Biologists think that endosymbiosis gave rise to mitochondria before plastids partly because
a. The products of photosynthesis could not be metabolized without mitochondrial enzymes.
b. All eukaryotes have mitochondria (or their remnants), whereas many eukaryotes do not have plastids.
c. Mitochondrial DNA is less similar to prokaryotic DNA than is plastid DNA.
d. Without mitochondrial CO₂ production, photosynthesis could not occur.

Verified step by step guidance

1. Understand the concept of endosymbiosis: Endosymbiosis is a theory that explains how eukaryotic cells may have evolved from prokaryotic cells. It suggests that certain organelles, like mitochondria and plastids, originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.
2. Consider the role of mitochondria and plastids: Mitochondria are involved in cellular respiration, converting oxygen and nutrients into ATP, the energy currency of the cell. Plastids, such as chloroplasts, are involved in photosynthesis, converting light energy into chemical energy stored in glucose.
3. Evaluate the presence of mitochondria and plastids in eukaryotes: All eukaryotic cells have mitochondria or their remnants, which are essential for energy production. However, not all eukaryotic cells have plastids, as plastids are primarily found in plants and some protists.
4. Analyze the evolutionary timeline: The presence of mitochondria in all eukaryotes suggests that they were acquired earlier in evolutionary history than plastids. This is because mitochondria are essential for basic cellular functions, whereas plastids are specialized for photosynthesis.
5. Consider the implications of mitochondrial enzymes: The products of photosynthesis, such as glucose, require mitochondrial enzymes for further metabolism through cellular respiration. This interdependence supports the idea that mitochondria were established before plastids in eukaryotic cells.

Key Concepts

Endosymbiosis Theory

Endosymbiosis is a theory that explains the origin of eukaryotic cells from prokaryotic organisms. It suggests that mitochondria and plastids, such as chloroplasts, originated as free-living bacteria that were engulfed by a host cell. This symbiotic relationship eventually led to the bacteria becoming integral organelles within the eukaryotic cell, providing essential functions like energy production and photosynthesis.

Mitochondria and Plastids

Mitochondria are organelles responsible for producing energy through cellular respiration, while plastids, including chloroplasts, are involved in photosynthesis. The presence of mitochondria in all eukaryotic cells, as opposed to plastids, which are only found in plants and some protists, supports the idea that mitochondria were acquired earlier in evolutionary history. This distinction is crucial for understanding the sequence of endosymbiotic events.

Eukaryotic Diversity

Eukaryotic diversity refers to the wide range of organisms that possess eukaryotic cells, including animals, plants, fungi, and protists. The fact that all eukaryotes have mitochondria, while only some have plastids, suggests that the acquisition of mitochondria was a more fundamental and earlier event in eukaryotic evolution. This helps explain why biologists believe mitochondria appeared before plastids in the evolutionary timeline.