The world of cellular life is broadly divided into two fundamental groups: prokaryotes and eukaryotes. Bacteria represent the prokaryotic domain, characterized by a simpler cellular architecture that arose early in life’s history. Eukaryotes, which include all animals, plants, fungi, and protists, feature a far more complex internal organization. While all cells share basic requirements, the way these two groups fulfill those requirements reveals a fundamental difference in biological design.
Genetic Material and Compartmentalization
The organization of the cell’s genetic material represents the most profound distinction between bacteria and eukaryotes. In a bacterial cell, the primary genetic instruction set is typically contained within a single, circular chromosome. This chromosome is located in a dense, irregularly shaped region of the cytoplasm known as the nucleoid, which is not enclosed by a membrane. Bacteria often possess smaller, non-essential loops of DNA called plasmids, which can carry genes for traits like antibiotic resistance.
Eukaryotic cells, by contrast, store their multiple, linear chromosomes within a true nucleus, a specialized compartment surrounded by a double-membrane envelope. This nuclear membrane physically separates the genetic information from the rest of the cell’s activities. The DNA within this nucleus is extensively packaged, tightly wound around specialized proteins called histones to form complex structures known as nucleosomes. This association between DNA and histone proteins enables the massive length of genetic material to be compacted efficiently.
The difference in genetic organization has a direct effect on how the cell operates. Because the bacterial chromosome is not separated from the cytoplasm, the processes of reading the DNA (transcription) and building the corresponding protein (translation) can occur almost simultaneously. In eukaryotes, the nuclear envelope ensures that transcription occurs inside the nucleus, and the resulting genetic message must be transported out to the cytoplasm before translation can begin. This spatial separation allows for more complex regulation and modification of the genetic message before a protein is produced.
Internal Membrane Systems and Organelles
A hallmark of the eukaryotic cell is its extensive system of internal membranes, which create specialized, functional compartments known as organelles. These membrane-bound structures allow for a division of labor, enabling the cell to carry out incompatible chemical reactions simultaneously. For example, the endoplasmic reticulum (ER) forms a network of sacs and tubes responsible for synthesizing and folding proteins (rough ER) and producing lipids (smooth ER). Proteins and lipids then travel to the Golgi apparatus, where they are modified, sorted, and packaged into vesicles for transport or secretion outside the cell.
Organelles are dedicated to energy conversion and waste management. Mitochondria use a double-membrane system to perform cellular respiration, generating the majority of the cell’s energy (ATP). Plant and algae cells contain chloroplasts, which capture light energy to produce sugars through photosynthesis. Lysosomes contain potent digestive enzymes for breaking down worn-out cellular components or engulfed particles.
Bacteria lack these complex internal organelles and therefore do not exhibit extensive compartmentalization. The functions that occur on the membranes of eukaryotic organelles, such as cellular respiration and photosynthesis, are instead carried out on the bacterial plasma membrane. In many photosynthetic bacteria, such as cyanobacteria, the plasma membrane folds inward to form specialized, flattened sacs called thylakoids. These infoldings greatly increase the membrane surface area, providing the necessary space for the light-harvesting machinery to function efficiently.
External Structures and Cell Boundaries
All cells possess a plasma membrane, a lipid bilayer that acts as a boundary and controls the passage of substances. Eukaryotic plasma membranes contain sterol molecules, such as cholesterol, which help maintain membrane fluidity and mechanical stability. Bacterial membranes generally do not contain these sterols, with the exception of a few specialized groups like Mycoplasma.
Many cells also possess a cell wall, a layer outside the plasma membrane that provides shape and protection. The bacterial cell wall is chemically unique, composed primarily of a cross-linked polymer called peptidoglycan. This structure distinguishes Gram-positive bacteria (thick layer) from Gram-negative bacteria (thin layer situated between two membranes). The cell wall is not a universal feature of eukaryotes; animal cells lack one entirely. Where present (plants, algae, fungi), it is made of different materials, including cellulose or chitin.
The outermost layer is often a sticky, carbohydrate-rich layer called the glycocalyx. In bacteria, this layer can be highly organized (a capsule) or a loose slime layer. The bacterial capsule helps the cell resist dehydration and evade engulfment by immune cells. Eukaryotic cells also have a glycocalyx, a coating of glycoproteins and glycolipids involved in cell recognition and adhesion.
Shared Components with Unique Structures
The ribosome is a shared cellular complex responsible for protein synthesis. Bacterial ribosomes are classified as 70S, reflecting their sedimentation rate in a centrifuge, and are composed of smaller subunits. Eukaryotic ribosomes are noticeably larger and more complex, classified as 80S. This difference in size and composition allows certain antibiotics to selectively target and inhibit the 70S bacterial ribosomes without affecting the larger 80S ribosomes in human cells.
Motility structures, such as flagella, are present in both cell types but operate using different mechanisms. A bacterial flagellum is a simple, slender filament composed of the protein flagellin that extends from the cell surface. This filament is connected to a complex basal body motor, which drives the flagellum in a rotary, propeller-like motion powered by a proton gradient. Eukaryotic flagella are complex appendages enclosed by the cell membrane and are built from the protein tubulin. Internally, they feature a characteristic “9+2” arrangement of microtubules, and their movement is a bending, whip-like motion powered by the hydrolysis of ATP.