The two fundamental classifications of cellular life are bacteria (prokaryotes) and the cells of animals, plants, fungi, and protists (eukaryotes). Prokaryotes are structurally simpler, single-celled organisms, while eukaryotes are typically more complex and can be single-celled or multicellular. The distinction between these two groups is based entirely on the presence or absence of specific internal structures, particularly those enclosed by membranes. This structural comparison highlights a major evolutionary divergence in cellular organization.
Genetic Material Organization
The most defining structural difference between these cell types is how the hereditary material, deoxyribonucleic acid (DNA), is housed and organized. Eukaryotic DNA is contained within the nucleus, a structure surrounded by a double membrane known as the nuclear envelope. This envelope physically separates the processes of transcription and translation, which allows for complex genetic regulation.
Within the eukaryotic nucleus, the DNA is organized into multiple, linear chromosomes. The DNA is tightly coiled around specific proteins called histones. This combination of DNA and protein forms a highly compacted structure known as chromatin, enabling efficient storage and access to the genome.
In contrast, bacteria lack a nucleus, and their DNA is instead located in a region of the cytoplasm referred to as the nucleoid. This region is not enclosed by a membrane, meaning the genetic material is in direct contact with the cellular contents. The primary bacterial chromosome is typically a single, circular molecule of DNA.
While bacteria do not use histones like eukaryotes, their DNA is still compacted by various associated proteins to fit within the nucleoid. Many bacteria also possess small, extra-chromosomal pieces of circular DNA called plasmids, which replicate independently of the main chromosome. These smaller genetic elements contrast with the primary linear chromosomal structure of eukaryotes.
Internal Compartmentalization
The internal structure of eukaryotic cells is defined by a high degree of compartmentalization, utilizing internal membranes to create specialized environments for various biochemical reactions. This is achieved through membrane-bound organelles, which spatially separate functions. Mitochondria are specialized for cellular respiration, while the endoplasmic reticulum (ER) forms an interconnected network dedicated to protein and lipid synthesis.
The Golgi apparatus modifies, sorts, and packages proteins and lipids received from the ER, preparing them for transport or secretion. Lysosomes and vacuoles are other membrane-bound sacs that manage waste and storage, maintaining distinct chemical conditions for their enzymatic activities. This internal division allows eukaryotic cells to grow larger and achieve greater functional complexity.
Bacterial cells generally lack these membrane-bound organelles. Metabolic processes that occur on internal membranes in eukaryotes, such as cellular respiration, often take place on the inner surface of the bacterial plasma membrane. While some bacteria contain specialized internal membranes or protein-bound microcompartments, they do not possess the extensive, diverse systems found in eukaryotes.
One structure both cell types share is the ribosome, the molecular machine responsible for synthesizing proteins. Eukaryotic ribosomes are larger (80S sedimentation coefficient), while bacterial ribosomes are smaller (70S coefficient). This structural distinction is medically relevant, as it allows certain antibiotics to target bacterial protein synthesis without significantly affecting the host’s eukaryotic cells.
External Boundaries and Size
The outermost boundaries of bacteria and eukaryotes show significant differences in composition and complexity, alongside a considerable variation in overall cell size. Bacterial cells typically measure between 0.1 and 5.0 micrometers (µm) in diameter, making them significantly smaller than eukaryotic cells, which commonly range from 10 to 100 µm. This smaller size in bacteria allows nutrients and waste products to diffuse quickly throughout the cell interior.
The cell wall, which provides structural support, is a major point of difference in composition. Most bacteria possess a rigid cell wall primarily composed of peptidoglycan, a unique polymer of sugars and amino acids. Eukaryotic cell walls, found in plants and fungi, are chemically distinct, consisting of substances like cellulose or chitin, and are entirely absent in animal cells.
The plasma membrane also shows structural variation. Eukaryotic plasma membranes contain sterols, such as cholesterol, which help regulate membrane fluidity and stability. Most bacterial membranes generally lack these sterols, contributing to differences in membrane dynamics.
Motility structures used for movement also differ structurally, despite serving the same purpose. Eukaryotic flagella are complex, membrane-enclosed projections powered by adenosine triphosphate (ATP) that move in a whip-like manner. Internally, they feature a characteristic “9+2” arrangement of microtubules. Bacterial flagella are simpler, composed primarily of flagellin protein, and function like a propeller, rotating using energy from a proton gradient.