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Cells:: FUNDAMENTAL UNIT OF LIFE
Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. Leeuwenhoek (1674), with the improved microscope, discovered the free-living cells in pond water for the first time. It was Robert Brown in 1831 who discovered the nucleus in the cell.
FUNDAMENTAL UNIT OF LIFE DISCOVERY OF CELL
While, examining a thin slice of cork, Robert Hooke saw that the cork
resembled the structure of a honeycomb consisting of many little
compartments. Cork is a substance which comes from the bark of a
tree. This was in the year 1665 when Hooke made this chance
observation through a selfdesigned microscope. Robert Hooke called
these boxes cells. Cell is a Latin word for ‘a little room’.
This may seem to be a very small and insignificant incident but it is very
important in the history of science. This was the very first time that
someone had observed that living things appear to consist of separate
units. The use of the word ‘cell’ to describe these units is used till this
day in biology.• COMPOUND MICROSCOPE CELLS OF ONION PEEL
Cell MORE TO KNOW ABOUT CELLS
Cells were first discovered by Robert Hooke in 1665. He
observed the cells in a cork slice with the help of a primitive
microscope. Leeuwenhoek (1674), with the improved
microscope, discovered the free living cells in pond water for the
first time. It was Robert Brown in 1831 who discovered the
nucleus in the cell.Invention of magnifying lenses discovery of
The invention of magnifying lenses led to the discovery of the
microscopic world. It is now known that a single cell may
constitute a whole organism as in Amoeba, Chlamydomonas,
Paramoecium and bacteria.
These organisms are called unicellular organisms (uni = single).
On the other hand, many cells group together in a single body
and assume different functions in it to form various body parts
in multicellular organisms (multi = many) such as some fungi,
plants and animals.
Every multicellular organism has come from a single cell. Cells
divide to produce cells of their own kind. All cells thus come
from preexisting cells.The shape and size of cells are related to the specific function they perform.
Some cells like Amoeba have changing shapes. In some cases the cell shape
could be more or less fixed and peculiar for a particular type of cell; for
example, nerve cells have a typical shape.•
Plant cells, in addition to the plasma membrane, have another rigid outer
covering called the cell wall. The cell wall lies outside the plasma membrane. The
plant cell wall is mainly composed of cellulose. Cellulose is a complex substance
and provides structural strength to plants.
When a living plant cell loses water through osmosis there is shrinkage or
contraction of the contents of the cell away from the cell wall. This phenomenon
is known as plasmolysis.NUCLEUS,CHROMOSOMES DNA DEOXYRIBO OR
DEFINED NUCLIEC ACID
The nucleus has a double layered covering called nuclear membrane. The
nuclear membrane has pores which allow the transfer of material from inside the
nucleus to its outside, that is, to the cytoplasm.
The nucleus contains chromosomes, which are visible as rodshaped structures
only when the cell is about to divide. Chromosomes contain information for
inheritance of features from parents to next generation in the form of DNA
(Deoxyribo\Defined Nucleic Acid) molecules.
Chromosomes are composed of DNA and protein. DNA molecules contain the
information necessary for constructing and organizing cells. Functional segments
of DNA are called genes.
In a cell which is not dividing, this DNA is present as part of chromatin material.
Chromatin material is visible as entangled mass of thread like structures.
Whenever the cell is about to divide, the chromatin material gets organized into
In some organisms like bacteria, the nuclear region of the cell may be poorly
defined due to the absence of a nuclear membrane. Such an undefined nuclear
region containing only nucleic acids is called a nucleoid. Such organisms, whose cells
lack a nuclear membrane, are called prokaryotes (Pro = primitive or primary;
karyote ≈ karyon = nucleus). For ex. Bacteria , Algae etc.
Prokaryotic cells also lack most of the other cytoplasmic organelles present in
eukaryotic cells. Many of the functions of such organelles are also performed by
poorly organized parts of the cytoplasm (see section 5.2.4). The chlorophyll in
photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag
Cell organelles are enclosed by membranes. In prokaryotes, beside the absence of
a defined nuclear region, the membranebound cell organelles are also absent.EUKARYOTIC CELLS
The Cells having nuclear membrane and well defined nuclear region
are known as eukaryotic cells.eu = true karyote = nucleus means
good or true nuclei. Eukaryotic cells can be easily distinguished
through a membranebound nucleus. For ex. Animal cell , Plant cell.
Eukaryotic cells are membranebound organelles to carry out specific
cell tasks. They have different internal membranes, which are known
as organelles. These organelles play a vital role in cell maintenance
and other functions. These organelles generally consist of cell wall,
plasma membrane, nucleus, mitochondria, chloroplasts (plastids),
endoplasmic reticulum, ribosome, Golgi apparatus, lysosomes,
vacuoles, cytoplasm and chromosomes.
The eukaryotic cells have nuclear membrane as well as membrane
enclosed organelles.Fill the spaces with
correct answers about
eukaryotic cells.CELL ORGANELLES ANIMAL CELL PLANT CELL
Cell wall Absent Present
Usually round or spherical. Rectangular
(irregular in shape absence of cell wall ) (fixed shape presence of cell wall)
Nucleus Present Present
Plasma membrane Present Present
Nucleolus Present Present
Endoplasmic reticulum Present Present
Golgi apparatus Present Present
Lysosomes Present Present
Vacuoles Present (Small and more in number) Present (Large and few in number)
Mitochondria Present Present
Plastids Absent Present
Chromosomes Present Present
Ribosome Present Present
Centrosomes Present Absent
Chloroplast Absent Present
Cytoplasm Present PresentPLANT ANIMAL CELLS
As we know that plant cells are found in plants ,and animal cells in animals
human being they have some characteristics some of them are :
Endoplasmic reticulum (ER)
•Soft endoplasmic reticulum
Rough endoplasmic reticulum (RER)ROUGH ENDOPLASMIC RETICULUM (RER) (SER)
SOFT ENDOPLASMIC RETICULUM
RER looks rough under a microscope because it has particles called ribosomes
attached to its surface. The ribosomes, which are present in all active cells, are the
sites of protein manufacture. The manufactured proteins are then sent to various
places in the cell depending on need, using the ER.
The SER helps in the manufacture of fat molecules, or lipids, important for cell
function. Some of these proteins and lipids help in building the cell membrane. This
process is known as membrane biogenesis. Some other proteins and lipids function
as enzymes and hormones.
SER plays a crucial role in detoxifying many poisons and drugs.Golgi Apparatus
• The Golgi apparatus, first described by Camilo Golgi, consists of a system of
membranebound vesicles arranged approximately parallel to each other in
stacks called cisterns. These membranes often have connections with the
membranes of ER and therefore constitute another portion of a complex
cellular membrane system.
• The material synthesised near the ER is packaged and dispatched to
various targets inside and outside the cell through the Golgi apparatus. Its
functions include the storage, modification and packaging of products in
vesicles. In some cases, complex sugars may be made from simple sugars in
the Golgi apparatus. The Golgi apparatus is also involved in the formation
• Lysosomes are a kind of waste disposal system of the cell. Lysosomes help
to keep the cell clean by digesting any foreign material as well as wornout
cell organelles. Foreign materials entering the cell, such as bacteria or
food, as well as old organelles end up in the lysosomes, which break them
up into small pieces. Lysosomes are able to do this because they contain
powerful digestive enzymes capable of breaking down all organic
material. During the disturbance in cellular metabolism, for example,
when the cell gets damaged, lysosomes may burst and the enzymes
digest their own cell.Therefore, lysosomes are also known as the ‘suicide bags’ of a cell. Structurally, lysosomes
are membranebound sacs filled with digestive enzymes. These enzymes are made by RER.
• Mitochondria are known as the power houses of the cell. The energy required
for various chemical activities needed for life is released by mitochondria in the
form of ATP (Adenosine triphosphate) molecules. ATP is known as the energy
currency of the cell. The body uses energy stored in ATP for making new
chemical compounds and for mechanical work. Mitochondria have two
membrane coverings instead of just one. The outer membrane is very porous
while the inner membrane is deeply folded. These folds create a large surface
area for ATPgenerating chemical reactions.
• Mitochondria are strange organelles in the sense that they have their own DNA
and ribosomes. Therefore, mitochondria are able to make some of their own
Plastids are present only in plant cells. There are two types of plastids –
Chromoplasts (coloured plastids) and leucoplasts (white or colourless
plastids). Plastids containing the pigment chlorophyll are known as
Chloroplasts are important for photosynthesis in plants. Chloroplasts also
contain various yellow or orange pigments in addition to chlorophyll.
Leucoplasts are primarily organelles in which materials such as starch, oils
and protein granules are stored.• The internal organisation of the plastids consists of numerous membrane layers
embedded in a material called the stroma. Plastids are similar to mitochondria in
external structure. Like the mitochondria, plastids also have their own DNA and
Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in
animal cells while plant cells have very large vacuoles. The central vacuole of some
plant cells may occupy 5090 of the cell volume.
In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
Many substances of importance in the life of the plant cell are stored in vacuoles.
These include amino acids, sugars, various organic acids and some proteins. In single
celled organisms like Amoeba, the food vacuole contains the food items that the
Amoeba has consumed. In some unicellular organisms, specialized vacuoles also play
important roles in expelling excess water and some wastes from the cell.Made by :Anju Bala mam
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