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THE FUNDAMENTAL UNIT OF LIFE

CHAPTER 1

Our planet Earth is inhabited by different types of living organisms, some that look similar while some look very different from each other. Irrespective of the type of living organisms, each organism is made up of a basic structural and functional unit called “Cell”. Let us learn in detail about the

discovery and the function of a cell in the subsequent sections below.

5.1 WHAT ARE LIVING ORGANISMS MADE UP OF?

5.1.1 Discovery of a “Cell”

ROBERT HOOKE: (1635-1703)

In 1665, an English Scientist Robert Hooke discovered a very small but significant entity that became the foundation of “life”. He observed a thin shaving of cork (cork is the outer covering or the bark of a tree) under a microscope and found that it is made up of thousands of little compartments which he imagined to resemble little rooms. He aptly named these small structures as “cells”. However Robert’s research did not provide any further insights on the structure of a cell.

The word ‘cell’ is derived from the Latin word ‘cella’, which literally means ‘a small room’). It was first coined in 1665, by the English physicist Robert Hooke, to refer to the microscopic structure of cork. The microscopic structure closely resembled a honey-comb with porous structures (Fig 1) resembling the little rooms of a monastery and hence he called these units as cells. This discovery by Hooke was done by a simple self-designed microscope (Fig 2)

Objectives :

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ANTON VON LEEUWENHOEK (1632-1723)

Through the simple microscope, Robert Hooke could only see the outer thickened walls of the cork and could not provide any further insights. Anton Van Leeuwenhoek, a Dutch microscopist made a technologically advanced microscope (Fig 3). He observed the free floating ‘Algae Spirogyra’ swimming in pond water and discovered that these little animals were actually single cells themselves. He is considered the father of microscopy because of the advances he made in microscope design and use. It was he who discovered the bacteria, yeast plants and much more. Anton Von Leeuwenhoek is considered the father of microscope and his researches, which were widely circulated, opened up an entire world of microscopic life to the awareness of scientists.

Eyepiece

Barrel

Focusing ScrewObjective

Specimen holder

Water Flask

Oil lamp

Fig. 1: Cork cells as seen under a microscope Fig. 2: Robert Hooke’s microscope

Fig. 3: Anton Von Leeuwenhoek’s microscope

MICROSCOPES: INSTRUMENTS FOR STUDYING CELLSCells are basically too small to be seen by the naked eye without any technological aid. They can only be studied with the help of instruments called as ‘Microscopes’.Microscope gets its name from the Greek word “small” and “to look”. It is essentially an instrument that is used to view objects that are too small for the naked eye and the science of investigating these small objects under a microscope is called ‘Microscopy’.

Types of Microscopes:

1. Light or Compound Microscope

The light microscope is so called because it employs visible light (generally sunlight) to illuminate small objects. It is probably the most well-known and commonly used instrument in biology labs today. Optical or light microscopy involves passing this visible light transmitted through a single or combination of lenses to allow a magnified view of the sample. The resulting image can be detected directly by the eye or imaged on a photographic plate

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Referring to figure 4 above, a good quality microscope has a built-in reflector (mirror), adjustable condenser below the stage, mechanical stage, and binocular eyepiece tube. The sample or specimen is mounted on a glass slide and kept on the mechanical stage under the binocular eye piece tube. The condenser is used to focus light on the specimen through an opening in the stage. Sharp image can be obtained by carefully focusing the side knobs. The larger upper knob is meant for coarse adjustments while the lower small knob is used to for fine adjustments and getting a perfect image of the specimen. After passing through the specimen, the light is displayed to the eye with an apparent field that is much larger than the area illuminated. The magnification of the image can be obtained by changing the objective lens magnification (usually stamped on the lens body).

2. Electron Microscope

Electron microscope is a significant alternative to light microscopy which uses electrons rather than light to generate the image. It was developed by Ernst Ruska and later by Max Knoll of Germany and uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolution power than a light-powered optical microscope as electrons have wavelengths about 100,000 times shorter than visible light (photons), and hence can achieve better magnifications of up to about 500,000 times, as compared to light microscopes which are limited by magnifications below 2000x.Due to this property Electron microscopes are helpful in observing sub cellular structured which cannot be clearly seen through a light microscope. Internal vacuum is essential for the working and care must be taken to ensure that the sample being used in an Electron microscope is ultra thin and absolutely dry.

Eyepiece

Bodytube

Revolving NosepieceArmObjectives

Stage clip StageDiaphragmCoarse

adjustment knobLight source

Fine adjustment knob

Base

Fig 4 : A light microscope

Fig. 6: Parts of an Electron Microscope

Fig. 5: An Electron Microscope

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Table 1: Differences between light microscope and electron microscope

Light Microscope Electron Microscope

It uses a beam of visible light to illuminate the object

It uses a beam of electrons to illuminate the object

It uses a single or combination of lenses Uses electromagnets

Magnification obtained is usually 300 to 1500 times

Magnification obtained is 100,00 to 500,000 times

Internal vacuum is not required Internal vacuum is essential

5.1.2: Cell Theory

As mentioned earlier, Robert Hooke had only seen the thickened walls of the cells and could not view the inner contents of the cell. However, the first sightings of the internal action of the cell were made by a Scottish botanist Robert Brown in 1831.He discovered and named the central substance of a plant cell as the ‘Nucleus’. Later in 1839, J.E. Purkinje, a Czech physiologist named the living fluid substance present inside the cell as ‘protoplasm’.Advancement in technology and continued research in this field helped Scientists develop a very important foundation theories of Biology called – “The Cell Theory”In 1838 Scientists Matthias Schleiden proposed that all plants comprise of cells. Later in 1839, Theodor Schwann (1810-1882) a German Zoologist independently confirmed that all living things-both plants and animals are made up of cells. Thus cell is the basic fundamental and structural unit of life. In 1855, another German biologist Rudolf Virchow, further extended and refined the cell theory by proving that all cells arise from pre-existing cells by dividing themselves i.e ‘Omnis cellulae a cellula’

The “Cell Theory” has three main aspects to it:

1. All living organisms are made up of single or many cells.

2. All cells arise from pre-existing cells by division. No cell can originate spontaneously on its own, instead comes into existence only by division of an already existing cell.

3. The cell is the fundamental unit of structure in all living organisms. All metabolic reactions take place in a cell.

Thus to conclude this discussion “What makes a Living Organism”; any self replicating living form comprises of a basic fundamental structure called Cell.

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ACTIVITY 1Objective: Observing Cells under a MicroscopeResources: Microscopes, slides, cover slips, source of plant cells e.g. onion epidermis, leaf surface or petals or even potato and tomato scrapings and appropriate stainsNow that we have understood what is a “cell” let us observe this under a microscope ourselves and see if the shape and size or structure of cells within the same organism is the same or different? For this we can use any plant material like onion peel layer (called epidermis) or surface of a leaf or petals. For our experimental purpose we have considered the thin onion epidermis.

1. Take an onion peel layer and immerse it immediately in a watch glass containing water to avoid dryness.

2. Prepare a microscope slide by gently placing a drop of water on it. Then with the help of a thin paintbrush transfer the onion epidermis from the watch glass on this slide.

3. A drop of stain solution like safranine will colour these cells in the onion layer to make it more visible to us under the microscope.

4. Place a cover slip on this mount taking care that no air bubbles emerge on the slide. The air bubbles will cause hindrance while observing the mount under the microscope.

5. Now our slide is ready to be observed under a compound microscope.

6. Draw the structures as is on the sheet.

7. Repeat this experiment with different size of the onion peel taken from different parts of the same onion.

What do you observe when you look through the lens of the microscope? Does it resemble the figure 7 as shown above?

Interestingly, we observe that with different peels that may have been taken from either small or big onions the basic structure of the cell does not change. These small structures (called cells) together form a big structure called as the onion bulb. Thus, cell is the basic structure and the building block of all the onion bulbs. This can be extrapolated to any living organism.

Fig. 7: Cells of the onion peel