Section 2: Molecular make up of Cells
The practical Life Science techniques are an important part of the Life Science assessment program. This section addresses some of these important skills. Learners are introduced to the various microscopes and this enables the skills of drawing, labelling and annotating diagrams and micrographs, using microscopes as well as calculating magnification of cells. (This will be covered later in this Chapter).
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. The human body is made up of 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, (Figure 2.5) 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 Figures Figure 2.1 to Figure 2.3.
Sections for TEM have to be so thin that they have to be prepared using a special piece of equipment called an ultramicrotome.
The world through a microscope | ||
![]() SEM: A natural community of bacteria growing on a single grain of sand. | ![]() SEM: These pollen grains show the characteristic depth of field of SEM micrographs. | ![]() TEM: Image of chloroplast, showing thylakoid discs within a eukaryotic cell. |
Transmission electron microscopes can magnify an image 50 million times.
Figure 2.4: Transmission electron microscope in use.
The apparatus most commonly used in lab microscopy exercises is a simple light microscope. Table 2.1 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. |
Objective | 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. |
Frame | - 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. |
Stage | - 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.
How to use a microscope correctly
Remember that microscopes are expensive scientific equipment and need to be handled with care to prevent damaging them. Proper lens paper should be used when cleaning dust or dirt off any lenses. Avoid getting moisture on the objective lenses. Dust and moisture are the biggest enemies of microscopes.
If using a mirror for illumination instead of a light bulb, never reflect direct sunlight as you could damage your eyes.
Differences between the light microscope and transmission electron microscope
Property | Light Microscope | Transmission Electron Microscope |
Source | Light | Beam of electrons |
Resolution (how far apart two objects must be in order to be distinguished as separate) | Under optimal conditions (clean lenses, oil immersion), the resolution is micrometres or 2 thousands of a millimetre | Resolution of a transmission electron microscope is about nanometres () which is about millionth of a millimetre. This means that a transmission electron microscope has about times the resolving power of a light instrument |
Material (alive/ dead) | Alive or dead. Bright field or phase contrast microscopes enable viewer to observe living cells. Specimens need to be stained. | Dead. Electron microscope images are produced by passing an electron beam through tissues stained with heavy metals. |
Example of microscope image | ![]() Bacterial spores as seen under light microscope. | ![]() Chlamydomonas reinhardtii, a single celled green algae, as seen under the transmission electron microscope. |
Microscopes magnify an image using a lens found in the eye-piece, which is also known as the ocular lens. The image is further magnified by the objective lens. Thus the magnification of a microscope is: magnification power of the eye-piece x the power of the objective lens
Example: if the eyepiece magnification is 5X and the objective lens' magnification is 10X, the image of the object viewed under the microscope is 50X bigger than the object:
Calculating the field of view
When viewing an object through a microscope, the diameter of the circle through which you view the object is known as the field of view.
As the magnification increases, the field of view decreases.
To measure the field of view, use a microscope slide with a tiny ruler printed on it. For example, the size of the field of view shown below under low power magnification is approximately 1 mm.
Figure 2.6: Field of view is approximately 1 mm.
Once the size of the field of view is known, we can estimate the size of the objects being viewed under the microscope. At 10 X magnification, the field of view is mm. If the magnification is increased to 100 X, what will the new field of view be?
mm at 10 X magnification
mm at 100 X magnification
If magnification is increased 10-fold, the field of view will decrease 10-fold. Thus it will become 0.1 mm. What this means is that at higher magnification, we are able to see objects of smaller and smaller size within our field of view. This is why at higher magnification, the field of view becomes smaller.
At 500 X magnification, the field of view of a microscope is mm. What will the field of view be at 100X magnification?
Calculating magnification and using scale bars
When drawing cells or cellular structures, your diagrams will usually be much larger than the actual size of the structures you will be drawing. The magnification is given by:
When a scale bar is provided with the diagram, the magnification is given by:
Calculate the overall magnification of a compound light microscope with a magnification of 10 X due to the eyepiece and a magnification of a 100X due to the objective lens.
Using the formula:
If the measured length of the magnified beetle larva image shown below was 2 centimetres (20 mm), the ocular magnification of the microscope is 5 X and you are using an objective lens magnification of 10 X, what is the actual length of the larva in millimetres?
Use the same formula as above
If the image is 50 X larger than the object, what is the size of the object? Calculate this by simple proportion given in the formula below.
Calculate the actual length of AB from the image shown in the micrograph given with the scale bar given below.
This should be approximately
Given that the measured length of the scale bar is approximately , work out the length AB:
Activity 3.2: Investigation of cell size
Learners to be given photomicrographs to practice this exercise.
Activity 3.3: Drawing diagrams of scale
Learners to prepare slides to view under the microscope. Using the skills from the previous activity they are to do cell calculations.