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 symbiotic relationship where one organism lives inside the other. Scientists believe this relationship led to the development of mitochondria and plastids (like chloroplasts) in eukaryotic cells.
2. Identify the key differences between mitochondria and plastids: Mitochondria are involved in energy production through cellular respiration, while plastids, such as chloroplasts, are involved in photosynthesis.
3. Analyze the options given: Option (b) states that all eukaryotes have mitochondria (or remnants of them), but not all eukaryotes have plastids. This suggests a fundamental role for mitochondria in eukaryotic cells.
4. Consider the evolutionary implications: If all eukaryotes have mitochondria, it implies that mitochondria were acquired early in the evolution of eukaryotes, possibly before plastids. This supports the idea that endosymbiosis leading to mitochondria occurred before the endosymbiosis leading to plastids.
5. Conclude with the most likely answer: Based on the analysis, option (b) provides a strong evolutionary reason why biologists think mitochondria arose before plastids through endosymbiosis.

Key Concepts

Endosymbiotic Theory

The endosymbiotic theory posits that certain organelles, specifically mitochondria and plastids, originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. This theory explains how these organelles have their own DNA and replicate independently of the cell, supporting the idea that they were once separate organisms that formed a symbiotic relationship with host cells.

Mitochondria vs. Plastids

Mitochondria are organelles responsible for energy production through aerobic respiration, while plastids, such as chloroplasts, are involved in photosynthesis. All eukaryotic cells possess mitochondria or their remnants, indicating their essential role in cellular metabolism, whereas plastids are found primarily in plants and some protists, highlighting a divergence in evolutionary necessity.

Mitochondrial DNA

Mitochondrial DNA (mtDNA) is distinct from nuclear DNA and is inherited maternally. It is more similar to prokaryotic DNA than to nuclear DNA, supporting the endosymbiotic theory. The differences in DNA structure and sequence between mitochondrial and plastid DNA provide insights into their evolutionary origins and the relationships between eukaryotes and prokaryotes.