Question 1

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.

Accepted Answer

(a) Diffusion and Osmosis Diffusion Osmosis1. Diffusion is the passive movement of particles, ions, and molecules along the concentration gradient. 1. Osmosis is the process in which the diffusion of a solvent (water) occurs across a semi-permeable membrane.2. It can occur in solids, liquids, and gases.  2. It occurs in the liquid medium.3. It does not require a semi-permeable membrane. 3. It requires a semi-permeable membrane.(b) Transpiration and Evaporation Transpiration Evaporation 1. It occurs in plants. 1. It occurs from any free surface and involves living and non-living surfaces.2. It is a physiological process. 2. It is a physical process. 3. It occurs mainly through the stomatal pores on plant leaves. 3. It is occurs through any free surface. 4. It is controlled by environmental factors as well as physiological factors of plants such as root-shoot ratio and number of stomata.  4. It is entirely driven by environmental factors.(c) Osmotic pressure and Osmotic potentialOsmotic pressure Osmotic potential1. It is expressed in bars with a positive sign.1. It is expressed in bars with a negative sign.2. It is a positive pressure.2. It is a negative pressure.3. Its value increases with an increase in the concentration of solute particles.3. Its value decreases with an increase in the concentration of solute particles.(d) Imbibition and Diffusion:ImbibitionDiffusion1. Imbibition is a special type of diffusion. In this process, water is absorbed by solids and colloids, causing an enormous increase in volume.1. Diffusion is the passive movement of particles, ions, and molecules along the concentration gradient.2. It usually involves water.2. It involves solids, liquids, and gases. (e) Apoplast and Symplast pathways of water in plantsApoplast pathwaySymplast pathway1. The apoplast pathway involves the movement of water through the adjacent cell walls of the epidermis and cortex. The movement of water is restricted at the casparian strips of the root endodermis.1. The symplast pathway involves the movement of water through the interconnected protoplasts of the epidermis, cortex, endodermis, and root pericycle.2. It is a faster process of water movement and water moves through mass flow.2. It is a slower process of water movement.(f) Guttation and TranspirationGuttation Transpiration1. It occurs usually at night.1. It occurs usually during the day.2. Water is lost from the leaves in the form of liquid droplets.2. Water is lost from the leaves in the form of water vapour.3. It occurs through the vein endings of leaves.3. It occurs through the stomata.4. It is an uncontrolled process.4. It is a controlled process.

Question 2

Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?

Accepted Answer

Transpiration pull or cohesion-tension theory was originally proposed by Dixon and Joly in 1894 and further improved by Dixon in 1914. According to this theory, a continuouscolumn of water is present in the xylem channels of plant. The continuity of water column is maintained in the plant because of cohesive force of water molecules. There is another force of adhesion which holds water tp 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. As a result, the turgor pressure of these cells decreases and the diffusion pressure deficit (DPD) increases. Now these cells take water from adjoining cells and the turgor of those adjoining cells decreases. This process is repeated and ultimately water is absorbed from 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. Environmental factors:(i) Relative humidity - The rate of transpiration is inversely proportional to the relative humidity, i.e., the rate of transpiration is higher when the relative humidity is lower and lower when the relative humidity is higher.(ii)Atmospheric temperature - A high temperature opens stomata even 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.(iii)Light - Because most of the transpiration occurs through stomata, the rate of transpiration is quite high is light. It falls down appreciably in the darkness.(iv)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.(v) Atmospheric pressure - Low atmospheric pressure enhances evaporation, produces air currents and increases the rate of transpiration.(vi)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.Internal or plant factors :(i) Leaf area (transpiring area) - A plant with large leaf area will show more transpiration than another plant with less leaf area.(ii)Leaf structure - Leaf structure affects transpiration in following ways:(a) Cuticular transpiration decreases with the thickness of cuticle and cutinisation of epidermal walls.(b) Because most of the transpiration takes place through the stomata, their number and position influences the rate of transpiration.(c) The sunken stomata are device to reduce the rate of transpiration by providing an area where little air movement occurs.(iii)Root/shoot ratio - A low root/shoot ratio decreases the rate of transpiration while a high ratio increases the rate of transpiration.(iv)Mucilage and solutes - They decrease the rate of transpiration by holding water tenaciously.Transpiration is useful to plants in the following ways:(i) Removal of excess water - It has been held that plants absorb far more amount of water than is actually required by them. Transpiration, therefore, removes the excess of water.(ii)Root system - Transpiration helps in better development of root system which is required for support and absorption of mineral salts.(iii)Quality of fruits - The ash and sugar content of the fruit increases with the increase in transpiration.(iv)Temperature maintenance - Transpiration prevents overheating of leaves. However, plants growing in areas where transpiration is meagre do not show over-heating. Some succulents can endure a temperature of 60Â°C without any apparent damage.(v)Pole in ascent of sap and turgidity - Ascent of sap mostly occurs due to transpiration pull exerted by transpiration of water. This pull is important in the absorption of water. Further, transpiration maintains the shape and structure of plant parts by keeping cells turgid.(vi)Distribution of mineral salts- Mineral are mostly distributed by rising column of sap.(vii)Photosynthesis - Transpiration supplies water for photosynthesis.

Question 3

Discuss the factors responsible for ascent of xylem sap in plants.

Accepted Answer

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:(i) 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.(ii)Root pressure: It is positive pressure that pushes sap from below due to active absorption by root.(iii)Transpiration pull: 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.

Question 4

Explain pressure flow hypothesis of translocation of sugars in plants.

Accepted Answer

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 i.e., in leaves, (by photosynthesis) it is converted to sucrose (a dissacharide). The sugar is then moved in the form of sucrose into adjacent companion cells and then into the living phloem i.e., in sieve tube cells 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 thephloem 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.

Chapter 11 : Transport in Plants