Cell (biology)
The cell is the basic structural and functional unit of all forms of life or organisms. The term comes from the Latin word cellula meaning 'small room'. A biological cell basically consists of a semipermeable cell membrane enclosing cytoplasm that contains genetic material. Most cells are only visible under a microscope. Except for highly-differentiated cell types most cells are capable of replication, and protein synthesis. Some types of cell are motile. Cells emerged on Earth about four billion years ago.
All organisms are grouped into prokaryotes, and eukaryotes. Prokaryotes are single-celled and include archaea and bacteria. Eukaryotes can be single-celled or multicellular, and include protists, plants, animals, most species of fungi, and some species of algae. All multicellular organisms are made up of many different types of cell. The diploid cells that make up the body of a plant or animal are known as somatic cells, which excludes the haploid gametes.
Prokaryotes lack a membrane-bound nucleus and have a nucleoid instead. In eukaryotic cells, the nucleus is enclosed in the nuclear membrane. Eukaryotic cells contain other membrane-bound organelles such as mitochondria, which provide energy for cell functions, and chloroplasts, in plants that create sugars by photosynthesis. Other non-membrane-bound organelles may be proteinaceous, such as the ribosomes present in both groups. A unique membrane-bound prokaryotic organelle, the magnetosome has been discovered in magnetotactic bacteria.
Cells were discovered by Robert Hooke in 1665, who named them after their resemblance to cells in a monastery. Cell theory, developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all organisms, and that all cells come from pre-existing cells.
Types
Organisms are broadly grouped into eukaryotes, and prokaryotes. Eukaryotic cells possess a membrane-bound nucleus, and prokaryotic cells lack a nucleus but have a nucleoid region. Prokaryotes are single-celled organisms, whereas eukaryotes can be either single-celled or multicellular. Single-celled eukaryotes include microalgae such as diatoms. Multicellular eukaryotes include all animals, and plants, most fungi, and some species of algae.| Property | Archaea | Bacteria | Eukaryota |
| Cell membrane | Ether-linked lipids | Ester-linked lipids | Ester-linked lipids |
| Cell wall | Glycoprotein, or S-layer; rarely pseudopeptidoglycan | Peptidoglycan, S-layer, or no cell wall | Various structures; animal cells lack a cell wall |
| Gene structure | Circular chromosomes, similar translation and transcription to Eukaryota | Circular chromosomes, unique translation and transcription | Multiple, linear chromosomes, but translation and transcription similar to Archaea |
| Internal cell structure | No nucleus; rarely has membrane-bound organelles | No nucleus or membrane-bound organelles | Has nucleus and other membrane-bound organelles |
| Metabolism | Various, including diazotrophy, with methanogenesis unique to Archaea | Various, including photosynthesis, aerobic and anaerobic respiration, fermentation, diazotrophy, and autotrophy | Photosynthesis, cellular respiration, and fermentation; no diazotrophy |
| Reproduction | Asexual reproduction, horizontal gene transfer | Asexual reproduction, horizontal gene transfer | Sexual and asexual reproduction |
| Methionine | Formylmethionine | Methionine | |
| RNA polymerase | One | One | Many |
| EF-2/EF-G | Sensitive to diphtheria toxin | Resistant to diphtheria toxin | Sensitive to diphtheria toxin |
Prokaryotes
All prokaryotes are single-celled and include bacteria and archaea, two of the three domains of life. Prokaryotic cells were likely the first form of life on Earth, characterized by having vital biological processes including cell signaling. They are simpler and smaller than eukaryotic cells, lack a nucleus, and the other usually present membrane-bound organelles. Prokaryotic organelles are less complex, and are typically membrane-less. All prokaryotic cells secrete different substances from their membranes, including exoenzymes, and extracellular polymeric substances.Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter. The largest bacterium known, Thiomargarita magnifica, is visible to the naked eye with an average length of, but can be as much as
Bacteria
Bacteria are enclosed in a cell envelope, that protects the interior from the exterior. It generally consists of a plasma membrane covered by a cell wall which, for some bacteria, is covered by a third gelatinous layer called a bacterial capsule. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Mycoplasma only possess the cell membrane. The cell envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective mechanical and chemical filter. The cell wall consists of peptidoglycan and acts as an additional barrier against exterior forces. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. It also prevents the cell from expanding and bursting from osmotic pressure due to a hypotonic environment.The DNA of a bacterium typically consists of a single circular chromosome that is in direct contact with the cytoplasm in a region called the nucleoid. Some bacteria contain multiple circular or even linear chromosomes. The cytoplasm also contains ribosomes and various inclusions where transcription takes place alongside translation. Extrachromosomal DNA as plasmids, are usually circular and encode additional genes, such as those of antibiotic resistance. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including species of Borrelia which causes Lyme disease. The prokaryotic cytoskeleton in bacteria is involved in the maintenance of cell shape, polarity and cytokinesis.
Compartmentalization is a key feature of eukaryotic cells but some species of bacteria, have protein-based organelle-like microcompartments such as gas vesicles, and carboxysomes, and encapsulin nanocompartments. Certain membrane-bound prokaryotic organelles have also been discovered. They include the magnetosome of magnetotactic bacteria, and the anammoxosome of anammox bacteria.
Cell-surface appendages can include flagella, and pili, protein structures that facilitate movement and communication between cells. The flagellum stretches from the cytoplasm through the cell membrane and extrudes through the cell wall. Fimbriae are short attachment pili, the other type of pilus is the longer conjugative type. Fimbriae are formed of an antigenic protein called pilin, and are responsible for the attachment of bacteria to specific receptors on host cells.
Archaea
Archaea are enclosed in a cell envelope consisting of a plasma membrane and a cell wall.An exception to this is the Thermoplasma that only has the cell membrane. The cell membranes of archaea are unique, consisting of ether-linked lipids. The prokaryotic cytoskeleton has homologues of eukaryotic actin and tubulin. A unique form of metabolism in the archaean is methanogenesis. Their cell-surface appendage equivalent of the flagella is the differently structured and unique archaellum. The DNA is contained in a circular chromosome in direct contact with the cytoplasm, in a region known as the nucleoid. Ribosomes are also found freely in the cytoplasm, or attached to the cell membrane where DNA processing takes place.
The archaea are noted for their extremophile species, and many are selectively evolved to thrive in extreme heat, cold, acidic, alkaline, or high salt conditions. There are no known archaean pathogens.
Eukaryotes
Eukaryotes can be single-celled, as in diatoms, or multicellular, as in animals, plants, most fungi, and some algae. Multicellular organisms are made up of many different types of cell known overall as somatic cells. Eukaryotes are distinguished by the presence of a membrane-bound nucleus. The nucleus gives the eukaryote its name, which means "true nut" or "true kernel", where "nut" means the nucleus. A eukaryotic cell can be 2 to 100 times larger in diameter than a typical prokaryotic cell.Eukaryotic cells have a cell membrane that surrounds a gel-like cytoplasm. The cytoplasm contains the cytoskeleton, and surrounds the cell nucleus, the endoplasmic reticulum, ribosomes, the Golgi apparatus, mitochondria, lysosomes, peroxisomes, endosomes, vacuoles and vesicles, and may have a cell wall, chloroplasts, vaults, and cell-surface appendages. There are many cell variations among the different eukaryote groups.
The membranes of most of the organelles including the cell membrane are sometimes referred to as the endomembrane system. All of these membranes are involved in the secretory and endocytic pathways, modifying, packaging, and transporting proteins and lipids to and from the trans-Golgi network. In mammalian cells, endocytosis includes early, late, and recycling endosomes.
Most distinct cell types arise from a single totipotent cell, called a zygote, that differentiates into hundreds of different cell types during the course of development. Differentiation of cells is driven by different environmental cues and intrinsic differences.
Eukaryotic cell types include those that make up animals, plants, fungi, algae, and protists. All of which have many different species and cell differences.