Table of contents

1. Introduction to Biology2h 42m
2. Chemistry3h 37m
3. Water1h 26m
4. Biomolecules2h 23m
5. Cell Components2h 26m
6. The Membrane2h 31m
7. Energy and Metabolism2h 0m
8. Respiration2h 40m
9. Photosynthesis2h 49m
10. Cell Signaling59m
11. Cell Division2h 47m
12. Meiosis2h 0m
13. Mendelian Genetics4h 44m
14. DNA Synthesis2h 27m
15. Gene Expression3h 6m
16. Regulation of Expression3h 31m
17. Viruses37m
- Viruses
  37m
18. Biotechnology2h 58m
19. Genomics17m
- Genomes
  17m
20. Development1h 5m
21. Evolution3h 1m
22. Evolution of Populations3h 53m
23. Speciation1h 37m
24. History of Life on Earth2h 6m
25. Phylogeny2h 31m
26. Prokaryotes4h 59m
27. Protists1h 12m
28. Plants1h 22m
29. Fungi36m
- Fungi
  19m
- Fungi Reproduction
  17m
30. Overview of Animals34m
- Overview of Animals
  34m
31. Invertebrates1h 2m
32. Vertebrates50m
33. Plant Anatomy1h 3m
34. Vascular Plant Transport1h 2m
- Water Potential
  1h 2m
35. Soil37m
- Soil and Nutrients
  20m
- Nitrogen Fixation
  16m
36. Plant Reproduction47m
- Flowers
  33m
- Seeds
  14m
37. Plant Sensation and Response1h 9m
38. Animal Form and Function1h 19m
39. Digestive System1h 10m
- Digestion
  1h 3m
- Blood Sugar Homeostasis
  7m
40. Circulatory System1h 49m
41. Immune System1h 12m
42. Osmoregulation and Excretion50m
- Osmoregulation and Excretion
  50m
43. Endocrine System1h 4m
- Endocrine System
  1h 4m
44. Animal Reproduction1h 2m
- Animal Reproduction
  1h 2m
45. Nervous System1h 55m
- Neurons and Action Potentials
  1h 8m
- Central and Peripheral Nervous System
  46m
46. Sensory Systems46m
- Sensory System
  46m
47. Muscle Systems23m
- Musculoskeletal System
  23m
48. Ecology3h 11m
49. Animal Behavior28m
- Animal Behavior
  28m
50. Population Ecology3h 41m
51. Community Ecology2h 46m
52. Ecosystems2h 36m
53. Conservation Biology24m
- Conservation Biology
  24m

35. Soil

Nitrogen Fixation

General Biology

35. Soil

Nitrogen Fixation

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Multiple Choice

Why must atmospheric nitrogen ($N_2$) be 'fixed' before most organisms can use it, and which organisms are primarily responsible for this process?

Because $N_2$ is highly reactive and must be stabilized by fungi before use.

Because $N_2$ is inert and cannot be directly used by most living organisms; nitrogen-fixing bacteria convert it into usable forms.

Because $N_2$ is toxic to most organisms; only plants can convert it into ammonia.

Because $N_2$ is only present in water; algae are responsible for nitrogen fixation.

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Verified step by step guidance

Understand the nature of atmospheric nitrogen ($N_2$): Atmospheric nitrogen is a diatomic molecule with a triple bond between the two nitrogen atoms. This bond is extremely strong, making $N_2$ chemically inert and not readily usable by most organisms.

Recognize why nitrogen fixation is necessary: Most organisms cannot break the triple bond in $N_2$ to access nitrogen for building essential biomolecules like amino acids and nucleotides. Nitrogen must be converted into more reactive and usable forms, such as ammonia ($NH_3$) or nitrate ($NO_3^-$).

Identify the organisms responsible for nitrogen fixation: Nitrogen-fixing bacteria, such as those in the genera *Rhizobium* and *Azotobacter*, are primarily responsible for converting $N_2$ into usable forms. Some cyanobacteria also perform nitrogen fixation.

Understand the process of nitrogen fixation: These bacteria use the enzyme nitrogenase to catalyze the conversion of $N_2$ into ammonia ($NH_3$). The reaction requires energy, typically supplied by ATP, and occurs under anaerobic conditions to protect the nitrogenase enzyme from oxygen damage.

Recognize the ecological importance: Nitrogen fixation is a critical process in the nitrogen cycle, enabling the incorporation of atmospheric nitrogen into ecosystems. Plants often form symbiotic relationships with nitrogen-fixing bacteria to access fixed nitrogen directly.