All life on Earth is built from cells, the fundamental units of existence. Scientists classify all organisms into two primary types: prokaryotic cells and eukaryotic cells. While both share a common evolutionary origin and underlying biological machinery, they exhibit profound differences in complexity, internal structure, and organizational strategies. These distinctions influence everything from genetic organization to reproduction.
Core Components Shared by All Cells
Every cell must maintain a distinct internal environment and possess the tools for survival and replication. The plasma membrane serves as the outer boundary for both prokaryotic and eukaryotic cells. This selective barrier is composed of a lipid bilayer that regulates the passage of substances into and out of the cell.
Inside the membrane lies the cytoplasm, the jelly-like substance where all cellular components are suspended and most metabolic reactions take place. The liquid portion, called the cytosol, provides the medium for numerous biochemical processes, including the initial breakdown of glucose in glycolysis. Both cell types also possess deoxyribonucleic acid (DNA) as their genetic material, which holds the instructions for building and operating the cell.
To execute the instructions encoded in the DNA, all cells rely on ribosomes, which are molecular complexes responsible for synthesizing proteins. Ribosomes translate the genetic message carried by messenger RNA into the long chains of amino acids that form proteins. Although the ribosomes in prokaryotes are structurally smaller than those in eukaryotes, they perform the same essential function of protein construction in both cell types.
Architectural Differences in Internal Organization
The most significant distinction between the two cell types lies in their internal architecture, particularly the presence or absence of a nucleus and other membrane-bound compartments. Eukaryotic cells possess a true nucleus, a large organelle encased by a double membrane called the nuclear envelope. This separation of the genetic material provides a distinct, protective environment for the cell’s DNA, allowing for greater regulation of gene expression.
In contrast, prokaryotic cells lack a membrane-enclosed nucleus, with their genetic material concentrated instead within an irregularly shaped region of the cytoplasm known as the nucleoid. This absence of internal membranes means that transcription, the process of copying DNA into RNA, and translation, the synthesis of protein, can occur almost simultaneously. Eukaryotic cells achieve higher levels of functional organization through a system of membrane-bound organelles, a feature entirely missing in prokaryotes.
This internal division, known as compartmentalization, allows different biochemical processes that might interfere with one another to occur concurrently in specialized microenvironments. For example, the endoplasmic reticulum (ER) forms a network of membranes involved in synthesizing lipids and folding proteins destined for secretion or other organelles. Proteins and lipids then travel to the Golgi apparatus, which acts to modify, sort, and package these molecules for delivery to their final destinations.
Energy production in eukaryotes is primarily delegated to mitochondria, complex organelles with their own internal membranes that generate the cell’s chemical energy, adenosine triphosphate (ATP). By confining specific functions to these organelles, the eukaryotic cell manages a much larger volume and achieves a higher level of complexity than the simpler, non-compartmentalized prokaryotic cell. This organizational strategy allows for the high degree of cellular specialization seen in multicellular organisms.
Contrasts in Size, Genome, and Reproduction
Prokaryotes and eukaryotes differ significantly in their scale, genetic code, and methods of replication. Prokaryotic cells are small, with diameters ranging from 0.1 to 5.0 micrometers (µm). Eukaryotic cells are substantially larger, presenting diameters between 10 and 100 µm, often 10 to 100 times the size of a prokaryote.
The genetic material also exhibits a fundamental structural difference in the two cell types. Prokaryotic DNA is organized into a single, circular chromosome located in the nucleoid region. Eukaryotic DNA, conversely, is divided into multiple, linear chromosomes that are housed within the nucleus.
A further distinction is how the DNA is managed: eukaryotic DNA is extensively packaged by being wrapped around specialized proteins called histones, forming a dense structure known as chromatin. This complex arrangement allows for the compact storage of a much larger genome and provides an additional layer of control over gene activity. Prokaryotes do not use histones for packaging, instead relying on other proteins to supercoil their circular chromosome.
In reproduction, prokaryotes utilize a straightforward asexual process called binary fission, where the single chromosome is replicated, and the cell divides into two identical daughter cells. Eukaryotes employ two more complex forms of cell division: mitosis, which is used for growth, tissue repair, and asexual reproduction, ensuring daughter cells are genetically identical to the parent cell. For sexual reproduction, eukaryotes use meiosis, a specialized process that reduces the number of chromosomes by half to produce genetically diverse gametes.
Real-World Examples of Prokaryotes and Eukaryotes
The differences in cell structure correlate with the classification of all life into the three major domains. The prokaryotic structure defines the domains Bacteria and Archaea, while the eukaryotic structure defines the domain Eukarya. Organisms in the Bacteria domain include species such as Escherichia coli and Salmonella, which are found in diverse environments.
The Archaea domain consists of single-celled organisms that inhabit extreme environments, such as methanogens that produce methane or halophiles found in extremely salty water. Although Bacteria and Archaea are both prokaryotes, they are genetically and biochemically distinct from each other.
The Eukarya domain encompasses a broad array of life forms, ranging from single-celled organisms to complex multicellular beings. This domain includes the four kingdoms:
- Protists
- Fungi
- Plants
- Animals
Examples of eukaryotes are diverse, including the cells that make up trees, mushrooms, humans, and single-celled organisms like amoebas.