BSC 314 Lecture Notes - Lecture 3: Lipid Bilayer, Symbiogenesis, Cell Membrane
26 Jun 2018
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Cell Theory
The modern cell theory, one of the fundamental generalizations of biology, holds that:
All organisms are composed of one or more cells.
New cells come from pre existing cells; lifeforms today have descended in ‐
unbroken continuity from the first primitive cells that arose on earth more than 3.5
billion years ago.
Hereditary information passes from parent cell to daughter cell.
The fundamental biochemical reactions of life take place within cells.
Methods of studying cells
Microscopes. Cytology—the scientific study of cells—has progressed simultaneously
with the development of better, more powerful microscopes and cell preparation
techniques. Light microscopes, invented in the 1500s, today can magnify in the range
of 100x to 1000x and are at the maximum of their resolving power at about 0.2 m Φ
(micrometer = 1/ 1,000,000 meter). They are limited from further improvement by the
wavelength of visible light that is used to create the image (the shorter the wavelength,
the greater the resolution).
Beams of electrons have shorter wavelengths than visible light,
hence electron microscopes (which focus streams of electrons) are able to achieve
resolutions of 2–0.4 nm (nanometer = 1/ 1,000,000,000 meter = 1/1000 m) and Φ
magnifications of over 100,000x. One type of electron microscope, the transmission
electron microscope ( TEM), focuses a beam of electrons through an object much as
a light beam is used in light microscopes, whereas a scanning electron
microscope ( SEM) forms an image from electrons that are scattered from
the surface of a specimen.
Laboratory techniques. In light microscopy dyes that stain particular chemicals are
commonly used to identify cellular structures in thin sections of tissue mounted on
microscope slides. Specimens observed by electron microscopy are cut in much thinner
sections (electrons don't penetrate material well) and examined in a vacuum. These
preparations cause problems of interpretation of the microscopic image. Does the
material really look like this in living cells or has preparation altered its appearance?
Living material can not be examined using electron microscopes so the answer is moot.
Today's understanding of cells is based on two approaches: 1.) the sometimes very
difficult biochemical analyses of cells and 2.) the increasing greater magnification of
their constituent parts.
Cell size
Plant cells vary in size from about 7 m—the diameter of dividing cells at the tips of Φ
roots and shoots—to fiber cells a few micrometers wide but a meter or more in length.
Most plant (and animal) cells are between 10–100 m and thus too small to be seen Φ
without magnification since human eyes have a resolving power (resolution) of
about 100 m. (Resolving power is the ability to see adjacent objects as separate; Φ
magnification simply makes things larger.
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Document Summary
The modern cell theory, one of the fundamental generalizations of biology, holds that: All organisms are composed of one or more cells. New cells come from pre existing cells; lifeforms today have descended in unbroken continuity from the first primitive cells that arose on earth more than 3. 5 billion years ago. Hereditary information passes from parent cell to daughter cell. The fundamental biochemical reactions of life take place within cells. Cytology the scientific study of cells has progressed simultaneously with the development of better, more powerful microscopes and cell preparation techniques. Light microscopes, invented in the 1500s, today can magnify in the range of 100x to 1000x and are at the maximum of their resolving power at about 0. 2 m (micrometer = 1/ 1,000,000 meter). They are limited from further improvement by the wavelength of visible light that is used to create the image (the shorter the wavelength, the greater the resolution).
