BIOL 240W 1st Edition Lecture 3 Outline of Last Lecture I. Essential ElementsII. MacronutrientsIII. MicronutrientsIV. Nutrient DeficienciesV. Soil PropertiesOutline of Current Lecture I. Cation Exchangea. Capturing nutrientsb. Soil pHc. Problems with phosphateII. The Nitrogen Cyclea. Rhizobium bacteriaIII. Mycorrizal fungiCurrent LectureI. Cation Exchangea. Capturing nutrientsi. Cationic nutrients (Ca2+, K+, Mg+) are attracted to negative soil particles.ii. Cells respire and release CO2, which combine swith water to make carbonic acid. This releases a proton (H+)1. H2O + CO2 H2CO3 HCO3- + H+iii. Released proton acidifies soil and hydrogen ions are exchanged with minerals. Minerals are then taken up by rootsb. Soil pHi. pH can change a mineral to a form the roots cannot absorb. It can also cause minerals to be bound too tightly to clay particles.ii. Most plants thrive in slightly lower pH (acidic soil) because more H+ ions are available, leading to a greater displacement of minerals.iii. Disadvantage: some minerals may be more readily available while another becomes less available.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.iv. Anions in the soil (NO3-, SO42-) are lost rapidly during rain and irrigation because the negatively charged ions are not attracted to the negative soil particles like cations. PO4- is an exception.c. Problems with Phosphate (PO4-)i. Most limiting rock-derived nutrientii. Forms highly insoluble aggregates with many substances. The roots cannot take in these insoluble aggregates.iii. Only a limited fraction of phosphate is soluble, and this can be easily lost during rainfall.iv. Potential shortages in the future (phosphate mines making fertilizer are not widespread)d. Aluminum toxicity- at pH <5, Al becomes soluble. Even if plant does not need Al, it will take it up through roots, eventually damaging them. Intensive agriculture increases the cation exchange (thus, increasing H+) leading to acidic soil.II. The Nitrogen cyclea. Microorganisms important to plant nutrition in soilb. Most nutrients are derived from fragmentation of rocks except for nitrogenc. Majority of atmosphere is made up of nitrogen, but N2 is very stable in the atmosphere, causing it to be the most limiting macronutrient.d. Nitrogen gas must be reduced to a form usable by plants (nitrogen fixation)i. N2 NH3 NH4+ii. Carried out by prokaryotesiii. Requires a lot of energy (8ATP per NH3)e. Role of ammonifying bacteriai. Proteins derived from humus (dead organic material) are decomposed to amino acids.ii. Ammonifying bacteria further decompose amino acids to NH3.iii. NH3 converted to NH4+ by H+ in soilf. Role of nitrogen-fixing bacteriai. Create NH3 differently from ammonifying bacteria because the starting material is N2 in the atmosphere.ii. Both create the same productsg. Chemosynthetic autotrophs (bacteria in soil) oxidize NH4+ to nitrite (Nitrosomonas) and then nitrate (Nitrobacter).i. Nitrate is the main form taken up by plants, but it is an anion (NO3-) so it iseasily leeched from soils.ii. Plants must spend energy to re-reduce NO3- NH4+iii. Only NH4+ is used to synthesize compounds containing nitrogenh. Industrial fixation of nitrogen has revolutionized farming because it is delivered inthe form of NH4+. However, expends large amounts of fossil fuels and the runoffs disrupts aquatic ecosystems.i. Problems with oxygeni. Nitrogenase is the enzyme required for nitrogen fixationii. This enzyme is inhibited by oxygen, limiting nitrogen fixation to anaerobic environments. However, in these environments, ATP production is also limited (oxygen is necessary for ATP production).iii. Plants overcome this by developing root nodules with Rhizobium bacteria.1. Contain a barrier that tightly regulates oxygen movement.2. Leghemoglobin is present. This binds to O2 before it can deactivate enzyme.3. Bacteroid cells have also optimized their electron transport chains to deal with low O2.4. Symbiosis: Host plant gets source of NH4+; Rhizobium gets sugar and a protective environment.III. Mycorrhizal fungia. Associated with majority of plant rootsb. Important to acquire phosphatec. Two types: arbuscular mycorrhizae and ectomycorrhizaei. Arbuscular mycorrhizal fungi are the most common type of the mycorrhizal associations (~70% of vascular plants). Arbuscles provide huge surface area for nutrient exchange by forming a dense web around outside of root. ii. Ectomycorrhizal fungi does not involve the penetration of cells, but instead grows through extracellular spaces.d. Almost all terrestrial plants have mycorrhizal associations with
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