Bacteria is important because this bacteria binds hydrogen molecules with the gaseous nitrogen to form ammonia in the soil. During assimilation, or when plants take up nitrates from the soil, bacteria aid in the process with the plants in making ammonia. Animal wastes is also a major place where bacteria thrives and produces ammonia. The process in which assimilation occurs in plants, and then bacteria converts the nitrates to ammonia is called ammonification. From the conversion of ammonia to nitrites, bacteria also aids in this process called nitrification. The nitrifying bacteria mostly present in soils, oxidize ammonia into nitrites, and from nitrites to nitrates.
Finally, the process of denitrification also has bacteria present to aid in converting nitrates back into a gaseous form of nitrogen in the atmosphere.
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The breathing process occurs as follows: Inhalation increases the volume of the lungs, which reduces internal pressure and causes air to enter. Then the exhalation makes the opposite process. Gaseous exchanges occur in the lung alveoli and are responsible for eliminating the gases we do not need and preparing the oxygen to be spread throughout the body.
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The reasons are:
- the blood is very low on nutrition
- the leech is not feeding itself regularly
The leech slows it metabolism a lot, so it can survive on very low nutrition. The blood is very low on nutrition, and it is the leech's food source, so the evolution made it preserve energy.
The leach doesn't have the opportunity to feed itself on regular basis, so it takes maximum advantage of any opportunity and consumes as much blood as possible so that it can survive long term when it will not have food at disposal.
Answer : A : They live in extreme environmental conditions.