Every living organism on Earth is categorized into one of two groups: prokaryotes or eukaryotes. The fundamental difference lies in their cellular structure. Prokaryotes are single-celled organisms lacking a nucleus and other membrane-bound organelles. They are generally smaller and simpler in structure, encompassing bacteria and archaea. Eukaryotes, conversely, are often multicellular, possessing a nucleus and membrane-bound organelles that compartmentalize and organize cellular functions. This group includes animals, plants, fungi, algae, and protozoans.
This article will delve deeper into the definitions of prokaryotes and eukaryotes, highlighting the key similarities and, more importantly, the distinctions between these two fundamental cell types. Understanding these differences is crucial in grasping the diversity and complexity of life itself.
Contents
Comparing Prokaryotes and Eukaryotes
Key Similarities Between Prokaryotes and Eukaryotes
What Are the Key Differences Between Prokaryotes and Eukaryotes?
- Transcription and Translation in Prokaryotes vs Eukaryotes
Prokaryote Definition - Prokaryotic Cell Features
- Examples of Prokaryotes
- Do Prokaryotes Have a Nucleus?
- Do Prokaryotes Have Mitochondria?
Eukaryote Definition - Eukaryotic Cell Features
- Examples of Eukaryotes
Comparing Prokaryotes and Eukaryotes
Prokaryotes are considered the earliest form of life, with scientists estimating that eukaryotes evolved from prokaryotic ancestors approximately 2.7 billion years ago.1 The prevailing theory regarding the origin of eukaryotes suggests a symbiotic relationship between two prokaryotic cells, leading to a merger known as endosymbiosis.2 These endosymbiotic events are thought to have given rise to membrane-bound organelles like mitochondria, providing eukaryotic ancestors with the energy necessary to evolve into the more complex eukaryotic cells we know today.
Recent research, however, challenges some aspects of this theory. A study from the University of Jena, published in mBio, identified prokaryotic bacteria capable of “eating” other cells.3 This discovery contradicts the long-held belief that endocytosis, the process of engulfing substances into a cell to form vesicles, was exclusive to eukaryotes. These findings indicate a need to reconsider existing theories about eukaryotic origins and the evolutionary pathways of cellular complexity.
The most significant distinction between prokaryotes and eukaryotes is the presence of a membrane-bound nucleus in eukaryotic cells, which is absent in prokaryotes. The nucleus serves as the control center of eukaryotic cells, housing their genetic information. In prokaryotes, DNA is located in a nucleoid region, a centralized area within the cytoplasm, but it is not enclosed within a nuclear membrane.
Beyond the nucleus, eukaryotes are characterized by a multitude of membrane-bound organelles, each performing specific functions within the cell. Prokaryotes, in contrast, lack these organized compartments. Another fundamental difference lies in DNA structure and organization.4 Eukaryotic DNA comprises multiple linear, double-stranded molecules located in the nucleus, whereas prokaryotic DNA is typically a single, circular, double-stranded molecule situated in the cytoplasm. It is worth noting that exceptions exist, with linear plasmids and chromosomes found in some prokaryotic species.5
Key Similarities Between Prokaryotes and Eukaryotes
Despite their significant differences, prokaryotic and eukaryotic cells share fundamental characteristics essential for life. These commonalities highlight the universal principles of cellular biology and underscore their shared ancestry. As illustrated in Figure 1, both cell types possess the following four key components:
- DNA: Deoxyribonucleic acid, the universal genetic material that carries the instructions for cell function and inheritance. In both cell types, DNA dictates the characteristics and activities of the cell.
- Plasma Membrane: Also known as the cell membrane, this outer boundary acts as a selective barrier, controlling the passage of substances in and out of the cell. It maintains the cell’s internal environment and interacts with the external world.
- Cytoplasm: The gel-like substance filling the cell, cytoplasm is where various biochemical reactions occur. It houses the cellular components and provides a medium for transport and metabolic processes.
- Ribosomes: These are the cellular machinery responsible for protein synthesis. Ribosomes translate genetic information from DNA into proteins, which carry out essential functions within the cell.
What Are the Key Differences Between Prokaryotes and Eukaryotes?
Prokaryotes and eukaryotes diverge significantly in various aspects of their cellular organization and function. These differences, summarized in Table 1, primarily revolve around structural variations, particularly the presence or absence of a nucleus and membrane-bound organelles. These variations reflect the distinct evolutionary paths and functional capabilities of these two fundamental cell types.
Table 1: Key Differences Between Prokaryotic and Eukaryotic Cells
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | Absent | Present (membrane-bound nucleus enclosing DNA) |
| Membrane-bound Organelles | Absent | Present (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes in animal cells; vacuoles, chloroplasts, cell wall in plant cells) |
| Cell Structure | Unicellular | Mostly multicellular; some unicellular (e.g., yeast, protists) |
| Cell Size | Typically smaller (0.1–5 μm), with the exception of Thiomargarita magnifica, a giant bacterium reaching centimeter lengths. | Larger (10–100 μm) |
| Complexity | Simpler | More complex, exhibiting higher levels of internal organization and compartmentalization. |
| DNA Form | Often circular, single chromosome; may also contain plasmids (small, circular DNA molecules). Some exceptions with linear chromosomes exist. | Linear, multiple chromosomes; organized within the nucleus and associated with histone proteins to form chromatin. |
| Transcription & Translation | Coupled in the cytoplasm | Uncoupled; transcription in the nucleus, translation in the cytoplasm |
| Ribosomes | Smaller (70S) | Larger (80S) |
| Cell Wall | Present in most, chemically complex (peptidoglycan in bacteria, pseudopeptidoglycan in archaea) | Present in plant cells (cellulose), fungi (chitin), and some protists; absent in animal cells |
| Cytoskeleton | Less complex, rudimentary cytoskeleton | Complex cytoskeleton composed of microtubules, microfilaments, and intermediate filaments, providing structural support, cell shape, and intracellular transport. |
| Cell Division | Binary fission | Mitosis and meiosis |
| Examples | Bacteria, Archaea | Animals, Plants, Fungi, Protists |
Transcription and Translation in Prokaryotes vs Eukaryotes
A significant functional difference between prokaryotes and eukaryotes lies in the coupling of transcription and translation, the processes of gene expression.
In prokaryotic cells, transcription and translation are coupled. This means that as messenger RNA (mRNA) is being synthesized from a DNA template (transcription), ribosomes can immediately begin translating this mRNA into protein (translation). Both processes occur in the cytoplasm because there is no nucleus to separate them.6 This coupling allows for rapid gene expression and response to environmental changes.
In eukaryotic cells, transcription and translation are uncoupled. Transcription takes place within the nucleus, where DNA resides. The newly synthesized mRNA then undergoes processing before being transported out of the nucleus into the cytoplasm. Translation occurs in the cytoplasm on ribosomes, which can be free-floating or attached to the endoplasmic reticulum. This separation in space and time allows for more complex regulation of gene expression in eukaryotes.
Prokaryote Definition
Prokaryotes encompass two domains of life: Bacteria and Archaea. These are unicellular organisms characterized by the absence of membrane-bound organelles. Prokaryotic cells are generally small and structurally simple, typically ranging from 0.1 to 5 μm in diameter.7 Despite their simplicity, prokaryotes are incredibly diverse and adaptable, inhabiting a wide range of environments.
Although lacking membrane-bound organelles, prokaryotic cells are not without internal organization. DNA in prokaryotes is concentrated in a region called the nucleoid (Figure 2). The cytoplasm of prokaryotes is a dynamic environment containing DNA, ribosomes, proteins, and metabolites. Some bacteria also possess primitive organelles, acting as micro-compartments to introduce a degree of organization.8
Prokaryotic Cell Features
Here are the key structural components commonly found in a prokaryotic bacterial cell (Figure 2):
- Nucleoid: The central region containing the cell’s DNA, which is typically a circular chromosome.
- Ribosomes: Sites of protein synthesis, translating mRNA into proteins.
- Cell Wall: A rigid outer layer providing structural support and protection. In bacteria, it’s primarily composed of peptidoglycans.
- Plasma Membrane: The inner membrane enclosing the cytoplasm, regulating the passage of substances.
- Capsule: An outer layer of carbohydrates in some bacteria, aiding in attachment and protection against harsh conditions.
- Pili (Fimbriae): Hair-like appendages involved in attachment to surfaces and DNA transfer.
- Flagella: Tail-like structures facilitating cell movement.
Examples of Prokaryotes
The two primary domains of prokaryotic life are Bacteria and Archaea. Bacteria are incredibly diverse and ubiquitous, playing crucial roles in ecosystems, human health, and various industrial processes. Archaea, often found in extreme environments, are also diverse and possess unique metabolic pathways.
Do Prokaryotes Have a Nucleus?
No, prokaryotes do not have a nucleus. Their DNA is located in the nucleoid region, which is not separated from the cytoplasm by a membrane. This lack of a nucleus is a defining characteristic of prokaryotic cells. Their DNA is typically a single circular chromosome. They also lack other membrane-bound organelles like the endoplasmic reticulum and Golgi apparatus.
Do Prokaryotes Have Mitochondria?
No, prokaryotes do not have mitochondria. Mitochondria, the powerhouses of the cell responsible for generating energy through cellular respiration, are exclusively found in eukaryotic cells. This absence extends to all membrane-bound organelles, including the nucleus and Golgi apparatus, which are hallmarks of eukaryotic cellular organization.
Eukaryote Definition
Eukaryotes are organisms whose cells are characterized by the presence of a nucleus and other membrane-bound organelles (Figure 3). These organelles are internal compartments within the cell, each specialized to perform specific functions, such as energy production, protein synthesis, and waste breakdown. Eukaryotic cells are more complex and larger than prokaryotic cells, typically ranging from 10 to 100 μm in diameter.9 While most eukaryotes are multicellular, some are unicellular, such as yeast and protists.
Eukaryotic Cell Features
Eukaryotic cells exhibit a high degree of internal organization, with various membrane-bound organelles performing specialized functions. Here’s an overview of the primary components of eukaryotic cells:
- Nucleus: The control center of the cell, containing the cell’s genetic material in the form of chromatin.
- Nucleolus: A structure within the nucleus responsible for ribosomal RNA (rRNA) production.
- Plasma Membrane: The outer boundary of the cell, a phospholipid bilayer regulating substance passage.
- Cytoskeleton: A network of protein fibers providing cell shape, support, and organelle positioning.
- Cell Wall: Present in plant cells, fungi, and some protists, providing structural support and protection. Absent in animal cells.
- Ribosomes: Sites of protein synthesis, larger and more complex (80S) than prokaryotic ribosomes.
- Mitochondria: The “powerhouses” of the cell, generating ATP through cellular respiration.
- Cytoplasmic Space: The region between the nuclear envelope and plasma membrane.
- Cytoplasm: The total inner-cellular volume excluding the nucleus, encompassing cytosol and organelles.
- Cytosol: The gel-like substance within the cytoplasm, excluding organelles.
- Endoplasmic Reticulum (ER): A network involved in protein and lipid synthesis and transport.
- Vesicles and Vacuoles: Membrane-bound sacs for transport and storage.
Other common organelles in eukaryotes include the Golgi apparatus (protein processing and packaging), chloroplasts (photosynthesis in plant cells and algae), and lysosomes (waste breakdown and recycling).
Examples of Eukaryotes
The domain Eukaryota encompasses a wide range of organisms, including animals, plants, fungi, algae, and protozoans. These diverse groups share the common characteristic of eukaryotic cell structure, despite their varied lifestyles and complexities.
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.
