BIOLOGY - CLUTCH CH.37 - SOIL.

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2 Plants produce sugars from photosynthesis, but still have many nutritional requirements C, H, and O account for ~95% of a plant s dry weight, and are obtained from CO2 and H2O Vascular plants require 17 essential nutrients Macronutrients nutrients required in large quantities, include N, P, K, Ca, S, and Mg Limiting nutrients availability of these nutrients limits plant growth, usually N, P, and/or K Micronutrients nutrients needed in small quantities, include B, Cl, Mn, Fe, Zn, Cu, Na, Mo, and Ni Present only in trace amounts in plants, yet essential to life Nutrients can potentially be toxic to plants in high concentration Some nutrients are mobile, and can be transported around the plant, others are immobile Old leaves will often transport nutrients to sustain the young leaves, dying off in the process Young leaves are the first to show nutrient deficiencies Page 2

3 Soil is composed of inorganic minerals, organic matter, trapped gases, liquids like water, and many living organisms Weathered rock breaks up into gravel, sand, silt, and clay, composing the base of soil Humus decaying organic matter, organisms add dead cells and feces to the soil, enriching it Texture proportions of soil components like gravel, sand, silt, and clay - Affects roots ability to penetrate and absorb nutrients - Affects ability of soil to hold water and oxygen Loam contains roughly equal portion of sand, silt, and clay, with lots of humus, very good soil Topsoil outermost layer of soil with the highest concentration of humus and microorganisms Composed of many living organisms like bacteria, archaea, fungi, algae, nematodes, protists, insects, and worms - Worms help move soils around, cycle nutrients, and break it up for better water and gas retention Soil horizons soil layers like topsoil, subsoil, and bedrock Soil ph varies greatly and affects nutrient absorption Acidic soil lots of decaying organic matter producing organic acids Alkaline soil limestone, CaCO3, leads to bicarbonate (HCO3 - ) formation Soil erosion wind and water carrying soil away from a site Roots helps prevent soil erosion, and the excrete acids, lowering ph Page 3

4 Plants extract soil nutrients as ions Most nutrient uptake occurs in the zone of maturation, behind the root tip, where root hairs are found Root hairs significantly increase the surface area available for water and nutrient absorption Anions negatively charged, dissolved in water in soil and readily available to plants for absorption - Ions dissolved in water are also easily leached form soil - Leaching loss of nutrients through the movement of water - Phosphate is an anion, but is not dissolved in water in soil, forms complexes with Fe and Ca cations Cations dissolve in water, but usually interact with clay anions or organic acids, harder for plants to extract Cation exchange soluble cations, like protons, bind to negatively charge soil particles, causing cations like Mg +2 and Ca +2 to be released from soil, allowing plants to absorb them Humus has higher cation exchange capacity that does clay Plants influence cation exchange by releasing CO2, a byproduct of cellular respiration CO2 forms carbonic acid in water, releasing protons to cause cation exchange If soil is too acidic, rain can wash cations away Page 4

5 Nutrients can easily pass through the cell wall, but the plasma membrane acts as a filter (selective permeability) Plants use proton pumps to create electrochemical gradients outside of the cell to allow ions to enter through transporters Electrochemical gradient strong enough to overpower counteracting forces, like ph gradient Cations like K + move through channels Anions like NO3 - use cotransporters that often use the proton gradient, and bring H + into the cell as well Ion exclusion plants filter harmful ions and poisonous metals Passive exclusion membrane lacks the transporter necessary to allow ion to pass through - Casparian strip forces ions through endodermal cells, from apoplast to symplast Active exclusion antiporters at tonoplast (vacuole membrane) remove material from the cytoplasm - H + pumps and Na/H + antiporters help prevent sodium from poisoning plant cell Metallothioneins cysteine-rich proteins that bind metals to prevent them from poisoning the organism Page 5

6 CONCEPT: NITROGEN FIXATION Nitrogen cycle chemical cycle in which nitrogen is converted into a variety of forms, and moves through ecosystems Plants can t absorb N2 even though its nearly 80% the atmosphere Nitrogen fixation conversion of N2 gaseous nitrogen into NH3 by bacteria and archaea NH4 + and NO3 - are the main forms of nitrogen absorbed by plants Carnivorous plants make their own sugar, and use carnivory to supplement nitrogen intake Epiphytes plants that live upon other plants absorb their water and nutrients from the air, rain, and debris around them Nitrogenase large multienzyme complex that reduces N2 à NH3, and facilitates nitrogen fixation Extremely energy intensive process to convert 1 N2 à 2 NH3, requires 8 high-energy electrons and 16 ATP Page 6

7 CONCEPT: NITROGEN FIXATION Bacteria carry out nitrogen fixation inside plant roots, mostly from the legume family Legumes family of flowering plants (Fabaceae) that carry rhizobia bacteria in nodules in the roots Rhizobia Gram-negative soil bacteria that perform nitrogen fixation in the roots of legumes - Endophytes fungi or bacteria that live inside other plants Nodules swollen nodes in the roots where bacteria have infected the plant Plants release flavonoids to signal rhizobia, which in turn release Nod factors Nod factors touch root hair, cause morphological change that allows bacteria to infect cortex via infection thread Mutualistic relationship à plant provides carbohydrates and protection, bacteria provides usable nitrogen - Leghemoglobin binds oxygen in nodules, protects nitrogenase from oxygen poisoning Mycorrhizae symbiotic fungus-root, fungi and roots that live in close association Hyphae increase surface area for absorption, and help plants acquire nutrients from organic matter in the soil Ectomycorrhizae hyphae on extracellular surface of plants that do not penetrate the cell wall Arbuscular mycorrhizae endophytes that penetrate the cortical cells in the roots of plants Page 7