Today the cell theory includes four more ideas
the cells are the building block of structures in living things
the cell is derived from other cells by division
the cell contains information that is used as instructions for growth, development and functioning
the cell is the functioning unit of life; the chemical reactions of life take place within cells.
The idea and concept of cell biology evolved during the 19th century as a result of gradual advancement in the field of microscopy and biochemistry. Today the study of the structure of cells (cytology) is part of a major branch of biology known as cell biology. Due to its wide application many new branches have sprung up in biology. Some of the new branches related to cytology are, Cytotaxonomy, Cytogenetics, Cell physiology, Cytochemistry, Molecular Biology, Cytopathology and Cytoecology.
- Microscopy
- Cytological Techniques
- Plasma Membrane
- Mitochondria
- Ribosomes
- Endoplasmic Reticulum. (ER)
- Golgi apparatus
- Lysosomes
- Centrioles
- Nucleus
- Cancer biology
Microscopy : The cells are very minute and complex organisations. The small dimensions and transparent nature of cell and its organelles pose problems to cell biologists trying to understand its organisation and functioning. Various instruments and techniques have been developed to study cell structure, molecular organization and function.
1 metre (m) = 1000 millimetres (mm)
1 mm (10-3m) = 1000 micrometres (μm)
1 μm (10-6m) = 1000 nanometres (nm)
1 nm (10-9m) = 1000 picometres (pm)
The Angstrom (Å) is 10-10 m. It is sometimes used to record the thickness of cell membranes and the sizes of macromolecules.
While viewing objects, human eyes have limited distinguishing or resolving power. The ability to reveal minute details is expressed in terms of limit of resolution. It is “the smalllest distance that may separate two points on an object and still permit their observation as distinct separate points”. The resolving power of naked human eye is 0.1 mm or 100 μm. It means that we cannot differentiate any points that are closer than this. Hence we need instruments capable of high resolution to see smaller objects.
Power of magnification is different from resolving power. Magnification is ‘the increase in size of optical image over the size of the object being viewed’. Increased magnification without improved resolution results only in a large blurred image. The human eye has no power of magnification
The first useful compound microscope was invented by Francis Janssen and Zacharias Janssen in 1590. It had two lenses with magnification powers between 10x and 30x. Galileo Galilei (1564-1642) invented a simple microscope to study the compound eye of insects. His microscope had only one magnifying lens. Marcello Malpighi (1628-1694) an Italian microanatomist used a microscope to study organ tissues of animals. Robert Hooke an English microscopist in 1665 examined a slice of cork tissue under a compound microscope built by him. He coined the term “cells” to honey comb of cells in cork tissue.
Compound light microscope This microscope uses visible light for illuminating the object. It
contains glass lenses that magnify the image of the object and focus the light on the retina of the observer’s eye. It has two lenses one at each end of a hollow tube. The lens closer to the object being viewed is called objective lens. The lens closer to the eye is called ocular lens or eyepiece. The object is illuminated by light beneath it. A third lens called condenser lens is located between the object and the light source and it serve to focus the light on the object.
Dark field microscope : This type of microscope is useful for viewing suspensions of bacteria. It has a special condenser that allows only rays of light scattered by structures within specimen. The result is an image that appears bright against a dark background, with a high degree of contrast. The process is similar to seeing dust particles floating in a sunbeam.
Phase contrast microscope The phase contrast microscope has special fitments to the objective
lens and sub stage condenser, the effect of which is to exaggerate the structural differences between the cell components. As a consequence, the structures within living, unstained cells become visible in high contrast and with good resolution. Phase contrast microscopy avoids the need to kill cells or to add dye to a specimen before it is observed microscopically
Oil - immersion microscopy In oil-immersion microscopy the light gathering properties of the objective lens are enhanced by placing oil in the space between the slide and objective lens. Normally the technique is used to view permanently mounted specimens. The oil immersion lens gives higher magnification than the normal high power objective lens.
to achieve resolutions as low as 3Å. Electromagnetic coils (ie., magnetic lenses) are used to control and focus a beam of electrons accelerated from a heated metal wire by high voltages, in the range of 20,000 to 100,000 volts. The electrons are made to pass through the specimen. Electrons that do
passes out of the object are focused by an objective coil (‘lens’). Finally a magnified image is produced by a projector coil or ‘lens’. The final image is viewed on a screen or is recorded on photographic film to produce electron micrograph. This type of electron microscope is called transmission electron microscope (TEM) In a compound light microscope, the image is formed due to
differences in light absorption. The electron microscope forms images as a result of differences in the way electrons are scattered by various regions of the object.
Scanning electron microscopy (SEM) This microscope has less resolution power than the TEM (ie., about 200Å). However it is a very effective tool to study the surface topography of a specimen. The whole specimen is scanned by a beam of electrons. An image is created by the electrons reflected from the surface of the specimen. Scanning electron micrographs show depth of focus and a three dimensional image of the object.
