Protists and bacteria are grouped into different domains because:

• A. protists have a membrane-bound nucleus, while bacteria do not
• B. bacteria are multicellular, while protists are always unicellular
• C. bacteria perform photosynthesis, but protists cannot
• D. protists can only survive in extreme environments, unlike bacteria

Answer: (A)

Verified step by step guidance

1. Understand the fundamental difference between the domains Bacteria and Eukarya (which includes protists). The key distinction lies in cellular organization, particularly the presence or absence of a membrane-bound nucleus.
2. Recall that bacteria are prokaryotes, meaning they lack a membrane-bound nucleus and other membrane-bound organelles, whereas protists are eukaryotes and have a true nucleus enclosed by a membrane.
3. Evaluate the options given: consider whether bacteria are multicellular or unicellular (bacteria are unicellular), whether bacteria or protists perform photosynthesis (some bacteria and some protists can perform photosynthesis), and whether protists only survive in extreme environments (protists inhabit diverse environments).
4. Focus on the defining characteristic that separates these groups at the domain level: the presence of a membrane-bound nucleus in protists versus its absence in bacteria.
5. Conclude that the correct reason for grouping protists and bacteria into different domains is that protists have a membrane-bound nucleus, while bacteria do not.