Part of a series on Biochemistry Key components Biomolecules Metabolism Index Outline History and topics History Biochemistry Cell biology Bioinformatics Enzymology Genetics Immunology Molecular biology Plant biochemistry Structural biology Branches of biochemistry List of biochemists Glossaries Glossary of biology Glossary of chemistry Portals: Biology, MCB v t e Cell biology Cell biology (formerly called cytology, from the Greek κυτος, kytos, "vessel") is a branch of biology that studies the different structures and functions of the celland focuses mainly on the idea of the cell as the basic unit of life. Cell biology explains the structure and organization of the organelles they contain. It includes the physiological properties, metabolic processes, signaling pathways, life cycle, and interactions with their environment. This is done both on a microscopic and molecular level as it encompasses prokaryotic cells and eukaryotic cells. Knowing the components of cells and how cells work is fundamental to all biological sciences; it is also essential for research in bio-medical fields such as cancer, and other diseases. Research in cell biology is closely related to genetics, biochemistry, molecular biology, immunology, and developmental History Cells, that were once invisible to the naked eye, became visible in 17th century Europe with the invention of the compound microscope. Robert Hooke was the first person to term the building block of all living organisms as "cells" after looking at cork.[1] The cell theory states that all living things are made up cells.[2] The theory also states that both plants and animals are composed of cells which was confirmed by plant scientist, Matthias Schleiden and animal scientist, Theodor Schwann in 1839.[3] 19 years later, Rudolf Virchow contributed to the cell theory,arguing that all cells come from the division of preexisting cells.[4] In recent years, there have been many studies which question the cell theory. Scientists have struggled to decide whether viruses are alive or not. Viruses lack common characteristics of a living cell, such as membranes, cell organelles, and the ability to reproduce by themselves.[5] Viruses range from 0.005 to .03 microns in size whereas Bacteria range from 1-5 microns.[6] Modern day 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, to derive treatments and other medications, etc. The techniques by which cells are studied have evolved. Advancement in microscopic techniques and technology such as fluorescence microscopy, phase-contrast microscopy, dark field microscopy, confocal microscopy, cytometry, transmission electron microscopy, etc. have allowed scientists to get a better idea of the structure of cells. Cell structure There are two fundamental classifications of cells: prokaryotes and eukaryotes. The major difference between the two is the presence and/or absence of organelles. Other factors such as size, the way in which they reproduce, and the number of cells distinguish them from one another.[8] Eukaryotic cells include animal, plant, fungi, and protozoa cells which all have a nucleus enclosed by a membrane. Prokaryotic cells, lacking an enclosed nucleus, include bacteria and archaea. Prokaryotic cells are much smaller than eukaryotic cells, making prokaryotic cells the smallest form of life.[9] Cytologists typically focus on eukaryotic cells whereas prokaryotic cells are the focus of microbiologists, but this is not always the case. Internal cellular structures The study of the cell is done on a molecular level; however, most of the processes within the cell are made up of a mixture of small organic molecules, inorganic ions, hormones, and water. Approximately 75-85% of the cell’s volume is due to water making it an indispensable solvent as a result of its polarity and structure.[10] These molecules within the cell, which operate as substrates, provide a suitable environment for the cell to carry out metabolic reactions and signalling. The cell shape varies among the different types of organisms, and are thus then classified into two categories: eukaryotes and prokaryotes. In the case of eukaryotic cells - which are made up of animal, plant, fungi, and protozoa cells - the shapes are generally round and spherical,[11] while for prokaryotic cells – which are composed of bacteria and archaea - the shapes are: spherical (cocci), rods (bacillus), curved (vibrio), and spirals (spirochetes).[12] Cell biology focuses more on the study of eukaryotic cells, and their signalling pathways, rather than on prokaryotes which is covered under microbiology. The mainconstituents of the general molecular composition of the cell includes: proteins and lipids which are either free flowing or membrane bound, along with different internal compartments known as organelles. This environment of the cell is made up of hydrophilic and hydrophobic regions which allows for the exchange of the above-mentioned molecules and ions. The hydrophilic regions of the cell are mainly on the inside and outside of the cell, while the hydrophobic regions are within the phospholipid bilayer of the cell membrane. The cell membrane consists of lipids and proteins which accounts for its hydrophobicity as a result of being non-polar substances.[10] Therefore, in order for these molecules to participate in reactions, within the cell, they need to be able to cross this membrane layer to get into the cell. They accomplish this process of gaining access to the cell via: osmotic pressure, diffusion, concentration gradients, and membrane channels. Inside of the cell are extensive internal sub-cellular membrane-bounded compartments called organelles. Processes The growth process of the cell does not refer to the size of the cell, but instead the density of the number of cells present in the organism at a given time. Cell growth pertains to the increase in the number of cells present in an organism as it grows and develops; as the organism gets larger so too does the number of cells present. Cells are the foundation of all organisms, they are the fundamental unit of life. The growth and development of the cell are essential for the maintenance of the host, and survival of the organisms. For this process the cell goes through the steps of the cell cycle and development which involves cell growth, DNA replication, cell division, regeneration, specialization, and cell death. The cell cycle is divided into four distinct phases, G1, S, G2, and M. The G phases – which is the cell growth phase - makes up approximately 95% of the cycle.[13] The proliferation of cells is instigated by progenitors, the cells then differentiate to become specialized, where specialized cells of the same type aggregate to form tissues, then organs and ultimately systems.[10] The G phases along with the S phase – DNA replication, damage and repair - are considered to be the interphase portion of the cycle. While the M phase (mitosis and cytokinesis) is the cell division portion of the cycle.[13] The cell cycle is regulated by a series of signalling factors and complexes such as CDK's, kinases, and p53. to name a few. When 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 cause to the organism’s survival. Techniques used to study cells Cells may be observed under the microscope, using several different techniques; these include optical microscopy, transmission electron microscopy, scanning electron microscopy, fluorescence microscopy, correlative light-electron microscopy, and confocal microscopy. There are several different methods used in the study of cells: 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 [14] 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 The binding of an antibody to a target protein Collection of the target protein through elution[15] Proteins extracted from cell membranes by detergents and salts or other kinds of chemic