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The cell is the basic structural and functional unit of all life or . 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 ) most cells are capable of Self-replication, and protein synthesis. Some types of cell are cell motility. Cells Abiogenesis about four billion years ago.
All organisms are grouped into , and . Prokaryotes are single-celled, and include archaea, and bacteria. Eukaryotes can be single-celled or multicellular, and include , , , most types of fungi, and some species of algae. All multicellular organisms are made up of many different Cell type. The that make up the body of a plant or animal are known as , and in animals excludes the haploid .
Prokaryotes lack a membrane-bound Cell nucleus (unlike eukaryotic cells) and have a nucleoid region 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 , in plants that create sugars by photosynthesis. Other non-membrane-bound organelles may be proteinaceous such as the 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 Monastic cell 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.
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
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 , 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, cell polarity and cytokinesis.
Compartmentalization is a key feature of eukaryotic cells but some species of bacteria, have protein-based organelle-like microcompartments such as , and , 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 Pilus, 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.
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.
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, , , , 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 secretion and endocytosis pathways, modifying, packaging, and transporting proteins and lipids to and from the trans-Golgi network. In mammalian cells, endocytosis includes Early endosome, late endosome, and recycling .
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 during cell division).
Eukaryotic cell types include those that make up , , fungi, algae, and . All of which have many different species and cell differences.
The ectoderm gives rise to several different types of epithelial tissues including the skin, and , and to the nervous tissue. Epithelium as mesothelium forms the lining of many organs, and Body cavity. Epithelial cells are joined together in sheets by way of ; adherens junctions, and bind the cells together, and bind the cells to the basement membrane. All three types are linked to the cell cytoskeleton. (2026). 9780815334811, Garland. ISBN 9780815334811
There are an estimated 200 different in the human body. The estimated cell count in a typical adult human body is around 30 trillion cells, 36 trillion in an adult male, and 28 trillion in a female.
Underlying, and attached to the cell membrane is the cell cortex, the outermost part of the actin cytoskeleton.
The cytosol is a gel-like substance made up of water, ions, and non-essential biomolecules, and is the main site of protein synthesis, and degradation. The acidity (pH) of the cytosol is near neutral, and is regulated by transporters in the cell membrane. Different proteins in the cytoplasm operate optimally at different pHs. (2026). 9780815344643, Garland science, Taylor and Francis group. ISBN 9780815344643 The cytosol forms of the cell's volume. (2026). 9783540787648, Springer Science & Business Media. ISBN 9783540787648
The DNA of each cell is its genetic material, and is organized in multiple linear molecules, called , that are coiled around histone and housed in the cell nucleus. 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.
Many animal cells are cilium and most cells except red blood cells have primary cilia. Primary cilia play important roles in Chemoreceptor and mechanosensation. 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." The cilia in other cells are motile organelles, and in the respiratory epithelium play an important role in the movement of mucus. In the reproductive system ciliated epithelium in the move the egg from the uterus to the ovary. Motile cilia also known as flagella, drive the sperm cells. Invertebrate have ciliated excretory . Other excretory cells also found in planarians are that are long and flagellated.
Plastids divide by binary fission. in plant cells store water, and are surrounded by a membrane. The vacuoles of plant cells are usually larger than those of animal cells. The vacuole membrane transports ions against concentration gradients.
The plant cytoskeleton is a dynamic structure that has a scaffold of microtubules and microfilaments, but not the intermediate filaments. The microtubule organizing center in plant cells is often sited underneath the cell membrane where nucleated microtubules often form sheet-like semi-parallel arrays.
There are two types of peroxisomes in plants. One type is in the leaves where it takes part in photorespiration. The other type is in germinating seeds where they take part in the conversion of fatty acids into sugars for the plant's growth. In this peroxisome type the enzymatic content is so different than in other groups that it has an alternative name of glyoxysome, their enzymes are of the glyoxylate cycle.
Alginate is a polysaccharide found in the matrix of the cell walls of brown algae, and has many important uses in the food industry, and in pharmacology.
The cell walls of fungi are uniquely made of a chitin-glucan complex.
Some protists such as amoebae may heterotroph and ingest food by phagocytosis. Vacuoles known as in the cytoplasm may be used to draw in and incorporate the captured particles. Other types of protists are , providing themselves with energy by photosynthesis. Most single-celled protists are motile, and generate movement with cilia, flagella, or pseudopodia.
have two different sorts of cell nucleus: a tiny, diploid micronucleus (the "generative nucleus", which carries the germline of the cell), and a large, ampliploid macronucleus (the "vegetative nucleus", which takes care of general cell regulation.
DNA replication, or the process of duplicating a cell's genome, always happens when a cell divides through mitosis or binary fission. This occurs during the S 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. Replication, like all cellular activities, requires specialized proteins.
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. Errors in signaling interactions may cause diseases such as cancer, autoimmunity, and diabetes.
Proteins can be targeted to the inner space of an organelle, different intracellular membranes, the cell membrane, or to the exterior of the cell via secretion. Information contained in the protein itself directs this delivery process. Correct sorting is crucial for the cell; errors or dysfunction in sorting have been linked to multiple diseases.
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), a molecule that possesses readily available energy, through two different pathways. In plant cells, create sugars by photosynthesis, using the energy of light to join molecules of water and carbon dioxide.
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 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. The new polypeptide chain then folds into a functional three-dimensional protein molecule.
A separate mode of cellular death is known as a mitotic catastrophe, which occurs during mitosis, following the improper progression of, or entrance to the cell cycle. This mechanism operates to prevent genomic instability. Other cell death pathways are described, and include anoikis, pyroptosis, mitoptosis, parthanatos, and necroptosis.
Cells emerged around 4 billion years ago. The first cells were most likely . The early cell membranes were probably simpler and more permeable than later ones, with only a single fatty acid chain per lipid. Lipids spontaneously form bilayered vesicles in water, and could have preceded RNA.
Eukaryote cells were created some 2.2 billion years ago in a process called eukaryogenesis. This is widely agreed to have involved symbiogenesis, in which an and a bacterium came together to create the first eukaryotic common ancestor. It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor, gaining capabilities along the way.
This cell had a new level of complexity, with a nucleus and facultatively aerobic Mitochondrion.
The first evidence of multicellularity in an organism comes from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago. Cyanobacteria are variable in morphology, filamentous forms exhibit functional cell differentiation such as (for nitrogen fixation), akinetes (resting stage cells), and hormogonia (reproductive, motile filaments). These, together with the intercellular connections they possess, are considered the first signs of multicellularity.
Multicellularity was made possible by the development of the extracellular matrix (ECM) similar in function to the bacterial EPS that consists of extracellular polymeric substances. EPS enables microbial cell adhesion, and is believed to be the first evolutionary step toward multicellular organisms. Basement membranes are a type of specialized extracellular matrix that surrounds most , and are essential in their formation. Extracellular matrix components of laminin domains, integrated with other proteins such as have been described in single-celled motile that pre-dates the evolutionary emergence of basement membranes, one of the two types of ECM. The emergence of the basement membrane coincided with the origin of multicellularity. The other type of ECM is the Interstitium.
The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.
Motility
Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia, and the projection of pseudopodia in amoeboid movement. Cells in multicellular organisms can move during processes such as wound healing, the immune response, and cancer metastasis. In wound healing in animals, white blood cells move to the wound site to kill the causing infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. 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. (2026). 9780815340720, Garland Science. ISBN 9780815340720
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 Chemotaxis efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused which enable them to sense upcoming maze junctions before reaching them, including around corners.
Cell death
Cell death occurs when a cell ceases to carry out its functions, as a result of ageing, or types of cell injury (necrosis). Programmed cell death, including apoptosis, and autophagy is a natural process of replacing dead cells with new ones.
Origins
The origin of cells has to do with the Abiogenesis, which began the history of life on Earth. 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. This process required an enzyme to Catalysis the RNA reactions, which may have been the early that formed in hydrothermal vents.
(2026). 9780521761314, Cambridge University Press. ISBN 9780521761314 It featured at least one centriole and cilium, sex (meiosis and syngamy), , and a dormant cyst with a cell wall of chitin and/or cellulose. The last eukaryotic common ancestor gave rise to the eukaryotes' crown group, containing the ancestors of , Fungus, , and a diverse range of single-celled organisms. The were created around 1.6 billion years ago with a second episode of symbiogenesis that added , derived from cyanobacteria.
Multicellularity
Multicellular behavior is demonstrated by that are cloned from a single cell and form visible microbial colonies. A microbial consortium of two or more species can form a biofilm by the secretion of extracellular polymeric substances (EPSs). consist of different groups of microorganisms grouped together in a multicellular-like fashion.
History of research
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". 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.
(1980). 9783777603643 ISBN 9783777603643 By 1935, he had built an electron microscope with twice the resolution of a light microscope, revealing previously unresolvable organelles.
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