Exam 2: Topic 10

Question 1

Contrast the carbon cycle and nitrogen cycle in terms of importance of microbes
in cycling the material.

Answer 1

1. Carbon cycle: microbes act as primary decomposers of organic matter, which means they are essential for recycling carbon back into the atmosphere as CO2. Microbes break down dead plants and animals, releasing CO2 back into the atmosphere through respiration.
2. Nitrogen cycle: microbes drive the majority of nitrogen transformations through processes like nitrogen fixation. Certain bacteria, like Rhizobium, can directly convert atmospheric nitrogen (N2) into ammonia (NH3), a form usable by plants, which is a key step in the nitrogen cycle and largely dependent on microbes.

Question 2

What does “fixing” nitrogen mean?

Answer 2

The process by which nitrogen is taken from its molecular form (N2) in the atmosphere and converted into nitrogen compounds useful for other biochemical processes.
• N2 + 3H2 —(nitrogenase)—-> 2NH3
• Nitrogenase: inhibited by O2

Question 3

Define the terms: Nitrogen-fixing bacteria, nitrogenase, and heterocyst.

Answer 3

• Nitrogen fixing bacteria: Nitrogen-fixing bacteria are microorganisms that can convert atmospheric nitrogen gas into a usable form, like ammonia
• Nitrogenase: an enzyme found in certain bacteria and archaea that catalyzes the process of biological nitrogen fixation
• Heterocyst: a specialized cell found in certain filamentous cyanobacteria that functions as the site for nitrogen fixation.

Question 4

Describe two obstacles that makes fixing atmospheric nitrogen difficult.

Answer 4

1. The nitrogenase enzyme, responsible for nitrogen fixation, is quickly deactivated by oxygen, forcing nitrogen-fixing bacteria to develop specialized structures to maintain low oxygen levels around the enzyme.
2. Fixing nitrogen into ammonia requires a significant amount of ATP, limiting the growth rate of nitrogen-fixing organisms.

Question 5

Explain how cyanobacteria are capable of fixing nitrogen, despite being aerobic?

Answer 5

By creating specialized cells called heterocysts which provide an anaerobic environment necessary for the oxygen-sensitive nitrogenase enzyme to function, effectively separating the process of nitrogen fixation from oxygen-producing photosynthesis within the same organism; this allows them to fix atmospheric nitrogen even in oxygen-rich environments.