Plants and Nitrogen

Goals and Objectives:

  1. Be able to describe or illustrate the nitrogen cycle and define ammonification, nitrification, and denitrification.
  2. Be able to explain why nitrogen fixation is purely done by prokaryotes.
  3. Be able to describe, in a general sense, the process of rhizobial infection and nodule formation in legumes.
  4. Be able to describe the biochemical reactions involved in nitrogen fixation by symbiotic bacteria.
  5. Be able to describe the process of infection and nodule development by nitrogen fixing bacteria in legumes.
  6. Be able to describe how nitrogen is assimilated and transported once it has been taken up by the plant in the form of ammonia.
  7. Be able to describe how nitrate is assimilated by plants.
  8. Be able to describe how plants cycle nitrogen.
  9. Be able to discuss the importance of nitrogen in agriculture and in plant communities in general.

Study Questions:
  1. Most plants obtain their  bulk of their nitrogen from the soil in the form of _____________ or _______________.
  2. What is the nitrogen cycle?  What three pools of nitrogen make up the nitrogen cycle?
  3. Give a definition for each of the following and explain where each occurs in the nitrogen cycle.
  4. List the non-biological sources of fixed nitrogen.
  5. What does the textbook mean when it says the "Biological nitrogen fixation is exclusively a prokaryote domain"  page 101.
  6. Give a general description of the free-living nitrogen fixers.  (You will not be required to learn the genera).
  7. Give a general description of the nodule forming symbiotic nitrogen fixers.  List the three major genera involved in this process.
  8. What other symbiotic nitrogen fixers are mentioned in the text?
  9. Describe the process by which symbiotic nitrogen fixing bacteria set up infection in legumes.
  10. Write the equation for the enzymatic reaction involving dinitrogenase.  You do not need to know the specifics about how the enzyme works.
  11. Compare the cost of nitrogen fixation by the amount of ATP used and the amount of carbon required to fix a gram of dinitrogen.
  12. What does molecular oxygen do to the process of nitrogen fixation?
  13. List the strategies for regulating oxygen that have evolved.
Skip sections on Hydrogen Production (pages 108-112) and Genetics of Nitrogen Fixation (pages 112-113).
  1. Once dinitrogen has been fixed by symbiotic bacteria, how is the ammonia handled within the plant? (See figure 6.12 on page 114)
  2. What are the functions of the following enzymes?
  3. How is nitrogen exported from the nodules?
  4. How do plants that do not form nitrogen-fixing associations take up nitrogen from the soil?
  5. How are the enzymes involved in nitrate assimilation controlled?
  6. As plants age, nitrogen is moved from one place to another.  How is nitrogen usually stored in the plant?  What occurs when a plant is under nitrogen stress (too little nitrogen)?
  7. How are fertilizers used in agricultural situations and what are the results or effects of this usage?
  8. In natural ecosystems, what controls the availability of nitrogen to the plant?