Life on Earth is broadly categorized into two major cellular domains: prokaryotes and eukaryotes. Prokaryotic cells, which include bacteria and archaea, are generally smaller and lack a nucleus or other membrane-bound internal compartments. Eukaryotic cells, which make up animals, plants, fungi, and protists, are structurally more complex, housing their genetic material within a nucleus and featuring numerous specialized organelles. Despite these differences in complexity, both cell types share four fundamental structures that are universally required to carry out the basic functions of life. These shared components illustrate a common evolutionary ancestry.
The Universal Boundary (Plasma Membrane)
The existence of any cell is defined by its separation from the external environment, a function universally performed by the plasma membrane. This membrane is a flexible, double-layered structure known as the phospholipid bilayer, which is present in both prokaryotic and eukaryotic cells. The basic composition of this barrier is identical across all domains of life, featuring phospholipids with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails pointing inward.
The primary function of this boundary is selective permeability, meaning it controls which substances can enter or leave the cell. Small, nonpolar molecules like oxygen can often pass directly through the lipid portion, while larger or charged molecules require specialized protein channels embedded within the membrane. This careful regulation maintains a stable internal condition, a process known as homeostasis. The plasma membrane acts as the cell’s interface, managing the transport of nutrients and waste products.
The Cellular Fluid (Cytoplasm)
Inside the plasma membrane of both cell types lies the cytoplasm, a semi-fluid substance that fills the cell interior. The core component of the cytoplasm is the cytosol, an aqueous solution composed primarily of water, salts, ions, and organic molecules. In prokaryotes, the cytoplasm contains everything within the plasma membrane, including the genetic material.
The cytoplasm serves as the medium in which all other cellular components are suspended. It is the universal site for many fundamental metabolic reactions that sustain the cell. For example, the initial stages of cellular respiration, such as glycolysis, take place within the cytoplasm of both prokaryotes and eukaryotes.
The Machinery for Protein Synthesis (Ribosomes)
Ribosomes are complex structures found in all living cells, and their function is protein synthesis. They are responsible for translating the genetic instructions encoded in messenger RNA (mRNA) into sequences of amino acids, forming polypeptide chains that eventually fold into functional proteins. Without these structures, a cell cannot construct the enzymes and structural components it needs to survive.
While their function is identical, the structure of ribosomes shows a slight variation. Prokaryotic cells possess smaller 70S ribosomes, while eukaryotic cells have larger 80S ribosomes. Despite this size distinction, both 70S and 80S ribosomes are composed of a large and a small subunit, and both perform the same chemical reaction of forming peptide bonds between amino acids.
The Shared Genetic Code (DNA)
Deoxyribonucleic acid, or DNA, serves as the hereditary material common to all cellular life. DNA stores the complete set of instructions required for building and operating the cell, a function that is identical in both prokaryotes and eukaryotes. Its fundamental chemical composition is universally a double helix structure composed of four nucleotide bases: adenine, guanine, cytosine, and thymine.
Although the chemical nature of DNA is shared, its organization differs significantly between the two domains. Prokaryotes typically organize their DNA into a single, circular chromosome located in a region of the cytoplasm called the nucleoid. In contrast, eukaryotic cells house their DNA within a membrane-bound nucleus, organizing it into multiple, linear chromosomes that are tightly wound around proteins called histones to form chromatin.