Punjab State Board PSEB 11th Class Biology Book Solutions Chapter 11 Transport in Plants Textbook Exercise Questions and Answers.
PSEB Solutions for Class 11 Biology Chapter 11 Transport in Plants
PSEB 11th Class Biology Guide Transport in Plants Textbook Questions and Answers
What are the factors affecting the rate of diffusion?
Factors affecting the rate of diffusion are as follows:
- Gradient of concentration
- Permeability of membrane
What are porins? What role do they play in diffusion?
The porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through. Porins facilitate diffusion.
Describe the role played by protein pumps during active transport in plants.
Protein pumps use energy to carry substances across the cell membrane. These pumps can transport substances from a low concentration to a high concentration (‘uphill’ transport). Transport rate reaches a maximum when all the protein transporters are being used or are saturated. Like enzymes, the carrier protein is very specific in what it carries across the membrane. These proteins are sensitive to inhibitors that react with protein side chains.
Explain why pure water has the maximum water potential.
Water molecules possess kinetic energy. In liquid and gaseous form, they are in random motion that is both rapid and constant. The greater the concentration of water in a system, the greater is its kinetic energy or ‘water potential’. Hence, pure water will have the greatest water potential. Water potential is denoted by the Greek symbol psi or \p and is, expressed in pressure units such as pascals (Pa).
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
(a) Differences between Diffusion and Osmosis
|1. It is a movement of molecules from high concentration to low concentration.
|It is a movement of molecules from high concentration to low concentration
|2. It does not require any driving force.
|It occurs in response to a driving force.
(b) Differences between Transpiration and Evaporation
|1. It is the loss of water through the aerial parts of plants.
|It is the loss of water from free surface of water.
|2. It occurs in living tissues.
|It occurs in non-living surfaces.
|3. It is both physical and physiological process.
|It is only a physical process, controlled by environmental factors.
(c) Differences between Osmotic Pressure and Osmotic Potential
|1. It is the pressure required to stop the movement of water molecules through a semipermeable
|It is the amount by which water potential is reduced by the presence of solute.
|2. Osmotic pressure is the positive pressure.
|Osmotic potential is negative.
(d) Differences between Imbibition and Diffusion
|It is a special type of diffusion, where water is absorbed by solids-colloids causing them to increase in volume. For example, absorption of water by dry seeds and dry wood.
|In diffusion, molecules move in a random fashion. It is not dependent on a living system.
(e) Differences between Apoplast and Symplast Pathways of Movement of Water in Plants
|1. It is the system of adjacent cell walls that is continuous throughout the plant except casparian strips of the endodermis of the roots.
|It is the system of interconnected protoplast.
|2. Water moves through the intercellular spaces and the walls of cells.
|Water travels through the cytoplasm
|3. Movement does not involve crossing the cell membrane.
|Water has to move in cells through the cell membrane.
(f) Differences between Guttation and Transpiration
|1. It occurs through hydathodes, present at the vein ends.
|It occurs through general surface stomata and lenticles.
|2. It occurs in leaves only.
|It can occur through all aerial parts.
|3. It does not occur in deficient water conditions and never leads to wilting.
|It can occur in water deficient conditions leading to wilting.
|4. It is regulated by humidity, temperature and presence of water in soil.
|It is regulated by a number of external and internal factors such as relative humidity, temperature, opening and closing of stomata, etc.
Briefly describe water potential. What are the factors affecting it?
Water potential is the potential energy of water relative to pure free water (e.g., deionised water). It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mecanical pressure or matrix effects including surface tension. Water potential is measured in units of pressure and is commonly represented by the Greek letter (psi). This concept has proved especially useful in understanding water movement within plants, animals and soil.
Water potential of a cell is affected by both solute and pressure potential. The relationship between them is as follows:
Ψw = Ψs + Ψp
What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
If a pressure greater than atmospheric pressure is applied to pure water or a solution its water potential increases. It is equivalent to pumping water from one place to another. Pressure can be build up in a plant system when water enters a plant cell due to diffusion causing a pressure build up against the cell wall. It makes the cell turgid, this increases the pressure potential. Pressure potential is usually positive. It is denoted by Ψs.
(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.
(a) Plasmolysis occurs when water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall. This occurs when the cell is kept in a solution that is hypertonic (has more solutes) to the protoplasm. Water moves out from the cell through diffusion and causes the protoplasm to shrink away from the walls. In such situation, cell becomes plasmolysed.
When the cell is placed in an isotonic solution. There is not flow of water towards the inside or outside. If the external solution balances the osmotic pressure of the cytoplasm, it is said to be isotonic. When the water flow into the cell and out of the cells are in equilibrium the cell is called flaccid.
(b) When the plant cell is kept in a solution having high water potential (hypotonic solution or dilute solution as compared to cytoplasm), water diffuses into the cell causing the cytoplasm to build up a pressure against the wall, called turgor pressure. The pressure exerted by the protoplasts due to entry of water against the rigid walls is called pressure potential (Ψp). Because of the rigidity of the cell wall, the cell does not rupture. This turgor pressure is ultimately responsible for enlargement of cells.
How is the mycorrhizal association helpful in absorption of water and minerals in plants?
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 that perhaps a root cannot do. The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.
What role does root pressure play in water movement in plants?
Root pressure can provide only modest push during water transport in plants. The main role of root pressure is in re-establishing the continuous chain of water molecules in the xylem. The continuous chain often breaks due to enormous tension created by transpiration pull.
Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Transpiration occurs mainly through the stomata in the leaves. 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. This creates a transpiration pull.
Factors Affecting Transpiration: Temperature, light, humidity and wind speed.
Importance of Transpiration: Transport of liquids and minerals is facilitated because of transpiration.
Discuss the factors responsible for ascent of xylem sap in plants.
The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:
- Cohesion: Mutual attraction between water molecules.
- Adhesion: Attraction of water molecules to polar surfaces (such as the surface of tracheary elements).
- Surface Tension: Water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
These properties give water high tensile strength, i.e., an ability to resist a pulling force and high capillarity, i.e., the ability to rise in thin tubes. In plants, capillarity is aided by the small diameter of the tracheary elements, the tracheids and vessel elements.
What essential role does the root endodermis play during mineral absorption in plants?
The endodermis of roots have many transport proteins embedded in their plasma membrane. They let some solutes cross the membrane but not all. Transport proteins in endodermis cells enable plant cells lo adjust the quantity and types of solutes to be absorbed from the soil. It regulates the quantity and type of minerals and ions, that reach the xylem tissue of plants.
Explain why xylem transport is unidirectional and phloem transport bi-directional?
The source sink (food making tissue-tissue which stores food) relationship is variable in plants so, the direction of movement in the phloem can be upwards downwards, i.e., bi-directional. It is opposite to xylem, where the movement is always unidirectional. Hence, unlike one-way flow of water in transpiration, food in phloem sap can be transported in any required direction so long there is a source of sugar and a sink is able to use, store or remove the sugar. Here, in case of unidirectional flow in xylem tissue, it is important to note that root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.
Explain pressure flow hypothesis of translocation of sugars in plants.
The Pressure Flow or Mass Flow Hypothesis: The accepted mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis. As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a disaccharide). The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport.
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 decreases and water moves out of the phloem.
Hydrostatic pressure in the phloem sieve tube increases, pressure flow begins and the sap moves through the phloem. Meanwhile, at the sink, incoming sugars are actively transported out of the phloem and removed as complex carbohydrates. The loss of solute produces a high water potential in the phloem and water passes out, returning eventually to xylem.
What causes the opening and clog” T of guard cells of stomata during transpiration?
The immediate cause of the opening or closing-of the stomata is a change in the turgidity of the guard cells. The inner wall of each guard cell, 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. The opening of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally making it easier for the stoma 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 stoma closes.