane that actively pump ions from the epidermis into the cytoplasm of the endodermal cells. They let some solutes cross the membrane but not others thus acting as control points. This helps the plant control the quantity and types of solutes that reach the xylem. Root endodermis also allows transportation of ions only in one direction due to presence of a waxy layer made of suberin (Casparian Strip).
14. Explain why xylem transport is unidirectional and phloem transport bi-directional.
Answer: Xylem transport water from roots to leaves. Transpiration of water through leaves generates a transpiration pull that causes continuous upward movement of water. That’s why transport in xylem is unidirectional. Phloem is the transport vessels for food. Food synthesize by leaves by photosynthesis is transported from leaves to other parts of the body by phloem (for utilization and storage). In spring, food is transported from storage places to developing bud. That’s why transport in phloem is bidirectional.
15. Explain pressure flow hypothesis of translocation of sugars in plants.
Answer: Pressure flow hypothesis best explains the flow of food in phloem. Plants synthesize glucose in photosynthesis that is converted into sucrose for transport through phloem. Phloem consists of companion cells and sieve tube cells. Sucrose is transported into companion cell then into sieve tube by active transport. Loading of sucrose in phloem cells make them hypertonic that causes movement of water into the phloem cells from adjacent xylem. It increases osmotic pressure in phloem cells and phloem sap will move to the area of low pressure (this causes movement of sap from source to sink along the phloem). At the sink sucrose is taken out of the phloem cells by active transport. As the sugars decrease there is a decrease in osmotic pressure that results into movement of water out of the phloem cells. Removal of water from phloem at the sink again creates a pressure gradient that causes continuous movement of sap from source to sink.
16. What causes the opening and closing of guard cells of stomata during transpiration?
Answer: Transpiration and gaseous exchange (exchange of CO2 and O2) take place through stomata. Stomata are small opening on leaves. Stomatal pores are surrounded by guard cells which are thick and elastic at the side of pore and thin at opposite side of pore. Stomata are open during day and are closed at night. Turgor pressure plays important role in opening and closing of stomata. Whenever there is increase in turgidity of guard cell they expand. Because of thin outer boundary of guard cells, they bulged out opening the pore of stomata. When turgidity of guard cells decreases they go back to their normal shape and that causes closing of stomatal aperture.