Microscopy

    The study of the cell, and the development of the cell theory were

    possible through the advances in microscopy. Light microscopes focus

    visible light through a condenser lens and that light is refracted through

    an objective and an ocular lens to magnify the image (up to about 1500

    times with resolving power of about 200 nm).

 

    Magnification in light microscopes is limited by the wavelenghts of

    visible light. Electron microscopes were developed in the 1940s that

    dramatically improved the resolving, and thus, magnifying powers.

    Electron beams - with much shorter wavelengths than light - have a

    resolving power of about 0.2 nm.  Electron microscopy uncovered

    cellular ultrastructure.

 

    Transmission EM (TEM) - for cross-sectional studies

    Scanning EM (SEM) - for surface features (3-dimensional)>

 

    Types of cells

 

    All cells have in common: cell membrane, cytoplasm (protoplasm), DNA,

    and ribosomes.

    Prokaryotic cells are found only in the Monera kingdom (bacteria), have

    no true nucleus, and no membrane bound organelles.

    Eukaryotic cells are found in the remaining four kingdoms, have a

    well-defined membrane-bound nucleus containing the genetic material,

    and have specialized organelles many of which are membrane-bound.

 

    Cell size

    Most bacteria range in sizes between one and 10 microns. Most

    eukaryotic cells range from 10-100 microns.  The lower limit to cell size

    is guided by the need to house enough DNA to run the cell and to have

    enough cellular components to sustain life and reproduce. The upper

    limit is guided by the need to keep the surface area to volume ratio

    small (correction: high) to provide for efficient trafficking of nutrients

    into the cell and wastes out of the cell. Cytoplasmic volume must be kept

    small enough for DNA to maintain control.

 

    Eukaryotic cells are large and specialized. Internal membrane-bound

    compartments organized into organelles help to maintain control:

    a. partitioning specialized functions in the cell

    b. each organelle may have unique lipid and protein components for

    unique functions

    c. keeping functions separate. One enzyme in one organelle may be

    dangerous or incompatible with the functioning of another organelle.