All living cells can be classified as either prokaryotic or eukaryotic. Bacteria are prokaryotic or prenuclear organisms (organisms without a true nucleus), while fungi, protozoa, helminths, and other organisms are eukaryotic.
Viruses depend on host cells for survival, so they are not considered cellular organisms but infectious agents. Prions (abnormal infectious proteins) are also not considered living cells.
A notable characteristic of eukaryotic cells is the presence of membrane-enclosed subcellular organelles with specialized cellular functions such as mitochondria (sites of aerobic respiration) and chloroplasts (sites of photosynthesis in green plants).
- Endoplasmic reticulum processes and transport proteins.
- Lysosomes provide an environment for controlled enzymatic degradation of intracellular substances.
- Mitochondria is the powerhouse of the cell; it generates energy (ATP)
- The nucleus provides a membrane enclosure for chromosomes.
- The Golgi body transports substances throughout the cell, including internal delivery and exocytosis or secretion of molecules.
Prokaryotic cells, such as bacteria, do not contain organelles. All functions take place in the cytoplasm or cytoplasmic membrane of the cell. The cell wall composed of peptidoglycan is the notable structure present only in prokaryotic bacterial cells.
Eukaryotic cells have a cytoskeleton that supports cellular structure, organization, and movement. The cytoskeleton plays an essential role in immunology by mediating phagocytosis.
Eukaryotic and prokaryotic cells differ considerably at the macromolecular level, including chromosomal organization, gene expression, and protein synthesis machinery. For example;
- Eukaryotic cells contain a nucleus with a nuclear membrane enclosing multiple chromosomes, while prokaryotic cells have a single chromosome (nucleoid) that is not enclosed in a nuclear membrane.
- Another major difference between bacterial DNA and eukaryotic DNA is that bacterial DNA has no introns, whereas eukaryotic DNA does.
- A key genetic difference between prokaryotes and eukaryotes is that eukaryotes typically contain two copies of each gene and are, thus, genetically diploid.
Prokaryotic and eukaryotic cells differ substantially in many other characteristics, some of which are tabulated here:
| Features | Prokaryotic cells | Eukaryotic cells |
| Size | 1-2 by 1-4 micrometer or less | Greater than 5 micrometers in width or diameter |
| Genetic system | ||
| Location | Nucleoid, chromatin body or nuclear material | Nucleus, mitochondria, chloroplasts |
| Nuclear membrane | Absent | Present |
| Chromosome | Single, closed, circular double-stranded DNA. | Multiple, linear chromosomes. |
| Histones | Chromosome does not contain histones. | Chromosomes have histones. |
| Sexuality | Zygote nature is merozygotic (partial diploid) | Zygote is diploid |
| Cytoplasmic nature and Structures | ||
| Cytoplasmic streaming | Absent | Present |
| Pinocytosis | Absent | Present |
| Gas vacuoles | Can be present | Absent |
| Mesosome | Present | Absent |
| Ribosomes | 70S, distributed in the cytoplasm. | 80S arrayed on membranes as in endoplasmic reticulum; 70S in mitochondria and chloroplasts |
| Mitochondria | Absent | Present |
| Chloroplasts | Absent | May be present |
| Golgi structures | Absent | Present |
| Endoplasmic reticulum | Absent | Present |
| Membrane-bound (true) vacuoles | Absent | Present |
| Outer Cell Structure | ||
| Presence of sterols | Generally do not contain sterols; except the wall-less Mycoplasma, which has sterols in its membranes. | Eukaryotic cell membranes contain sterols |
| Cell Wall | Peptidoglycan (murein or mucopeptide) as component | Absence of peptidoglycan |
| Locomotor organelles | Simple fibril (flagella) | Multifibrilled with “9+2” microtubules |
| Pseudopodia | Absent | Present in some |
| Metabolic mechanisms | Wide variety, particularly that of anaerobic energy-yielding reactions; some fix nitrogen gas; some accumulate poly-β-hydroxybutyrate as reserve material | Glycolysis is a pathway for energy-yielding mechanism |
| DNA base ratios as moles % of guanine + cytosine (G+C%) | 28 to 73 | About 40 |
| Examples | Bacteria, blue-green algae. | Algae, fungi, protozoa, plants, and animals. |
