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Cell (biology)
Basic unit of many life forms From Wikipedia, the free encyclopedia
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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 (examples include red blood cells and gametes) 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 types 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, and in animals excludes the haploid gametes.
Prokaryotic cells lack the membrane-bound nucleus present in eukaryotic cells, and instead have a nucleoid region. 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 (though different) 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.
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Types
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tree of life]]
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.[1] 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.[2][3][4]
Prokaryotes
All prokaryotes are single-celled and include bacteria and archaea, two of the three domains of life.[5] Prokaryotic cells were likely the first form of life on Earth,[6][7] 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.[8] Prokaryotic organelles are simple structures typically non-membrane-bound.[9]
Bacteria
Bacteria are enclosed in a cell envelope, that protects the interior from the exterior.[10] 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.[11] 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.[12] The cell wall consists of peptidoglycan and acts as an additional barrier against exterior forces.[13][12] 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 (cytolysis) from osmotic pressure due to a hypotonic environment.[14]
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.[15][16][17] The cytoplasm also contains ribosomes and various inclusions where transcription takes place alongside translation.[18][19] Extrachromosomal DNA as plasmids, are usually circular and encode additional genes, such as those of antibiotic resistance.[20] Linear bacterial plasmids have been identified in several species of spirochete bacteria, including species of Borrelia which causes Lyme disease.[21] The prokaryotic cytoskeleton in bacteria is involved in the maintenance of cell shape, polarity and cytokinesis.[22]
Compartmentalization is a feature of eukaryotic cells but some species of bacteria, have protein-based organelle-like compartments such as encapsulin nanocompartments, and gas vesicles,[23][24][25] and certain membrane-bound prokaryotic organelles have been discovered. They include the magnetosome of magnetotactic bacteria,[23] and the anammoxosome of anammox bacteria.[26][27]
Cell-surface appendages can include flagella, and pili, protein structures that facilitate movement and communication between cells.[28] The bacterial flagellum stretches from the cytoplasm through the cell membrane and extrudes through the cell wall. They are long and thick thread-like proteinaceous appendages.[29][30] A fimbria is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of an antigenic protein protein called pilin, and are responsible for the attachment of bacteria to specific receptors on host cells. There are special types of pili involved in bacterial conjugation.[31]
Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter.[32] The largest bacterium known, Thiomargarita magnifica, is visible to the naked eye with an average length of 1 cm, but can be as much as 2 cm[33] [34]
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.[11] The cell membranes of archaea are unique, consisting of ether-linked lipids. The prokaryotic cytoskeleton has homologues of eukaryotic actin and tubulin.[22] 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.[35][30] 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.[18][36]
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.[37] There are no known archaean pathogens.[38]
Eukaryotes
Eukaryotes can be single-celled, as in diatoms (microscopic algae), or multicellular. Animals, plants, most fungi, and some algae are multicellular.[39] Eukaryotes are distinguished by the presence of a membrane-bound nucleus.[40] The nucleus gives the eukaryote its name, which means "true nut" or "true kernel", where "nut" means the nucleus.[41] Eukaryotic cells can be 2 to 1000 times larger in diameter than a typical prokaryote.[42]
Multicellular organisms are made up of many different types of cell known overall as somatic cells.[45] Typical plant cells include parenchyma cells including transfer cells, and collenchyma cells. Animal cells include all those that make up the four main tissue types of epithelium – a number of different epithelial cells; connective tissue such as osteoblasts in bone, and chondrocytes in cartilage; nervous tissue including different brain cells and nerves, and muscle tissue having different muscle cells.[1] The number of cells in these tissues vary with species. Studies on the human have estimated a total cell count at around 30 trillion cells (~36 trillion cells in the male, and ~28 trillion in the female).[46][47]
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Eukaryotic cells
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The cells of eukaryotes have a cell membrane that surrounds a gel-like cytoplasm; it contains the cytoskeleton, the cell nucleus, the endomembrane system, and organelles including mitochondria, and the Golgi apparatus.
Cell membrane
Diagram of cell membrane detailing the lipid bilayer
Diagram of cell membrane detailing membrane proteins
The cell membrane, or plasma membrane, is a selectively permeable membrane as an outer boundary of the cell that encloses the cytoplasm.[48] The membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a lipid bilayer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic), and is sometimes referred to as a fluid mosaic membrane.[49] Embedded within the cell membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell.[18] The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (molecule or ion) pass through freely, to a limited extent or not at all.[50] Cell surface receptors embedded in the membrane allow cells to detect external signaling molecules such as hormones.[51]
Underlying, and attached to the cell membrane is the cell cortex, the outermost part of the actin cytoskeleton.[52] Its thickness varies with cell type and physiology.
Cytoplasm
The membrane encloses the cytoplasm of the cell. It is made up of two main components, the cytosol, and the protein filaments that make up the cytoskeleton.[53][54] The cytosol is a gel-like substance made up of water, ions, and non-essential biomolecules. The network of filaments and microtubules of the cytoskeleton gives shape and support to the cell, and has a part in organising the cell components. The cytoplasm surrounds all the organelles of the cell.[53][54]
Cytoskeleton
The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. The cytoskeleton is composed of microtubules, intermediate filaments and microfilaments. In a neuron the intermediate filaments are known as neurofilaments. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The outermost part of the cytoskeleton is the cell cortex, or actin cortex, a thin layer of cross-linked actomyosins.[52]
The centrosome is the cytoskeleton organizer in the animal cell that produces the microtubules of a cell—a key component of the cytoskeleton.[55] It directs the transport through the ER and the Golgi apparatus.[56] Centrosomes are composed of two centrioles which lie perpendicular to each other in which each has an organization like a cartwheel, which separate during cell division and help in the formation of the mitotic spindle.[55]
Organelles
Organelles are parts of the cell that are specialized for carrying out one or more vital functions.[18] There are several types of organelles in a cell held in the gelatinous cytosol of the cytoplasm that fills the cell and surrounds the organelles, forming 30%–50% of a cell volume.[57] Most organelles vary in size and/or number based on the growth of the host cell.[58] The nucleus and Golgi apparatus are typically solitary,[citation needed] while others (such as mitochondria, peroxisomes and lysosomes) can be numerous – from hundreds to thousands. Lysosomes contain enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides, Lysosomes are optimally active in an acidic environment. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.[18][59]
Vacuoles sequester waste products. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water.[60]
Mitochondria
Mitochondria generate energy for the cell. Mitochondria are self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of the cell.[18] Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP (aerobic respiration).[61] Mitochondria multiply by binary fission.[62]
Nucleus
The cell nucleus is the largest organelle in the animal cell.[63] It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope, space between these two membrane is called perinuclear space. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled.[18] Cells use DNA for their long-term information storage that is encoded in its DNA sequence.[18] RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.[64] Mitochondria have their own DNA (mitochondrial DNA).[65] The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes,[18] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.[66]
The DNA of each cell is its genetic material, and is organized in multiple linear molecules, called chromosomes, that are coiled around histone proteins and housed in the cell nucleus.[40][67] In humans, the nuclear genome is divided into 46 linear chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The nucleus is a membrane-bound organelle. Other organelles in the cell have specific functions such as mitochondria which provide the cell's energy.[68]
Endomembrane system
The endomembrane system consists of all the different internal membranes of the cell. These membranes are held in the cell's cytoplasm and divide the various organelles.
Endoplasmic reticulum
The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough endoplasmic reticulum (RER), which has ribosomes on its surface that secrete proteins into the ER, and the smooth endoplasmic reticulum (SER), which lacks ribosomes.[18]
A ribosome is a large complex of RNA and protein molecules.[18] They each consist of two subunits, one larger than the other, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane of the rough endoplasmatic reticulum.[69]
The smooth ER plays a role in calcium sequestration and release, and helps in synthesis of lipid.[70]
Golgi apparatus
The Golgi apparatus processes and packages the macromolecules, such as proteins and lipids, that are synthesized by the cell. It is organized as a stack of plate-like structures known as cisternae.[71]
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Cells of major eukaryote groups
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Animal cells
All the cells in an animal body develop from one diploid cell called a zygote. During the development of an animal, the cells differentiate into the specialised tissues and organs of the organism. (An exception is the simple sponge). The estimated cell count in a typical adult human body is around 30 trillion cells. Different groups of cells differentiate from the three germ layers. Differentiation results in structural or functional changes to the typical eukaryotic cell.
Some types of specialised cell are localised to a particular animal group. Vertebrates for example have specialised, structurally changed cells including muscle cells. The cell membrane of a skeletal muscle cell or of a cardiac muscle cell is termed the sarcolemma.[72] And the cytoplasm is termed the sarcoplasm. Skeletal muscle cells also become multinucleated. Populations of animal groups evolve to become distinct species, where sexual reproduction is isolated. The many species of vertebrates for example have other unique characteristics by way of additional specialised cells.
Organelles are parts of the cell that are specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function).[18] In addition to the organelles shared by all eukaryotes, animal cells often have cilia or flagella.[73][74] Many animal cells are ciliated and most cells except red blood cells have primary cilia. Primary cilia play important roles in chemosensation and mechanosensation.[75] Each cilium may be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."[76] Ciliated cells in the respiratory epithelium play an important role in the movement of mucus. Some animal cells have flagella such as flagellated protists and sperm cells, that enable movement.[73]
Invertebrate planarians have ciliated excretory flame cells.[77] Other excretory cells also found in planarians are solenocytes that are long and flagellated.
Plant cells
Other types of organelle specific to plant cells, are pigment-containing plastids, especially chloroplasts that contain chlorophyll, and large water-storing vacuoles.
Chloroplasts capture the sun's energy to make carbohydrates through photosynthesis.[78]Chromoplasts contain fat-soluble carotenoid pigments such as orange carotene and yellow xanthophylls which helps in synthesis and storage. Leucoplasts are non-pigmented plastids and helps in storage of nutrients.[79] Plastids divide by binary fission. Vacuoles in plant cells store water. They are liquid filled spaces surrounded by a membrane.[80]The vacuoles of plant cells are usually larger than those of animal cells.They are described as liquid filled spaces and are surrounded by a membrane.[80] Vacuoles of plant cells are surrounded by a membrane which transports ions against concentration gradients.[81]
Algal cells
Algae members are photoautotrophs able to use photosynthesis to produce energy. Photosynthesis is made possible by the use of plastids, organelles in the cytoplasm known as chloroplasts. Algal photoautotrophs include red algae.[82]
Fungal cells
The cells of fungi have in addition to the shared eukaryotic organelles a spitzenkörper in their endomembrane system, associated with hyphal tip growth. It is a phase-dark body that is composed of an aggregation of membrane-bound vesicles containing cell wall components, serving as a point of assemblage and release of such components intermediate between the Golgi and the cell membrane. The spitzenkörper is motile and generates new hyphal tip growth as it moves forward.[83]
Protist cells
The cells of protists may be bounded only by a cell membrane, or may in addition have a cell wall, or may be covered by a pellicle (in ciliates), a test (in testate amoebae), or a frustule (in diatoms).
Some protists such as amoebae may feed on other organisms and ingest food by phagocytosis. Vacuoles known as phagosomes in the cytoplasm may be used to draw in and incorporate the captured particles. Other types of protists are photoautotrophs, providing themselves with energy by photosynthesis.[84] Most protists are motile and generate movement with cilia, flagella, or pseudopodia.[84]
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Physiology
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Replication
During cell division, part of the cell cycle, a single cell, the mother cell divides into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.[85]
DNA replication, or the process of duplicating a cell's genome,[18] always happens when a cell divides through mitosis or binary fission.[85] This occurs during the S (synthesis) phase of the cell cycle.
In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II.[86] Replication, like all cellular activities, requires specialized proteins.[18]
DNA repair
All cells contain enzyme systems that scan for DNA damage and carry out repair. Diverse repair processes have evolved in all organisms. Repair is vital to maintain DNA integrity, avoid cell death and errors of replication that could lead to mutation. Repair processes include nucleotide excision repair, DNA mismatch repair, non-homologous end joining of double-strand breaks, recombinational repair and light-dependent repair (photoreactivation).[87]
Growth and metabolism
Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.[88]
Complex sugars can be broken down into simpler sugar molecules called monosaccharides such as glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP),[18] a molecule that possesses readily available energy, through two different pathways. In plant cells, chloroplasts create sugars by photosynthesis, using the energy of light to join molecules of water and carbon dioxide.[89]
Protein synthesis
Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.[64]
Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate into the cytoplasm. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome.[64] The new polypeptide then folds into a functional three-dimensional protein molecule.
Motility
Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia.[30]
In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.[90] The process is divided into three steps: protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.[91][90]
Navigation, control and communication
In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners.[92][93][94]
Death
Cell death is the event of a biological cell ceasing to carry out its functions. This may be the result of the natural process of old cells dying and being replaced by new ones, as in programmed cell death, or may result from factors such as diseases, localized injury, exposure to a toxic substance, or the death of the host organism. Apoptosis or Type I cell-death, and autophagy or Type II cell-death are both forms of programmed cell death, while necrosis is a non-physiological process that occurs as a result of infection or injury.[95][96]
Cell ancestry traces back in an unbroken lineage for over 3 billion years. The immortality of a cell lineage depends on the maintenance of cell division potential,[97] which may be lost because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death during development. Maintenance of division potential over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly DNA damage. Sexual processes provide an opportunity for effective repair of DNA damage in the germ line by homologous recombination.[97][98]
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Multicellularity
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Cell differentiation
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms.[99] Microorganisms cloned from a single cell can form visible microbial colonies. A microbial consortium of two or more species of microorganisms can form a biofilm community,[100] such as dental plaque. The cell-to-cell adhesion found in microbial colonies may have been the first evolutionary step toward more complex multicellular organisms.[101]
In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions.[102] In animals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others. Cell types differ both in appearance and function, yet are genetically identical. Cells are able to be of the same genotype but of different cell type due to the differential expression of the genes they contain.[103]
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 (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division).[104]
Signaling
Cell signaling is the process by which a cell interacts with itself, other cells, and the environment. Typically, the signaling process involves three components: the first messenger (the ligand), the receptor, and the signal itself.[105] Most cell signaling is chemical in nature, and can occur with neighboring cells or more distant targets. Signal receptors are complex proteins or tightly bound multimer of proteins, located in the plasma membrane or within the interior.[106]
Each cell is programmed to respond to specific extracellular signal molecules, and this process is the basis of development, tissue repair, immunity, and homeostasis. Individual cells are able to manage receptor sensitivity including turning them off, and receptors can become less sensitive when they are occupied for long durations.[106] Errors in signaling interactions may cause diseases such as cancer, autoimmunity, and diabetes.[107]
Origin of multicellularity
Multicellularity has evolved independently at least 25 times,[108] including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants.[109] It evolved repeatedly for plants (Chloroplastida), once or twice for animals, once for brown algae, and perhaps several times for fungi, slime molds, and red algae.[110] Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships.[111]
The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago.[108] Other early fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon.[112]
The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.[108]
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Origins
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The origin of cells has to do with the origin of life, which began the history of life on Earth.
Origin of life
Small molecules needed for life may have been carried to Earth on meteorites, created at deep-sea vents, or synthesized by lightning in a reducing atmosphere. There is little experimental data defining what the first self-replicating forms were. RNA may have been the earliest self-replicating molecule, as it can both store genetic information and catalyze chemical reactions.[113] This process required an enzyme to catalyze the RNA reactions, which may have been the early peptides that formed in hydrothermal vents.[114]
Cells emerged around 4 billion years ago.[115][116] The first cells were most likely heterotrophs. The early cell membranes were probably simpler and more permeable than modern ones, with only a single fatty acid chain per lipid. Lipids spontaneously form bilayered vesicles in water, and could have preceded RNA.[117][118]
First eukaryotes
Eukaryotic cells were created some 2.2 billion years ago in a process called eukaryogenesis. This is widely agreed to have involved symbiogenesis, in which archaea and bacteria came together to create the first eukaryotic common ancestor.[119] However, the sequence of the steps involved has been disputed.[citation needed] It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor, gaining capabilities along the way.[120][121]
This cell had a new level of complexity and capability, with a nucleus[122][120] and facultatively aerobic mitochondria.[119] It featured at least one centriole and cilium, sex (meiosis and syngamy), peroxisomes, and a dormant cyst with a cell wall of chitin and/or cellulose.[123][121] The last eukaryotic common ancestor gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.[124][125] The plants were created around 1.6 billion years ago with a second episode of symbiogenesis that added chloroplasts, derived from cyanobacteria.[119]
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History of research
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In 1665, Robert Hooke examined a thin slice of cork under his microscope, and saw a structure of small enclosures. He wrote "I could exceeding plainly perceive it to be all perforated and porous, much like a honeycomb, but that the pores of it were not regular".[126] To further support his theory, Matthias Schleiden and Theodor Schwann studied cells of both animal and plants. What they discovered were significant differences between the two types of cells. This put forth the idea that cells were fundamental to both plants and animals.[127]
- 1632–1723: Antonie van Leeuwenhoek taught himself to make lenses, constructed basic optical microscopes and drew protozoa, such as Vorticella from rain water, and bacteria from his own mouth.[128]
- 1665: Robert Hooke discovered cells in cork, then in living plant tissue using an early microscope. In his book Micrographia he coined the term cell (from Latin cellula, meaning "small room") since they resembled the cells of a monastery[129][130][131][132][128]
- 1839: Theodor Schwann[133] and Matthias Jakob Schleiden elucidated the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, founding the cell theory.[134][135]
- 1855: Rudolf Virchow stated that new cells come from pre-existing cells by cell division (omnis cellula ex cellula).
- 1931: Ernst Ruska built the first transmission electron microscope at the University of Berlin.[136] By 1935, he had built an electron microscope with twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing how eukaryotic cells were created by symbiogenesis.[137]
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See also
References
External links
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