Final Exam Study Guide Lectures: 1 - 5Lecture 1Aristotle once suggested that plants are soil-eaters. The Van Helmot experiment tested a willow seed that was planted in a pot of 90.9 kg of soil. After 5 years, the plant weighed 76.8 kg,but only .06 kg of soil was missing. This concluded that plants are not soil eaters. They obtain nutrients elsewhere.Most of a plant’s mass is from water. 96% of the dry mass is derived from CO2 (and H2O), which is assimilated into carbohydrates during photosynthesis. 4% is inorganic substances from soil (discussed in future lectures this week). Plants obtain inorganic materials (from air and soil) and synthesize organic matter (from inorganic precursors). Lecture 2An essential Element is a chemical element that is required for a plant to complete its life cycle and produce another generation. Researchers determine essential elements by using a hydroponic culture, which grow plants in mineral solutions instead of soil. There are 17 essential elements needed by plants (found in table 37.1 of text). Nine of the essential elements are macronutrients. They are called macronutrients because plants require them in large amounts. The big three micronutrients are Nitrogen, Phosphorous, and Potassium. Nitrogen is the biggest limiting factor in the growth of plants (not readily available in large amounts). It isa component of nucleic acids, protein, chlorophyll, etc. Nitrogen is scavenged from soil as NO3- or NH4+.Phosphorous is a component of nucleic acids, phospholipids, ATP, etc. It is scavenged from soil in the form of PO42-. Potassium is scavenged in the soil as K+ (potassium ion). It is a cofactor for protein synthesis, and an osmoregulator to determine the flow of water in and out of plant cells.The other eight essential elements are only needed in small amounts. These are the micronutrients. They are rarely limiting to the plant growth. Examples include chlorine, iron, manganese, etc. They function as co-factors, or nonprotein “helpers” in enzymatic reaction. An example is the iron in the hemecompound that is critical for electron transport in photosynthesis and respiration (cytochromes).Most deficiencies involve nitrogen, potassium, and phosphorous. Micronutrient shortages are less common. They are normally due to soil composition, and only a small amount of mineral is needed to correct a deficiency. Symptoms can be based on mineral’s function as a nutrient (ex: deficiency in magnesium, a component of chlorophyll, causes yellowing of leaves). Other deficiencies can be determined based on the leaves of the plant. Symptoms also depend on the mineral’s mobility in the plant. Highly mobile nutrients are seen first in older tissues. Young, growing tissues preferentially accept nutrients over older tissues(ex: Mg). Immobile nutrients appear in young tissues because older tissues may have adequate amounts that they retain. BIOL 240W 1st EditionYoung seedlings do not show mineral deficiency symptoms because minerals released from stored reserves in the seed itself meet their mineral requirements. When a mineral is deficient, genes may be turned on or off to compensate (molecular response). In lab, we will be investigating the changes in microRNA gene expression in response to nutritional deficiencies. Nutrients are scavenged from the soil in the form of ions. Its composition and the role of living organismsin the top layer of soil are important. Loams are the topsoil supporting most abundant growth. They are composed of equal amounts of sand (course particles), silt (medium-sized particles), and clay (small particles). They have ample surface area for adhesion and retention of minerals and water while enabling diffusion of oxygen to roots. Sandy soils do not retain enough water to support plant growth, while clay soils retain too much water.Lecture 3Cationic nutrients (Ca2+, K+, Mg+) are attracted to negative soil particles. Cells respire and release CO2, which combine swith water to make carbonic acid. This releases a proton (H+)H2O + CO2  H2CO3  HCO3- + H+Released proton acidifies soil and hydrogen ions are exchanged with minerals. Minerals are then taken up by roots. 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. Most plants thrive in slightly lower pH (acidic soil) because more H+ ions are available, leading to a greater displacement of minerals.A disadvantage is that some minerals may be more readily available while another becomes less available. 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.Phosphate (PO4-) is the most limiting rock-derived nutrient. It forms highly insoluble aggregates with many substances. The roots cannot take in these insoluble aggregates. Only a limited fraction of phosphate is soluble, and this can be easily lost during rainfall. Aluminum toxicity at pH <5 occurs because 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.Most nutrients are derived from fragmentation of rocks except for nitrogen. The majority of the atmosphere is made up of nitrogen, but N2 is very stable in the atmosphere, causing it to be the most limiting macronutrient. Nitrogen gas must be reduced to a form usable by plants (nitrogen fixation).N2  NH3  NH4+This is carried out by prokaryotes. It requires a lot of energy (8ATP per NH3). Proteins derived from humus (dead organic material) are decomposed to amino acids by ammonifying bacteria. Ammonifying bacteria further decompose amino acids to NH3, which is then converted to NH4+ by H+ in soilNitrogen-fixing bacteria reate NH3 differently from ammonifying bacteria because the starting material isN2 in the atmosphere. Both create the same products.Chemosynthetic autotrophs (bacteria in soil) oxidize NH4+ to nitrite (Nitrosomonas) and then nitrate (Nitrobacter). Nitrate is the main form taken up by plants, but it is an anion (NO3-) so it is easily leeched from soils. Plants must spend energy to re-reduce NO3-  NH4+. However, Only NH4+ is used to synthesize compounds containing nitrogenDuring these processes, there are problems with oxygen. Nitrogenase is the enzyme required for nitrogen fixation