Gujarat Board GSEB Textbook Solutions Class 11 Biology Chapter 11 Transport in Plants Textbook Questions and Answers.
Gujarat Board Textbook Solutions Class 11 Biology Chapter 11 Transport in Plants
GSEB Class 11 Biology Transport in Plants Text Book Questions and Answers
What are the factors affecting the rate of diffusion?
Factors affecting the rate of diffusion
- 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. An extracellular molecule bound to the transport protein; the transport protein then rotates and releases the molecule inside the cell e.g. water channels made up of 8 different types of aquaporins.
Describe the role played by protein pumps during active transport in plants.
Active transport uses energy to pump molecules against a concentration gradient. Active transport is carried out by membrane proteins. Hence different proteins in the membrane play a major role in both active as well as passive transport. Pumps are proteins that use energy to carry substances across the cell membrane.
These pumps can transport substances from a low concentration to a high concentration. The 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 its ‘water potential’. Hence it is obvious that pure water will have the greatest water potential. The water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero.
Differentiate between the following:
(a) Diffusion and Osmosis
Diffusion is a form of passive transport which takes place anywhere and the flow happens from high concentration to low concentration. Osmosis happens across a semipermeable membrane. For diffusion, the semi-permeable membrane is not a precondition.
(b) Transpiration and Evaporation
Transpiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems flowers and roots. Leaf surface are dotted with openings called stoma that are bordered by guard cells. Collectively these structures are called stomata. Leaf transpiration occurs through stomata and can be thought of as a necessary cost associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.
(c) Osmotic Pressure and Osmotic Potential
The osmotic potential is defined as the capability of a solution to suck water in if it was separated from another solution by a semipermeable membrane. It is always a negative number. Higher the negative number of the osmotic potential of a solution, the more it will suck water in.
The terms isotonic, hypotonic and hypertonic describe the difference in osmotic pressure between the solutions with a certain osmotic potential. Two solutions are isotonic when the osmotic potentials are equal. When they are different, the one with the higher potential will have less pressure and the one with lower potential will have more pressure.
(d) Imbibition and Diffusion
Imbibition is a special type of diffusion when water is absorbed by solids colloids causing them to enormously increase in volume. The classical examples of imbibition are absorption of water by seeds and dry wood.
Imbibition is also diffusion since water movement is along a concentration gradient, the seeds and other such materials have almost no water hence they absorb water easily. Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition. In addition, for any substance to imbibe any liquid, affinity between the adsorbent and the liquid is also a prerequisite.
(e) Apoplast and Symplast pathways of movement of water in plants.
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the casparian strip in roots, air spaces between plant cells and the cuticula of the plant. Structurally the apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces, forming a tissue level compartment comparable to the symplast. The apoplastic route facilitates the transport of water and solutes across a tissue or organ. This process is known as apoplastic transport.
The symplast of a plant is the inner side of the plasma membrane in which water can freely diffuse. The plasmodesmata allow the direct flow of small molecules such as sugar, amino acids and ions between cells. Larger molecules, including transcription factors and plant viruses, can also be transported through with the help of actin structures.
(f) Guttation and Transpiration.
Guttation is the appearance of drops of vascular plants such as grasses. At night, transpiration usually does not occur because most plants have their stomata closed. When there is a high soil moisture level, water will enter plant roots, because the water potential of roots is lower than soil solution. The water will accumulate in the plant, creating slight root pressure. The root pressure forces some water to exude through a special leaf tip or edge structured called hydathodes forming drops. Root pressure provides the impetus for this flow. Transpiration, on the other hand, happens because of transpiration pull.
(g) Briefly describe water potential. What are the factors affecting it?
Nater potential is the potential energy of water relative to pure free water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure or matrix effects including surface tension. Water potential is measured in units of pressure and is commonly represented by the Greek letter 141’ (Psi).
Briefly describe water potential. What are the factors affecting it?
Water potential is a concept fundamental to understanding water movement. Solute potential and pressure potential are the two main, components that determine 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 its ‘water potential’. Hence it is obvious that pure water will have the greatest water potential. If two systems containing water are in contact, random movement of water molecules will result in a net movement of water molecules from the system with higher energy to the one with lower energy.
This process of movement of substances down a gradient of free energy is called diffusion. Water potential is denoted by the Greek symbol Psi or Ψs and is expressed in pressure units such as pascals (Pa). By convention, the water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero.
All solutions have a lower water potential than pure water; the magnitude of this lowering due to dissolution of a solute is called solute potential or Ψs is always negative. The more the solute molecules, the lower (more negative) is the Ψs For a solution at atmospheric pressure water potential = solute potential Ψs
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 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, though in plants negative potential or tension in the water column in the xylem plays a major role in water transport up a stem. Pressure potential is denoted as Ψs
The 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.
(a) With the help of well-labeled 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 (or tissue) is placed in a solution that is hypertonic (has more solutes) to the protoplasm. Water moves out; it is first lost from the cytoplasm and then from the vacuole. The water when drawn out of the cell through diffusion into the extracellular (outside the cell) fluid causes the protoplast to shrink away from the walls.
The cell is said to be plasmolyzed. The movement of water occurred across the membrane moving from an area of high water potential (i.e. the cell) to an area of lower water potential outside the cell. When the cell is placed in an isotonic solution, there is no net flow of water towards the inside or the outside. If the external solution balances the osmotic pressure of the cytoplasm it is said to be isotonic. When the water flows into the cell and out of the cell are in equilibrium the cells are said to the flaccid.
(b) When the ceils are placed in a hypotonic solution (higher water . potential 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. The pressure exerted by the protoplasts due to the 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 the enlargement and extension growth of cells.
How is mycorrhizal association helpful in the absorption of water and minerals in plants?
Some plants have additional structures associated with them that help in water and mineral absorption. 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 absorbs 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 Nitrogen-containing compounds to the mycorrhizae.
What role does root pressure play in water movement in plants?
As various ions from the soil are actively transported into the roof’s vascular tissue, 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 up to small heights in the stem.
Root pressure can only provide a modest push in the overall process of water transport. They obviously do not play a major role in water movement up 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. Root pressure does not account for the majority of water transport; most plants meet their need by the transpiratory pull.
Describe the transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?
Transpiration is the evaporative loss of water by plants. It occurs mainly through the stomata in the leaves. The exchange of oxygen and carbon dioxide in the leaf also occurs through stomata. Normally stomata are open in the daytime and close during the night. The cause is a change in the turgidity of the guard cells. The inner wall of each guard cell towards the stomatal aperture is thick and elastic.
When turgidity increases, 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. 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 the ascent of xylem sap in plants.
The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:
- Cohesion – the mutual attraction between water molecules.
- Adhesion – the 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.
The process of photosynthesis requires water. The system of xylem vessels from root to leaf vein can supply the needed water. But what force does a plant use to move water molecules into the leaf parenchyma cells where they are needed? As water evaporates through the stomata, since the thin film of water over the cells is continuous, it results in the pulling of water, molecule by molecule, into the leaf from the xylem.
Also, because of the lower concentration of water vapor in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air. This creates a ‘puli’. Measurements reveal that the forces generated by transpiration can create pressures sufficient to lift a xylem-sized column of water over 130 meters high.
What essential role does the root endodermis play during mineral absorption in plants?
Unlike water, all minerals cannot be passively absorbed by the roots. Therefore most minerals must enter the root by active absorption into the cytoplasm of epidermal cells. This needs energy in the form of ATP. The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis. Some ions also move into the epidermal cells passively.
Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasms of the epidermal cells. Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem. It is important to note that the root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.
Explain why xylem transport is unidirectional and phloem transport bi-directional.
Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e. bi-directional. This contrasts with that of the xylem where the movement is always unidirectional i.e. upwards. Hence, unlike transpiration’s one-way flow of water, food in phloem sap can be transported in any direction needed so long as there is a source of sugar and a sinkable to use, store or remove the sugar. Phloem sap is mainly water and sucrose, but other sugars, hormones, and amino acids are also transported or translocated through the phloem.
Explain the pressure-flow hypothesis of translocation of sugars in plants?
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, for example) 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. This process of loading at the source produces a hypertonic condition 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 decreases and water moves out of the phloem.
To summarise, the movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the mass movement in the phloem.
Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments. As 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 the xylem.
What causes the opening and closing of guard cells of stomata during transpiration?
Normally stomata are open in the daytime and close during the night. 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, is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture 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.
The opening of the stoma v 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.