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Mineral salt uptake 5B.1 and 2B.2 By CSE
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Mineral culture solutions Each lack a certain mineral Enables deficiency symptoms to be seen easily Chlorosis – poor growth and yellowing of leaves due to lack of nitrate or Mg Phosphate deficiency leads to poor root growth
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Nitrates Nitrogen needed for protein synthesis, nucleic acids, coenzymes, vitamins, chlorophyll Absorbed as nitrates from soil or from surrounding water if aquatic or as ammonium ions Macronutrient Deficiency – poor growth, yellowing of leaves = chlorosis
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Nitrate is reduced to nitrite and then to ammonia, using either NADH produced in mitochondria or NADPH produced in chloroplasts: NO 3 - NO 2 - NH 3 Nitrate reductase Nitrite reductase
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Ammonia then combines with the keto acid ∂ ketoglutaric acid to form the amino acid, glutamic acid. This then donates its amino group to other keto acids in transamination reactions to form the corresponding amino acid. Eg it can donate its amino group to oxaloacetic acid ( a keto acid) to form aspartic acid (an amino acid)
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Phosphates Absorbed in large amounts from soil solution Absorbed as hydrogen phosphate (HPO 4 2- ) in normal soils or dihydrogen phosphate (H 2 PO 4 - ) in acid soil Assimilated into organic ATP during respiration then into nucleic acids, sugar phosphates and phospholipids Deficiency means poor growth particularly of roots
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Magnesium Absorbed as Mg 2+ Needed in small amounts, micronutrient Is a component of chlorophyll Is an activator of some enzymes such as ATPase Deficiency = chlorosis of leaves
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Other minerals Eg Copper, molybdenum, manganese, zinc Many act as enzyme activators Called micronutrients or trace elements Molybdenum is a component of nitrate reductase which catalyses the reduction of nitrate to nitrite. Iron is a constituent of some of the electron carriers in respiration and photosynthesis. Some micronutrients are only needed by particular plants eg silicon is needed by grasses as silica is a constituent of cell walls. Sodium is needed for C4 plants.
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Mineral uptake difficulties Some minerals are even more dilute in soil than CO 2 is in the atmosphere eg phosphate concentrations are usually only a few parts per million. Minerals move slowly through the soil. Air currents take CO 2 to the leaf but the only movement of soil water is by very slow capillary flow brought about by water uptake through root. Minerals in solution diffuse more slowly than CO 2 diffuses in air
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Uptake Root hairs – long hair like extensions of the outer epidermal cells of the root Penetrate between soil particles Increase surface area of root. The root system of a 50g rye plant has a surface area / mass ratio over 2000 times greater than that of a 70kg human. Main entry point for water and mineral ions Ions enter by diffusion or active transport, depending on its electrochemical gradient The cells of outer part of the root have a membrane potential, the outside being 100mV positive to the inside, as all cells
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Uncharged solutes uptake depends on concentration gradient of the solute Movement of a charged ion depends on its concentration inside and outside the cell and the electric potential difference across the cell membrane = electrochemical gradient is combination of these 2. All anions (nitrate, sulphate, phosphate) enter root hair cells against their electrochemical and concentration gradient by active transport, so needs energy Cations eg sodium, calcium, magnesium have an electrochemical gradient which favours their entry by passive diffusion (need to be moved out of cells by active transport) as they are positive so electric attraction greater than concentration gradient difference as they are already inside root. It depends on active processes as the generation of the membrane potential is due to the pumping out of positively charged H +. The energy comes from respiration.
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On entering root, ions may stay in cytoplasm or vacuole or pass out to the cellulose walls Then pass through outer parts of the root by the apoplast or symplast pathway Apoplast = movement through adjacent cell walls Symplast = water moves along same water potential gradient across interconnected cytoplasm of the cells of the root system
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Apoplast route Cellulose is loose, open network and can be half full of water As water/ salts drawn into xylem, more water/salts attracted from adjacent cell wall Move across root until reach endodermis where casparian strip of suberin stops it Water/salts have to enter cytoplasm by active transport or diffusion
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Symplast route Cytoplasm of adjacent plant cells connected by strands called plasmodesmata which go through pores in the cellulose walls. Water moves from the soil across the cytoplasm of adjacent cells along the water potential gradient from the root hair cells to the xylem
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