Chapter 29 Plant Nutrition Answers to Study Questions 1 Some lawnmowers collect clippings for easy disposal and to prevent clumps from inhibiting photosynthesis What is a possible drawback of this practice with respect to plant nutrition The grass clippings contain many nutrients both organic and inorganic that could benefit the growing grass plants if they were left to decompose and compost ultimately this is preferable to artificial fertilizers 2 How would adding clay to loamy soil affect the soil s capacity to exchange cations and retain water Explain Clay consists of the smallest sized soil particles and therefore provide a huge total surface area for ion exchange with positively charged nutrient ions in the soil water Clay also retains water to a far greater degree than larger soil particles also owing to the particle small size and large surface area to volume ratio Soil particles are negatively charged and therefore attract positively charged ions or cations 3 Why is organic fertilizer better for the soil and environment than synthetic fertilizer Organic fertilizer provides nitrogen N phosphorus P and potassium K in a form that is complexed with large carbon containing molecules such as that found in humus and animal waste This causes nutrient release to be slow and less likely to leach away with each rainfall or watering Industrially produced synthetic fertilizers are highly enriched and immediately available to plants but also less likely to be retained by the soil and are thus easily leached out 4 How is the statement Plants are nourished mostly by air supported by the list of micronutrients and macronutrients Table 37 1 Ninety percent of the plant dry weight is derived from CO2 in the air which is fixed to organic carbon compounds that form the bulk of the plant body 5 Are some essential elements more important than others Explain No all essential elements are equally important because a plant cannot complete its life cycle without all essential elements regardless of quantity this need defines the term essential in this regard 6 Identify the macronutrients and name a function for each Carbon organic compounds Oxygen organic compounds aerobic respiration Hydrogen organic compounds Nitrogen nucleic acids proteins ATP hormones chlorophyll coenzymes Potassium intracellular solute for cellular solute potential stomatal opening enzyme cofactor Calcium cytoskeletal regulator second messenger enzyme activation maintains integrity of cell wall cell membrane Magnesium component of chlorophyll enzyme activation Phosphorus proteins nucleic acids ATP coenzymes Sulfur proteins coenzymes 7 How do soil bacteria and mycorrhizae contribute to plant nutrition Soil bacteria like Rhizobium Klebsiella and Frankia are nitrogen fixers they reduce N2 to NH3 making atmospheric nitrogen available to the plant Mycorrhizae are symbiotic fungal root associations that facilitate nutrient exchange between the two organism types the fungi obtain fixed carbon from the plant and the plant benefits from the huge absorptive surface area for water and minerals provided by the fungi as well as enhanced phosphate uptake 8 What is the importance of leghemoglobin Leghemoglobin is an oxygen binding protein produced by the roots of leguminous plants that limits the oxygen concentration in the region of the nodules Nitrogen fixing bacteria reside in the nodules and produce nitrogenase an enzyme that is inactivated in the presence of high levels of oxygen 9 The structure and function of leghemoglobin is very similar to that of myoglobin an oxygen storing protein found in mammalian muscle tissue What is the best explanation for the existence of two very similar oxygen binding proteins in two very different organisms This is an example of convergent evolution which gave rise to an optimum oxygen binding protein in two very different organisms Leghemoglobin and myoglobin are therefore analogous proteins or homoplasies 10 Identify the chemical processes that give rise to the different forms of nitrogen Nitrogen fixation reduces N2 with H2 to give rise to NH3 nitrification oxidizes NH3 to NO2 converts nitrate to N2 ammonification converts organic nitrogen containing molecules to NH3 nitrate denitrification nitrite and NO3 11 Distinguish between an epiphyte and a parasitic plant An epiphyte is a plant that lives on top of another plant at most to gain an advantage in height that it otherwise would not have A parasitic plant lives on another plant not only for the advantage of height but also for nutrients it can gain from the host plant An epiphytic plant is usually not harmful to its plant companion but a parasitic plant can often kill its host 12 Usually the first thing we think about plants is that they are photosynthetic Consider the non photosynthetic parasites dodder and Indian pipe why are they still classified as plants These organisms are still considered plants because they possess many homologies that link them to flowering plants the loss of chlorophyll can be thought of as an evolutionary reversal 13 How does a carnivorous or insectivorous plant attract its prey Carnivorous plants attract their prey by secreting sweet nectar as a lure to their various shaped traps