Prokaryotic cells are single-celled organisms, such as bacteria and archaea, that lack a true nucleus, distinguishing them from more complex cells. The term “organelle” typically refers to subunits encased in a lipid membrane. Because prokaryotic cells lack membrane-bound compartments like mitochondria or a Golgi apparatus, they are generally considered to lack true organelles. However, they possess internal and external structures that perform all necessary functions for survival and reproduction.
Essential Internal Components
The interior of a prokaryotic cell is not compartmentalized by internal membranes, but it does have specialized regions for genetic and metabolic activities. The cell’s genetic material is concentrated in a central area known as the nucleoid region, which is not separated from the cytoplasm by an envelope. This region contains the cell’s single, large, circular chromosome.
Scattered throughout the cytoplasm are numerous ribosomes, which are non-membrane-bound structures responsible for synthesizing proteins. Prokaryotic ribosomes are structurally simpler and smaller than those in complex cells, designated as 70S ribosomes. The cytoplasm also frequently contains inclusion bodies, or storage granules, which serve as reservoirs for nutrients. These granules store substances like glycogen for energy, polyphosphate, or sulfur, allowing the cell to survive periods of nutrient scarcity.
The Protective and Permeable Barrier
The prokaryotic cell is enclosed by a cell envelope, a structure composed of several layers. The innermost layer is the plasma membrane, a lipid bilayer that regulates the passage of substances into and out of the cell, making it selectively permeable. This membrane is also the site for energy generation, carrying out processes like respiration that occur in mitochondria in more complex cells.
Surrounding the plasma membrane is the cell wall, which provides structural support and helps the cell maintain its shape, preventing bursting due to osmotic pressure. The cell wall’s chemical composition, primarily peptidoglycan, is the basis for classifying bacteria as Gram-positive or Gram-negative. Outside the cell wall, some prokaryotes secrete a gelatinous layer called the glycocalyx, which can be a loose slime layer or a dense capsule. The capsule primarily serves a protective role, shielding the cell from desiccation and hindering engulfment by host immune cells.
Structures for Movement and Attachment
Prokaryotes utilize external appendages that extend beyond the cell envelope to interact with their surroundings. The flagellum is a long, whip-like filament composed of the protein flagellin, which rotates like a propeller to provide motility. This rotational mechanism allows the cell to move toward or away from chemical stimuli, a behavior known as chemotaxis.
Other hair-like structures called pili and fimbriae are shorter and more numerous, functioning in adhesion. Fimbriae allow the cell to attach to surfaces, such as host tissues, which is important for colonization and biofilm formation. A specialized type of pilus, the sex pilus, is an elongated tubular structure that facilitates conjugation, a process where genetic material, such as plasmids, is transferred between two cells.
Defining the Difference: Prokaryotic vs. Eukaryotic Organization
The confusion surrounding “organelles” in prokaryotes stems from the strict definition of the term in cell biology. An organelle is formally defined as a specialized subunit enclosed by a lipid membrane. The simple internal organization of prokaryotic cells lacks this extensive internal membrane system, which is why they are considered to have no true organelles.
The fundamental distinction lies in the principle of compartmentalization. Eukaryotic cells rely on membrane-bound structures like the nucleus and endoplasmic reticulum to isolate and concentrate specific biochemical reactions, increasing cellular efficiency. Prokaryotes, conversely, achieve all their functions within the cytoplasm and at the plasma membrane, maintaining a simpler, less compartmentalized structure. This difference reflects the evolutionary divergence between prokaryotes and the larger, structurally complex eukaryotic cells.