Cell Compartmentalization

Cell compartmentalization in biology

Cell compartmentalization is a major characteristic feature of eukaryotic cells, which have several differentiated compartments formed by membrane-bounded organelles and internal membranes (Fig. 1). This allows the cell to perform more efficiently, diverse metabolic reactions in these specialized compartments.

Fig. 1: Observe all the internal membranes and organelles that eukaryotic cells have, like the animal and plant cells pictured here, which represent separated cell compartments.

Metabolic reactions and processes require specific local environments to take place, with the appropriate molecules, enzymes, and specific conditions, such as a different pH.

The cell membranes not only enclose the cell and the organelles that form the internal compartments, but also actively participate in their functions. The membrane controls the passage of material into and out of the cell and the organelles, and contains built-in enzymes with specific functions. The basic structure of most biological membranes is a double layer of phospholipids, where diverse proteins are embedded (Fig. 2). The composition of lipids and proteins varies according to the cell and organelle function.

Although prokaryotes generally lack internal membranes and membrane-bounded organelles, they have internal regions where specific molecules and cell materials concentrate. There are some groups of prokaryotes with membrane delimited compartments or cell membrane invaginations, however, they mostly serve for material storage and are not as complex as in eukaryotes.

Fig. 3: Prokaryotic cells, like the bacteria pictured here, are not compartmentalized as eukaryotic cells (although cell materials and molecules can concentrate in some regions).

Cell compartmentalization examples

Basically, every component of the cell that has a distinctive function can be a compartment. Based on their main function and interrelations we can classify them into five groups, representing different cell compartmentalization examples:

Table 1: Examples of cell compartments.

A cell compartment usually contains the material and molecules required only for certain cellular processes. it can also maintain a specific proton concentration for a required pH environment.

Origins of cell compartmentalization

Compartmentalization in cells represented a major change from prokaryotic to eukaryotic cell structure, organization, and function. Still, being such an important event in cell evolution, the origins of cell compartmentalization are not well understood and this is currently a prolific field of research and discussion. While the endosymbiont bacterial origin of the mitochondrion and the chloroplast is now widely accepted, the development of cell compartments as the endomembrane system and the nucleus is more of a mystery.

Fig. 4: This diagram of the endosymbiotic theory falls under the late endosymbiosis models, which generally hypothesize that the archaeal host was already complex when it acquired a bacterial symbiont. Thus, according to these models, the development of internal membranes was somewhat previous to the endosymbiotic event.

Models of cell compartmentalization

Many hypotheses propose different models for the evolutionary processes that led to the Last Eukaryote Common Ancestor (LECA), the last single-cell organism that possessed all the characteristic features of a eukaryotic cell and from which all modern eukaryotes originated.

The debate revolves mostly around the sequence and timeline in which different compartments developed. An important issue is when the acquisition of the ancestral mitochondrial endosymbiont occurred and the level of complexity the ancestral archaeon host had at that time. This matters because depending on the order of occurrence, the origin and formation of the internal membranes and the way of acquiring and maintaining an endosymbiont might have been completely different.

We can classify most of the existing models into two major groups, based on the proposed timing of the endosymbiosis event, the early-endosymbiosis models, and the late-endosymbiosis models:

• The models that propose the early merging of an ancestral archaeon and an ancestral bacterium gave rise to the new eukaryote cell. The key point is that the archaeon host had a simple prokaryotic structure and the endosymbiosis event itself enhanced the development of internal membranes (thus these models are commonly under symbiogenesis). Some more recent models even suggest that the eukaryotic internal membranes formed from vesicles secreted by the ancestral mitochondrion itself into the host cytosol.

• The models that suggest a late endosymbiosis propose that a proto-eukaryote evolved from an ancestral archaeon, developing a more complex structure before the endosymbiosis event. There are different proposals for the nature of this proto-eukaryote, but it would have had simpler forms of some of the modern eukaryote features (internal membranes and a cytoskeleton, some propose a nucleus too). These early features would have allowed an ancestral host to phagocyte a bacterial endosymbiont. The models assume that the internal membranes were formed by invaginations of the archaeon host cell membrane, and therefore they are commonly called autogenous models.

Cell compartmentalization advantages

Cell compartmentalization probably gave many benefits to early eukaryotes as they evolved in an incredible variety of organisms (unicellular and multicellular) with an equal variety of cell types and functions. The advantages of cell compartmentalization probably allowed eukaryotes to compete for resources in a different way than prokaryotes, and to exploit new resources as well.

Table 2: Some advantages that compartments can give to eukaryotic cells compared to prokaryotic ones.

However, prokaryotes have existed for longer than eukaryotes and, even if all prokaryotes have a similar basic morphology, they are incredibly diverse in the way they obtain energy, the places they inhabit, and they reproduce extremely fast.

Thus, we should see these differences as distinct ways to exist, both with advantages and disadvantages depending on the circumstances, but in the end, both types have been successful at living and thriving on earth.

Disadvantages of cell compartmentalization

While eukaryotic cells can be more efficient in some activities because they perform several reactions at the same time in different compartments, they also need more control checkpoints and regulatory mechanisms, thus slowing some processes compared to prokaryotes. Some disadvantages of cell compartmentalization in eukaryotic cells are the following:

• More energy requirements to maintain a bigger cell with more complex biological components and organelles.
• Slower cell division.
• Gene expression takes more time.