Question

(three systems), indicate whether it most likely arose from an ingested bacterium or from the nucleated host. Explain your reasoning. (b) Describe one structure or metabolic process that purple bacteria might have dispensed with once they became endosymbionts in a eukaryotic cell. How would loss of this feature prevent the bacteria from living outside the host? (c) Describe one structure or metabolic process that cyanobacteria might have dispensed with once they became endosymbionts in a eukaryotic cell. How would loss of this feature prevent the bacteria from living outside the host? (d) Peroxisomes, unlike mitochondria or chloroplasts, scarcely resemble free- living organisms. Assuming peroxisomes evolved from ingested bacteria, describe three features mitochondria retain but peroxisomes apparently lost over hundreds of millions of years. Describe one advantage peroxisomes might have conferred on ancient nucleated cells.

Short answer

The three systems most likely originated from ingested bacteria. Purple and cyanobacteria would have lost some structures or processes required for independent living once they became endosymbionts. Peroxisomes, though different from mitochondria, may have aided ancient nucleated cells by breaking down long-chain fatty acids.

Step-by-step solution

Origin of the Three Systems

A eukaryotic cell is more complex in structure than a bacterium and contains structures such as the mitochondria and chloroplasts which arose from bacterial endosymbionts. From the evolutionary history, mitochondria were derived from an ingested purple bacterium while chloroplasts originated from an ingested cyanobacterium.

Modifications in Purple Bacteria

Once purple bacteria become endosymbionts in a eukaryotic cell, they were able to lose certain structures and processes that were required for independent living but not within the host, such as flagella for movement. Without such features, these bacteria would be unable to survive outside in diverse environmental conditions.

Changes in Cyanobacteria

Similar to purple bacteria, cyanobacteria that became endosymbionts might have dispensed with some structures or processes such as the thick cell wall that was vital for survival in external habitats but not inside a host. This feature, if lost, would prevent these bacteria from living outside the host because they would be more vulnerable to environmental changes.

Peroxisomes vs. Mitochondria

Unlike mitochondria, peroxisomes do not resemble free-living organisms, indicating a different evolutionary history. Over hundreds of millions of years, peroxisomes appear to have lost features mitochondria retains such as the double membrane structure, their own DNA, and their own ribosomes. However, peroxisomes may have conferred an advantage on ancient nucleated cells by aiding in the breakdown of long-chain fatty acids.

Key concepts

Mitochondria

Mitochondria are often referred to as the powerhouses of the cell because they generate the energy that cells need to function. They have a fascinating origin story tied to the Endosymbiotic Theory. This theory suggests that mitochondria were once free-living purple bacteria. These bacteria were ingested by a host cell and evolved over time to become a permanent resident of the eukaryotic cell.
Mitochondria have retained several features from their bacterial ancestors:

• They have their own DNA, separate from the DNA found in the cell's nucleus.
• Mitochondria possess a double-membrane structure, which is believed to result from the engulfing process.
• They can reproduce independently within the cell.

These features distinctly highlight their prokaryotic origins, making them unique compared to other cell organelles. The process of energy production in mitochondria is also linked to their bacterial past.

Chloroplasts

Chloroplasts are known for their role in photosynthesis, where they convert sunlight into nutritional energy for the plant cells in which they reside. Like mitochondria, chloroplasts have roots in the Endosymbiotic Theory. They are believed to have evolved from cyanobacteria that were engulfed by an ancestral eukaryotic cell.
Chloroplasts have a unique set of characteristics:

• They contain their own genetic material, similar to mitochondria and ancient bacteria.
• Chloroplasts have a double membrane, supporting the idea of their evolutionary origins as a once free-living organism.
• Within chloroplasts, thylakoid membranes contain chlorophyll, crucial for capturing sunlight.

The double membrane and the presence of their own DNA portray their similarity to cyanobacteria and reinforce their evolutionary pathway from free-living organisms to essential cellular components.

Eukaryotic Cells

Eukaryotic cells are complex structures that include a defined nucleus and various membrane-bound organelles, marking them distinct from prokaryotic cells. The Endosymbiotic Theory provides an explanation for the origin of certain organelles within these cells. It infers that organelles such as mitochondria and chloroplasts were once independent prokaryotic organisms that formed a symbiotic relationship with early eukaryotic ancestors.
Key features of eukaryotic cells include:

• Nucleus: Contains the cell's genetic blueprint.
• Membrane-bound organelles: Includes mitochondria, chloroplasts, peroxisomes, and endoplasmic reticulum.
• Cytoskeleton: Provides structural support and facilitates cell movement.

This complex architecture allows eukaryotic cells to perform more sophisticated functions, contributing to the diverse forms of life seen in plants, animals, fungi, and protists. The endosymbiotic relationships that led to such developments highlight the remarkable nature of cellular evolution.

Peroxisomes

Peroxisomes are specialized organelles within eukaryotic cells, distinct from mitochondria and chloroplasts due to their simpler structure and lack of independent DNA. Unlike their counterparts, peroxisomes do not closely resemble any free-living bacteria, making their evolutionary journey slightly more enigmatic.
However, the presumed evolutionary advantage of peroxisomes lies in their metabolic functions:

• They play a key role in breaking down fatty acids through beta-oxidation.
• Peroxisomes help detoxify harmful substances like hydrogen peroxide, maintaining cellular health.
• They contribute to lipid metabolism and biosynthesis of specific lipids like plasmalogens, crucial for normal cellular functions.

Peroxisomes' benefits likely supported the survival and efficient functioning of ancient nucleated cells, possibly leading to their retention throughout evolution, despite the loss of features that indicate a symbiotic origin.