The Physiology of Flowering Plants - KHAM PHA MOI

116 MINERAL NUTRITIONMechanisms of membrane transportThe two main characteristics that determine the ease with which aparticle can diffuse through a biological membrane are the lipidsolubilityof the particle and its molecular size.The more lipid-soluble(lipophilic) and the smaller the particle, the easier the penetration.The mineral ions are very hydrophilic, so they do not dissolve in thelipid bilayer. The atomic weights of some of the nutrient ions arequite low. However, the electric charge on the ions attracts hydrationshells of water molecules; e.g. K + (mass 39 Da) carries 4 molecules ofwater whereas the divalent Ca 2+ (mass 40 Da) has about 12 associatedwater molecules. These hydration shells increase the effective size ofthe ion considerably. The permeability of biological membranestowards ions is therefore very low. The flux of ions across membranesis enabled by specific transport proteins in the membranes whichfacilitate the movement of ions and not only provide the physicalmeans of passage for the ions, but utilize the energy of ATP totransport the ions against their electrochemical potential gradients.The total number of ion transport proteins in plant membranes ismuch greater than the number of nutrient ions, there often beingmore than one transport protein for the same ion. In Arabidopsis, 16genes have been identified coding for proteins involved with nitrateuptake, and the same number for phosphate (Vance 2001). The specificityof transport proteins varies; some are highly specific to singleions, but often they can transport several related ions, i.e. ions ofsimilar physicochemical properties such as valency and size. Therubidium ion, for instance, is transported by a number of cellularsystems for K + transport, and the radioactive 86 Rb is often used inexperiments as a substitute for K + , there being no convenient Kradioisotope available.The ion transport proteins can be divided into the pumps, theporters (carriers) and the channels; their main characteristics aresummarized in Table 4.2 and Fig. 4.4. The term ‘carrier’ was originallyused for all membrane transport proteins, before different typeswere distinguished.(1) Pumps.These are transport proteins which hydrolyse ATPand simultaneously transfer an ion across the membrane. The energyis derived from the ATP hydrolysis, directly. Pumps are vectorial, i.e. aTable 4.2 Plant ion transport systems and their basic features. Porters and channels exist also for organicchemicals. All cellular membranes contain transport proteins, mostly specific for a particular membrane.Pump Porter ChannelEnergetics Uses ATP directly Uses H + gradient fromH + pumpMovement along freeenergy gradientIons moved s –1 up to 510 2 510 2 to 10 4 10 6 to 10 8Examples of ionstransportedH + ,Ca 2+ Most if not all Ca 2+ ,K + ,Cl – ,H +