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The Cell
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Part A
CELL BIOLOGY
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Biotechnology
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The Cell
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1
THE CELL
The cell is the smallest structural and functional unit of the all living
organisms. Cell biology is the study of the cell. Cell biology is also
known as cytology. The word cytology was derived from Greek word
Kitos (cytos) means a halo vessel or a compartment and Logus means to
discover or to study. Therefore cytology literally means a branch of
biology which studies the different aspects of cells, like study of
phenomenon of heredity, variation, evolution, neutrition, metabolism,
growth, reproduction of cell, etc.
History
1. In 1665, Robert Hook first time observed the cell. He observed a
thin slice of cork under his own microscope. Under microscope,
he observed a honey-comb like structure showing hallow empty
spaces or compartments surrounded by firm cell wall. To each
hallow space he called a ‘cell’. Actually what Robert Hook
observed was dead cell.
2. Antony Van Leeuwenhoek (1632–1723) observed first time the
microorganisms (prokaryotic cell) under his compound
microscope. He observed tiny microbes in a drop of pond water
which he called animacules.
3. In 1831, Robert Brown, first time observed a nucleus in the cell.
4. In 1855 Rudolph Virchow observed that new cells arise from
preexisting cells.
5. In 1938, M.J. Schleiden, a German botanist studied many plants
section under microscope and came to conclusion that, “all
plants are ultimately made up of cell”.
6. In 1938 T.S. Schwann, a German Zoologist came to the similar
conclusion that, “all the animals are ultimately made up of cell”.
Later, Schleiden and Schwann both together put forth a theory
which is known as a cell theory. According to this theory all the
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organisms i.e. plants and animals are made up of cells. In twentieth
century, after the invention of modern microscopes and various
technique, our knowledge of cell increased and a statement of cell theory
was slightly modified. According to the latest cell theory, “all the living
organisms consists of cells and cell products”.
Therefore all the living organisms i.e. prokaryotic and eukaryotic
organisms are made up of cells. Viruses are not made up of cells. In
unicelluar organisms, all functions are carried out by the same cell, while
in multicellular organisms, different functions are performed by different
types of cells.
The cell theory may be summarised as
(a) Cells of organisms are differentiated into distinct cell types.
(b) A cell is a feature of all organisms with the exception of
viruses.
(c) The protoplasm is the living content of the cell and determines
the activity of the cell and thus the whole organism.
(d) New cells originate from preexisting cells through division. In
any case, cell never arise de novo.
All living cells/organisms have a common ancestor. Therefore, all
cells shows common simplicity in the molecular organisation and
principle of molecular economy.
Cell Size—There seems no generalisation in size, shape and
structure of the cell. The variety and diversity of cells are as many as
their functions. The cell size broadly ranges from 0.2 µm to 2 mm
(Table 1.1).
Table 1.1
VARIOUS CELL TYPES AND THEIR SIZE
Cell type
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Mycoplasma (PPLO)
Bacteria
Small lymphocytes
Erythrocytes
Amoeba proteus
Liver cells
Euglena
Human oocyte
Ostrich egg
Neurons (length)
Size (µ)
0.10
0.20–2.50
4.0
8.0
8.0–15.0
20.00
100–200
250
75 × 103
2 × 106
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The Cell
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Cell shape—Cell shape is controlled by certain physical, chemical
and physiological factors. Majority of cells have characteristic fixed
shape. While certain cells have no fixed shape called variable cells, for
e.g., amoeba and leucocytes. Generally multicellular organisms are made
up of different shaped cells (Table 1.2).
Table 1.2
VARIOUS CELL SHAPES AND THEIR EXAMPLES
Cell shape
1. Variable cells
2. Fixed cells
(i) Spherical cells
(ii) Flattened cells
(iii)
(iv)
(v)
(vi)
(vii)
(viii)
Cuboidal cells
Columner cells
Discoidal cells
Spindle shaped cells
Elongated cells
Branched cells
Example
Amoeba, Leucocytes, Myxomycetes
egg of many animals
Squamous epithelium, endotheliums, and
upper layer of epidermis
Thyroid gland follicles
The cells lining the intestine.
Erythrocytes
Smooth muscle fibres
Nerve cells
Pigment cells of skin
Depending upon their organisation, cells are basically of two types
i.e., prokaryotic cells and eukaryotic cells. Prokaryotic cells are primitive
type of cells. These cells lacks true nucleus and other cell organelle like
mitochondria, chloroplasts, endoplasmic reticulum, golgi complex, etc.
The word prokaryotes is derived from Greek word pro means primitive
or old and karyotes means nucleus or main. Eukaryotic cells have true
nucleus and it also contains cell organelles. The word eukaryotes is
derived from Greek word eu means true and karyotes means nucleus.
The main differences between prokaryotic and eukaryotic cells is
enlisted in Table 1.3.
Table 1.3
DIFFERENCE BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS
1.
2.
3.
4.
5.
Prokaryotic cell
Eukaryotic cell
A true nucleus is absent.
Nuclear membrane is absent.
Nucleolus is absent.
Chromosome is single, circular.
Chromosome present freely in
the cytoplasm.
A true nucleus is present.
Nuclear membrane is present.
Nucleolus is present.
Chromosomes are many and linear.
Chromosomes are enclosed inside the
nuclear membrane.
(Contd.)
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Prokaryotic cell
6. Histone proteins are absent in
the organisation of chromosome.
7. In photosynthetic cells, chlorophyll pigments are present in
cell membrane.
8. Ribosomes are 70s type.
9. Cell organells are absent.
10. Streaming or amoeboid movement of cytoplasm absent.
11. Flagella if present, do not have
typical 9 + 2 arrangement.
12. Enzymes necessary for respiration are present in plasmamembrane.
13. Cell division by the process of
amitosis i.e., mitosis and meiosis
is absent.
14. mRNA will not have the 5'
methyl cap and 3' poly A tail.
15. Transcription and translation are
combined process, both takes
place in cytoplasm.
16. In genes non-coding regions are
absent.
17. After transcription there is no
post transcriptional modification
in mRNA.
18. Polycistronic mRNA.
19. The cell wall contains amino
sugars and muramic acid.
Eukaryotic cell
Chromosomes are well organised with
histone protein.
In photosynthetic cells, the chlorophyll
pigments are present in the plastids—
chloroplast.
Ribosomes are 80s type.
Cell organells are present.
Streaming or amoeboid movement of
cytoplasm present.
Flagella shows typical 9 + 2
arrangement.
Enzymes necessary for respiration are
present in mitochondria.
Cell division by the process of mitosis
and meiosis.
mRNA having 5' methyl cap and 3'
poly A tail.
Transcription and translation are not
combined process. Transcription takes
place in nuclers, while translation
occurs in cytoplasm.
In genes non-coding region (i.e.,
introns) are present.
After transcription, post transcriptional
modifications occurs in mRNA.
Mono cistronic mRNA.
When cell wall present, it doesnot contain amino sugar and muramic acid.
Ultrastructure of Eukaryotic cell—Eukaryotic cells are highly
evolved cells and show striking characters in contrast to prokaryotes. It
contains specialised membrane bound cell organelles. The most
important cell organelle is nucleus, which is membrane bound and
includes filamentous chromosomes. Other membrane bound organelles
include chloroplants, mitochondria, lysosomes, golgi complex,
endoplasmic reticulum, etc. The cell organelles provides micro
environments to subcellular metabolic units.
The eukaryotic organisms are very diverse, few are unicellular,
where all the functions are carried out by same cell, while many are
multicellular organisms. Multicellular eukaryotes contains variety of cell
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The Cell
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types originate as a result of cell differentiation and it’s organised
aggregates, forming tissue systems. Although there is a basic pattern of
organisation, several functional specializations are acquired by cells.
Eukaryotes are grouped into phototrophs (e.g., plants) and
chemotrops (e.g., animals). The phototrophs have a photosynthetic
apparatus and a rigid cell wall of cellulose.
Photosynthetic Eukaryotes—The eukaryotes, which are able to
synthesize their own food in presence of the sunlight is called—
Photosynthetic Eukaryotes. As an example of this we can consider a
typical plant cell. A plant cell has a thick outer most coating called cell
wall, which protect the cell from various osmatic stresses. Cell wall is
made up of cellulose. Inside the cell wall, there is a plasma membrane,
which encloses the cytoplasm. At several locations, two adjacent cells
have intercellular channels called plasmodesmata, which help cellular
communication.
The cytoplasm of plant cell is semifluid, in which many cell
organelles micro and macroneutrients and enzymes are suspended. All
cell organelles are bound by double membrane. Different cell organelles
which are present in plant cell are as follows (Fig. 1.1)
(a) Nucleus—Nucleus is bounded by double layered nuclear
membrane. It contains filamentous chromosomes, which contains genetic
information.
(b) Endoplasmic reticulum—The endoplasmic reticulum forms a
network of tubules whose one end is connected with nuclear membrane.
There are two types of endoplasmic reticulum—smooth endoplasmic
reticulum and rough or granular endoplasmic reticulum. The main
function of endoplasmic reticulam is synthesis and storage of protein/
enzymes.
(c) Golgi complex—It is made up of cisternae and vesicles. It’s main
function is storage.
(d) Mitochondria—Mitochondria synthesize ATP molecule (energy
curency of cell) by oxidative phosphorylation in the absence of light by
breaking down organic molecules.
(e) plastids—Plastids are characteristic of plant cells only. Plastids
are of two types—leucoplasts (with out pigment) and chromoplasts (with
pigments). The most important plastid is with green chlorophyll pigment
called chloroplasts. Chloroplasts participate in the photosynthetic activity.
(f) Lysosomes—It contains hydrolytic enzymes for digestion.
(g) Peroxysomes—It neutralise metabolic peroxides in the cell.
(h) Glyoxysomes—It is involved in glyoxylate metabolism.
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(i) Ribosomes—Plant cell contains 80s types ribosomes. Ribosomes
participates in protein synthesis.
(j) Vacuoles—The plant cell contains a well defined, large vacuoles.
It is usually filled with liquid material. Unlike other cell organelles,
vacuoles are bound by single membrane called tonoplast. In plant cell,
large volume is made up of vacuole generally central and cytoplasm and
other cell organells occupy little space, generally peripheral.
Mitochondrion
Small vacuole
Cellulose cell
wall
Golgicomplex
(dictyosome)
Chloroplast
Nucleolus
Cell membrane
(plasmatemma)
Nucleoplasm
containing
chromatin
Endoplasmic
reticulum
with attached
ribosomes
Central vacuole
Leucoplast
containing
starch grain
Ground substance
of cytoplasm
with granular
inclusions
Plasmodesma
Fig. 1.1 A typical plant cell.
Chemosynthetic Eukaryotes—The eukaryotes which are not able
to synthesise their food in the presence of sunlight are called
chemosynthetic eukaryotes. e.g.. Animal cell. Animal cells are devoid of
a cell wall, and the plasma membrane is the outermost cell boundary,
which carries out catalytic as well as osmotic protection functions. The
animal cell contains all membrane-bound cell organelles in cytoplasm
similar to those of plant cells, with the exception of plastids. Other than
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The Cell
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cell organelles, animal cells have centrosomes, locomotory organelles
and an elaborate cytoskeleton (Fig. 1.2).
Region containing microtubules and microfibrils
Pinocytic vesicles
Golgi
apparatus
Secretion
vacuole
Tysosome
Centrosome
Nucleus
Ribosomes
attached
to endoplasmic
reticulum
Nucleolus
Phospholipid
storage granule
Nuclear
membrane
Neutral lipid
storage
granule
Cytoplasmic
matrix
Mitochondrion
Plasma membrane
Fig. 1.2 A typical animal cell.
Centrosomes are usually one per cell, which is located near the
nuclear surface. It is a complex structure with a dense outer granular
mantle that encloses a less granular material around two pairs of
cylindrical bodies called centrioles. The centrioles participate in mitosis
and also anchor the flagellum in sperm cell. The cytoplasmic matrix
consists of a complex network of microtubules and microfilaments,
forming the cytoskeleton. The cytoskeleton helps in the movement of
cell organelles, change in cell shape, amoeboid locomotion, cytokinesis,
etc.
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Summary
The cell is the smallest structural and functional unit of the all living
organisms. Cell biology is the study of cell. In 1938, M.J. Schleiden and
T.S. Schwann proposed the cell theory. There is no generalisation in cell
size, shape and structure. Basically cells are of two types namely
prokaryotic cell and eukaryotic cell. Prokaryotic cell is more primitive
cell. Eukaryotic cells depending upon its capacity to produce their own
food in presence of sunlight is divided into photosynthetic eukaryotes
(e.g. plant cell) and chemosynthetic eukaryotes (e.g. animal cell). The
main characteristic of plant cells is it contains chloroplasts and large
vacuoles. The main characteristic of animal cells is it contains
centrosomes, locomotory organelles, cytoskeleton. The remaining cell
organelles commonly present in both plant and animal cells are nucleus,
mitochondria, golgi complex, 80s ribosome, endoplasmic reticulum,
lysosomes, peroxysomes.
EXERCISE
I. 1. What is cell? Describe cell theory.
2. Explain broad classification of cell.
II. Differentiate between the following:
1. Eukaryotic cells and Prokaryotic cells.
2. Plant cells and animal cells.