Problem 2 - Campbell Biology

Question

Knowledge/Comprehension 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 \(\mathrm{CO}_{2}\) production, photosynthesis could not occur.

Step-by-Step Solution

Answer

The correct answer is B.

Step 1: Understanding the Problem

The question asks why biologists believe that mitochondria emerged before plastids through endosymbiosis. Four reasons are provided; one of them is correct.

Step 2: Analyze Statement A

Option A states mitochondria are necessary for metabolizing the products of photosynthesis. This is incorrect as both processes can occur independently in different organisms.

Step 3: Analyze Statement B

Option B asserts all eukaryotes have mitochondria, while not all eukaryotes have plastids. This is accurate and aligns with current biological knowledge.

Step 4: Analyze Statement C

Option C claims mitochondrial DNA is less similar to prokaryotic DNA compared to plastid DNA. This does not provide an evolutionary sequence reasoning.

Step 5: Analyze Statement D

Option D states that photosynthesis depends on mitochondrial \(\backslashmathrm{CO}_{2}\) production, which is incorrect since photosynthesis can occur without mitochondria.

Step 6: Conclusion

After evaluating all statements, option B is the most plausible reason.

Key Concepts

Mitochondria

Mitochondria are known as the powerhouses of the cell. They are small structures within cells that generate most of the cell's supply of ATP, which is used as a chemical energy source. Mitochondria are thought to have originated from a process called endosymbiosis, where a prokaryotic cell was engulfed by a larger cell and eventually evolved into a symbiotic relationship. This theory is supported by the fact that mitochondria have their own DNA, which is circular like bacterial DNA, and ribosomes that are more similar to those found in bacteria than those in eukaryotic cells.

Plastids

Plastids are another group of organelles in eukaryotic cells, mostly known for their role in photosynthesis. The most well-known plastids are chloroplasts, which capture light energy to produce glucose in a process called photosynthesis. Like mitochondria, chloroplasts and other plastids are believed to have arisen from endosymbiosis of a photosynthetic bacteria. This is because they also have their own DNA and ribosomes. Plastids can be found in plants and some algae, but not all eukaryotes have them.

Eukaryotes

Eukaryotes are organisms whose cells contain a nucleus and other organelles enclosed within membranes. They can be unicellular or multicellular, and they include animals, plants, fungi, and protists. The presence of mitochondria in all eukaryotes suggests that these organelles appeared early in the evolution of eukaryotic cells. By contrast, plastids are found in many, but not all, eukaryotes, suggesting that they evolved later. This difference is one reason why biologists believe that mitochondria came before plastids in the history of life.