The fundamental difference between the two primary cell types on Earth, prokaryotes and eukaryotes, lies in their internal architecture and organization. Eukaryotic cells, which make up animals, plants, fungi, and protists, are defined by their complex system of internal compartments, a feature absent in the simpler prokaryotic cells (bacteria and archaea). This difference in structure is related to cellular compartmentalization, which allows complex life forms to manage multiple, often conflicting, biochemical processes simultaneously.
Defining Membrane-Bound Organelles
A membrane-bound organelle is a specialized subunit within a cell that is distinctly enclosed by its own lipid bilayer membrane. This surrounding membrane acts as a selective barrier, maintaining a unique internal environment separated from the main cellular fluid, the cytosol. This separation allows for highly specific chemical reactions to occur without interference from processes happening elsewhere in the cell.
Eukaryotic cells rely on this compartmentalization for efficiency, as seen in structures like the nucleus, which safeguards the genetic material, and the mitochondria, which manage energy production. The presence of these encapsulated structures is a defining characteristic used to classify a cell as eukaryotic.
The Structural Organization of Prokaryotic Cells
Prokaryotic cells are fundamentally structured as a single, large compartment. The definitive answer is no: prokaryotes do not possess the membrane-bound organelles found in eukaryotes. Their genetic material is not contained within a nucleus but is concentrated in a region of the cytoplasm known as the nucleoid. This genetic material typically takes the form of a single, circular chromosome.
The entire internal volume of the cell is filled with the cytoplasm, a gel-like substance where all cellular processes occur. Prokaryotes contain structures like ribosomes, which are responsible for protein synthesis. However, these are not enclosed by a membrane and are not classified as membrane-bound organelles. This simple, non-compartmentalized structure is a hallmark of prokaryotes, allowing for rapid growth and reproduction.
Specialized Internal Compartments
Although prokaryotes lack true membrane-bound organelles, many species possess sophisticated internal structures that perform specialized functions, which are often called microcompartments. These structures are distinct from eukaryotic organelles because their boundaries are often formed by protein shells rather than lipid bilayers. A well-studied example is the carboxysome, which is a polyhedral protein shell found in many carbon-fixing bacteria.
Carboxysomes encapsulate the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) along with carbonic anhydrase, to concentrate carbon dioxide for efficient photosynthesis. Other examples include gas vesicles, which regulate buoyancy in aquatic prokaryotes, and various inclusion bodies, which are simple granules used for storing nutrients like polyphosphate or glycogen. A rare exception to the “no membrane” rule is the anammoxosome, a true membrane-bound compartment found only in certain Planctomycetes bacteria, which is used to carry out the anaerobic ammonium oxidation reaction.
Relying on the Plasma Membrane for Key Functions
Prokaryotes efficiently manage complex metabolic tasks, such as generating energy, by utilizing the plasma membrane itself. In eukaryotic cells, cellular respiration and photosynthesis occur within the dedicated membrane-bound mitochondria and chloroplasts, respectively. Prokaryotes achieve the same functional outcome by embedding the necessary enzyme systems, like the electron transport chain, directly into their cell membrane.
The plasma membrane thus serves as the functional equivalent for energy generation, with the separation of charges and creation of proton gradients occurring across this single boundary. In photosynthetic prokaryotes, such as cyanobacteria, the plasma membrane may form extensive, folded internal sheets called thylakoids. These membrane folds significantly increase the surface area available for the light-dependent reactions of photosynthesis, demonstrating a strategy of surface area modification rather than internal encapsulation.