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UNIT OF LIFE
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.
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)
Anton Von Leeuwenhoek (1632-1723)
In 1665, an English Scientist Robert Hooke
discovered a very small but signiﬁcant entity
Through the simple microscope, Robert Hooke
could only see the outer thickened walls of the
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
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 Leeuwenhoek’s microscope
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
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
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’
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
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
The “Cell Theory” has three main aspects to it:
1. All living organisms are made up of single or
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.
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 stains
The Fundamental Unit of Life
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.
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.
Fig. 7: Cells of the onion peel
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.
The above activity can also be done by preparing temporary mounts of different leaf peels, tips of
roots of onion or even peels of onion of different sizes. A temporary mount of the specimen can be
taken off with a jerk, stained with a suitable stain and observed under the microscope.
After performing Activities 1 and 2, we should be able to provide the answers to the following questions
regarding a cell.
1. Do all cells look alike in terms of their shape and size?