Cell
From Encyclopædia
The concept that all living material is made of cells or the chemical products of cells is a
Central, unifying one in modern BIOLOGY. The concept--of the cell as the fundamental, irreducible unit of life--is the basis for an understanding of living organisms as well as the foundation of modern research in the life
sciences.HistoryThe word cell was introduced by Robert HOOKE in 1665. In the course of microscopic studies of cork, Hooke noticed that the material under study was made up of many "little boxes." Ten years later, Anton van LEEUWENHOEK, using a
hand-held
lens, described several tiny microscopic creatures later found to be bacteria and
Protozoa. Over the next 150 years, numerous investigators, using the MICROSCOPE, turned to the examination of living tissue and developed detailed descriptions of it.The cell theory of life is a concept that was developed by many scientists over a period of about 150 years. The initial statement of the cell theory was given by Lorenz Oken (1779-1851) in 1805. In 1839, Theodore SCHWANN stated the theory essentially in its contemporary form. Subsequent research in the origin of cells led Rudolf VIRCHOW to state (1858) the generalization that every cell is formed from a preexisting cell.The current theory of cellular biology may be presented as a set of six statements: (1) all living material is made up of cells or the products of cells; (2) all cells are derived from previously existing cells; most cells arise by cell division, but in sexual organisms they may be formed by the fusion of a sperm and an egg; (3) a cell is the most elementary unit of life; (4) every cell is bounded by a plasma membrane, an extremely thin
skin separating it from the environment and from other cells; (5) all cells have strong biochemical similarities; and (6) most cells are small, about 0.001 cm (0.00004 in) in length; for example, the smallest cells of the microorganism mycoplasma are 0.3 microns in size, whereas some giant algal cells may be several centimeters long.Cell ClassesIn recent years it has become clear that two classes of cells exist: the PROKARYOTES and the
eukaryoteS. The Prokaryotes include the bacteria and the blue-green algae (the Monera kingdom). These are all single-celled organisms that lack both a true
nucleus and other membrane-bounded cellular substructures. The genetic material responsible for the transmission of characteristics from one generation to another is contained in a single large
molecule of circular deoxyribose
nucleic acid, or DNA. The
eukaryotes include
plants, animals,
Protozoa, and
fungi. These cells contain nuclei and other membrane-bounded cell components, or organelles, such as mitochondria and plastids. The genetic material is organized into chromosomes.Methods of StudyThe study of cell structure includes the fields of CYTOLOGY (for cells) and HISTOLOGY (for tissues), whereas the function of cells is studied in CELL PHYSIOLOGY, BIOCHEMISTRY, and cytogenetics. The first instrument used in studying cell structure was the
light microscope, which remains an important tool today. The
electron microscope and the scanning
electron microscope have extended the range of observed detail as well as the kinds of cell structures that can be observed.In examining biological specimens, using microscopic techniques, it is necessary to stain the material and cut it into samples thin enough for a
light beam or an
electron beam to penetrate them. First, the tissue is treated, to "fix" the structures so they will not be altered by the staining and slicing. Usually this is done by using chemicals such as
alcohol and
formaldehyde. Machines for slicing embedded tissue preparations are called microtomes, and the procedure is known as sectioning.Stains have been developed that react differently with cell structures, depending on their chemical composition or enzymatic activity. The
science of analyzing structures by their chemical reactions is called cytochemistry, and the study of th? staining properties of tissues is part of histochemistry. The use of stains containing radioactive
atoms, known as autoradiography, may be combined with the study of cell physiology by feeding cells specific compounds with radioactive
atoms and then microscopically observing on a photographic emulsion the distribution of radioactive events.The generalized functions of most cells are maintenance, synthesis of cell products, and cell division. These require that the cell take in nutrients and excrete waste products. The nutrients are used either as building blocks in synthesizing large
molecules, or they are oxidized, producing energy for powering the cell's activities. Because synthesis, maintenance, and mechanical and electrical activity all require energy, a major chemical activity in nearly all types of cells is the energy-linked
conversion of metabolites. Adenosine triphosphate (ATP) is the universal energy-transfer
molecule; it is constantly utilized and regenerated by energy-yielding chemical reactions.
[edit] COMPONENTS OF CELLS
A cell is bounded by a cell membrane. The material known as the cytoplasm lies within the membrane and contains several organelles and granules in suspension. plant and bacteria cells have an additional membrane, or wall, that lies outside the cell membrane and is not essential to the functioning and growth of the cell.Plasma MembraneCells are surrounded by a thin membrane of lipid (fat) and protein, about 100 angstroms (0.0000004 in) thick. It Controls the transport of molecules in and out of the cell and thus serves as a line of demarkation between the cell and the surrounding tissue. Outside the plasma membrane may be other envelopes such as the outer membrane, the cell wall (in plants), and the extracellular material. Unit membranes also occur in a eukaryote cell's interior, for example, as part of the endoplasmic reticulum, nuclei, and mitochondria. The exterior portions of the cell surfaces determine cell-to-cell interactions and are thus important in the formation and Control of tissue. The extracellular material also acts as a glue that holds cells together in tissues. The molecules outside the cell are usually composed of protein and carbohydrate.NucleiMost cells have a single nucleus bounded by a nuclear envelope, or membrane, with pores. Pores provide continuity between the nucleus and the cytoplasm. The nucleus contains one or more discrete structures, known as nucleoli, which are sites of ribosomal ribonucleic acid (RNA) synthesis. Hereditary information is carried in the DNA contained within the chromosomes in the nucleus. This information is transcribed into the RNA in the nucleus, which serves as a messenger. The messenger then moves outside of the nucleus to the ribosomes where it guides the synthesis of proteins. Thus, the nucleus directs the activity of the cell.RibosomesRibosomes are tiny particles within the cell. Made of RNA and protein, they are present in large numbers in most cells and are the site of protein synthesis (the manufacture of large protein molecules from AMINO ACID subunits).Endoplasmic ReticulumWithin most eukaryotic cells is a complex set of membranous structures. When viewed in the electron microscope, the membranes are either rough (covered with granules or ribosomes) or smooth. Generally, the rough ENDOPLASMIC RETICULUM is highly developed in cells that synthesize large amounts of protein.Golgi ApparatusA special type of membrane aggregate is often found near the nucleus. This collection of membranes is called the GOLGI APPARATUS. It is visible when viewed under the microscope because the outer portion stains with the metal osmium. In cells that synthesize and secrete products, the Golgi apparatus is the site of the material that is accumulated.MitochondriaMitochondria (see mitochondrion) are composed of an outer membrane and a highly convoluted inner membrane. A series of chemical reactions that occur on the inner membrane convert the energy of oxidation into the chemical energy of ATP. In this process, called oxidative phosph?rylation, the predominant energy transfer molecule is ATP. Almost all of the energy passes through this molecule before being utilized in cell function. Cells with high rates of metabolism usually have a large number of mitochondria.ChloroplastsPlant cells contain organelles of photosynthesis known as chloroplasts (see chloroplast). These structures have a membranous outer envelope and a high laminated inner membranous structure. The interior membranes contain the chlorophyll and are responsible for photosynthesis. chloroplasts are viewed as an example of a more general class of organelles called plastids.CentriolesMost cells have two cylindrical bodies, called centrioles, located near the nucleus. The centrioles appear as sets of triple tubules. Centrioles play a part in cell division.Other OrganellesThe material containing the organelles is called ground substance, or cytoplasm. It contains proteins, small molecules, and a group of entities organized as microfilaments and microtubules. Microfilaments are long, thin, contractile rods that appear to be responsible for the movement of cells, both external and internal. Microtubules are hollow, cylindrical aggregates of tubelike structure that help give the cell shape and form; they are also involved in other cell processes.lysosomes (see LYSOSOME) are small bodies where large numbers of enzymes are stored. Many cells, particularly those of plants, contain large liquid-filled areas known as vacuoles; the vacuoles are believed to be involved in digestion or excretion, or both.
