Every form of life on Earth is categorized into one of two primary groups: prokaryotes and eukaryotes. This classification hinges on their fundamental cellular structure. Prokaryotes are characterized as unicellular organisms lacking a nucleus and membrane-bound organelles. Generally smaller and simpler in structure, prokaryotes encompass bacteria and archaea. Conversely, eukaryotes are often multicellular, boasting a nucleus and membrane-bound organelles that intricately organize and compartmentalize cellular functions. This group includes a diverse range of organisms from animals and plants to fungi, algae, and protozoans.
This article delves into a comprehensive comparison of prokaryotes and eukaryotes, highlighting both their similarities and the key distinctions that set them apart.
Delving into Prokaryotic and Eukaryotic Cells: Similarities and Differences
Scientists believe that prokaryotes represent the earliest forms of life, with eukaryotes evolving from prokaryotic ancestors approximately 2.7 billion years ago.1 The prevailing theory regarding the origin of eukaryotes posits that endosymbiosis, a process where two prokaryotic cells entered a symbiotic relationship and merged, played a crucial role.2 These endosymbiotic events are thought to have spurred the development of membrane-bound organelles, notably mitochondria. Mitochondria provided eukaryotic ancestors with the necessary energy to evolve into the more complex eukaryotic cells we recognize today.
However, groundbreaking research from the University of Jena, published in mBio, has unveiled prokaryotic bacteria capable of “consuming” other cells.3 This discovery challenges the long-held belief that endocytosis, a cellular process involving the internalization of substances into intracellular vesicles, was exclusive to eukaryotes. These recent findings prompt a re-evaluation of existing theories surrounding the origins of eukaryotes.
The most significant difference between prokaryotes and eukaryotes lies in the presence of a membrane-bound nucleus. Eukaryotic cells possess a nucleus where their genetic material is housed. In contrast, prokaryotic DNA resides in the nucleoid region, a central area not enclosed by a membrane.
Beyond the nucleus, eukaryotes are distinguished by a multitude of membrane-bound organelles, which are absent in prokaryotes. Another critical distinction lies in DNA structure and location.4 Eukaryotic DNA is organized into multiple linear double-stranded molecules located within the nucleus. Prokaryotic DNA, conversely, is typically a single, circular, double-stranded molecule situated in the cytoplasm. It’s important to note, however, that linear plasmids and chromosomes have been observed in some prokaryotes.5
Fundamental Similarities Between Prokaryotic and Eukaryotic Cells
Despite their differences, prokaryotic and eukaryotic cells share several fundamental characteristics, as illustrated in Figure 1. These common features underscore the universal principles of life at the cellular level.
Figure 1: A comparative illustration highlighting the common and distinct features of prokaryotes and eukaryotes. Credit: Technology Networks.
These shared features include:
- DNA: Both cell types utilize DNA as their genetic material, carrying the instructions for cellular function and inheritance.
- Plasma Membrane: A plasma membrane encloses both prokaryotic and eukaryotic cells, acting as a selective barrier that regulates the passage of substances in and out of the cell.
- Cytoplasm: The cytoplasm, the gel-like substance filling the cell, is present in both cell types. It houses the cellular components and facilitates biochemical reactions.
- Ribosomes: Ribosomes, the molecular machines responsible for protein synthesis, are found in both prokaryotes and eukaryotes.
Key Distinctions: Prokaryotes Versus Eukaryotes
Prokaryotes and eukaryotes diverge significantly in several key aspects, encompassing structural variations such as the presence or absence of a nucleus and membrane-bound organelles. Table 1 provides a concise summary of these differences.
Table 1: A comparative table outlining the key differences between prokaryotes and eukaryotes.
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | Absent | Present |
| Membrane-bound Organelles | Absent | Present |
| Cell Structure | Unicellular | Predominantly multicellular; some unicellular forms exist |
| Cell Size | Typically smaller (0.1–5 μm), with the exception of a recently discovered centimeter-long bacterium in a mangrove swamp. discovered | Larger (10–100 μm) |
| Complexity | Simpler | More complex |
| DNA Form | Predominantly circular, although linear plasmids and chromosomes have been observed in certain prokaryotes. | Linear |
| Examples | Bacteria, Archaea | Animals, Plants, Fungi, Protists |
Transcription and Translation: Divergent Processes
Transcription and translation, the fundamental processes of gene expression, exhibit notable differences between prokaryotes and eukaryotes.
In prokaryotic cells, transcription and translation are coupled processes. This means that translation, the synthesis of proteins from mRNA, commences while mRNA synthesis (transcription) is still underway.6 This coupling is facilitated by the absence of a nucleus, allowing ribosomes direct access to the nascent mRNA.
In contrast, transcription and translation are spatially and temporally separated in eukaryotic cells. Transcription takes place within the nucleus, where DNA is transcribed into mRNA. The mature mRNA then exits the nucleus and enters the cytoplasm, where translation occurs at ribosomes. This separation provides eukaryotes with additional levels of gene regulation.
Prokaryote Definition and Characteristics
Prokaryotes, encompassing bacteria and archaea, are unicellular organisms characterized by the absence of membrane-bound structures. These simple cells are typically small, ranging from 0.1 to 5 μm in diameter.7
Figure 2: Illustrative diagram showcasing the key structural components of a prokaryotic cell. Credit: Technology Networks.
While lacking membrane-bound organelles, prokaryotic cells exhibit distinct regions within their cellular structure. DNA in prokaryotes is concentrated in the nucleoid region (Figure 2). Proteins, DNA molecules, and metabolites coexist within the cytoplasm. Primitive organelles, found in bacteria, serve as micro-compartments, introducing a degree of organization to the cellular arrangement.8
Key Features of Prokaryotic Cells
Prokaryotic bacterial cells possess a set of characteristic features (Figure 2), including:
- Nucleoid: A central region containing the cell’s DNA.
- Ribosomes: Sites of protein synthesis.
- Cell Wall: An outer layer providing structural integrity and protection, often composed of peptidoglycans.
- Cell Membrane (Plasma Membrane): A universal feature, separating the cell from its external environment.
- Capsule: A carbohydrate layer surrounding the cell wall in some bacteria, aiding in surface attachment and protection.
- Pili (Fimbriae): Rod-shaped structures involved in attachment and DNA transfer.DNA transfer
- Flagella: Tail-like appendages facilitating cellular movement.
Examples of Prokaryotes
Bacteria and archaea represent the two primary domains of prokaryotic life.
Nucleus and Mitochondria in Prokaryotes
Prokaryotes lack a nucleus; their DNA resides in the nucleoid region. Similarly, they do not possess mitochondria or other membrane-bound organelles such as the endoplasmic reticulum. Prokaryotic DNA is typically a single circular chromosome.
Eukaryote Definition and Characteristics
Eukaryotes encompass organisms whose cells contain a nucleus and other organelles enclosed within a plasma membrane (Figure 3). Organelles are specialized internal structures responsible for diverse cellular functions, including energy generation and protein synthesis.
Figure 3: Diagrammatic representation of a eukaryotic cell and its key organelles. Credit: Technology Networks.
Eukaryotic cells are generally larger (10–100 μm) and structurally more complex than prokaryotic cells. While multicellularity is prevalent among eukaryotes, unicellular eukaryotic organisms also exist.9
Key Features of Eukaryotic Cells
Eukaryotic cells are characterized by membrane-bound organelles, each performing specific cellular tasks. Key components include:
- Nucleus: The control center, housing genetic information in the form of chromatin.
- Nucleolus: A substructure within the nucleus, responsible for ribosomal RNA (rRNA) production.
- Plasma Membrane: A phospholipid bilayer enclosing the cell and its organelles.
- Cytoskeleton: A network of protein fibers providing cell shape and organelle positioning.
- Cell Wall: Present in plant cells and some other eukaryotes, providing structural support and protection.
- Ribosomes: Sites of protein synthesis.
- Mitochondria: The “powerhouses” of the cell, responsible for energy production through cellular respiration.Mitochondria
- Cytoplasmic Space: The region between the nuclear envelope and plasma membrane.
- Cytoplasm: The total intracellular volume excluding the nucleus, encompassing the cytosol and organelles.
- Cytosol: The gel-like substance within the cytoplasm, excluding organelles.
- Endoplasmic Reticulum (ER): An organelle involved in protein maturation and transport.
- Vesicles and Vacuoles: Membrane-bound sacs involved in transport and storage.
Other common organelles in many eukaryotes include the Golgi apparatus, chloroplasts (in plant cells and algae), and lysosomes.
Examples of Eukaryotes
Animals, plants, fungi, algae, and protozoans are all classified as eukaryotes.
References
- Cooper GM. The Cell: A Molecular Approach. 2nd ed. Sunderland, MA: Sinauer Associates; 2000. https://www.ncbi.nlm.nih.gov/books/NBK9841/. Accessed January 29, 2025.
- Archibald JM. Endosymbiosis and eukaryotic cell evolution. Curr Biol. 2015;25(19):R911-R921. doi: 10.1016/j.cub.2015.07.055
- Wurzbacher Carmen E., Hammer Jonathan, Haufschild Tom, Wiegand Sandra, Kallscheuer Nicolai, Jogler Christian. “Candidatus Uabimicrobium helgolandensis”—a planctomycetal bacterium with phagocytosis-like prey cell engulfment, surface-dependent motility, and cell division. mBio. 2024;15(10):e02044-24. doi: 10.1128/mbio.02044-24
- Karlin S, Mrázek J. Compositional differences within and between eukaryotic genomes. PNAS. 1997;94(19):10227-10232. doi: 10.1073/pnas.94.19.10227
- Hinnebusch J, Tilly K. Linear plasmids and chromosomes in bacteria. Mol Microbiol. 1993;10(5):917-922. doi: 10.1111/j.1365-2958.1993.tb00963.x
- Webster MW, Weixlbaumer A. The intricate relationship between transcription and translation. PNAS. 2021;118(21):e2106284118. doi: 10.1073/pnas.2106284118
- Secaira-Morocho H, Chede A, Gonzalez-de-Salceda L, Garcia-Pichel F, Zhu Q. An evolutionary optimum amid moderate heritability in prokaryotic cell size. Cell Rep. 2024;43(6):114268. doi: 10.1016/j.celrep.2024.114268
- Cole LA. Biology of Life. Academic Press; 2016:93-99. https://www.sciencedirect.com/science/article/abs/pii/B9780128096857000137. Accessed January 29, 2025.
- Simon M, Plattner H. International Review of Cell and Molecular Biology. Academic Press; 2014:141-198. https://www.sciencedirect.com/science/article/abs/pii/B978012800255100003X. Accessed January 29, 2025.
