Which of the following statements about the cell is correct?
A. Prokaryotic cells contain mitochondria.
B. Viruses are cells.
C. All cells have a nucleus.
D. The cell is the basic unit of life.
Answer: Option D
Solution (By JKExamLibrary)
The cell theory states that the cell is the fundamental unit of structure and function. Not all cells have a nucleus (prokaryotes), prokaryotes lack mitochondria, viruses are not cells.
Explanation:
Biological nitrogen fixation, carried out by symbiotic (Rhizobium) and free-living (Azotobacter) bacteria, reduces atmospheric N₂ to ammonia. Nitrification converts ammonia to nitrates, denitrification returns N₂ to air, ammonification releases ammonia from organic matter.
Explanation:
Charles Darwin's theory of evolution, presented in 'On the Origin of Species' (1859), is based on two main concepts: branching descent (all organisms are related through common ancestry) and natural selection (survival of the fittest). Natural selection states that individuals with traits better suited to their environment are more likely to survive and reproduce, passing those traits to the next generation.
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