Allium cells in different phases of the cell cycle

The cells of eukaryotes (left) and prokaryotes (right)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life.^[1] Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals). Humans contain about 10 trillion (10¹³) cells. Most plant and animal cells are between 1 and 100 µm and therefore are visible only under the microscope.^[2]

The cell was discovered by Robert Hooke in 1665. In 1835, before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.^[3]

The word cell comes from the Latin cellula, meaning "a small room". The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.^[4]

Anatomy[link]

There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.

Prokaryotic cells[link]

Diagram of a typical prokaryotic cell

The prokaryote cell is simpler, and therefore smaller, than a eukaryote cell, lacking a nucleus and most of the other organelles of eukaryotes. There are two kinds of prokaryotes: bacteria and archaea; these share a similar structure.

Nuclear material of prokaryotic cell consist of a single chromosome that is in direct contact with cytoplasm. Here, the undefined nuclear region in the cytoplasm is called nucleoid.

A prokaryotic cell has three architectural regions:

• On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells;
• Enclosing the cell is the cell envelope – generally consisting of a cell wall covering a plasma membrane though some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea). The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Some eukaryote cells (plant cells and fungi cells) also have a cell wall;
• Inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Though not forming a nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids enable additional functions, such as antibiotic resistance.

Eukaryotic cells[link]

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. This nucleus gives the eukaryote its name, which means "true nucleus." Other differences include:

• The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
• The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also contain some DNA.
• Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."^[5]
• Eukaryotes can move using motile cilia or flagella. The flagella are more complex than those of prokaryotes.

Structure of a typical animal cell

Structure of a typical plant cell

Subcellular components[link]

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary components of the cell, then briefly describe their function.

Membrane[link]

The cytoplasm of a cell is surrounded by a cell membrane or plasma membrane. The plasma membrane in plants and prokaryotes 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 double layer of lipids (hydrophobic fat-like molecules) and hydrophilic phosphorus molecules. Hence, the layer is called a phospholipid bilayer. It may also be called a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.

Cytoskeleton[link]

Bovine Pulmonary Artery Endothelial cell: nuclei stained blue, mitochondria stained red, and F-actin, an important component in microfilaments, stained green. Cell imaged on a fluorescent microscope.

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 microfilaments, intermediate filaments and microtubules. There is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.^[6]

Genetic material[link]

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long-term information storage, but some viruses (e.g., retroviruses) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA) in organisms that use DNA for the genetic code itself. Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.

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

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 23 pairs of linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also 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 also insert their genetic material into the genome.

Organelles[link]

The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs," called organelles, that are adapted and/or specialized for carrying out one or more vital functions. Both eukaryotic and prokaryotic cells have organelles but organelles in eukaryotes are generally more complex and may be membrane bound.

There are several types of organelles in a cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.

Diagram of a cell nucleus

• Cell nucleus – eukaryotes only - 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. 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.

• Mitochondria and Chloroplasts – eukaryotes only - the power generators: Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by splitting in two. Respiration occurs in the cell mitochondria.

Diagram of an endomembrane system

• Endoplasmic reticulum – eukaryotes only: The endoplasmic reticulum (ER) is the 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 and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER plays a role in calcium sequestration and release.^{[citation needed]}

• Golgi apparatus – eukaryotes only : 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.^{[citation needed]}

• Ribosomes: The ribosome is a large complex of RNA and protein molecules. 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).^[7]

• Lysosomes and Peroxisomes – eukaryotes only: 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. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.^{[citation needed]}

• Centrosome – the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.^{[citation needed]}

• Vacuoles: Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of eukaryotic cells are usually larger in those of plants than animals.^{[citation needed]}

Structures outside the cell membrane[link]

Many cells also 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 impermeable cell membrane. In order to assemble these structures export processes to carry macromolecules across the cell membrane must be used.

Cell wall[link]

Many types of prokaryotic and eukaryotic cell have a 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 pectin, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan.

Prokaryotic[link]

Capsule[link]

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.^{[citation needed]} Capsules are not marked by normal staining protocols and can be detected by special stain.^{[citation needed]}

Flagella[link]

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. Are most commonly found in bacteria cells but are found in animal cells as well.

Fimbriae (pili)[link]

They are short and thin hair like filaments, formed of protein called pilin (antigenic). Fimbriae are responsible for attachment of bacteria to specific receptors of human cell (adherence). There are special types of pili called (sex pili) involved in conjunction.^{[citation needed]}

Functions[link]

Growth and metabolism[link]

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. Complex sugars consumed by the organism can be broken down into a less chemically complex sugar molecule called glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form of energy, through two different pathways.

The first pathway, glycolysis, requires no oxygen and is referred to as anaerobic metabolism. Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy.

The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and can generate enough ATP to run all the cell functions.^{[citation needed]}

An overview of protein synthesis.
Within the nucleus of the cell (light blue), genes (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.

Creation[link]

Cell division involves a single cell (called a mother cell) dividing 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. Eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.

DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job.

Protein synthesis[link]

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.

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 through the cell. 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. The new polypeptide then folds into a functional three-dimensional protein molecule.

Movement or motility[link]

Cells can move during many processes: such as wound healing, the immune response and cancer metastasis. For wound healing to occur, white blood cells and cells that ingest bacteria move to the wound site to kill the microorganisms that cause infection.
At the same time fibroblasts (connective tissue cells) move there to remodel damaged structures. In the case of tumor development, cells from a primary tumor move away and spread to other parts of the body. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.^[8] 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.^[9][10]

Origins[link]

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

Origin of the first cell[link]

There are several theories about the origin of small molecules that could lead to life in an early Earth. One is that they came from meteorites (see Murchison meteorite). Another is that they were created at deep-sea vents. A third is that they were synthesized by lightning in a reducing atmosphere (see Miller–Urey experiment); although it is not clear if Earth had such an atmosphere. There are essentially no experimental data defining what the first self-replicating forms were. RNA is generally assumed the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (see RNA world hypothesis). But some other entity with the potential to self-replicate could have preceded RNA, like clay or peptide nucleic acid.^[11]

Cells emerged at least 4.0–4.3 billion years ago. The current belief is that these cells were heterotrophs. An important characteristic of cells is the cell membrane, composed of a bilayer of lipids. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids are known to spontaneously form bilayered vesicles in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form.^[12]

Origin of eukaryotic cells[link]

The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearing organelles like the mitochondria and the chloroplasts are almost certainly what remains of ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, where the rest of the cell appears derived from an ancestral archaean prokaryote cell—an idea called the endosymbiotic theory.

There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or viceversa: see the hydrogen hypothesis for the origin of eukaryotic cells.

Sex, as the stereotyped choreography of meiosis and syngamy that persists in nearly all extant eukaryotes, may have played a role in the transition from prokaryotes to eukaryotes. An 'origin of sex as vaccination' theory suggests that the eukaryote genome accreted from prokaryan parasite genomes in numerous rounds of lateral gene transfer. Sex-as-syngamy (fusion sex) arose when infected hosts began swapping nuclearized genomes containing co-evolved, vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts.^[13]

History of research[link]

• 1632–1723: Antonie van Leeuwenhoek teaches himself to grind lenses, builds a microscope and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth.
• 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.^[4]
• 1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
• The belief that life forms can occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822–1895) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
• 1855: Rudolf Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula).
• 1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University of Berlin. By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
• 1953: Watson and Crick made their first announcement on the double-helix structure for DNA on February 28.
• 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory.

See also[link]

Wikimedia Commons has media related to: Cell biology

• Cell biology
• Cell culture
• Cell type
• Cellular component
• Cytorrhysis
• Cytotoxicity
• Plasmolysis
• Stem cell
• Syncytium

References[link]

•  This article incorporates public domain material from the NCBI document "Science Primer".

External links[link]

• Inside the Cell
• Virtual Cell's Educational Animations
• The Inner Life of A Cell, a flash video showing what happens inside of a cell. Daniel Reda of Singularity University narrates (beginning at 22:24) [1]
• The Virtual Cell
• Cells Alive!
• Journal of Cell Biology
• The Biology Project > Cell Biology
• Centre of the Cell online
• The Image & Video Library of The American Society for Cell Biology, a collection of peer-reviewed still images, video clips and digital books that illustrate the structure, function and biology of the cell.
• HighMag Blog, still images of cells from recent research articles.
• New Microscope Produces Dazzling 3D Movies of Live Cells, March 4, 2011 - Howard Hughes Medical Institute.
• WormWeb.org: Interactive Visualization of the C. elegans Cell lineage - Visualize the entire cell lineage tree of the nematode C. elegans

Textbooks[link]

• Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). Molecular Biology of the Cell (4th ed.). Garland. ISBN 0-8153-3218-1. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.TOC&depth=2. 
• Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipurksy SL, Darnell J (2004). Molecular Cell Biology (5th ed.). WH Freeman: New York, NY. ISBN 978-0-7167-4366-8. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.TOC. 
• Cooper GM (2000). The cell: a molecular approach (2nd ed.). Washington, D.C: ASM Press. ISBN 0-87893-102-3. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=cooper.TOC&depth=2. 

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2011)

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Cell biology (formerly cytology, from the Greek kytos, "contain") is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level. Cell biology research encompasses both the great diversity of single-celled organisms like bacteria and protozoa, as well as the many specialized cells in multicellular organisms such as humans.

Knowing the components of cells and how cells work is fundamental to all biological sciences. Appreciating the similarities and differences between cell types is particularly important to the fields of cell and molecular biology as well as to biomedical fields such as cancer research and developmental biology. These fundamental similarities and differences provide a unifying theme, sometimes allowing the principles learned from studying one cell type to be extrapolated and generalized to other cell types. Therefore, research in cell biology is closely related to genetics, biochemistry, molecular biology, immunology, and developmental biology.

Understanding cells in terms of their molecular components.

Processes[link]

Movement of proteins[link]

Endothelial cells under the microscope. Nuclei are stained blue with DAPI, microtubles are marked green by an antibody and actin filaments are labelled red with phalloidin.

Each type of protein is usually sent to a particular part of the cell. An important part of cell biology is the investigation of molecular mechanisms by which proteins are moved to different places inside cells or secreted from cells.

Most proteins are synthesized by ribosomes in the rough endoplasmic reticulum.Ribosomes contain the nucleic acid RNA, which assembles and joins amino acids to make proteins. They can be found alone or in groups within the cytoplasm as well as on the RER. This process is known as protein biosynthesis. Biosynthesis (also called biogenesis) is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products (also simply known as protein translation). Some proteins, such as those to be incorporated in membranes (known as membrane proteins), are transported into the "rough" endoplasmic reticulum (ER) during synthesis. This process can be followed by transportation and processing in the Golgi apparatus. The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. The Golgi apparatus is somewhat like a post office. It receives items (proteins from the ER), packages and labels them, and then sends them on to their destinations (to different parts of the cell or to the cell membrane for transport out of the cell).^[1] From the Golgi, membrane proteins can move to the plasma membrane, to other sub-cellular compartments, or they can be secreted from the cell. The ER and Golgi can be thought of as the "membrane protein synthesis compartment" and the "membrane protein processing compartment", respectively. There is a semi-constant flux of proteins through these compartments. ER and Golgi-resident proteins associate with other proteins but remain in their respective compartments. Other proteins "flow" through the ER and Golgi to the plasma membrane. Motor proteins transport membrane protein-containing vesicles along cytoskeletal tracks to distant parts of cells such as axon terminals.

Some proteins that are made in the cytoplasm contain structural features that target them for transport into mitochondria or the nucleus. Some mitochondrial proteins are made inside mitochondria and are coded for by mitochondrial DNA. In plants, chloroplasts also make some cell proteins.

Extracellular and cell surface proteins destined to be degraded can move back into intracellular compartments upon being incorporated into endocytosed vesicles some of which fuse with lysosomes where the proteins are broken down to their individual amino acids. The degradation of some membrane proteins begins while still at the cell surface when they are separated by secretases. Proteins that function in the cytoplasm are often degraded by proteasomes.

Other cellular processes[link]

• Active transport and Passive transport - Movement of molecules into and out of cells.
• Autophagy - The process whereby cells "eat" their own internal components or microbial invaders.
• Adhesion - Holding together cells and tissues.
• Reproduction - Made possible by the combination of sperm made in the testiculi (contained in some male cells' nuclei) and the egg made in the ovary (contained in the nucleus of a female cell). When the sperm breaks through the hard outer shell of the egg a new cell embryo is formed, which, in humans, grows to full size in 9 months.
• Cell movement: Chemotaxis, Contraction, cilia and flagella.
• Cell signaling - Regulation of cell behavior by signals from outside.
• DNA repair and Cell death
• Metabolism: Glycolysis, respiration, Photosynthesis
• Transcription and mRNA splicing - gene expression.

Internal cellular structures[link]

Electron micrograph.

• Chloroplast - key organelle for photosynthesis (only found in plant cells)
• Cilia - motile microtubule-containing structures of eukaryotes
• Cytoplasm - contents of the main fluid-filled space inside cells
• Cytoskeleton - protein filaments inside cells
• Endoplasmic reticulum - major site of membrane protein synthesis
• Flagella - motile structures of bacteria, archaea and eukaryotes
• Golgi apparatus - site of protein glycosylation in the endomembrane system
• Lipid bilayer - fundamental organizational structure of cell membranes
• Lysosome - break down cellular waste products and debris into simple compounds (only found in animal cells)
• Membrane lipid and protein barrier
• Mitochondrion - major energy-producing organelle by releasing it in the form of ATP
• Nucleus - holds most of the DNA of eukaryotic cells and controls all cellular activities
• Organelle - term used for major subcellular structures
• Ribosome - RNA and protein complex required for protein synthesis in cells
• Vesicle - small membrane-bounded spheres inside cells

Techniques used to study cells[link]

Cells may be observed under the microscope. This includes the Optical Microscope, Transmission Electron Microscope, Scanning Electron Microscope, Fluorescence Microscope, and by Confocal Microscopy.

Several different techniques exist to study cells are

• Cell culture is the basic technique of growing cells in a laboratory independent of an organism.
• Immunostaining, also known as immunohistochemistry, is a specialized histological method used to localize proteins in cells or tissue slices. Unlike regular histology, which uses stains to identify cells, cellular components or protein classes, immunostaining requires the reaction of an antibody directed against the protein of interest within the tissue or cell. Through the use of proper controls and published protocols (need to add reference links here), specificity of the antibody-antigen reaction can be achieved. Once this complex is formed, it is identified via either a "tag" attached directly to the antibody, or added in an additional technical step. Commonly used "tags" include fluorophores or enzymes. In the case of the former, detection of the location of the "immuno-stained" protein occurs via fluorescence microscopy. With an enzymatic tag, such as horse radish peroxidase, a chemical reaction is carried out that results in a dark color in the location of the protein of interest. This darkened pattern is then detected using light microscopy.
• Computational genomics is used to find patterns in genomic information ^[2]
• DNA microarrays identify changes in transcript levels between different experimental conditions.
• Gene knockdown mutates a selected gene.
• In situ hybridization shows which cells are expressing a particular RNA transcript.
• PCR can be used to determine how many copies of a gene are present in a cell.
• Transfection introduces a new gene into a cell, usually an expression construct

Purification of cells and their parts Purification may be performed using the following methods:

• Cell fractionation
  • Release of cellular organelles by disruption of cells.
  • Separation of different organelles by centrifugation.
• Flow cytometry
• Immunoprecipitation
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See also[link]

Wikimedia Commons has media related to: Cell biology

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Notable cell biologists[link]

• Peter Agre
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References[link]

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• Aging Cell

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This article is about the philanthropist. For institutions named after him, see Johns Hopkins University and Johns Hopkins Hospital.

Johns Hopkins
Born	(1795-05-19)May 19, 1795 Anne Arundel County, Maryland, US
Died	December 24, 1873(1873-12-24) (aged 78) Baltimore, Maryland, US
Occupation	Entrepreneur, investor, philanthropist, abolitionist
Net worth	USD $10 million at the time of his death (approximately 1/944th of US GNP)[1]
Religion	Quaker

Johns Hopkins (May 19, 1795^[2] – December 24, 1873) was an American entrepreneur, abolitionist and philanthropist of 19th-century Baltimore, Maryland.

His bequests founded numerous institutions bearing his name, including Johns Hopkins Hospital, Johns Hopkins University, Johns Hopkins School of Nursing, Johns Hopkins School of Medicine, and Johns Hopkins Bloomberg School of Public Health.

A biography entitled Johns Hopkins: A Silhouette written by his cousin, Helen Hopkins Thom, was published in 1929 by the Johns Hopkins University Press.

Early life[link]

Johns Hopkins was born on May 19, 1795, to Samuel Hopkins (1759–1814) of Anne Arundel County, Maryland, and Hannah Janney (1774–1864), of Loudoun County, Virginia. Home was Whitehall, a 500-acre (two km²) tobacco plantation in Anne Arundel County.^[3] His first name derives from a maternal great-grandmother, Margaret Johns, who passed it on to her son.

The Hopkins family were members of the Society of Friends (Quakers). In 1807 they emancipated their slaves in accordance with their local Society decree, which called for freeing the able-bodied and caring for the others, who would remain at the plantation and provide labor as they could.^[4] The second eldest of eleven children, 12 year-old Johns was required to work on the farm, interrupting his formal education. From 1806 to 1809, he likely attended The Free School of Anne Arundel County, which was located in today's Davidsonville, Maryland.

In 1812, at the age of 17, Hopkins left the plantation to work in his uncle Gerard Hopkins' Baltimore wholesale grocery business. While living with his uncle's family, Johns and his cousin, Elizabeth, fell in love; however, the Quaker taboo against marriage of first cousins was especially strong, and neither Johns nor Elizabeth ever married.^[3]

As he became able, Hopkins provided for his extended family, both during his life and posthumously through his will. He bequeathed a home for Elizabeth, where she lived until her death in 1889.

Whitehall Plantation is located in today's Crofton, Maryland. Its home, since restored and modified, is on Johns Hopkins Road, adjacent to Reidel Road. The heavily landscaped property is surrounded by Walden Golf Course, and bears a historic marker.

Business years[link]

Hopkins' early experiences and successes in business came when he was put in charge of the store while his uncle was away during the War of 1812. After seven years with his uncle, Hopkins went into business together with Benjamin Moore, a fellow Quaker. The business partnership was later dissolved with Moore purporting Hopkins' penchant for capital accumulation as the cause for the divide.^[3]

After Moore's withdrawal, Hopkins partnered with three of his brothers and established Hopkins & Brothers. The company prospered by selling various wares in the Shenandoah Valley from Conestoga wagons, sometimes in exchange for corn whiskey, which was then sold in Baltimore as "Hopkins' Best". The bulk of Hopkins' fortune however was made by his judicious investments in a myriad of ventures, most notably the Baltimore and Ohio Railroad (B&O), of which he became a Director in 1847 and Chairman of the Finance Committee in 1855. He was also President of Merchants' Bank as well as director of a number of other organizations.^[5]

A charitable individual, Hopkins put up his own money more than once to not only aid Baltimore City during times of financial crises, but also to twice bail the railroad out of debt, in 1857 and 1873.^[6] In 1996, Johns Hopkins ranked 69th in "The Wealthy 100: From Benjamin Franklin to Bill Gates - A Ranking of the Richest Americans, Past and Present."^[7]

Civil War[link]

One of the first campaigns of the American Civil War was planned at Johns Hopkins' summer estate, Clifton, where he had also entertained a number of foreign dignitaries including the future King Edward VII.^[8] Hopkins was a strong supporter of the Union, unlike most Marylanders, who sympathized with and often supported the South and the Confederacy.^[9] During the Civil War, Clifton became a frequent meeting place for local Union sympathizers, and federal officials.

Hopkins' support of Abraham Lincoln also often put him at odds with some of Maryland's most prominent people, particularly Supreme Court Justice Roger B. Taney, who continually opposed Lincoln's presidential decisions, such as his policies of limiting habeas corpus and stationing troops in Maryland. In 1862 Hopkins wrote a letter to Lincoln requesting the President not to heed the detractors' calls and continue to keep soldiers stationed in Maryland. Hopkins also pledged financial and logistic support to Lincoln, in particular the free use of the B&O railway system.^[10][11]

Abolitionism[link]

Johns Hopkins is described as being an "abolitionist before the word was even invented", having been represented as such both prior to the Civil War period, as well as during the Civil War and Reconstruction Era.^[12][13][14] There are several accounts that describe the abolitionist influence Hopkins was privy to as a 12 year old participant in his parents' emancipation of their family's slaves in 1807.^[3] Prior to the Civil War Johns Hopkins worked closely with two of America's most famous abolitionists, Myrtilla Miner^[15] and Henry Ward Beecher.^[16] During the Civil War Johns Hopkins, being a staunch supporter of Lincoln and the Union, was instrumental in bringing fruition to Lincoln's emancipatory vision.^[17][18]

After the Civil War and during Reconstruction, Johns Hopkins' stance on abolitionism infuriated many prominent people in Baltimore.^[19][20] During the American Reconstruction period to his death ^[21] his abolitionism was expressed in the documents founding the Johns Hopkins Institutions, and reported in newspaper articles before, during, and after the founding of these institutions. Before the war, there was significant written opposition to his support for Myrtilla Miner's founding of a school for African American females^[22] (now the University of the District of Columbia). Similarly, opposition (and some support) was expressed during Reconstruction, such as in 1867, the same year he filed papers incorporating the Johns Hopkins Institutions, when he attempted unsuccessfully to stop the convening of the Constitutional Convention where the Democratic Party came into power and where a new Constitution, the Constitution still in effect, was voted to replace the Constitution of the Radical Republicans previously in power.^[23]

Apparent also in the literature of the times was opposition, and support for, the various other ways he expressed opposition to the racial practices that were beginning to emerge, and re-emerge as well, in the city of Baltimore, the state of Maryland, the nation and in the posthumously constructed and founded institutions that would carry his name,^[24] A Baltimore American journalist praised Hopkins for founding three institutions, a university, a hospital and an orphan asylum, specifically for colored children, adding that Hopkins was a "man (beyond his times) who knew no race" citing his provisions for both blacks and whites in the plans for his hospital. The reporter also pointed to similarities between Benjamin Franklin's and Johns Hopkins' views on hospital care and construction, such as their shared interest in free hospitals and the availability of emergency services without prejudice. This article, first published in 1870, also accompanied Hopkins' obituary in the Baltimore American as a tribute in 1873. Cited in many of the newspaper articles on him during his lifetime and immediately after his death were his provisions of scholarships for the poor, and quality health services for the underserved, the poor without regard to their age, sex and color, the colored children asylum and other orphanages, the mentally ill and convalescents.

Philanthropy[link]

Living his entire adult life in Baltimore, Hopkins made many friends among the city's social elite, many of them Quakers. One of these friends was George Peabody, who was also born in 1795, and who in 1857 founded the Peabody Institute in Baltimore. Other examples of public giving were evident in the city, as public buildings housing free libraries, schools and foundations sprang up along the city's widening streets. On the advice of Peabody, some believe, Hopkins determined to use his great wealth for the public good.

The Civil War had taken its toll on Baltimore, however, as did the yellow fever and cholera epidemics that repeatedly ravaged the nation's cities, killing 853 in Baltimore in the summer of 1832 alone. Hopkins was keenly aware of the city's need for medical facilities, particularly in light of the medical advances made during the war, and in 1870 he made a will setting aside seven million dollars - mostly in B&O stock - for the incorporation of a free hospital and affiliated medical and nurse's training colleges, as well as an orphanage for colored children and a university. Many board members were on both boards. The hospital and orphan asylum would each be overseen by the 12-member hospital board of trustees, and the university by the 12-member university board of trustees. Johns Hopkins' bequest was used to posthumously found the Johns Hopkins Colored Children Orphan Asylum ^[25] first as he requested, in 1875; the Johns Hopkins University in 1876; the Johns Hopkins Press, the longest continuously operating academic press in America, in 1878; the Johns Hopkins Hospital and the Johns Hopkins School of Nursing in 1889; the Johns Hopkins University School of Medicine 1893; and the Johns Hopkins School of Hygiene and Public Health in 1916.

Johns Hopkins' views on his bequests, and on the duties and responsibilities of the two board of trustees, especially the hospital board of trustees led by his friend and fellow Quaker Francis King, were formally stated primarily in four documents, the incorporation papers filed in 1867, his instruction letter to the hospital trustees dated March 12, 1873, his will, which was quoted from extensively in his Baltimore Sun obituary,^[26] and in his will's two codicils, one dated 1870 and the other dated 1873.^[27]

In these documents, Hopkins also made provisions for scholarships to be provided for poor youths in the states where Johns Hopkins had made his wealth, as well as assistance to orphanages other than the one for African American children, to members of his family, to those he employed, black and white, his cousin Elizabeth, and, again, to other institutions for the care and education of youths regardless of color and the care of the elderly, and the ill, including the mentally ill, and convalescents.

John Rudolph Niernsee, one of the most famous architects of the time, designed the orphan asylum and helped to design the Johns Hopkins Hospital. The original site for the Johns Hopkins University had been chosen personally by Hopkins. According to his will, it was to be located at his summer estate, Clifton. However, a decision was made not to found the university there. The property, now owned by the city of Baltimore, is the site of a golf course and a park named Clifton Park. While the Johns Hopkins Colored Children Orphan Asylum was founded by the hospital trustees, the other institutions that carry the name of "Johns Hopkins" were founded under the administration of the first president of the Johns Hopkins University and Johns Hopkins Hospital, Daniel Coit Gilman and his successors.

Colored Children Orphan Asylum[link]

As per Johns Hopkins' instruction letter, the Johns Hopkins Colored Children Orphan Asylum (JHCCOA)^[28] was founded first, in 1875, a year before Gilman's inauguration, now the founding date of the university. The construction of the asylum, including its educational and living facilities, was praised by The Nation and the Baltimore American, the latter stating that the orphan asylum was a place where "nothing was wanting that could benefit science and humanity". As was done for other Johns Hopkins Institutions, it was planned after visits and correspondence with similar institutions in Europe and America.

The Johns Hopkins Orphan Asylum opened with 24 boys and girls. Under Gilman and his successors, this orphanage was later changed to serve as an orphanage and training school for black female orphans principally as domestic workers, and next as an "orthopedic convalescent" home and school for "colored crippled" children and orphans. The asylum was eventually closed in 1924 nearly fifty years after it opened, and was never reopened.

Hospital, University, Press, and Schools of Nursing and Medicine[link]

As per Hopkins' March 1873 Instruction Letter, the School of Nursing was founded alongside the Hospital in 1889 by the hospital board of trustees in consultation with Florence Nightingale. Both the nursing school and the hospital were founded over a decade after the founding of the orphan asylum in 1875 and the university in 1876. Hopkins' instruction letter explicitly stated his vision for the hospital; first, to provide assistance to the poor of "all races", no matter the indigent patient's "age, sex or color"; second, that wealthier patients would pay for services and thereby subsidize the care provided to the indigent; third, that the hospital would be the administrative unit for the orphan asylum for African American children which was to receive $25,000 in annual support out of the hospital's half of the endowment; and fourth, that the hospital and orphan asylum should serve 400 patients and 400 children respectively, fifth, that the hospital should be part of the university, and, sixth, that religion but not sectarianism should be an influence in the hospital.

By the end of Gilman's presidency, Johns Hopkins University, Johns Hopkins Press, Johns Hopkins Hospital and Johns Hopkins School of Nursing, and Johns Hopkins School of Medicine, and Johns Hopkins Colored Children Orphan Asylum had been founded, the latter by the trustees, and the others in the order listed under the Gilman administration. "Sex" and "color" were major issues in the early history of the Johns Hopkins Institutions. The founding of the School of Nursing is usually linked to Johns Hopkins' statements in his March 1873 instruction letter to the trustees that "I desire you to establish, in connection with the hospital, a training school for female nurses. This provision will secure the services of women competent to care for those sick in the hospital wards, and will enable you to benefit the whole community by supplying it with a class of trained and experienced nurses".

Women's most well known success after the founding of the nursing school was their requirement that they be allowed to attend Johns Hopkins medical institutions after they provided funds that made possible the opening of the School of Medicine in 1893. Five African American women were among the first women enrolled in the Johns Hopkins University's undergraduate school in 1970. Furthermore Kelly Miller and Frederick Scott were the first persons of African descent to attend the Johns Hopkins University's graduate and undergraduate schools, respectively. Frederick Scott was the first graduate of African descent from Johns Hopkins University, and he, Robert Gamble, and Kenyan-born James Nabwangu were the first graduates of African descent from Johns Hopkins University's undergraduate school, and Johns Hopkins' medical schools respectively. Those employees holding jobs in the service sector are those of African descent who have the longest and most continuous history at the Johns Hopkins Institutions.

Legacy[link]

Johns Hopkins Monument

(at Johns Hopkins University)

Following Hopkins' death, the Baltimore Sun wrote a lengthy obituary which closed thus: "In the death of Johns Hopkins a career has been closed which affords a rare example of successful energy in individual accumulations, and of practical beneficence in devoting the gains thus acquired to the public." His contribution to the university that has become his greatest legacy was, by all accounts, the largest philanthropic bequest ever made to an American educational institution.

Johns Hopkins' Quaker faith and his early life experiences, in particular the 1807 emancipation, had a lasting influence throughout his life and his posthumous legacy as a businessman, railroad man, banker, investor, ship owner,^[29] philanthropist and a founder of several Institutions. From very early on, Johns Hopkins had looked upon his wealth as a trust to benefit future generations. He is said to have told his gardener that, "like the man in the parable, I have had many talents given to me and I feel they are in trust. I shall not bury them but give them to the lads who long for a wider education"; his philosophy quietly anticipated Andrew Carnegie's much publicized Gospel of Wealth by more than 25 years.^[30]

His philanthropy, banking and other business practices were founded neither on slavery nor on the separate but unequal racism of the post Civil War years of his life. In this vein, integral parts of his legacy, as an emancipator, a founder of an orphan asylum for African-American youths, a staunch advocate of abolitionism and of quality care not just for those physically ill, but also for the elderly, the poor, no matter their age, gender, or skin color, and the mentally ill, have by and large been overlooked, even in the institutions that carry his name.

In 1973 Johns Hopkins was cited prominently in the Pulitzer Prize winning book The Americans: The Democratic Experience by Daniel Boorstin, former head of the Library of Congress. From November 14, 1975 to September 6, 1976 Hopkins portrait was displayed at the National Portrait Gallery in an exhibit on the democratization of America based on Boorstin's book. In 1989, the United States Postal Service issued a $1 postage stamp in Johns Hopkins' honor, as part of the Great Americans series.^[31]

References[link]