Biologists think that endosymbiosis gave rise to mitochondria before plastids partly because:

• A. mitochondria are only present in plant cells
• B. plastids are found in both prokaryotes and eukaryotes
• C. plastids evolved before the origin of eukaryotic cells
• D. all eukaryotes have mitochondria, but not all have plastids

Answer: (D)

Verified step by step guidance

1. Understand the concept of endosymbiosis: Endosymbiosis is a theory that explains how certain organelles, such as mitochondria and plastids, originated from free-living prokaryotic cells that were engulfed by ancestral eukaryotic cells. These prokaryotes formed a symbiotic relationship with the host cell, eventually becoming integral parts of the cell.
2. Recognize the distribution of mitochondria and plastids: Mitochondria are found in all eukaryotic cells, regardless of whether they are plant, animal, or fungal cells. Plastids, on the other hand, are primarily found in plant cells and some protists, but not in all eukaryotic cells.
3. Analyze the evolutionary timeline: Mitochondria are thought to have evolved earlier in the history of eukaryotic cells because they are universally present in all eukaryotes. Plastids, which are involved in photosynthesis, likely evolved later, as they are not found in all eukaryotic cells.
4. Consider the implications of universal presence: The fact that all eukaryotic cells have mitochondria suggests that the acquisition of mitochondria was a critical step in the evolution of eukaryotic cells. This universal presence supports the idea that mitochondria were established before plastids.
5. Conclude based on evidence: The reasoning behind the statement 'all eukaryotes have mitochondria, but not all have plastids' aligns with the idea that mitochondria were acquired first during endosymbiosis, followed by plastids in specific lineages such as plants and algae.