The objective of this chapter is to make students familiar with the basic concepts of microscopy and why they are important in the advancement of biology. Here are the important points that you should familiarize at the end of this unit.

The cell is the basic structural and functional unit of life. Some of the vital processes of life take place inside a cell. Due to its small size, it cannot be observed with naked eyes. It was after the discovery of a microscope by Robert Hooke that scientists came to know of the presence of the cell.

In 1662 Robert Hooke, one of the founders of Royal society London, devised a microscope to observe a piece of cork. What he saw was a small compartment like structures which he called a cell. Later on, with the advancement of microscopes scientists found out other components or organelles that make up the whole cell.

The development of the microscope led to the discovery of cells. With this scientists came up with cell theory. The main postulates of cell theory are as follows:

• All living things are made up of cells.

• New cells arise from the division of preexisting cells.

• The cell is the basic structural and functional unit of life.

With the advancement in microscopes, we came to know more about the structure of cells. All living cells comprises of 3 main components. The outermost boundary is called the cell membrane. It is made up of phospholipid molecules that separate the intracellular compartment to an extracellular compartment. In the center is the nucleus which is a membrane-bound organelle and controls all the functions of the cell. In between the nucleus and the cell membrane is cytoplasm. It is the site for all chemical reactions of life. These are the basic structures that can be seen with a light microscope. The invention of the electron microscope made it possible to visualize smaller structures that are present inside the cell. These structures are called organelles. They help the cell in performing all the functions.

Plants and animals both are living organisms but they both have different functions to do and have a different mode of life. This is possible because of the different structures that their cell have.

The animal cell has a cell membrane as the outermost boundary while plant cells have an additional layer called the cell wall. Cell wall helps in giving a definite shape to the plant cell as well as it helps them in surviving different ph levels. Besides this, the animal cell has a nucleus in the center while in plant cells a vacuole is present which pushes the nucleus to the periphery. Plant cells also have one additional organelle called chloroplast which is the site for photosynthesis. Plant cells lake centrosome which is present in animal cells close to the nucleus. They are responsible for spindle formation in animal cells during cell division.

A microscope is an instrument that magnifies the image of an object. It magnifies it at a scale which convenient for the observer to study it closely. There are two types of microscope mostly used.

• Compound microscope

• Electronic microscope

A compound microscope comprises of two sets of lenses to magnify the image. (fig 1.)

The components can be divided into two parts.

It has three parts:

• Head/Body: It consists of optical lenses.

• Base: It gives support to the microscope and has the illuminator.

• Arm: it connects the head to the base.

The following are the optical parts of the microscope.

1) EYEPIECE

The eyepiece has the lens through which the observer views the specimen.

2) EYEPIECE TUBE

It contains the eyepiece and objective lenses.

3) NOSE PIECE

Nose piece has the objective lens which is placed near the object.

4) OBJECTIVE LENSES

There are different objective lenses with different focal lengths. Thus, the observer can change the magnification.

5) COARSE AND FINE FOCUS KNOB

They are used to adjust the focus to make the image clear.

6) STAGE

The stage is where the specimen is mounted.

7) ILLUMINATOR

The illuminator is the light source for the microscope.

The image of the cell or specimen when seen in a microscope is magnified numerous times. That is not the actual size of the object. Here is how we can calculate the actual size of the object.

There are two ways of how object size can be calculated. One is when the magnification of the microscope is given. There are times when magnification is not mentioned. Then the method of calculation will be different.

Magnification= size of image/ size of specimen

• Use a ruler to measure the size of the image.

• For example, the size of the image is 19um. And the magnification is X100.

• Use the above equation to find the size of the specimen.

100=19/size of specimen

Size of specimen=19/100

Size of specimen=0.0019 micrometer = 1.9 nm

There are times when the magnification of the microscope is not mentioned. Instead, a scale bar is given. In that case, calculations are slightly different.

In this case, magnification is to be calculated first.

• Use a ruler to measure the size of the image.

• For example: The image size is for example 2.6cm=26000micrometer

• The specimen size is 10um. (this is taken from the scale bar. Obviously, the original size of the specimen will be different from this.)

• Convert the units to make the same.

• Use the equation to calculate magnification:

Magnification= size of image/ size of specimen

M= 26000/10

M=2600

Once the magnification is known now we can easily calculate the size of the object.

M= size of image/size of specimen

Size of specimen= size of image/M

Size of specimen= 26000/2600

Size of specimen=10 micrometer.

Unit conversion:

 10-2 centi

10-3 mili

10- 6 micro

 10-9 nano

Objects can also be measured directly on a microscope by using graticle. Graticle is a transparent scale present on an eyepiece. When the object is viewed the graticle is also present. Using it the dimension can be marked in an arbitrary unit. After that, a stage micrometer is placed instead of the slide on the stage. When both the graticle and scale are in place the dimensions can be noted down in micrometer.

Here are steps how it can be done:

• Place the graticle on the eyepiece.

• Align the slid with the graticle to measure the size.

• Then remove the slide and place the micrometer.

• Align both the graticle and stage micrometer.

• Note the size of the object in micrometers.

Magnification is defined as the ability of an optical device to increase the size of the image.

The resolution, on the other hand, is the ability of a microscope to separate two objects from each other. So, the better the resolution of a microscope the finer the image quality will be.

In the light microscope, the resolution is determined by its wavelength. The resolution will be half of the wavelength. Visible light has a wavelength of 400-700 nm. Thus, the resolution of the light microscope will be calculated as 200nm. Any object smaller than this is difficult to be focused on a light microscope.

Electron microscope works by focusing the beam of electrons from the specimen. Since electrons have a shorter wavelength. The resolution achieved by an electron microscope is much better than a light microscope.

It is due to the electron microscope that we have discovered the different organelles present inside the cell.