Do Eukaryotic Cells Have Mitochondria?

Eukaryotic cells, which include cells from plants, animals, fungi, and protists, typically contain mitochondria. These organelles are responsible for generating most of the energy in the form of ATP (adenosine triphosphate) needed for the cell’s activities. Mitochondria have their own DNA, distinct from the nuclear DNA, and are typically depicted as sausage-shaped structures with a double membrane. They are crucial for various cellular processes, including cell division, signaling, and apoptosis (programmed cell death).

The Exceptions: Eukaryotic Cells Without Mitochondria

However, there are exceptions to the rule. Some eukaryotic cells, such as the protist Monocercomonoides, do not have mitochondria or any evidence that they ever did. This discovery challenges the evolutionary assumption that all eukaryotic cells have mitochondria or their degenerate forms.

Monocercomonoides: A Mitochondria-Free Eukaryote

Monocercomonoides is a protist that belongs to the supergroup Metamonada, which includes several other mitochondria-free eukaryotes. This organism has been found to rely on alternative energy-generating systems, using proteins with biochemical structures similar to those used by bacteria, archaea, and plastids during the first few steps of iron-sulfur cluster synthesis.

The Evolutionary Implications

The discovery of Monocercomonoides and other mitochondria-free eukaryotes challenges the evolutionary assumption that all eukaryotic cells have mitochondria or their degenerate forms. This finding suggests that the presence of mitochondria may not have been an absolute requirement for the transition to larger cell sizes and the evolution of complex eukaryotic cells.

The Scaling of ATP Synthase Molecules

In terms of quantifiable data, the number of ATP synthase molecules available to supply ATP energy for cell growth and maintenance has been found to scale with a cell’s volume. Researchers have examined how the respiratory requirements of a cell, measured by the number of ATP synthase molecules, scale with a cell’s volume.

The Volume Limit

The researchers found that, at some point of increasing cell size, there will be a volume limit where the ATP synthases cannot supply enough ATP for the cell to divide at a certain rate. This suggests that eukaryotes have overcome this barrier, possibly with the help of mitochondria, although mitochondria may not have been absolutely necessary for this transition to larger volumes.

The Importance of Mitochondria

While most eukaryotic cells contain mitochondria, the discovery of mitochondria-free eukaryotic cells highlights the complexity and diversity of cellular structures and energy-generating systems. Mitochondria play a crucial role in cellular processes, but their presence may not have been an absolute requirement for the evolution of complex eukaryotic cells.

Conclusion

In summary, while eukaryotic cells typically contain mitochondria, there are exceptions to this rule. The discovery of mitochondria-free eukaryotic cells, such as Monocercomonoides, challenges evolutionary assumptions and provides insights into the diversity of cellular structures and energy-generating systems. The scaling of ATP synthase molecules with cell volume also suggests that mitochondria may not have been an absolute necessity for the transition to larger cell sizes in eukaryotes.

References:
– Eukaryote Without Mitochondria Not a Product of Evolution
– Eukaryotic Cells Can Thrive Without Mitochondria
– The Scaling of Respiratory Complexity in Eukaryotic Cells