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History | Cell Membrane | Cytoplasm

Cellular Biology

Cells are the structural units of all living things (with the possible exceptions of viruses and prions). Prokaryotic cells lack a formed nucleus to house the genetic material (DNA) and nuclear proteins called histones. These cells, found only among bacteria, are small (1-5 microns), have a cell wall outside the cell membrane and lack membrane bound organelles. Eukaryotic cells are larger, have a membrane bound nucleus and various cellular organelles. They also have histones in association with the DNA in the nucleus. Except where specified, this site is dedicated to animal eukaryotic cell biology.

Cells arise in the body from progenitor or stem cells and become specialized for one or more distinct functions such as contraction, nerve conduction, secretion, absorption, or protection. This process of cell specialization is known as cell differentiation. Structural or morphological modifications during differentiation are accompanied by biochemical changes. For example, formation of red blood cells requires the differentiating cells to make specialized proteins for oxygen transport and jetison the nucleus.

Milestones in Cell Biology

1626 Redi postulated that living things do not arise from spontaneous generation.
1655 Hooke described 'cells' in cork.
1674 Leeuwenhoek discovered protozoa. He saw bacteria some 9 years later.
1833 Brown described the cell nucleus in cells of the orchid.
1838 Schleiden and Schwann proposed cell theory.
1855  Virchow postulated that new cells come from preexisting cells.
1857 Kolliker described mitochondria.
1869 Miescher isolated DNA for the first time.
1879  Flemming described chromosome behavior during mitosis.
1883  Germ cells are haploid, chromosome theory of heredity.
1898 Golgi described the golgi apparatus.
1926  Svedberg developed the first analytical ultracentrifuge.
1938  Behrens used differential centrifugation to separate nuclei from cytoplasm.
1939 Siemens produced the first commercial transmission electron microscope.
1941 Coons used fluorescent labeled antibodies to detect cellular antigens.
1952 Gey and coworkers established a continuous human cell line.
1953 Crick, Wilkins and Watson proposed structure of DNA double-helix.
1955 Eagle systematically defined the nutritional needs of animal cells in culture.
1957 Meselson, Stahl and Vinograd developed density gradient centrifugation in cesium chloride solutions for separating nucleic acids.
1965  Ham introduced a defined serum-free medium. Cambridge Instruments produced the first commercial scanning electron microscope.
1976 Sato and colleagues publish papers showing that different cell lines require different mixtures of hormones and growth factors in serum-free media.
1981 Transgenic mice and fruit flies are produced. Mouse embryonic stem cell line established.
1987 First knockout mouse created.
1998 Mice are cloned from somatic cells.
2000 Human genome DNA sequence draft.

Cellular Components
Plasma (cell) membrane
Each eukaryotic cell has as its boundary to the outside a cell membrane (7.5 to 10 nm in thickness) that envelopes the cytoplasmic matrix containing specialized membrane-bound components called organelles. The cell or plasma membrane is a lipid bilayer containing proteins, cholesterol, and oligosaccharides that functions as a selective barrier for entry and exit of substances. The plasma membrane, by limiting the transport of some things and facilitating the movement of others helps to maintain the internal environment of the cell, which is different from the extracellular fluid. Under an electron microscope membranes appear to have a trilaminar structure. This is because the lipid bilayers are arranged such that hydrophilic phospholipid groups are oriented toward the outside of the membrane while the more hydrophobic lipid fatty acid chains form the middle of the trilaminar structure. The molecular make up of each half of the membrane is different in that different lipids and proteins are more abundant in one side over the other.

Proteins are a very important part of the cell membrane. Basically they can be classified into two groups based on physical distribution. Integral proteins are embedded within the cell membrane and may in fact pass multiple times through the membrane. Peripheral proteins are loosely associated with membrane surfaces. Carbohydrate portions of glycoproteins and glycolipids are found on the external surface of the cell membrane where they are important parts of receptor molecules. Receptors are necessary to cellular signalling, adhesion, and recognition. Most proteins are fixed in place within the cell membrane by interactions with the cytoskeleton. However, some integral proteins can move about and sometimes will accumulate on one region of the membrane in a process called capping.

The cell membrane is not static. It is remodeled by the addition of new membrane vesicles from the Golgi while removal takes place in the form of endocytotic, phagocytotic and pinocytotic vesicles being formed and then fused with lysozomes for processing. Membrane receptors and membrane are often conserved and recycled to the plasma membrane. This membrane trafficking is important in the cell economy.

The cytoplasmic matrix is not an unstructured liquid gel as once was thought. The cytosol makes up some 50% of the cell volume. It contains numerous cytoskeletal elements, organelles, vesicles, metabolic enzymes and sometimes pigment deposits. The matrix coordinates the movement of intracellular organelles, and provides a framework for the organization of enzyme pathways such as those within the glycolytic series. All of the necessities of protein synthesis are found within the cytosol and it contains numerous enzymes that build large molecules and break down small molecules. Protein motors are present in the cytosol to help transport things along the cytoskeletal framework.

Cellular Organelles
Cellular Communication
Cell Division

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