Page 61 - Understanding NCERT Science 09

We can try preparing temporary mounts Chlamydomonas, Paramoecium and bacteria.
of peels of onions of different sizes. What do These organisms are called unicellular
we observe? Do we see similar structures or organisms (uni = single). On the other hand,
different structures? many cells group together in a single body
and assume different functions in it to form
What are these structures?
various body parts in multicellular organisms
These structures look similar to each other. (multi = many) such as some fungi, plants
Together they form a big structure like an and animals. Can we find out names of some
onion bulb! We find from this activity that more unicellular organisms?
onion bulbs of different sizes have similar Every multi-cellular organism has come
small structures visible under a microscope. from a single cell. How? Cells divide to
The cells of the onion peel will all look the produce cells of their own kind. All cells thus
same, regardless of the size of the onion they come from pre-existing cells.
came from.
These small structures that we see are Activity ______________ 5.2
the basic building units of the onion bulb.
• We can try preparing temporary
These structures are called cells. Not only
mounts of leaf peels, tip of roots of
onions, but all organisms that we observe
onion or even peels of onions of different
around are made up of cells. However, there sizes.
are also single cells that live on their own. • After performing the above activity, let
us see what the answers to the following
Cells were first discovered by questions would be:
Robert Hooke in 1665. He observed (a) Do all cells look alike in terms of
shape and size?
the cells in a cork slice with the help
(b) Do all cells look alike in structure?
of a primitive microscope.
(c) Could we find differences among
Leeuwenhoek (1674), with the
cells from different parts of a
improved microscope, discovered the
plant body?
free living cells in pond water for the (d) What similarities could we find?
first time. It was Robert Brown in
Some organisms can also have cells of
1831 who discovered the nucleus in different kinds. Look at the following picture.
More to know term ‘protoplasm’ for the fluid
the cell. Purkinje in 1839 coined the
It depicts some cells from the human body.
substance of the cell. The cell theory,
that all the plants and animals are
composed of cells and that the cell is
the basic unit of life, was presented
by two biologists, Schleiden (1838)
and Schwann (1839). The cell theory
was further expanded by Virchow
Blood
(1855) by suggesting that all cells cells Nerve Cell
arise from pre-existing cells. With the Smooth
muscle
discovery of the electron microscope cell
in 1940, it was possible to observe and
understand the complex structure of
the cell and its various organelles.
Bone
Fat cell
cell
The invention of magnifying lenses led to
the discovery of the microscopic world. It is
Ovum Sperm
now known that a single cell may constitute
a whole organism as in Amoeba, Fig. 5.3: Various cells from the human body

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