The main nitrogen reservoir is the atmosphere, which is about 80 percent nitrogen gas. Despite the universal need for nitrogen and the abundance of atmospheric nitrogen, no eukaryote can make use of nitrogen gas. Why? How do eukaryotes obtain nitrogen?
What will be an ideal response?
Nitrogen gas consists of two atoms of nitrogen held together by a triple covalent bond. A triple bond holds atoms together very strongly. Eukaryotes do not have an enzyme that can break the strong bond between the two nitrogen atoms.
Certain types of bacteria and archaea can break the triple bond. They carry out nitrogen fixation, combining nitrogen atoms in nitrogen gas with hydrogen to produce ammonia (NH3). Ammonia dissolves to form ammonium (NH4+). Plants can take up ammonium from soil water and use it in metabolic reactions. Animals meet their nitrogen needs by eating plants or one another.
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Modify the following statement to make it correct: Ribozymes are ribonucleosomes used by bacteria and eukaryotic cells to modify DNA.
A. Ribozymes are ribonucleases used by bacteria and eukaryotic cells to modify DNA. B. Ribozymes are ribonucleosomes used by bacteria and eukaryotic cells to modify RNA. C. Ribozymes are ribonucleases used by bacteria and eukaryotic cells to modify RNA. D. Ribozymes are ribonucleosomes used only by eukaryotic cells to modify RNA.
Stromatolites are ________
a. mineral features in caves that extend downward from the ceiling b. the earliest type of seed plant c. mat-like remains of bacteria found in extinct hydrothermal vents d. dome-shaped, layered structures formed by photosynthetic bacteria
A loud sound can be distinguished from a soft whisper due to
a. more neurons depolarize. b. each receptor depolarizes more fully. c. the frequency of depolarizing neurons increases. d. variations in the signals each sound produces. e. more neurons depolarize, each receptor depolarizes more fully, and the frequency of depolarizing neurons increases.
Most humans have a preference for sweet, fatty, and ________ tastes.
Fill in the blank(s) with the appropriate word(s).