Evolution is one of the Biology Topics that has been debated and studied for centuries, exploring the process by which species change over time.
How Cells are Studied – Concepts of Biology
Cells are too small to be seen with the naked eye. Cells are studied with the help of microscopes. Microscopes are high-resolution instruments that are used for observing the fine details of very minute objects. Two common types of microscopes are light microscopes and electron microscopes.
1. Light or Compound Microscope
The simple student’s microscope which is often used in schools is called a compound microscope. In these microscopes many lenses are combined together and their magnification power range from 300 to 1500 times. These microscopes use light (generally sunlight) to illuminate the object, so these compound microscopes are called light microscopes. Let us try to get acquainted with various parts of a light microscope.
As shown in Fig, the object or specimen on a glass slide is kept on a stage under an objective piece (having lenses) almost in the middle of the microscope. Light is passed through the object/specimen with the help of a mirror (called a reflector) and a condenser from below the stage. From the eyepiece on the top one can see a magnified image of the object/specimen. A sharp image
forms by focusing the side knobs properly.
The upper and large knob is meant for coarse adjustments and it is used for rapid and precise focussing of the object. The lower and small knob is used for fine adjustments (i.e., for getting a perfect image of the object). The magnification of an image can be increased or decreased by changing the objectives of high or low power (5 X, 10 X, 15 X, etc.) accordingly.
2. Electron Microscope
As shown in Fig. 2.5, an electron microscope is a very large instrument that uses electromagnets for magnification and electrons for illumination. This remarkable instrument was developed by Knoll and Ruska of Germany in 1932 and it was put to use in 1940. It uses very high-voltage electricity.
An Electron microscope helps in observing subcellular structures which cannot be seen through a compound microscope. An internal vacuum is essential for its working. The object must be ultra-thin and dry. It is impregnated with some metal to enhance contrast. The image of the object is obtained on a photographic film or screen. Magnification is 100,000 to 500,000.
Differences between Light Microscope and Electron Microscope
| Light Microscope | Electron Microscope |
| 1. It uses glass lenses. | 1. It uses electromagnets. |
| 2. It uses a beam of light to illuminate the object. | 2. It uses a beam of electrons instead of light. |
| 3. Internal vacuum is not required. | 3. Internal vacuum is essential. |
Let us take the inner fleshy leaf of the onion bulb. With the help of a set of forceps, we can peel off the skin (called the epidermis; it is one cell thick) from the concave side (inner layer) of the onion. This layer can be put immediately in a Petri dish (a glass dish) or watch glass containing water. This will avoid the peel from getting folded and getting dry also. What do we do with this onion peel?
Let us take a glass slide, put a drop of clean water on it, and transfer a piece of peel from the Petri dish or watch glass to the slide. Make sure that peel is perfectly flat on the slide. You may need a thin camel hair paint brush to help you to transfer the peel. At this stage put a drop of iodine solution on the piece of onion peel followed by a cover slip. In this way, you have prepared a temporary mount of onion peel. You can observe this slide under the low and high power of a light compound microscope.
What do you observe as you look through the lens? Can you draw the structures that you are able to see through the microscope in your observation sheet? Does it look like Figure B?
If you prepare temporary mounts of onion peels of different sizes, you will observe that they all have similar small structures. These structures look similar to each other and together they form a big structure such as an onion bulb. These small microscopic structures that you see in an onion peel are the basic building units of an onion bulb. These structures are called cells. Not only onions but all organisms that you see around are made up of cells.
Basic similarities among all cells of higher organisms are as follows: they have a plasma membrane, a cytoplasm with organelles, and a nucleus.
- Protoplasm: The contents of a living cell, contained within the plasma membrane, form protoplasm. Protoplasm is usually differentiated into the nucleus and cytoplasm.
- Gene: It is a distinct unit of hereditary information. Gene is inherited from one generation to the next and determines an observable characteristic or trait of an organism. Genes have to carry coded information of parents to their children or progeny so that children remain exactly like their parents.
- Generally, a gene is made of DNA molecules, but sometimes it is made of RNA molecules as observed in Tobacco Mosaic Virus (TMV).
- DNA: Deoxyribonucleic acid; a polymeric nucleic acid.
- RNA: Ribonucleic acid; a polymeric nucleic acid.