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151-466: Cell biology (also cellular biology or cytology ) is a branch of biology that studies the structure , function , and behavior of cells . All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and functioning of organisms. Cell biology is the study of the structural and functional units of cells. Cell biology encompasses both prokaryotic and eukaryotic cells and has many subtopics which may include

302-535: A last universal common ancestor that lived about 3.5 billion years ago . Geologists have developed a geologic time scale that divides the history of the Earth into major divisions, starting with four eons ( Hadean , Archean , Proterozoic , and Phanerozoic ), the first three of which are collectively known as the Precambrian , which lasted approximately 4 billion years. Each eon can be divided into eras, with

453-440: A lipid bilayer , including cholesterols that sit between phospholipids to maintain their fluidity at various temperatures. Cell membranes are semipermeable , allowing small molecules such as oxygen, carbon dioxide, and water to pass through while restricting the movement of larger molecules and charged particles such as ions . Cell membranes also contain membrane proteins , including integral membrane proteins that go across

604-443: A nucleus , and prokaryotic cells, which do not. Prokaryotes are single-celled organisms such as bacteria , whereas eukaryotes can be single-celled or multicellular. In multicellular organisms , every cell in the organism's body is derived ultimately from a single cell in a fertilized egg . Every cell is enclosed within a cell membrane that separates its cytoplasm from the extracellular space . A cell membrane consists of

755-508: A number of shapes , ranging from spheres to rods and spirals . Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats . Bacteria inhabit soil, water, acidic hot springs , radioactive waste , and the deep biosphere of the Earth's crust . Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of

906-418: A better understanding of the structure and function of cells. Many techniques commonly used to study cell biology are listed below: There are two fundamental classifications of cells: prokaryotic and eukaryotic . Prokaryotic cells are distinguished from eukaryotic cells by the absence of a cell nucleus or other membrane-bound organelle . Prokaryotic cells are much smaller than eukaryotic cells, making them

1057-628: A broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of at least one cell that processes hereditary information encoded in genes , which can be transmitted to future generations. Another major theme is evolution , which explains the unity and diversity of life. Energy processing is also important to life as it allows organisms to move , grow, and reproduce . Finally, all organisms are able to regulate their own internal environments . Biologists are able to study life at multiple levels of organization , from

1208-431: A cell binds a molecule that is attached to the membrane of another cell. Endocrine signaling occurs through molecules secreted into the bloodstream. Paracrine signaling uses molecules diffusing between two cells to communicate. Autocrine is a cell sending a signal to itself by secreting a molecule that binds to a receptor on its surface. Forms of communication can be through: Cells are the foundation of all organisms and are

1359-461: A cell lineage depends on the maintenance of cell division potential. This potential may be lost in any particular lineage because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death ( apoptosis ) during development. Maintenance of cell division potential over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly DNA damage . In sexual organisms, continuity of

1510-474: A cell's size, shape, membrane potential , metabolic activity , and responsiveness to signals, which are largely due to highly controlled modifications in gene expression and epigenetics . With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Thus, different cells can have very different physical characteristics despite having the same genome . Morphogenesis, or

1661-408: A chemical (e.g., nitrous acid , benzopyrene ) or radiation (e.g., x-ray , gamma ray , ultraviolet radiation , particles emitted by unstable isotopes). Mutations can lead to phenotypic effects such as loss-of-function, gain-of-function , and conditional mutations. Some mutations are beneficial, as they are a source of genetic variation for evolution. Others are harmful if they were to result in

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1812-423: A different part of the cell membrane. When the cell begins to pull apart, the replicated and original chromosomes are separated. The consequence of this asexual method of reproduction is that all the cells are genetically identical, meaning that they have the same genetic material (barring random mutations ). Unlike the processes of mitosis and meiosis used by eukaryotic cells, binary fission takes place without

1963-1233: A few archaea have very different shapes, such as the flat and square cells of Haloquadratum walsbyi . Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably for the enzymes involved in transcription and translation . Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes , including archaeols . Archaea use more energy sources than eukaryotes: these range from organic compounds , such as sugars, to ammonia , metal ions or even hydrogen gas . Salt-tolerant archaea (the Haloarchaea ) use sunlight as an energy source, and other species of archaea fix carbon , but unlike plants and cyanobacteria , no known species of archaea does both. Archaea reproduce asexually by binary fission , fragmentation , or budding ; unlike bacteria, no known species of Archaea form endospores . The first observed archaea were extremophiles , living in extreme environments, such as hot springs and salt lakes with no other organisms. Improved molecular detection tools led to

2114-403: A final electron acceptor, which is usually the oxidized form of NADP , which is reduced to NADPH, a process that takes place in a protein complex called photosystem I (PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in

2265-400: A hollow sphere of cells , the blastula , during embryonic development . Over 1.5 million living animal species have been described —of which around 1 million are insects —but it has been estimated there are over 7 million animal species in total. They have complex interactions with each other and their environments, forming intricate food webs . Binary fission Organisms in

2416-769: A loss of function of genes needed for survival. Gene expression is the molecular process by which a genotype encoded in DNA gives rise to an observable phenotype in the proteins of an organism's body. This process is summarized by the central dogma of molecular biology , which was formulated by Francis Crick in 1958. According to the Central Dogma, genetic information flows from DNA to RNA to protein. There are two gene expression processes: transcription (DNA to RNA) and translation (RNA to protein). The regulation of gene expression by environmental factors and during different stages of development can occur at each step of

2567-417: A more successful evolutionary theory based on natural selection ; similar reasoning and evidence led Alfred Russel Wallace to independently reach the same conclusions. The basis for modern genetics began with the work of Gregor Mendel in 1865. This outlined the principles of biological inheritance. However, the significance of his work was not realized until the early 20th century when evolution became

2718-457: A new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. Homologous chromosomes are separated in the first division ( meiosis I ), and sister chromatids are separated in the second division ( meiosis II ). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in

2869-511: A potential strategy for the prevention and treatment of various disorders. Many of these disorders are prevented or improved by consuming polyphenol in the meal. As a result, natural compounds with the ability to modify the autophagy mechanism are seen as a potential therapeutic option. The creation of the double membrane (phagophore), which would be known as nucleation, is the first step in macro-autophagy. The phagophore approach indicates dysregulated polypeptides or defective organelles that come from

3020-409: A process known as allopatric speciation . A phylogeny is an evolutionary history of a specific group of organisms or their genes. It can be represented using a phylogenetic tree , a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a lineage of descendants of a particular species or population. When a lineage divides into two, it

3171-408: A region of deoxyribonucleic acid (DNA) that carries genetic information that controls form or function of an organism. DNA is composed of two polynucleotide chains that coil around each other to form a double helix . It is found as linear chromosomes in eukaryotes , and circular chromosomes in prokaryotes . The set of chromosomes in a cell is collectively known as its genome . In eukaryotes, DNA

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3322-541: A role in the synthesis and packaging of proteins, respectively. Biomolecules such as proteins can be engulfed by lysosomes , another specialized organelle. Plant cells have additional organelles that distinguish them from animal cells such as a cell wall that provides support for the plant cell, chloroplasts that harvest sunlight energy to produce sugar, and vacuoles that provide storage and structural support as well as being involved in reproduction and breakdown of plant seeds. Eukaryotic cells also have cytoskeleton that

3473-436: A separate clade as some protists may be more closely related to plants, fungi, or animals than they are to other protists. Like groupings such as algae , invertebrates , or protozoans , the protist grouping is not a formal taxonomic group but is used for convenience. Most protists are unicellular; these are called microbial eukaryotes. Plants are mainly multicellular organisms , predominantly photosynthetic eukaryotes of

3624-423: A single carbon atom can form four single covalent bonds such as in methane , two double covalent bonds such as in carbon dioxide (CO 2 ), or a triple covalent bond such as in carbon monoxide (CO). Moreover, carbon can form very long chains of interconnecting carbon–carbon bonds such as octane or ring-like structures such as glucose . The simplest form of an organic molecule is the hydrocarbon , which

3775-512: A single parent cell. Isolates of the human parasite Blastocystis hominis were observed to begin such a process within 4 to 6 days. Cells of the fish parasite Trypanosoma borreli have also been observed participating in both binary and multiple fission. In the apicomplexans , a phylum of parasitic protists, multiple fission, or schizogony , is manifested either as merogony , sporogony , or gametogony . Merogony results in merozoites , which are multiple daughter cells that originate within

3926-446: A specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts —they allow a reaction to proceed more rapidly without being consumed by it—by reducing the amount of activation energy needed to convert reactants into products . Enzymes also allow

4077-399: A unified theory as the modern synthesis reconciled Darwinian evolution with classical genetics . In the 1940s and early 1950s, a series of experiments by Alfred Hershey and Martha Chase pointed to DNA as the component of chromosomes that held the trait-carrying units that had become known as genes . A focus on new kinds of model organisms such as viruses and bacteria, along with

4228-421: A varied mix of traits, and reproduction is able to increase any population, Darwin argued that in the natural world, it was nature that played the role of humans in selecting for specific traits. Darwin inferred that individuals who possessed heritable traits better adapted to their environments are more likely to survive and produce more offspring than other individuals. He further inferred that this would lead to

4379-492: A water molecule again. In pure water , the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in a pH that is neutral. Organic compounds are molecules that contain carbon bonded to another element such as hydrogen. With the exception of water, nearly all the molecules that make up each organism contain carbon. Carbon can form covalent bonds with up to four other atoms, enabling it to form diverse, large, and complex molecules. For example,

4530-430: A σ protein that assists only with initiation. For instance, in a process termed conjugation , the fertility factor allows the bacteria to possess a pilus which allows it to transmit DNA to another bacteria which lacks the F factor, permitting the transmittance of resistance allowing it to survive in certain environments. Eukaryotic cells are composed of the following organelles: Eukaryotic cells may also be composed of

4681-444: Is a central feature of sexual reproduction in eukaryotes, and the most fundamental function of meiosis appears to be conservation of the integrity of the genome that is passed on to progeny by parents. Two aspects of sexual reproduction , meiotic recombination and outcrossing , are likely maintained respectively by the adaptive advantages of recombinational repair of genomic DNA damage and genetic complementation which masks

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4832-427: Is a form of asexual reproduction or cloning, where an organism is split into fragments. Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. In echinoderms , this method of reproduction is usually known as fissiparity . Any splitting of a single population of individuals into discrete parts may be considered fission. A population may undergo fission process for

4983-641: Is a large family of organic compounds that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other elements such as oxygen (O), hydrogen (H), phosphorus (P), and sulfur (S), which can change the chemical behavior of that compound. Groups of atoms that contain these elements (O-, H-, P-, and S-) and are bonded to a central carbon atom or skeleton are called functional groups . There are six prominent functional groups that can be found in organisms: amino group , carboxyl group , carbonyl group , hydroxyl group , phosphate group , and sulfhydryl group . In 1953,

5134-721: Is a self-degradative mechanism that regulates energy sources during growth and reaction to dietary stress. Autophagy also cleans up after itself, clearing aggregated proteins, cleaning damaged structures including mitochondria and endoplasmic reticulum and eradicating intracellular infections. Additionally, autophagy has antiviral and antibacterial roles within the cell, and it is involved at the beginning of distinctive and adaptive immune responses to viral and bacterial contamination. Some viruses include virulence proteins that prevent autophagy, while others utilize autophagy elements for intracellular development or cellular splitting. Macro autophagy, micro autophagy, and chaperon-mediated autophagy are

5285-480: Is a small polar molecule with a bent shape formed by the polar covalent bonds of two hydrogen (H) atoms to one oxygen (O) atom (H 2 O). Because the O–H bonds are polar, the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge. This polar property of water allows it to attract other water molecules via hydrogen bonds, which makes water cohesive . Surface tension results from

5436-918: Is a term of convenience as not all algae are closely related. Algae comprise several distinct clades such as glaucophytes , which are microscopic freshwater algae that may have resembled in form to the early unicellular ancestor of Plantae. Unlike glaucophytes, the other algal clades such as red and green algae are multicellular. Green algae comprise three major clades: chlorophytes , coleochaetophytes , and stoneworts . Fungi are eukaryotes that digest foods outside their bodies, secreting digestive enzymes that break down large food molecules before absorbing them through their cell membranes. Many fungi are also saprobes , feeding on dead organic matter, making them important decomposers in ecological systems. Animals are multicellular eukaryotes. With few exceptions, animals consume organic material , breathe oxygen , are able to move , can reproduce sexually , and grow from

5587-525: Is also known as the S-phase. During mitosis, which is also known as the M-phase, the segregation of the chromosomes occur. DNA, like every other molecule, is capable of undergoing a wide range of chemical reactions. Modifications in DNA's sequence, on the other hand, have a considerably bigger impact than modifications in other cellular constituents like RNAs or proteins because DNA acts as a permanent copy of

5738-403: Is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation . The ATP generated in this process

5889-519: Is controlled by the temporal activation of Cdks, which is governed by cyclin partner interaction, phosphorylation by particular protein kinases, and de-phosphorylation by Cdc25 family phosphatases. In response to DNA damage, a cell's DNA repair reaction is a cascade of signaling pathways that leads to checkpoint engagement, regulates, the repairing mechanism in DNA, cell cycle alterations, and apoptosis. Numerous biochemical structures, as well as processes that detect damage in DNA, are ATM and ATR, which induce

6040-424: 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 but also in the diagnosis of some infectious diseases and other inflammatory conditions. For example, a common application of cytopathology

6191-463: Is known about how bacteria that naturally don't grow a cell wall divide, but it is thought to resemble the L-form's budding -like division process of extrusion and separation. Binary fission is generally rapid, though its speed varies between species. For E. coli , cells typically divide about every 20 minutes at 37 °C. Because the new cells will, in turn, undergo binary fission on their own,

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6342-441: Is made by substrate-level phosphorylation , which does not require oxygen. Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organism's metabolic activities via cellular respiration. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water. In most cases, oxygen

6493-535: Is made up of microtubules , intermediate filaments , and microfilaments , all of which provide support for the cell and are involved in the movement of the cell and its organelles. In terms of their structural composition, the microtubules are made up of tubulin (e.g., α-tubulin and β-tubulin ) whereas intermediate filaments are made up of fibrous proteins. Microfilaments are made up of actin molecules that interact with other strands of proteins. All cells require energy to sustain cellular processes. Metabolism

6644-526: Is mainly in the cell nucleus . In prokaryotes, the DNA is held within the nucleoid . The genetic information is held within genes, and the complete assemblage in an organism is called its genotype . DNA replication is a semiconservative process whereby each strand serves as a template for a new strand of DNA. Mutations are heritable changes in DNA. They can arise spontaneously as a result of replication errors that were not corrected by proofreading or can be induced by an environmental mutagen such as

6795-399: Is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox reactions. Although cellular respiration is technically a combustion reaction , it clearly does not resemble one when it occurs in a cell because of the slow, controlled release of energy from the series of reactions. Sugar in

6946-407: Is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered by FtsZ polymerization and "Z-ring" formation). The new cell wall ( septum ) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, ribosomes , and plasmids . Meiosis

7097-399: Is released as a waste product. Most plants, algae , and cyanobacteria perform photosynthesis, which is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth. Photosynthesis has four stages: Light absorption , electron transport, ATP synthesis, and carbon fixation . Light absorption is

7248-412: Is represented as a fork or split on the phylogenetic tree. Phylogenetic trees are the basis for comparing and grouping different species. Different species that share a feature inherited from a common ancestor are described as having homologous features (or synapomorphy ). Phylogeny provides the basis of biological classification. This classification system is rank-based, with the highest rank being

7399-399: Is simply the exact opposite of respiration as it ultimately produces molecules of glucose. Cell signaling or cell communication is important for cell regulation and for cells to process information from the environment and respond accordingly. Signaling can occur through direct cell contact or endocrine , paracrine , and autocrine signaling . Direct cell-cell contact is when a receptor on

7550-547: Is strongly linked to the cell mitochondrial channel's ongoing reconfiguration through a range of mechanisms known as mitochondrial membrane dynamics, including endomembrane fusion and fragmentation (separation) and ultrastructural membrane remodeling. As a result, mitochondrial dynamics regulate and frequently choreograph not only metabolic but also complicated cell signaling processes such as cell pluripotent stem cells, proliferation, maturation, aging, and mortality. Mutually, post-translational alterations of mitochondrial apparatus and

7701-711: Is the Pap smear , a screening test used to detect cervical cancer , and precancerous cervical lesions that may lead to cervical cancer. The cell cycle is composed of a number of well-ordered, consecutive stages that result in cellular division. The fact that cells do not begin the next stage until the last one is finished, is a significant element of cell cycle regulation. Cell cycle checkpoints are characteristics that constitute an excellent monitoring strategy for accurate cell cycle and divisions. Cdks, associated cyclin counterparts, protein kinases, and phosphatases regulate cell growth and division from one stage to another. The cell cycle

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7852-514: Is the cell growth phase – makes up approximately 95% of the cycle. The proliferation of cells is instigated by progenitors. All cells start out in an identical form and can essentially become any type of cells. Cell signaling such as induction can influence nearby cells to determinate the type of cell it will become. Moreover, this allows cells of the same type to aggregate and form tissues, then organs, and ultimately systems. The G1, G2, and S phase (DNA replication, damage and repair) are considered to be

8003-468: Is the process by which genes and traits are passed on from parents to offspring. It has several principles. The first is that genetic characteristics, alleles , are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on the law of dominance and uniformity , which states that some alleles are dominant while others are recessive ; an organism with at least one dominant allele will display

8154-485: Is the set of chemical reactions in an organism. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conversion of food/fuel to monomer building blocks; and the elimination of metabolic wastes . These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized as catabolic —the breaking down of compounds (for example,

8305-449: Is the simplest and most common method of asexual reproduction. Multiple fission at the cellular level occurs in many protists , e.g. sporozoans and algae . The nucleus of the parent cell divides several times by amitosis , producing several nuclei. The cytoplasm then separates, creating multiple daughter cells. Some parasitic, single-celled organisms undergo a multiple fission-like process to produce numerous daughter cells from

8456-410: Is the study of chemical processes within and relating to living organisms . Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including molecular synthesis, modification, mechanisms, and interactions. Life arose from the Earth's first ocean, which formed some 3.8 billion years ago. Since then, water continues to be

8607-493: Is to store, transmit, and express hereditary information. Cell theory states that cells are the fundamental units of life, that all living things are composed of one or more cells, and that all cells arise from preexisting cells through cell division . Most cells are very small, with diameters ranging from 1 to 100  micrometers and are therefore only visible under a light or electron microscope . There are generally two types of cells: eukaryotic cells, which contain

8758-414: Is vital for upholding the correct cellular balance. Autophagy instability leads to a variety of illness symptoms, including inflammation, biochemical disturbances, aging, and neurodegenerative, due to its involvement in controlling cell integrity. The modification of the autophagy-lysosomal networks is a typical hallmark of many neurological and muscular illnesses. As a result, autophagy has been identified as

8909-644: The Cambrian explosion . During the Permian period, synapsids , including the ancestors of mammals , dominated the land, but most of this group became extinct in the Permian–Triassic extinction event 252 million years ago. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. After

9060-495: The Cretaceous–Paleogene extinction event 66 million years ago killed off the non-avian dinosaurs, mammals increased rapidly in size and diversity . Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. Bacteria are a type of cell that constitute a large domain of prokaryotic microorganisms . Typically a few micrometers in length, bacteria have

9211-535: The Miller–Urey experiment showed that organic compounds could be synthesized abiotically within a closed system mimicking the conditions of early Earth , thus suggesting that complex organic molecules could have arisen spontaneously in early Earth (see abiogenesis ). Macromolecules are large molecules made up of smaller subunits or monomers . Monomers include sugars, amino acids, and nucleotides. Carbohydrates include monomers and polymers of sugars. Lipids are

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9362-492: The Precambrian about 1.5 billion years ago and can be classified into eight major clades : alveolates , excavates , stramenopiles , plants, rhizarians , amoebozoans , fungi , and animals. Five of these clades are collectively known as protists , which are mostly microscopic eukaryotic organisms that are not plants, fungi, or animals. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor ), protists by themselves do not constitute

9513-727: The activator is the transcription factor that stimulates transcription when it binds to the sequence near or at the promoter. Negative regulation occurs when another transcription factor called a repressor binds to a DNA sequence called an operator , which is part of an operon, to prevent transcription. Repressors can be inhibited by compounds called inducers (e.g., allolactose ), thereby allowing transcription to occur. Specific genes that can be activated by inducers are called inducible genes , in contrast to constitutive genes that are almost constantly active. In contrast to both, structural genes encode proteins that are not involved in gene regulation. In addition to regulatory events involving

9664-592: The bacterial phyla have species that can be grown in the laboratory. Archaea constitute the other domain of prokaryotic cells and were initially classified as bacteria, receiving the name archaebacteria (in the Archaebacteria kingdom ), a term that has fallen out of use. Archaeal cells have unique properties separating them from the other two domains , Bacteria and Eukaryota . Archaea are further divided into multiple recognized phyla . Archaea and bacteria are generally similar in size and shape, although

9815-404: The cell . In 1838, Schleiden and Schwann began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, although they opposed the idea that (3) all cells come from the division of other cells, continuing to support spontaneous generation . However, Robert Remak and Rudolf Virchow were able to reify

9966-403: The cell membrane of another cell or located deep inside a cell. There are generally four types of chemical signals: autocrine , paracrine , juxtacrine , and hormones . In autocrine signaling, the ligand affects the same cell that releases it. Tumor cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling,

10117-416: The circulatory systems of animals or vascular systems of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with an inotropic receptor can alter the excitability of a target cell. Other types of receptors include protein kinase receptors (e.g., receptor for

10268-461: The cycling of nutrients and energy through their biophysical environment . The earliest of roots of science, which included medicine, can be traced to ancient Egypt and Mesopotamia in around 3000 to 1200 BCE . Their contributions shaped ancient Greek natural philosophy . Ancient Greek philosophers such as Aristotle (384–322 BCE) contributed extensively to the development of biological knowledge. He explored biological causation and

10419-528: The domain followed by kingdom , phylum , class , order , family , genus , and species . All organisms can be classified as belonging to one of three domains : Archaea (originally Archaebacteria), bacteria (originally eubacteria), or eukarya (includes the fungi, plant, and animal kingdoms). The history of life on Earth traces how organisms have evolved from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and all life on Earth, both living and extinct, descended from

10570-477: The domains of Archaea and Bacteria reproduce with binary fission. This form of asexual reproduction and cell division is also used by some organelles within eukaryotic organisms (e.g., mitochondria ). Binary fission results in the reproduction of a living prokaryotic cell (or organelle) by dividing the cell into two parts, each with the potential to grow to the size of the original. The single DNA molecule first replicates, then attaches each copy to

10721-481: The duplication of its DNA and some of its organelles , and the subsequent partitioning of its cytoplasm into two daughter cells in a process called cell division . In eukaryotes (i.e., animal, plant, fungal , and protist cells), there are two distinct types of cell division: mitosis and meiosis . Mitosis is part of the cell cycle, in which replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which

10872-487: The germline depends on the effectiveness of processes for avoiding DNA damage and repairing those DNA damages that do occur. Sexual processes in eukaryotes , as well as in prokaryotes , provide an opportunity for effective repair of DNA damages in the germ line by homologous recombination . The cell cycle is a four-stage process that a cell goes through as it develops and divides. It includes Gap 1 (G1), synthesis (S), Gap 2 (G2), and mitosis (M). The cell either restarts

11023-403: The kingdom Plantae, which would exclude fungi and some algae . Plant cells were derived by endosymbiosis of a cyanobacterium into an early eukaryote about one billion years ago, which gave rise to chloroplasts. The first several clades that emerged following primary endosymbiosis were aquatic and most of the aquatic photosynthetic eukaryotic organisms are collectively described as algae, which

11174-423: The microscope . It was then that scholars discovered spermatozoa , bacteria, infusoria and the diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in entomology and helped to develop techniques of microscopic dissection and staining . Advances in microscopy had a profound impact on biological thinking. In the early 19th century, biologists pointed to the central importance of

11325-412: The mitochondrial cristae . Oxidative phosphorylation comprises the electron transport chain, which is a series of four protein complexes that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH 2 that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane ( chemiosmosis ), which generates a proton motive force . Energy from

11476-445: The molecular biology of a cell to the anatomy and physiology of plants and animals, and evolution of populations. Hence, there are multiple subdisciplines within biology , each defined by the nature of their research questions and the tools that they use. Like other scientists, biologists use the scientific method to make observations , pose questions, generate hypotheses , perform experiments, and form conclusions about

11627-546: The phenotype of that dominant allele. During gamete formation, the alleles for each gene segregate, so that each gamete carries only one allele for each gene. Heterozygotic individuals produce gametes with an equal frequency of two alleles. Finally, the law of independent assortment , states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that are sex-linked . Test crosses can be performed to experimentally determine

11778-497: The regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells. Cellular respiration is a set of metabolic reactions and processes that take place in cells to convert chemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions , which break large molecules into smaller ones, releasing energy. Respiration

11929-557: The stationary phase of growth. Thermoproteota (formerly Crenarchaeota ) possess neither a cell wall nor the FtsZ mechanism. They use a primitive version of the eukaryotic ESCRT -III system (also known as Cdv ) to manipulate the membrane into separating, specifically by coming into the middle of the two soon-to-be daughter cells. Euryarchaeota use FtsZ like bacteria do. Some organelles in eukaryotic cells reproduce using binary fission. Mitochondrial fission occurs frequently within

12080-421: The DNA repair checkpoints The cell cycle is a sequence of activities in which cell organelles are duplicated and subsequently separated into daughter cells with precision. There are major events that happen during a cell cycle. The processes that happen in the cell cycle include cell development, replication and segregation of chromosomes.  The cell cycle checkpoints are surveillance systems that keep track of

12231-512: The OMM connects to other cellular organelles, such as the endoplasmic reticulum (ER), lysosomes, endosomes, and the plasma membrane. Mitochondria play a wide range of roles in cell biology, which is reflected in their morphological diversity. Ever since the beginning of the mitochondrial study, it has been well documented that mitochondria can have a variety of forms, with both their general and ultra-structural morphology varying greatly among cells, during

12382-481: The Phanerozoic eon that began 539 million years ago being subdivided into Paleozoic , Mesozoic , and Cenozoic eras. These three eras together comprise eleven periods ( Cambrian , Ordovician , Silurian , Devonian , Carboniferous , Permian , Triassic , Jurassic , Cretaceous , Tertiary , and Quaternary ). The similarities among all known present-day species indicate that they have diverged through

12533-696: The accumulation of favorable traits over successive generations, thereby increasing the match between the organisms and their environment. A species is a group of organisms that mate with one another and speciation is the process by which one lineage splits into two lineages as a result of having evolved independently from each other. For speciation to occur, there has to be reproductive isolation . Reproductive isolation can result from incompatibilities between genes as described by Bateson–Dobzhansky–Muller model . Reproductive isolation also tends to increase with genetic divergence . Speciation can occur when there are physical barriers that divide an ancestral species,

12684-589: The body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the Hox genes . Hox genes determine where repeating parts, such as the many vertebrae of snakes, will grow in a developing embryo or larva. Evolution is a central organizing concept in biology. It is the change in heritable characteristics of populations over successive generations . In artificial selection , animals were selectively bred for specific traits. Given that traits are inherited, populations contain

12835-450: The breaking down of glucose to pyruvate by cellular respiration ); or anabolic —the building up ( synthesis ) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy. The chemical reactions of metabolism are organized into metabolic pathways , in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by

12986-441: The cell cycle's integrity, accuracy, and chronology. Each checkpoint serves as an alternative cell cycle endpoint, wherein the cell's parameters are examined and only when desirable characteristics are fulfilled does the cell cycle advance through the distinct steps. The cell cycle's goal is to precisely copy each organism's DNA and afterwards equally split the cell and its components between the two new cells. Four main stages occur in

13137-448: The cell cycle, and in response to metabolic or cellular cues. Mitochondria can exist as independent organelles or as part of larger systems; they can also be unequally distributed in the cytosol through regulated mitochondrial transport and placement to meet the cell's localized energy requirements. Mitochondrial dynamics refers to the adaptive and variable aspect of mitochondria, including their shape and subcellular distribution. Autophagy

13288-502: The cell genome. When erroneous nucleotides are incorporated during DNA replication, mutations can occur. The majority of DNA damage is fixed by removing the defective bases and then re-synthesizing the excised area. On the other hand, some DNA lesions can be mended by reversing the damage, which may be a more effective method of coping with common types of DNA damage. Only a few forms of DNA damage are mended in this fashion, including pyrimidine dimers caused by ultraviolet (UV) light changed by

13439-453: The cell has completed its growth process and if it is found to be damaged or altered, it undergoes cell death, either by apoptosis or necrosis , to eliminate the threat it can cause to the organism's survival. The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the origin of life . It is not actually cells that are immortal but multi-generational cell lineages. The immortality of

13590-421: The cell membrane, Golgi apparatus, endoplasmic reticulum, and mitochondria. With the conclusion of the autophagocyte, the phagophore's enlargement comes to an end. The auto-phagosome combines with the lysosomal vesicles to formulate an auto-lysosome that degrades the encapsulated substances, referred to as phagocytosis. Biology Biology is the scientific study of life . It is a natural science with

13741-586: The cell theory, adding that all cells come from the division of pre-existing cells. Viruses are not considered in cell biology – they lack the characteristics of a living cell and instead are studied in the microbiology subclass of virology . Cell biology research looks at different ways to culture and manipulate cells outside of a living body to further research in human anatomy and physiology , and to derive medications. The techniques by which cells are studied have evolved. Due to advancements in microscopy, techniques and technology have allowed scientists to hold

13892-453: The cell, even when the cell is not actively undergoing mitosis, and is necessary to regulate the cell's metabolism . All chloroplasts and some mitochondria (not in animals), both organelles derived from endosymbiosis of bacteria, also use FtsZ in a bacteria-like fashion. Binary fission in organisms can occur in four ways: irregular , longitudinal , transverse , or oblique . For example: Binary fission means "division into two". It

14043-410: The cohesive force due to the attraction between molecules at the surface of the liquid. Water is also adhesive as it is able to adhere to the surface of any polar or charged non-water molecules. Water is denser as a liquid than it is as a solid (or ice). This unique property of water allows ice to float above liquid water such as ponds, lakes, and oceans, thereby insulating the liquid below from

14194-433: The cold air above. Water has the capacity to absorb energy, giving it a higher specific heat capacity than other solvents such as ethanol . Thus, a large amount of energy is needed to break the hydrogen bonds between water molecules to convert liquid water into water vapor . As a molecule, water is not completely stable as each water molecule continuously dissociates into hydrogen and hydroxyl ions before reforming into

14345-420: The components of cells and how cells work is fundamental to all biological sciences while also being essential for research in biomedical fields such as cancer , and other diseases. Research in cell biology is interconnected to other fields such as genetics , molecular genetics , molecular biology , medical microbiology , immunology , and cytochemistry . Cells were first seen in 17th-century Europe with

14496-421: The cycle from G1 or leaves the cycle through G0 after completing the cycle. The cell can progress from G0 through terminal differentiation. Finally, the interphase refers to the phases of the cell cycle that occur between one mitosis and the next, and includes G1, S, and G2. Thus, the phases are: The scientific branch that studies and diagnoses diseases on the cellular level is called cytopathology . Cytopathology

14647-501: The cytoplasm of a cell, there are many biomolecules such as proteins and nucleic acids . In addition to biomolecules, eukaryotic cells have specialized structures called organelles that have their own lipid bilayers or are spatially units. These organelles include the cell nucleus , which contains most of the cell's DNA, or mitochondria , which generate adenosine triphosphate (ATP) to power cellular processes. Other organelles such as endoplasmic reticulum and Golgi apparatus play

14798-455: The cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD for glycolysis. This waste product varies depending on

14949-418: The development of body form, is the result of spatial differences in gene expression. A small fraction of the genes in an organism's genome called the developmental-genetic toolkit control the development of that organism. These toolkit genes are highly conserved among phyla , meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect

15100-508: The development of transmembrane contact sites among mitochondria and other structures, which both have the potential to link signals from diverse routes that affect mitochondrial membrane dynamics substantially, Mitochondria are wrapped by two membranes: an inner mitochondrial membrane (IMM) and an outer mitochondrial membrane (OMM), each with a distinctive function and structure, which parallels their dual role as cellular powerhouses and signaling organelles. The inner mitochondrial membrane divides

15251-440: The developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells arise from less specialized cells such as stem cells . Stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. Cellular differentiation dramatically changes

15402-411: The discovery of archaea in almost every habitat , including soil, oceans, and marshlands . Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life . They are part of the microbiota of all organisms. In the human microbiome , they are important in the gut , mouth, and on

15553-420: The discovery of the double-helical structure of DNA by James Watson and Francis Crick in 1953, marked the transition to the era of molecular genetics . From the 1950s onwards, biology has been vastly extended in the molecular domain. The genetic code was cracked by Har Gobind Khorana , Robert W. Holley and Marshall Warren Nirenberg after DNA was understood to contain codons . The Human Genome Project

15704-598: The diversity of life. His successor, Theophrastus , began the scientific study of plants. Scholars of the medieval Islamic world who wrote on biology included al-Jahiz (781–869), Al-Dīnawarī (828–896), who wrote on botany, and Rhazes (865–925) who wrote on anatomy and physiology . Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought. Biology began to quickly develop with Anton van Leeuwenhoek 's dramatic improvement of

15855-474: The division plane between the two daughter cells. MinC and MinD function together as division inhibitors, blocking formation of the FtsZ ring. MinE stops the MinCD activity midcell, allowing FtsZ to take over for binary fission. More specifically, the following steps occur: Studies of bacteria made to not produce a cell wall, called L-form bacteria , shows that FtsZ requires a cell wall to work. Little

16006-634: The earliest terrestrial ecosystems , at least 2.7 billion years ago. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event . Ediacara biota appear during the Ediacaran period, while vertebrates , along with most other modern phyla originated about 525 million years ago during

16157-461: The eukaryotes. In G1, the cell is usually active and continues to grow rapidly, while in G2, the cell growth continues while protein molecules become ready for separation. These are not dormant times; they are when cells gain mass, integrate growth factor receptors, establish a replicated genome, and prepare for chromosome segregation. DNA replication is restricted to a separate Synthesis in eukaryotes, which

16308-481: The expression of deleterious recessive mutations . The beneficial effect of genetic complementation, derived from outcrossing (cross-fertilization) is also referred to as hybrid vigor or heterosis. Charles Darwin in his 1878 book The Effects of Cross and Self-Fertilization in the Vegetable Kingdom at the start of chapter XII noted “The first and most important of the conclusions which may be drawn from

16459-435: The following molecular components: Cell metabolism is necessary for the production of energy for the cell and therefore its survival and includes many pathways and also sustaining the main cell organelles such as the nucleus, the mitochondria, the cell membrane etc. For cellular respiration , once glucose is available, glycolysis occurs within the cytosol of the cell to produce pyruvate. Pyruvate undergoes decarboxylation using

16610-455: The form of glucose is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages: glycolysis , citric acid cycle (or Krebs cycle), electron transport chain , and oxidative phosphorylation . Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two pyruvates , with two net molecules of ATP being produced at

16761-428: The formation of a spindle apparatus on the cell. Like in mitosis (and unlike in meiosis), the parental identity is not lost. FtsZ is homologous to β-tubulin , the building block of the microtubule cytoskeleton used during mitosis in eukaryotes. FtsZ is thought to be the first protein to localize to the site of future division in bacteria, and it assembles into a Z ring, anchored by FtsZ-binding proteins and defines

16912-419: The fundamental units of life. The growth and development of cells are essential for the maintenance of the host and survival of the organism. For this process, the cell goes through the steps of the cell cycle and development which involves cell growth, DNA replication , cell division , regeneration, and cell death . The cell cycle is divided into four distinct phases : G1, S, G2, and M. The G phase – which

17063-426: The hormone insulin ) and G protein-coupled receptors . Activation of G protein-coupled receptors can initiate second messenger cascades. The process by which a chemical or physical signal is transmitted through a cell as a series of molecular events is called signal transduction . The cell cycle is a series of events that take place in a cell that cause it to divide into two daughter cells. These events include

17214-414: The initial step of photosynthesis whereby light energy is absorbed by chlorophyll pigments attached to proteins in the thylakoid membranes . The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a quinone designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach

17365-537: The insertion of methyl or ethyl groups at the purine ring's O6 position. Mitochondria are commonly referred to as the cell's "powerhouses" because of their capacity to effectively produce ATP which is essential to maintain cellular homeostasis and metabolism. Moreover, researchers have gained a better knowledge of mitochondria's significance in cell biology because of the discovery of cell signaling pathways by mitochondria which are crucial platforms for cell function regulation such as apoptosis. Its physiological adaptability

17516-481: The interphase portion of the cycle, while the M phase ( mitosis ) is the cell division portion of the cycle. Mitosis is composed of many stages which include, prophase, metaphase, anaphase, telophase, and cytokinesis, respectively. The ultimate result of mitosis is the formation of two identical daughter cells. The cell cycle is regulated in cell cycle checkpoints , by a series of signaling factors and complexes such as cyclins, cyclin-dependent kinase , and p53 . When

17667-469: The invention of the compound microscope . In 1665, Robert Hooke referred to the building blocks of all living organisms as "cells" (published in Micrographia ) after looking at a piece of cork and observing a structure reminiscent of a monastic cell ; however, the cells were dead. They gave no indication to the actual overall components of a cell. A few years later, in 1674, Anton Van Leeuwenhoek

17818-406: The last eukaryotic common ancestor. Prokaryotes (i.e., archaea and bacteria) can also undergo cell division (or binary fission ). Unlike the processes of mitosis and meiosis in eukaryotes, binary fission in prokaryotes takes place without the formation of a spindle apparatus on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it

17969-405: The ligand diffuses to nearby cells and affects them. For example, brain cells called neurons release ligands called neurotransmitters that diffuse across a synaptic cleft to bind with a receptor on an adjacent cell such as another neuron or muscle cell . In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through

18120-448: The lysosomal membrane to enclose the cytosol or organelles. The chaperone-mediated autophagy (CMA) protein quality assurance by digesting oxidized and altered proteins under stressful circumstances and supplying amino acids through protein denaturation. Autophagy is the primary intrinsic degradative system for peptides, fats, carbohydrates, and other cellular structures. In both physiologic and stressful situations, this cellular progression

18271-444: The membrane serving as membrane transporters , and peripheral proteins that loosely attach to the outer side of the cell membrane, acting as enzymes shaping the cell. Cell membranes are involved in various cellular processes such as cell adhesion , storing electrical energy , and cell signalling and serve as the attachment surface for several extracellular structures such as a cell wall , glycocalyx , and cytoskeleton . Within

18422-486: The mitochondrial lumen into two parts: the inner border membrane, which runs parallel to the OMM, and the cristae, which are deeply twisted, multinucleated invaginations that give room for surface area enlargement and house the mitochondrial respiration apparatus. The outer mitochondrial membrane, on the other hand, is soft and permeable. It, therefore, acts as a foundation for cell signaling pathways to congregate, be deciphered, and be transported into mitochondria. Furthermore,

18573-403: The mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells. The cell cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells , and some internal organs are renewed. After cell division, each of the daughter cells begin the interphase of

18724-437: The most abundant molecule in every organism. Water is important to life because it is an effective solvent , capable of dissolving solutes such as sodium and chloride ions or other small molecules to form an aqueous solution . Once dissolved in water, these solutes are more likely to come in contact with one another and therefore take part in chemical reactions that sustain life. In terms of its molecular structure , water

18875-401: The most prominent type, have several different shapes , although most are spherical or rod-shaped . Bacteria can be classed as either gram-positive or gram-negative depending on the cell wall composition. Gram-positive bacteria have a thicker peptidoglycan layer than gram-negative bacteria. Bacterial structural features include a flagellum that helps the cell to move, ribosomes for

19026-467: The multi-enzyme complex to form acetyl coA which can readily be used in the TCA cycle to produce NADH and FADH 2 . These products are involved in the electron transport chain to ultimately form a proton gradient across the inner mitochondrial membrane. This gradient can then drive the production of ATP and H 2 O during oxidative phosphorylation . Metabolism in plant cells includes photosynthesis which

19177-441: The observations given in this volume, is that generally cross-fertilisation is beneficial and self-fertilisation often injurious, at least with the plants on which I experimented.” Genetic variation , often produced as a byproduct of sexual reproduction, may provide long-term advantages to those sexual lineages that engage in outcrossing . Genetics is the scientific study of inheritance. Mendelian inheritance , specifically,

19328-517: The only class of macromolecules that are not made up of polymers. They include steroids , phospholipids , and fats, largely nonpolar and hydrophobic (water-repelling) substances. Proteins are the most diverse of the macromolecules. They include enzymes , transport proteins , large signaling molecules, antibodies , and structural proteins . The basic unit (or monomer) of a protein is an amino acid . Twenty amino acids are used in proteins. Nucleic acids are polymers of nucleotides . Their function

19479-404: The organism. In skeletal muscles, the waste product is lactic acid . This type of fermentation is called lactic acid fermentation . In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation

19630-428: The possibility of common descent . Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck , who presented a coherent theory of evolution. The British naturalist Charles Darwin , combining the biogeographical approach of Humboldt , the uniformitarian geology of Lyell , Malthus's writings on population growth, and his own morphological expertise and extensive natural observations, forged

19781-907: The process of evolution from their common ancestor. Biologists regard the ubiquity of the genetic code as evidence of universal common descent for all bacteria , archaea , and eukaryotes . Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean eon and many of the major steps in early evolution are thought to have taken place in this environment. The earliest evidence of eukaryotes dates from 1.85 billion years ago, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism . Later, around 1.7 billion years ago, multicellular organisms began to appear, with differentiated cells performing specialised functions. Algae-like multicellular land plants are dated back to about 1 billion years ago, although evidence suggests that microorganisms formed

19932-539: The process such as transcription , RNA splicing , translation , and post-translational modification of a protein. Gene expression can be influenced by positive or negative regulation, depending on which of the two types of regulatory proteins called transcription factors bind to the DNA sequence close to or at a promoter. A cluster of genes that share the same promoter is called an operon , found mainly in prokaryotes and some lower eukaryotes (e.g., Caenorhabditis elegans ). In positive regulation of gene expression,

20083-652: The production of spores . The species Metabacterium polyspora , a symbiont of guinea pigs , has been found to produce multiple endospores in each division. Some species of cyanobacteria have also been found to reproduce through multiple fission. Some protozoans reproduce by yet another mechanism of fission called plasmotomy . In this type of fission, a multinucleate adult parent undergoes cytokinesis to form two multinucleate (or coenocytic ) daughter cells. The daughter cells so produced undergo further mitosis. Opalina and Pelomyxa reproduce in this way. Fragmentation in multicellular or colonial organisms

20234-517: The promoter, gene expression can also be regulated by epigenetic changes to chromatin , which is a complex of DNA and protein found in eukaryotic cells. Development is the process by which a multicellular organism (plant or animal) goes through a series of changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle. There are four key processes that underlie development: Determination , differentiation , morphogenesis , and growth. Determination sets

20385-406: The proton motive force drives the enzyme ATP synthase to synthesize more ATPs by phosphorylating ADPs . The transfer of electrons terminates with molecular oxygen being the final electron acceptor . If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of fermentation . The pyruvate is not transported into the mitochondrion but remains in

20536-451: The same cell membrane; sporogony results in sporozoites , and gametogony results in micro gametes . Green algae can divide into more than two daughter cells. The exact number of daughter cells depends on the species of algae and is an effect of temperature and light. Most species of bacteria primarily undergo binary reproduction. Some species and groups of bacteria may undergo multiple fission as well, sometimes beginning or ending with

20687-449: The same time. Each pyruvate is then oxidized into acetyl-CoA by the pyruvate dehydrogenase complex , which also generates NADH and carbon dioxide. Acetyl-CoA enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH 2 , and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in

20838-680: The second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as ribulose bisphosphate (RuBP) in a sequence of light-independent (or dark) reactions called the Calvin cycle . Cell signaling (or communication) is the ability of cells to receive, process, and transmit signals with its environment and with itself. Signals can be non-chemical such as light, electrical impulses , and heat, or chemical signals (or ligands ) that interact with receptors , which can be found embedded in

20989-553: The skin. Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen cycling; organic compound turnover; and maintaining microbial symbiotic and syntrophic communities, for example. Eukaryotes are hypothesized to have split from archaea, which was followed by their endosymbioses with bacteria (or symbiogenesis ) that gave rise to mitochondria and chloroplasts, both of which are now part of modern-day eukaryotic cells. The major lineages of eukaryotes diversified in

21140-476: The smallest form of life. Prokaryotic cells include Bacteria and Archaea , and lack an enclosed cell nucleus.  Eukaryotic cells are found in plants, animals, fungi, and protists. They range from 10 to 100 μm in diameter, and their DNA is contained within a membrane-bound nucleus. Eukaryotes are organisms containing eukaryotic cells. The four eukaryotic kingdoms are Animalia, Plantae, Fungi, and Protista. They both reproduce through binary fission . Bacteria,

21291-479: The stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration. During the third stage of photosynthesis, the movement of protons down their concentration gradients from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase. The NADPH and ATPs generated by the light-dependent reactions in

21442-406: The study of cell metabolism , cell communication , cell cycle , biochemistry , and cell composition . The study of cells is performed using several microscopy techniques, cell culture , and cell fractionation . These have allowed for and are currently being used for discoveries and research pertaining to how cells function, ultimately giving insight into understanding larger organisms. Knowing

21593-471: The third tenet, and by the 1860s most biologists accepted all three tenets which consolidated into cell theory . Meanwhile, taxonomy and classification became the focus of natural historians. Carl Linnaeus published a basic taxonomy for the natural world in 1735, and in the 1750s introduced scientific names for all his species. Georges-Louis Leclerc, Comte de Buffon , treated species as artificial categories and living forms as malleable—even suggesting

21744-430: The three basic types of autophagy. When macro autophagy is triggered, an exclusion membrane incorporates a section of the cytoplasm, generating the autophagosome, a distinctive double-membraned organelle. The autophagosome then joins the lysosome to create an autolysosome, with lysosomal enzymes degrading the components. In micro autophagy, the lysosome or vacuole engulfs a piece of the cytoplasm by invaginating or protruding

21895-542: The time binary fission requires is also the time the bacterial culture requires to double in the number of cells it contains. This time period can, therefore, be referred to as the doubling time . Some species other than E. coli may have faster or slower doubling times: some strains of Mycobacterium tuberculosis may have doubling times of nearly 100 hours. Bacterial growth is limited by factors including nutrient availability and available space, so binary fission occurs at much lower rates in bacterial cultures once they enter

22046-439: The total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of interphase (during which the DNA is replicated) and is often followed by telophase and cytokinesis ; which divides the cytoplasm , organelles and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis all together define

22197-432: The translation of RNA to protein, and a nucleoid that holds all the genetic material in a circular structure. There are many processes that occur in prokaryotic cells that allow them to survive. In prokaryotes, mRNA synthesis is initiated at a promoter sequence on the DNA template comprising two consensus sequences that recruit RNA polymerase. The prokaryotic polymerase consists of a core enzyme of four protein subunits and

22348-431: The underlying genotype of an organism with a dominant phenotype. A Punnett square can be used to predict the results of a test cross. The chromosome theory of inheritance , which states that genes are found on chromosomes, was supported by Thomas Morgans 's experiments with fruit flies , which established the sex linkage between eye color and sex in these insects. A gene is a unit of heredity that corresponds to

22499-429: The world around them. Life on Earth, which emerged more than 3.7 billion years ago, is immensely diverse. Biologists have sought to study and classify the various forms of life, from prokaryotic organisms such as archaea and bacteria to eukaryotic organisms such as protists , fungi, plants, and animals. These various organisms contribute to the biodiversity of an ecosystem , where they play specialized roles in

22650-434: Was launched in 1990 to map the human genome . All organisms are made up of chemical elements ; oxygen , carbon , hydrogen , and nitrogen account for most (96%) of the mass of all organisms, with calcium , phosphorus , sulfur , sodium , chlorine , and magnesium constituting essentially all the remainder. Different elements can combine to form compounds such as water, which is fundamental to life. Biochemistry

22801-531: Was the first to analyze live cells in his examination of algae . Many years later, in 1831, Robert Brown discovered the nucleus . All of this preceded the cell theory which states that all living things are made up of cells and that cells are organisms' functional and structural units. This was ultimately concluded by plant scientist Matthias Schleiden and animal scientist Theodor Schwann in 1838, who viewed live cells in plant and animal tissue, respectively. 19 years later, Rudolf Virchow further contributed to

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