Biology XI: 11 Transport in plants(1 Means of Transport)

Means of Transport Transport in Plants Short distance transport: By diffusion and cytoplasmic streaming, supplemented by active transport Long distance transport: Translocation (occurs through vascular system) Transport of water and minerals through the xylem: Unidirectional Transport of organic and mineral nutrients through the phloem: Multidirectional Means of Transport Three means of transport in plants: Diffusion Facilitated Diffusion Active Transport Diffusion An important means of transport inside the plant body Movement of molecules in a random manner, across the concentration gradient Slower process, involving no expenditure of energy Not dependent on the living system Depends upon: Concentration gradient Permeability of the membrane Temperature Pressure Size of the substance Solubility in lipids Facilitated Transport Diffusion depends upon solubility in lipids. Therefore, substance having hydrophilic moiety finds it difficult to diffuse through the membrane. Hence, their movement has to be facilitated. In facilitated diffusion, the membrane proteins are involved. They provide a site for hydrophilic molecules to pass through the membrane. Concentration gradient is not provided through such proteins. It must already be present. In this case, no ATP (energy) expenditure is required. However, for diffusion against the concentration gradient, ATP expenditure is required. Proteins involved in the process form channels which may always be opened or controlled. Facilitated diffusion is very specific. Porins: Proteins that forms huge pores in the outer membranes of plastids, mitochondria, etc. Aquaporins: Proteins that facilitate diffusion of water molecules When two types of molecules move together through the proteins, transport can be of 3 types: Symport − both molecules move in the same direction Antiport − both molecules move in opposite directions Uniport − independent movement of molecules Maximum transport: When all proteins involved are saturated Active transport Requires energy to pump molecules against the concentration gradient Requires special proteins which are very specific and sensitive to inhibitors Pumps proteins, using energy to transport substances through uphill transport Maximum transport: When all proteins involved are saturated Comparison between simple diffusion, facilitated transport and active transport: Characteristic Simple diffusion Facilitated transport Active transport Requirement of special membrane proteins × √ √ Selectivity × √ √ Saturation of transport × √ √ Uphill transport × × √ Requirement of ATP × × √