From the Cell to the Cross

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Image from Purves et al. Diffusion is the net movement of a substance liquid or gas from an area of higher concentration to one of lower concentration. You are on a large 10 ft x 10 ft x10 ft elevator.

From the Cell to the Cross: A Gripping True-Life Story

An obnoxious individual with a lit cigar gets on at the third floor with the cigar still burning. You are also unfortunate enough to be in a very tall building and the person says "Hey we're both going to the 62nd floor! Eventually you are unable to escape the smoke! An example of diffusion in action.

Between the cross and the sword: the crisis of the gene concept

Nearer the source the concentration of a given substance increases. You probably experience this in class when someone arrives freshly doused in perfume or cologne, especially the cheap stuff. Since the molecules of any substance solid, liquid, or gas are in motion when that substance is above absolute zero 0 degrees Kelvin or degrees C , energy is available for movement of the molecules from a higher potential state to a lower potential state, just as in the case of the water discussed above.

The majority of the molecules move from higher to lower concentration, although there will be some that move from low to high. The overall or net movement is thus from high to low concentration. Eventually, if no energy is input into the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system. The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of organically produced chemicals inside the cell. Electron microscopic examinations of cell membranes have led to the development of the lipid bilayer model also referred to as the fluid-mosaic model.

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The most common molecule in the model is the phospholipid , which has a polar hydrophilic head and two nonpolar hydrophobic tails. These phospholipids are aligned tail to tail so the nonpolar areas form a hydrophobic region between the hydrophilic heads on the inner and outer surfaces of the membrane. This layering is termed a bilayer since an electron microscopic technique known as freeze-fracturing is able to split the bilayer. Diagram of a phospholipid bilayer. Phospholipids and glycolipids are important structural components of cell membranes. Phospholipids are modified so that a phosphate group PO 4 - replaces one of the three fatty acids normally found on a lipid.

The addition of this group makes a polar "head" and two nonpolar "tails".

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Structure of a phospholipid, space-filling model left and chain model right. Diagram of a cell membrane. This image is copyright Dennis Kunkel at www.

Cholesterol is another important component of cell membranes embedded in the hydrophobic areas of the inner tail-tail region. Most bacterial cell membranes do not contain cholesterol. Proteins are suspended in the inner layer, although the more hydrophilic areas of these proteins "stick out" into the cells interior as well as the outside of the cell. These integral proteins are sometimes known as gateway proteins.

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  • Proteins also function in cellular recognition, as binding sites for substances to be brought into the cell, through channels that will allow materials into the cell via a passive transport mechanism, and as gates that open and close to facilitate active transport of large molecules. The outer surface of the membrane will tend to be rich in glycolipids , which have their hydrophobic tails embedded in the hydrophobic region of the membrane and their heads exposed outside the cell.

    These, along with carbohydrates attached to the integral proteins, are thought to function in the recognition of self. Multicellular organisms may have some mechanism to allow recognition of those cells that belong to the organism and those that are foreign. Many, but not all, animals have an immune system that serves this sentry function. When a cell does not display the chemical markers that say "Made in Mike", an immune system response may be triggered. This is the basis for immunity, allergies, and autoimmune diseases.

    Organ transplant recipients must have this response suppressed so the new organ will not be attacked by the immune system, which would cause rejection of the new organ. Allergies are in a sense an over reaction by the immune system.

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    Autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythmatosis, happen when for an as yet unknown reason, the immune system begins to attack certain cells and tissues in the body. Note: Working electrode, reference electrode and gasket are sold separately. The dual glassy carbon electrode is considered to be a standard working electrode for cross flow cell.

    Anatomy of the Cell Membrane

    It is composed for two glassy carbon electrodes, of the 3 mm placed in series. Also it coul be rotated 90 degrees and applied as a parallel mode. The selectivity improves with the application of dual series electrode. In the parallel mode, identification of the substance, from the different applied voltage response ratio, is possible.

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    From the Cell to the Cross

    Continuous cell lines consist of cultured cells derived from a specific donor and tissue of origin that have acquired the ability to proliferate indefinitely. However, there are cautions to be aware of when using continuous cell lines, including the possibility of contamination, in which a foreign cell line or microorganism is introduced without the handler's knowledge.

    Two practical responses are suggested here. The list currently contains cell lines, drawn from 68 references. Continuous cell lines represent a readily accessible and easily studied resource for research into health and disease. These cell lines have acquired the ability to proliferate indefinitely if grown in the appropriate culture conditions; usually this is a rare event, since the majority of cells even in tumor tissue will cease proliferation after a limited number of cell divisions.

    Cell lines are particularly attractive models for studying malignant disease. Despite these advantages, numerous cautions have emerged from the literature regarding appropriate use of cell lines as model systems. Cell lines become contaminated when a foreign cell line or microorganism is introduced without the handler's knowledge. Simple errors during labeling of culture flasks, truncation of the cell line name or typographic errors in a published manuscript, can result in significant confusion for years after the event when another researcher attempts to use the same cell line for ongoing experimental work.

    These data suggest that poor practice within some laboratories results in contamination of multiple cell lines with multiple contaminants, which can then be disseminated more widely if these cultures are used by others. Rates of contamination for leukemia—lymphoma cell lines. Other studies have pointed out that testing of cell lines is often infrequent, resulting in the failure to detect contaminated samples.

    A practical example of the consequences of cell line contamination can be found in a recent study published by Berglind et al. In addition, many researchers obtain cell lines from one another, rather than approaching the originator or purchasing the cell line from a cell bank performing quality control testing. This may be faster or cheaper than obtaining cultures from a reputable source but the practice makes contamination more prevalent and harder to detect.

    Having defined the problems, it is time to focus on what can be done. Every researcher involved in cell culture will have cell lines currently in culture, stored in liquid nitrogen or may be commencing work on a new cell line. Check your cultured cells. Unless a cell line has come directly from a repository or other laboratory performing identification testing, it should be tested on arrival, and all cultures should be periodically tested while in use, before cryopreservation and when thawed from liquid nitrogen.

    Notes are also included for some cell lines. The list is made available in Excel spreadsheet or PDF format for easy accessibility. The cell lines listed within this database are divided into 2 tables.

    A full list of references is also given. Cell lines affected are primarily human, although cultures from at least 8 other species are included, and come from a wide spectrum of tissue types. The cell or tumor type is given within the list where known; extensive work has been done by some cell banks and laboratories in this area to characterize the actual cell type or tumor type.

    For the intraspecies contaminants, all of those detected were human but it is likely that this relates to the difficulty of detecting intraspecies contaminants for nonhuman species. It is important for such a list to be continually updated and feedback is welcome for this purpose. An earlier version of the database was released online by ECACC 31 ; 6 cell banks have now agreed to make the database available online and to update this information where necessary.

    In future, it is envisaged that the current list of misidentified cell lines will be included in a new initiative improving access to authentication data. Strict criteria for STR profiles derived from cancer cell lines are being developed.