Tiêu đề TRANSPORT IN PLANTS.pdf ✅

 Digital www.allendigital.in [ 3 ] 1. Introduction : In a flowering plant substance that would need to be transported are water, mineral nutrients, organic nutrients and plant growth regulators / hormones. The small distance transport means transport with in the cell or across the membrane or from cell to cell in a tissue occurs by diffusion, facilitated diffusion (passive transport) and by Active transport. Transport over longer distance proceeds through the vascular system (the xylem and the phloem) and is called translocation. Direction of Transport : (i) In rooted plants transport in xylem (of water and minerals) is unidirectional, from roots to stem. (ii) Organic and mineral nutrients however, undergo multidirectional transport. Organic compounds synthesised in the photosynthetic leaves are exported to all other parts of the plant including storage organs. From the storage organs they are later re-exported. The mineral nutrients are taken up by the roots and transported upwards into the stem, leaves and the growing regions. When any plant part undergoes senescence, nutrients may be withdrawn from such regions and moved to the growing parts. (iii) Hormones or plant growth regulators and other chemical stimuli are also transported, though in very small amounts, sometimes in a strictly polarised or unidirectional manner from where they are synthesised to other parts. Hence, in a flowering plant there is a complex traffic of compounds (but probably very orderly) moving in different directions, each organ receiving some substances and giving out some others. 2. Means or types of transport : Transport in plants is of two types – (A) Short distance transport (B) Long distance transport (A) Short distance transport : • If transport occurs within the cell or from one cell to another cell then it is considered as short distance transport. Further it is of two types – (i) Passive transport (ii) Active transport (i) Passive transport : • If transport occurs according to concentration gradient (High concentration to low concentration) without expenditure of ATP. It occurs by following methods – (a) Simple diffusion (b) Facilitated diffusion (a) Simple Diffusion : • "The movement of molecules or atoms or ions of a material from an area of higher concentration to an area of their lower concentration is called diffusion." e.g. Diffusion of odour of perfume, spreading of crystals of KMnO4 in water. Transport in Plants 01
NEET : Biology [ 4 ] www.allendigital.in  Digital • The diffusion is continued till the dynamic equilibrium is not established. At this stage the movement of molecules is equal in both the direction, so net movement is zero. • Diffusion is a slow process it can account for only short distance of molecule for e.g. for movement of a molecule across a typical plant cell (about 50 mm) takes approximately 2.5 second. • Diffusion is not dependent on a living system. • Diffusion is obvious in gases and liquids but diffusion in solids is more likely rather than of solids. Diffusion rate → Gas > Liquid > Solid • Diffusion of each substance is free from diffusion of other substance. Like diffusion of CO2 and O2 occurs at the same time in different directions through stomata. • Diffusion can occur in both manners - without any membrane or through the membrane. • Factors affecting diffusion rate - Concentration gradient, permeability of membrane, temperature and pressure. • The diffused molecules or ions exert a pressure on the substance or medium in which diffusion takes place, known as diffusion pressure. Significance of Diffusion : (1) Exchange of gases like CO2, O2 between plant and atmosphere, occur through diffusion. (2) It is the only means for gaseous movement within the plant body. (3) The process of transpiration is also a diffusion. The evaporation of water from the intercellular spaces is linked with diffusion during the transpiration. (b) Facilitated Diffusion : • Diffusion of any substance depends upon solubility in main constituent of membrane, lipid. Lipid soluble substance rapidly diffuse through the membrane. • Moiety of which substance is hydrophilic, diffuses difficultly through membrane. So need to simplify its movement. Membrane proteins provide space for transfer of these molecules. This process is called facilitated diffusion. • In facilitated diffusion, specific proteins help in transfer of substances across the membrane and no ATP is consumed. These specific proteins do not setup a concentration gradient, a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins. • Transport rate reaches maximum when all of the protein transporters are being used (saturation). • Facilitated diffusion is very specific, it allows cell to select substances for uptake. • It is sensitive to inhibitors which react with protein side chains. • The proteins form channels in the membrane for molecules to pass through. Some channels are always open, others can be controlled. Some are large, allowing a variety of molecules to cross. • 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. e.g. Water channels are made up of 8 different types of aquaporins. (i) In symport, two molecules move across the membrane in similar direction. (ii) Antiport they move in opposite directions. (iii) A molecule moves freely across the membrane then this method is called uniport.
Transport in Plants  Digital www.allendigital.in [ 5 ] (ii) Active Transport : • Active transport uses energy to transport and 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 ('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. Comparison different transport processes Property Simple diffusion Facilitated transport Active transport Requires special membrane proteins No Yes Yes Highly selective No Yes Yes Transport saturates No Yes Yes Sensitivity to inhibitors No Yes Yes Hormonal regulation No Yes Yes Uphill transport No No Yes Requires ATP energy No No Yes 3. Permeability : • The extent to which a membrane permits or restricts the movement of a substance is called membrane permeability. • The membrane are of four types on the basis of permeability: - (i) Permeable membrane : Such membrane is permeable for both - solutes and solvent. e.g. cellulosic cell wall of living cells, filter paper. (ii) Semipermeable Membrane : Such membrane allows diffusion of solvent molecules rapidly but does not allow solutes. e.g. Artificial membranes like Cellophane and Copper ferrocyanide membranes, parchment paper. Outer side of cell Membrane Outer side of cell Transport protein Transported molecule Inner side of the cell Antiport A A A B B Uniport Symport Membrane Facilitated diffusion Facilitated diffusion Transport protein Membrane
NEET : Biology [ 6 ] www.allendigital.in  Digital (iii) Selectively permeable Membrane or differentially permeable membrane: - Such membrane allows some selective solutes to pass through them along with the solvent molecules. e.g. Cell membrane, tonoplast, membrane of cell organelles. (iv) Impermeable membrane: - Membranes which do not allow both solvents to pass through it. e.g. cell wall of non-living cell like suberised cell wall. 4. Types of Solutions : (i) Isotonic solution : If solution in which a cell is placed, has equal concentration to that of cytoplasm and cell sap then solution is called isotonic solution. (ii) Hypotonic solution : If the concentration of outer solution is less than that of the cytoplasm and cell sap then solution is called hypotonic solution. (iii) Hypertonic solution : If the concentration of outer solution is higher than that of the cytoplasm and cell sap such solution is known as hypertonic solution. 5. Plant Water Relations : • Water is essential for all physiological activities of the plant and plays a very important role in all living organisms. It provides the medium in which most substances are dissolved. • The protoplasm of the cells is nothing but water in which different molecules are dissolved and (several particles) suspended. • A watermelon has over 92 percent water, most herbaceous plants have only about 10 to 15 percent of its fresh weight as dry matter. • Distribution of water within a plant varies - woody parts have relatively very little water, while soft parts mostly contain water. • A mature corn plant absorbs almost three litres of water in a day, while a mustard plant absorbs water equal to its own weight in about 5 hours. • Because of this high demand for water, it is not surprising that water is often the limiting factor for plant growth and productivity in both agricultural and natural environments. (A) Osmosis and Osmotic Pressure : • "Osmosis is defined as the diffusion of solvent (like water) from the solution of lower concentration (Hypotonic) to the solution of higher concentration (Hypertonic) when both are separated by a semipermeable membrane." • The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient. • Demonstration of osmosis by potato osmometer : If the potato tuber is placed in water, the cavity in the potato tuber containing a concentrated solution of sugar collects water due to osmosis. Potato Osmoscope Hypertonic solution Hypotonic solution Hypertonic solution Hypotonic solution Endosmosis Endosmosis Semipermeable membrane Demonstration of Osmos is by potato osmometer