Question

Assuming that mitochondria evolved from bacteria that entered cells by endocytosis, what are the likely origins of the outer and inner mitochondrial membranes?

Short answer

The outer mitochondrial membrane likely originated from the host eukaryotic cell's plasma membrane, while the inner mitochondrial membrane evolved from the engulfed bacterium's own plasma membrane.

Step-by-step solution

Understanding Endosymbiotic Theory

The endosymbiotic theory suggests that mitochondria evolved from free-living bacteria that were engulfed by ancestral eukaryotic cells through a process known as endocytosis. This process is characterized by a cell wrapping its membrane around a smaller cell or particle and then engulfing it.

Origin of the Outer Mitochondrial Membrane

According to the endosymbiotic theory, the outer mitochondrial membrane is derived from the host eukaryotic cell's plasma membrane. This would have been the initial point of contact and engulfment during endocytosis.

Origin of the Inner Mitochondrial Membrane

The inner mitochondrial membrane is believed to have evolved from the engulfed bacterium's own plasma membrane. After endocytosis, this inner membrane remained intact and eventually specialized into the mitochondrial inner membrane, maintaining its bacterial characteristics.

Key concepts

Mitochondrial Evolution

Delving into the mysteries of mitochondrial evolution uncovers a fascinating story of symbiosis that shaped the cells of nearly all complex organisms on Earth. The endosymbiotic theory outlines the origin of mitochondria as once free-living bacteria that were taken up by a larger host cell through endocytosis, an integral process to cellular function.

The host, likely an ancient eukaryotic cell, provided safety and nutrients to the bacteria, while the bacteria, equipped with the ability to efficiently produce energy through respiration, offered a substantial survival advantage to the host. Over time, the two organisms became metabolically interdependent, forging a permanent symbiotic relationship.

To understand this further, imagine this symbiosis as a merger between two companies, where each brings a unique set of strengths to the table, eventually interlinking so profoundly that they function as a single unit. This ancient partnership is essential to most life forms as it led to the formation of mitochondria, the powerhouse of the cell, changing the trajectory of life's evolution.

Endocytosis in Cells

Endocytosis is a cellular process by which a cell envelopes external materials using its membrane to bring them into its interior.

It's equivalent to a city building a wall around a resource-rich land, claiming it for its own use. This not only keeps the precious resources safe but also allows the city to control and exploit them efficiently. In biological terms, the process allows cells to internalize nutrients, filter out pathogens, and regulate the composition of their plasma membrane.

But endocytosis does more than that. It's pivotal for evolutionary marvels like the origin of mitochondria. The first step towards symbiosis, and ultimately mitochondrial evolution, involved a host cell engulfing a bacterium, nestling it inside. This critical event set the stage for these two separate entities to evolve into a complex eukaryotic cell with internal power generators - the mitochondria.

Mitochondrial Membranes

Mitochondria are double-membrane-bound organelles, each of these membranes having distinct roles and origins.

The outer mitochondrial membrane, an envelope keeping the organelle intact, is believed to be derived from the host eukaryotic cell's own plasma membrane. This is akin to a person wearing a protective coat from their environment. It serves as a gatekeeper, controlling what enters and leaves the mitochondrion.

The inner membrane, on the other hand, holds its origins in the engulfed bacterium's plasma membrane. Think of it as the original identity of the bacterium being preserved, but now repurposed. It's exceptionally folded into structures called cristae, which host the cellular machinery for producing energy. Here, the bacterium's prowess in energy production is harnessed and refined, leading to a specialized power factory within the host cell.