The living world includes innumerable types of plants and animals which show a large variation in their external features. But all of them resemble each other in one respect their bodies are made of numerous building blocks called cells.
In an organism, the cells are organized to perform different
functions and work together with a common aim to sustain life. Therefore, we
define a cell as a basic structural and functional unit of life.
Discovery of the cell
Cells are very small and invisible to the naked human eye. Our
knowledge of their structure was made possible only by the invention of the microscope and subsequent improvement in its technology.
The first microscope was invented by Antonie van Leeuwenhoek
(1632-1723). Basically, his microscope consisted of a single biconvex lens and
was known as a simple microscope.
In 1663, Robert Hooke, an English scientist, designed the
first compound microscope. He used two lenses instead of one to achieve a
greater magnification.
Using his compound microscope, Hooke examined a thin slice of
a cork and observed that it was made of tiny honeycomb compartment-like
structures piled up together. He named these structures as cells.
Diversity in Numbers,
Shapes and Sizes of Cells
Unicellular and multi-cellular
The organisms that are made up of a single cell, are called
unicellular organisms e.g. amoeba and paramecium. On the other hand, most of
the plants and animals are made up of numerous cells. They are called multicellular organisms e.g. human beings, insects, birds and plants like neem, rose
and wheat.
Different cell shapes
All living cells show a great variety in their shapes which
is determined by the function that a particular cell performs. For example, the
red blood cells are spherical or disc-shaped which enables them to hold more
oxygen. The cell of Chlamydomonas is oval-shaped. Spirogyra, a multi-cellular
alga has a cylindrical shaped body.
Different sizes of
cells
The size of the cells varies from microscopic to large. The bacterial cell is the smallest. The nerve cell is the longest and the egg cell is
the largest.
Basic Cell structures
Some basic structures, such as nucleus, cytoplasm and cell
membrane are common in plant cells as well as in animal cells.
The plant cells differ from animal cells in only one important
aspect. Outside the cell membrane, plant cells have a thick and rigid cell
wall.
Animal Cell and Plant Cell
|
Characteristics |
Plant cell |
Animal cell |
|
Size |
Comparatively
larger in size |
Usually smaller in
size |
|
Cell membrane |
In addition to a
cell membrane plant cells are also surrounded by a non-living, rigid cell wall |
Enclosed by a
living cell membrane |
|
Plastids |
Plastids are
present. The chloroplasts contain chlorophyll which is responsible for the
green colour of plants. |
Plastids are
absent. In some microscopic organisms like euglena and green algae,
chlorophyll is present. |
|
Vacuole |
A large vacuole is
present |
Vacuoles are
absent. If present, they are very small in size. |
|
Lysosome |
Lysosomes are
absent |
Lysosomes are
present and take part in destroying the harmful bacteria present in the cell. |
|
Centrosome |
Centrosome is
absent |
The centrosome is
present and initiates cell division |
Cell Inclusions
Apart from the living structures, a cell also contains some
non-living substances called cell inclusions. These are either found in the
cytoplasm or in a vacuole of the cell.
In plant cells, they are present in the form of starch,
proteins and waste materials like gum, mucilage, and tannin. In the animal
cells, they are present in the form of fat droplets, pigments and granules.
Cell Division – Growth
and Repair
All living cells are growing old and die. These cells are
replaced by some new cells formed as a result of cell division. For example, a
fractured bone can be healed only when the bone cells divide. Therefore, we can
say that cell division is needed for growth and repair in living organisms.
Cell Division
In cell division, a single cell grows bigger in size and
becomes a parent cell. The parent cell divides into two daughter cells. The
daughter cells further divide into four cells and this division goes on.
Along with the division of cells, their nucleus also divides. In animals, the typical growing regions are the end of bones and the layers of cells present under the skin. In plants, the root tips and apical buds are the main growing regions.