Cell (biology)

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Cell
Wilson1900Fig2.jpg
Onion (Allium cepa) root cells in different phases of the cell cycle (drawn by E. B. Wilson, 1900)
Celltypes.svg
A eukaryotic cell (left) and prokaryotic cell (right)
Identifiers
MeSH D002477
TH H1.00.01.0.00001
FMA 686465
Anatomical terminology

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 consists of cytoplasm enclosed within a membrane. 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.

Contents

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. The studies of cell biology (cytology), microbiology and its branches, and molecular biology are all combined in the field of cellular microbiology.

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. [1] Prokaryotes are single-celled organisms, whereas eukaryotes can be either single-celled or multicellular. [2] Animals, plants, most fungi, and some algae species are multicellular organisms. [3] [4]

Multicellular organisms are made up of many different types of cell known overall as somatic cells. [5] 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 muscle tissue having different muscle cells. [1] The number of cells in these groups vary with species. Studies on the human have estimated a total body count at around 30 trillion cells (~36 trillion cells in the male, and ~28 trillion in the female). [6] [7]

Prokaryotes

Structure of a typical bacterial cell, generally similar with the archaeal cell structure. The bacterial flagellum shown, differs from the archaellum in archaea Prokaryote cell.svg
Structure of a typical bacterial cell, generally similar with the archaeal cell structure. The bacterial flagellum shown, differs from the archaellum in archaea

All prokaryotes are single-celled and include bacteria and archaea, two of the three domains of life. [8] Prokaryotic cells were likely the first form of life on Earth, [9] [10] 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. [11]

Bacteria

Bacteria are enclosed in a cell envelope, that protects the interior from the exterior. [12] It generally consists of a plasma membrane covered by a cell wall which, for some bacteria, is covered by a third layer called a bacterial capsule. An exception to this is found in Mycoplasma that only possess the cell membrane. [13] The cell envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan and acts as an additional barrier against exterior forces. [14] It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. [15]

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. [16] [17] [18] The cytoplasm also contains ribosomes and various inclusions, [19] including extrachromosomal DNA as plasmids, which 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]

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, [22] [23] [24] and certain membrane-bound prokaryotic organelles have been discovered. They include the magnetosome of magnetotactic bacteria, [22] and the anammoxosome of anammox bacteria. [25] [26]

Cell-surface appendages can include bacterial flagella, and pili, protein structures that facilitate movement and communication between cells. [27]

Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter. [28] The largest bacterium known, Thiomargarita magnifica , is visible to the naked eye with a length of 1 to 2 cm. [29]

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. [13] Archaeal cells have unique properties separating them from bacteria and eukaryota, including: cell membranes made of ether-linked lipids; metabolisms such as methanogenesis; and a unique motility structure known as an archaellum. [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 in the cytoplasm.[ citation needed ] 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. [31]

Eukaryotes

Animal cell NIH.jpg
Structure of a typical animal cell
Plant cell structure-en.svg
Structure of a typical plant cell

Plants, animals, fungi, [32] slime moulds, [33] protozoa [34] and algae [35] are all eukaryotes. The main distinguishing feature of a eukaryotic cell is the presence of a cell nucleus, a membrane-bound organelle which houses most of the organism's genome, and coordinates processes such as protein biosynthesis, and cell division. [36] [37] Mitochondria contain some mitochondrial DNA. [38] Eukaryotic DNA is organized in multiple linear molecules, called chromosomes, that are coiled around histone proteins. [37] [39] The nucleus gives the eukaryote its name, which means "true nut" or "true kernel", where "nut" means the nucleus. [40] These cells can be 2 to 1000 times larger in diameter than a typical prokaryote. [41]

Another defining eukaryotic feature is the presence of other membrane-bound organelles sometimes called cellular compartments, in which specific activities take place. Some of these compartments, such as mitochondria, were likely acquired from symbiotic interaction with prokaryotes. [42]

Most eukaryotic cells are ciliated with primary cilia. [43] Primary cilia play important roles in chemosensation and mechanosensation. [44] 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." [45] Eukaryotic flagella are more complex than those of prokaryotes. [46]

Comparison of features of prokaryotic and eukaryotic cells
Prokaryotes Eukaryotes
Typical organisms bacteria, archaea protists, algae, fungi, plants, animals
Typical size~ 1–5  μm [11] ~ 10–100 μm [11]
DNA In nucleoid region In nucleus with double membrane [47]
Chromosomes Single, usually circular, in some prokaryotes multiple and/or linear Multiple paired linear chromosomes with histone proteins
RNA/protein synthesiscoupled in the cytoplasm [48] RNA synthesis in the nucleus
protein synthesis in the cytoplasm
Ribosomes 50S and 30S [49] 60S and 40S [49]
Cytoplasmic structureMicrocompartments with proteins, cytoskeleton Endomembrane system, cytoskeleton
Cell movement flagella flagella and cilia; lamellipodia and filopodia
Mitochondria none [50] Zero [50] to several thousand [51]
Chloroplasts nonein algae and plants
Organizationsingle cells, colonies, biofilms single cells, colonies, multicellular organisms with specialized cells
Cell division binary fission (simple division) mitosis (fission or budding)
meiosis
Membranes cell membrane Cell membrane and membrane-bound organelles

Subcellular components

All cells, whether prokaryotic or eukaryotic, have a cell membrane, cytoplasm, ribosomes, and DNA. [52] The defining difference between the two classes is the presence of a membrane-bound nucleus housing the DNA (and other membrane-bound organelles) in the eukaryotic cell; in the prokaryotic cell the DNA is not membrane-bound and other membrane-bound organelles are not typically present. [52] The exception is the eukaryotic red blood cell that does not have a nucleus or other organelle. [53]

Membrane

Diagram of the cell membrane detailing the lipid bilayer Cell membrane detailed diagram en.svg
Diagram of the cell membrane detailing the lipid bilayer

The cell membrane, or plasma membrane, is a selectively permeable biological membrane that surrounds the cytoplasm of all cells. [54] In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This 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. [55] Embedded within this 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. [19] 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. [56] Cell surface receptors embedded in the membrane allow cells to detect external signaling molecules such as hormones. [57]

Cytoskeleton

A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria are stained red, and microfilaments are stained green. DAPIMitoTrackerRedAlexaFluor488BPAE.jpg
A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria are stained red, and microfilaments are stained green.

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 eukaryotic cytoskeleton is composed of microtubules, intermediate filaments and microfilaments. In the cytoskeleton of 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. [19]

The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis. [58]

Genetic material

Deoxyribonucleic acid (DNA) DNA orbit animated.gif
Deoxyribonucleic acid (DNA)

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is encoded in its DNA sequence. [19] 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. [59]

Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, [19] linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory). [60]

A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans, the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, [19] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs. [61]

Foreign genetic material (most commonly DNA) can be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses can insert their genetic material into the genome via transformation. [62]

Organelles

Organelles are parts of the cell that are adapted and/or 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). [19] Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound. [63]

There are several types of organelles in a cell. Some (such as the nucleus and Golgi apparatus) are typically solitary,[ citation needed ] while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). Most organelles vary in size and/or number based on the growth of the host cell. [64] The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles, forming 30%–50% of a cell volume. [65]

Eukaryotic

Human cancer cells, specifically HeLa cells, with DNA stained blue. The central and rightmost cell are in interphase, so their DNA is diffuse and the entire nuclei are labelled. The cell on the left is going through mitosis and its chromosomes have condensed. HeLa cells stained with Hoechst 33258.jpg
Human cancer cells, specifically HeLa cells, with DNA stained blue. The central and rightmost cell are in interphase, so their DNA is diffuse and the entire nuclei are labelled. The cell on the left is going through mitosis and its chromosomes have condensed.
  • Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. 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. In prokaryotes, DNA processing takes place in the cytoplasm. [19]
  • Mitochondria and chloroplasts: 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 all eukaryotic cells. [19] 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). [66] Mitochondria and chloroplasts multiply by binary fission, like prokaryotes. [67] Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis. [68]
Diagram of the endomembrane system Endomembrane system diagram en.svg
Diagram of the endomembrane system
  • 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 ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. [19] The smooth ER plays a role in calcium sequestration and release, and helps in synthesis of lipid. [69]
  • Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell. It is typically organized as a stack of plate-like structures known as cisternae. [70]
  • Lysosomes and peroxisomes: Lysosomes contain digestive 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. [19] [71]
  • Centrosome: the cytoskeleton organizer: The centrosome produces the microtubules of a cell—a key component of the cytoskeleton. [72] It directs the transport through the ER and the Golgi apparatus. [73] 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. A single centrosome is present in the animal cells. [72] They are also found in some fungi and algae cells. [74]
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are described as liquid filled spaces and are surrounded by a membrane. [75] Some cells, most notably Amoeba , have contractile vacuoles, which can pump water out of the cell if there is too much water. [76] The vacuoles of plant cells and fungal cells are usually larger than those of animal cells. [75] Vacuoles of plant cells are surrounded by a membrane which transports ions against concentration gradients. [77]

Eukaryotic and prokaryotic

  • Ribosomes: The ribosome is a large complex of RNA and protein molecules. [19] They each consist of two subunits, 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 (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes). [78]
  • Plastids: Plastid are membrane-bound organelle generally found in plant cells and euglenoids and contain specific pigments, thus affecting the colour of the plant and organism. These pigments helps in food storage and tapping of light energy. There are three types of plastids based upon the specific pigments. Chloroplasts contain chlorophyll and some carotenoid pigments which helps in the tapping of light energy during photosynthesis. Chromoplasts contain fat-soluble carotenoid pigments like orange carotene and yellow xanthophylls which helps in synthesis and storage. Leucoplasts are non-pigmented plastids and helps in storage of nutrients. [79]

Structures outside the membrane

Many cells have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Wall

Many types of prokaryotic and eukaryotic cells have a carbohydrate-based cell wall. 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. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of cellulose, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan. [80]

Prokaryotes

Capsule

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Capsules are not marked by normal staining protocols and can be detected by India ink or methyl blue, which allows for higher contrast between the cells for observation. [81]

Flagella

Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. [82] Separate varieties of flagellum are found in archaea and in eukaryotes, having independently evolved their own structure, composition, and propulsion mechanism. [83]

Fimbriae

A fimbria (plural fimbriae also known as a pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called pilin (antigenic) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesion). There are special types of pili involved in bacterial conjugation. [84]

Physiology

Prokaryotes divide by binary fission, while eukaryotes divide by mitosis or meiosis. Three cell growth types.svg
Prokaryotes divide by binary fission, while eukaryotes divide by mitosis or meiosis.

Replication

Cell division is the process that involves a single cell's (called a mother cell) division 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, [19] 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. [19]

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), [19] 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. [59]

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. [59] 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. [83]

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]

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]

Multicellularity

Specialization/differentiation

Staining of a nematode Caenorhabditis elegans highlights the nuclei of its cells. C elegans stained.jpg
Staining of a nematode Caenorhabditis elegans highlights the nuclei of its cells.

Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. [95] 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, [96] 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. [97]

In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions. [98] 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. [99]

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). [100]

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. [101] 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. [102]

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. [102] Errors in signaling interactions may cause diseases such as cancer, autoimmunity, and diabetes. [103]

Origin of multicellularity

Multicellularity has evolved independently at least 25 times, [104] 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. [105] 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. [106] Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships. [107]

The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago. [104] 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. [108]

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure. [104]

Clinical significance

Cytopathology studies and diagnoses diseases on the cellular level. Cytopathology is generally used on samples of free cells or tissue fragments, in contrast to the pathology branch of histopathology, which studies whole tissues. Cytopathology is commonly used to investigate diseases involving a wide range of body sites, often to aid in the diagnosis of cancer, and in the diagnosis of some infectious diseases and other inflammatory conditions. For example, a common application of cytopathology is the Pap smear, a screening test used to detect cervical cancer, and precancerous cervical lesions that may lead to cervical cancer. [109]

Origins

The origin of cells has to do with the origin of life, which began the history of life on Earth.

Origin of life

Stromatolites are left behind by cyanobacteria, known as blue-green algae. They are among the oldest fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park in the United States. Stromatolites.jpg
Stromatolites are left behind by cyanobacteria, known as blue-green algae. They are among the oldest fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park in the United States.

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. [110] This process required an enzyme to catalyze the RNA reactions, which may have been the early peptides that formed in hydrothermal vents. [111]

Cells emerged around 4 billion years ago. [112] [113] 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. [114] [115]

First eukaryotes

In the theory of symbiogenesis, a merger of an archaean and an aerobic bacterium created the eukaryotes, with aerobic mitochondria, some 2.2 billion years ago. A second merger, 1.6 billion years ago, added chloroplasts, creating the green plants. Symbiogenesis 2 mergers.svg
In the theory of symbiogenesis, a merger of an archaean and an aerobic bacterium created the eukaryotes, with aerobic mitochondria, some 2.2 billion years ago. A second merger, 1.6 billion years ago, added chloroplasts, creating the green plants.

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. [116] 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. [117] [118]

This cell had a new level of complexity and capability, with a nucleus [119] [117] and facultatively aerobic mitochondria. [116] 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. [120] [118] 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. [121] [122] The plants were created around 1.6 billion years ago with a second episode of symbiogenesis that added chloroplasts, derived from cyanobacteria. [116]

History of research

Robert Hooke's drawing of cells in cork, 1665 RobertHookeMicrographia1665.jpg
Robert Hooke's drawing of cells in cork, 1665

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". [123] 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. [124]

Research methods

The studies of cell biology (cytology) is combined with microbiology and molecular biology in the field of cellular microbiology. [135] Research in cell biology is interconnected to other fields including genetics, molecular genetics, medical microbiology, immunology, and cytochemistry. Research methods in all these fields continue to be developed and include:

See also

References

  1. 1 2 Cooper, Geoffrey M. (2000). "The Origin and Evolution of Cells". The Cell: A Molecular Approach. 2nd edition. Sinauer Associates. Retrieved 17 September 2025.
  2. ↑ "Prokaryote structure". khanacademy. Retrieved 2025-08-16.
  3. ↑ Knoll, Andrew H. (2011). "The Multiple Origins of Complex Multicellularity". Annual Review of Earth and Planetary Sciences. 39: 217–239. Bibcode:2011AREPS..39..217K. doi:10.1146/annurev.earth.031208.100209.
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