Title Med-RM_Bot_SP-2_Ch-8-Transport in Plants.pdf ✅

Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents Means of Transport Plant-water Relations Long Distance Transport of Water Transpiration Uptake and Transport of Mineral Nutrients Phloem Transport: Flow from Source to Sink Chapter 8 Plants need to move molecules over very long distances, much more than animals do; they also do not have a circulatory system in place. Water taken up by the roots reaches each and every part of plant upto the tip of growing stem. The photosynthates or food synthesized by the leaves have also to be moved to all parts of plant including the root tips which remain embedded inside the soil. For instance, in a flowering plant the substances that would need to be transported are water, mineral nutrients, organic nutrients and plant growth regulators etc. Over short distances substances move by diffusion, and by cytoplasmic streaming supplemented by active transport while their long distance transport occurs through vascular system, i.e., xylem, phloem and is called translocation. MEANS OF TRANSPORT Various materials are transported into and out of a living cell, by a number of methods. Some of them are: (i) Diffusion, (ii) Facilitated diffusion and (iii) Active transport. (i) Diffusion The movement of molecules or ions from the region of their higher concentration to the region of their lower concentration, (along the concentration gradient) until the equilibrium is achieved is known as diffusion. The diffusing particles create a certain pressure called as diffusion pressure (DP) which is directly proportional to the number or concentration of diffusing particles. The molecules move from higher DP to lower DP. Characteristic of Diffusion (1) The direction of diffusion of one substance is independent of the movement of another substance. (2) It is very important to plants since it is the only means for gaseous movement within the plant body. Transport in Plants
40 Transport in Plants NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 40 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (3) It is a slow process and is not dependent on a living ‘system’. Diffusion is obvious in gases and liquids, but diffusion in solids rather than of solids is more common. Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature and pressure. (ii) Facilitated Diffusion The diffusion of hydrophilic substances along the concentration gradient through fixed membrane transport proteins without energy involvement is called facilitated diffusion. The diffusion of any substance across a membrane also depends on its solubility in lipids, the major constituent of the membrane. Substances soluble in lipids diffuse through the membrane faster. Substances that have a hydrophilic moiety, find it difficult to pass through the membrane; their movement has to be facilitated. There are certain proteins which provide the site for such molecules to pass through the membrane. They do not set up concentration gradient; a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins. Facilitated diffusion cannot cause net transport of molecules from a low to a high concentration, as this would require input of energy. Transport rate reaches a maximum when all of the protein transporters are being used (saturation). It is very specific. It is sensitive to inhibitors which react with protein side chains. The porins are proteins which form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules upto the size of small proteins to pass through. Given figure shows an extracellular molecules bound to the transport protein; the transport protein then rotates and releases the molecule inside the cell, e.g., Water channels are made up of eight different types of aquaporins. Outer side of cell Outer side of cell Membrane Membrane Transported molecule Transport protein Inner side of cell Fig. : Facilitated Diffusion Passive Symport and Antiports Membrane Carrier protein Uniport Antiport Symport A A A B B Fig. : Facilitated Diffusion Some carrier or transport proteins allow diffusion only if two types of molecules move together. They are of the following types (i) A symport is the transport of two types of molecules across the membrane in the same direction. (ii) An antiport is the transport of two different molecules or substances in opposite directions. Uniport : It is the transport of a molecule or substances across the membrane independent of other molecules or substances.
NEET Transport in Plants 41 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 41 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (iii) Active Transport It is the transport of materials across a membrane with the help of specific membrane proteins and ATP. Pumps are proteins that use energy to carry substances across the cell membrane. These pumps can transport substances from their low concentration to high concentration so it is an uphill transport (i.e., against concentration gradient) and is faster than passive transport. The rate of active transport reaches a maximum when all the protein pumps have been used in transport, this is called saturation effect. Carrier proteins are highly specific like enzymes. They are also sensitive to inhibitors that react with protein side chains. Table : Comparison of Different Transport Mechanisms 1. Requires special membrane proteins No Yes Yes 2. Highly selective nature No Yes Yes 3. Transport saturates No Yes Yes 4. Uphill transport No No Yes 5. Requires ATP energy No No Yes 6. Movement of transport proteins No No Yes 7. Response to protein inhibitors No Yes Yes Property Simple Diffusion Facilitated Transport Active Transport PLANT-WATER RELATIONS Water is essential for all physiological activities of plants. It plays a direct role in many useful reactions operating in cells. For eg. It plays a key role in photosynthesis and acts as a source of oxygen. A watermelon has over 92 per cent water; most herbaceous plants have only 10 to 15 per cent of its fresh weight as dry matter. Terrestrial plants take up huge amount water daily but most of it is lost to the air through evaporation from the leaves, i.e., transpiration. A mature corn plant absorbs almost three litres of water in a day, while mustard plant absorbs water equal to its own weight in about 5 hours. Because of this high demand of water, it is not surprising that water is often the limiting factor for plant growth and productivity in both agricultural and natural environments. Water Potential In thermodynamics, free energy represents the potential to do work. The free energy of water is referred to as water potential. Pressure is another source that provides energy to water. The increasing pressure increases the free energy and, hence, the water potential in a system. Water potential is also defined as chemical potential of water. The greater the concentrations of water in a system, the greater is its kinetic energy or ‘water potential’. Water moves from the point where water potential is greater, to the other point where water potential is less until the equilibrium is reached. Therefore, based on the concept of water potential, it is easier for the scientists to predict the way in which the water will move. Water potential is measured in terms of pressure. The common measurement unit of water potential is pascal, Pa (1 Megapascal, MPa = 10 bars) and is represented by the Greek letter, Psi ().
42 Transport in Plants NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 42 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 By convention, the water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero. If some solute is dissolved in pure water, the solution has fewer free water and the concentration of water decreases, reducing its water potential. Hence, 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 s . s is always negative. The more the solute molecules, the lower (more negative) is the s. For a solution at atmospheric pressure (water potential) w = (solute potential) s. If the pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. Pressure can build up in a plant system when water enters a plant cell due to diffusion causing a pressure built up against the cell wall, it makes the cell turgid; this increases the pressure potential or hydrostatic pressure of a solution. It 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 in a stem. It is denoted by p. Solute potential (s) and pressure potential (p) are the two main components that determine water potential. The relationship between them is as follows : w = s + p Osmosis The plant cell is surrounded by a cell membrane and a cell wall. The cell wall is freely permeable to water and substances in solution, hence is not a barrier to movement. The cell membrane of plant cells and tonoplast together play an important role in determining movement of molecules in and out of the cell. Osmosis can be defined as “the passage of solvent molecules from a region of their higher concentration to a region of their lower concentration through semi-permeable membrane”. It occurs spontaneously in response to a driving force. Osmosis is driven by two factors (i) Concentration of dissolved solutes in a solution (ii) Pressure gradient. Thistle Funnel Experiment : Let us discuss an experiment where a solution of sucrose in water taken in funnel is separated from pure water in a beaker through semi-permeable membrane. The level of the solution in the tube of the funnel is marked. After some time, the level in the tube increases. This is due to the entry of water molecules from beaker into the thistle funnel. The concentration of water molecules in the beaker is more than their concentration inside the thistle funnel. Therefore, water molecules move from the region of their higher concentration (i.e., from beaker) to the region of their lower concentration (i.e., inside the funnel). This will continue till the equilibrium is reached. External pressure can be applied from the upper part of the funnel such that no water diffuses into the funnel through the membrane. This pressure required to prevent water from diffusing is in fact, the osmotic pressure and this is the function of the solute concentration; more the solute concentration, greater will be the pressure required to prevent water from diffusing in. Numerically, osmotic pressure is equivalent to the osmotic potential (s), but the sign is opposite. Osmotic pressure () is the positive pressure applied, while osmotic potential is negative. So, s = – Factors affecting OP 1. Concentration of solute particles 2. Ionization of the solute molecule 3. Temperature 4. Hydration of the solute particles.