Endosymbiotic Theory: Evolution of Eukaryotic Cells

The endosymbiotic theory explains the process of moving from a prokaryotic cell, which lacks most organelles, through symbiogenetic additions.

The Three Stages of Endosymbiosis

1. A bacterium that uses sulfur as an energy source merges with a swimming bacterium to form a new entity. The result was the first eukaryotic cell. The nucleoplasm of animal cells, plants, and fungi would be the result of the union of these two bacteria. A baseline characteristic of both cells was added: a new, more complex morphology. DNA remained confined to an inner core separated from the rest of the cell by a membrane.
2. This new organization was still anaerobic, unable to metabolize oxygen, as this gas was poisonous to it. Therefore, they lived in environments where oxygen, increasingly present, was scarce. At this point, a new addition would lead to the ability to metabolize oxygen. This became the mitochondria and peroxisomes present in eukaryotic cells. Animals and fungi are the result of this second embodiment.
3. This third addition caused the plant kingdom. The newly acquired phagocytic cells, respiring oxygen and photosynthetic bacteria, some of them becoming resistant, would become part of the body, causing in turn a new organism capable of synthesizing energy from the sun. These new plants, with their success, helped contribute to the success of animals and fungi.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cell Characteristics:

• Specific to monerans.
• Have no defined nucleus (no nuclear envelope).
• Their genetic material is a circular DNA, and mitosis does not occur.
• No cytoskeleton.
• Have small ribosomes.
• Cell wall surrounded by a capsule.
• Respiratory enzymes found in the plasma membrane.
• Submicroscopic flagella present.

Eukaryotic Cell Characteristics:

• Typical of protoctists, fungi, plants, and animals.
• Nucleus surrounded by a membrane separating the genetic material.
• Multiple DNA molecules (perform mitosis).
• They have a cytoskeleton and a variety of organelles.
• Large ribosomes.
• Plants and protoctists have a cell wall, but it is easier to break down.
• Respiratory enzymes found in mitochondria.
• They have microscopic cilia and flagella.