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UNIT OF LIFE
ur 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)
Fig. 2: Robert Hooke’s microscope
Fig. 1: Cork cells as seen under a microscope
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.
Fig. 3: Anton Von
MICROSCOPES: INSTRUMENTS FOR STUDYING CELLS
Cells 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
The Fundamental Unit of Life
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).
Fig 4 : A light microscope
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.
Fig. 5: An Electron Microscope
Fig. 6: Parts of an Electron Microscope
Table 1: Differences between light microscope and electron microscope
It uses a beam of visible light to illuminate the
It uses a beam of electrons to illuminate the
It uses a single or combination of lenses
Magnification obtained is usually 300 to
Magnification obtained is 100,00 to 500,000
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
Thus to conclude this discussion “What makes a Living Organism”; any self replicating living form comprises
of a basic fundamental structure called Cell.
The Fundamental Unit of Life
Objective: Observing Cells under a Microscope
Resources: Microscopes, slides, cover slips, source of
plant cells e.g. onion epidermis, leaf surface or petals
or even potato and tomato scrapings and appropriate
Now 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.
Fig. 7: Cells of the onion peel
1. Take an onion peel layer and immerse it immediately in a watch glass containing water to avoid
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
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.