Molecular Makeup of Cells
Cells are the basic structural and functional units of all living organisms. Cells are made up of the compounds you learnt about in the previous chapter: carbohydrates, fats, proteins, nucleic acids and water. The word ‘cell’ was first used by the 17th century scientist Robert Hooke to describe the small pores in a cork that he observed under a microscope. Cells are very small structures. There is no simple consensus on the number of cells in human body. However, the human body is made up of about 1013 cells. Each of these is too small to see with the human eye and it is through the development of microscopic techniques that we have been better able to visualise and understand them.
Early attempts to magnify images of objects through grinding of glass lenses eventually gave rise to the earliest microscope. In 1600, Anton van Leeuwenhoek, a Dutch microbiologist used a simple microscope with only one lens to observe blood cells. He was the first scientist to describe cells and bacteria through observation under microscope. By combining two or more lenses, the magnification of the microscopes was improved, thus allowing scientists to view smaller structures.
The dissecting microscope is an optical microscope used to view images in three dimensions at low resolution. It is useful for low-level magnification of live tissue. The development of the light microscope, (see figure below) which uses visible light to magnify images allowed for up to 1000X magnification of objects through which scientists were able to view individual cells and internal cell structures such as the cell wall, membrane, mitochondria and chloroplasts.
However, although the light microscope allowed for 1000X magnification, in order to see even smaller structures such as the internal structure of organelles, microscopes of greater resolving power (with up to 10 000X magnification) were required.
With the development of electron microscopes the microscopic detail of organelles such as mitochondria and chloroplasts became easier to observe. The Transmission Electron Microscope (TEM) was developed first, followed by the Scanning Electron Microscope (SEM). TEM is used to view extremely thin sections of material. Beams of electrons pass through the material and are focused by electromagnetic lenses.
In SEM the electrons are bounced off the surface of the material and thus produce a detailed image of the external surface of the material. They produce a 3D image by picking up secondary electrons knocked off the surface with an electron collector. The image is then amplified and viewed on a screen. Examples of each of the image types produced by these microscopes are given in the figures below.
Sections for TEM have to be so thin that they have to be prepared using a special piece of equipment called an ultramicrotome.
The apparatus most commonly used in lab microscopy exercises is a simple light microscope. The figure below shows an annotated diagram of a light microscope with a description of the function of each part. The main parts are described in the table that follows and the function of each part is explained.
|Part of the microscope||Description|
|Ocular lens/ eyepiece|
– A cylinder containing two or more lenses.
– These lenses are held at the correct working distance.
– The ocular lens/eyepiece helps to bring the object into focus.
|Revolving nose piece|
The revolving nose piece holds the objectives in place so that they can rotate and can be changed easily.
The objective magnifies the objects.
There are normally three objectives present:
|Coarse adjustment screw|
The coarse adjustment screw is used for the initial focus of the object. By moving the stage up and down, bringing the object closer to or further away from the objective lens.
|Fine adjustment screw|
The fine adjustment screw is used for the final and clear focus of the object.
– A rigid structure for stability.
– The frame is supported by a U-shaped foot leading to the base of the microscope.
|Light source / mirror||– Provides a source of light so that the object can be viewed.|
|Diaphragm and condenser|
The diaphragm and condenser control the amount of light which passes through the slide.
– The microscope slide is placed here.
– The stage contains a clip or clips to prevent the slide from moving around.
– There is a hole in the stage which allows light through.
The ocular, rotating nosepiece and objectives are held above the stage by the arm.