CELL BIOLOGY - New Age International
CELL BIOLOGY - New Age International CELL BIOLOGY - New Age International
d:\data\newage~1\biote\bit-1.pm5/IIIrd proof/4-11-04 The Cell 1 Part A CELL BIOLOGY
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The Cell<br />
1<br />
Part A<br />
<strong>CELL</strong> <strong>BIOLOGY</strong>
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2 Biotechnology
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The Cell<br />
3<br />
1<br />
THE <strong>CELL</strong><br />
The cell is the smallest structural and functional unit of the all living<br />
organisms. Cell biology is the study of the cell. Cell biology is also<br />
known as cytology. The word cytology was derived from Greek word<br />
Kitos (cytos) means a halo vessel or a compartment and Logus means to<br />
discover or to study. Therefore cytology literally means a branch of<br />
biology which studies the different aspects of cells, like study of<br />
phenomenon of heredity, variation, evolution, neutrition, metabolism,<br />
growth, reproduction of cell, etc.<br />
History<br />
1. In 1665, Robert Hook first time observed the cell. He observed a<br />
thin slice of cork under his own microscope. Under microscope,<br />
he observed a honey-comb like structure showing hallow empty<br />
spaces or compartments surrounded by firm cell wall. To each<br />
hallow space he called a ‘cell’. Actually what Robert Hook<br />
observed was dead cell.<br />
2. Antony Van Leeuwenhoek (1632–1723) observed first time the<br />
microorganisms (prokaryotic cell) under his compound<br />
microscope. He observed tiny microbes in a drop of pond water<br />
which he called animacules.<br />
3. In 1831, Robert Brown, first time observed a nucleus in the cell.<br />
4. In 1855 Rudolph Virchow observed that new cells arise from<br />
preexisting cells.<br />
5. In 1938, M.J. Schleiden, a German botanist studied many plants<br />
section under microscope and came to conclusion that, “all<br />
plants are ultimately made up of cell”.<br />
6. In 1938 T.S. Schwann, a German Zoologist came to the similar<br />
conclusion that, “all the animals are ultimately made up of cell”.<br />
Later, Schleiden and Schwann both together put forth a theory<br />
which is known as a cell theory. According to this theory all the
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4 Biotechnology<br />
organisms i.e. plants and animals are made up of cells. In twentieth<br />
century, after the invention of modern microscopes and various<br />
technique, our knowledge of cell increased and a statement of cell theory<br />
was slightly modified. According to the latest cell theory, “all the living<br />
organisms consists of cells and cell products”.<br />
Therefore all the living organisms i.e. prokaryotic and eukaryotic<br />
organisms are made up of cells. Viruses are not made up of cells. In<br />
unicelluar organisms, all functions are carried out by the same cell, while<br />
in multicellular organisms, different functions are performed by different<br />
types of cells.<br />
The cell theory may be summarised as<br />
(a) Cells of organisms are differentiated into distinct cell types.<br />
(b) A cell is a feature of all organisms with the exception of<br />
viruses.<br />
(c) The protoplasm is the living content of the cell and determines<br />
the activity of the cell and thus the whole organism.<br />
(d) <strong>New</strong> cells originate from preexisting cells through division. In<br />
any case, cell never arise de novo.<br />
All living cells/organisms have a common ancestor. Therefore, all<br />
cells shows common simplicity in the molecular organisation and<br />
principle of molecular economy.<br />
Cell Size—There seems no generalisation in size, shape and<br />
structure of the cell. The variety and diversity of cells are as many as<br />
their functions. The cell size broadly ranges from 0.2 µm to 2 mm<br />
(Table 1.1).<br />
Table 1.1<br />
VARIOUS <strong>CELL</strong> TYPES AND THEIR SIZE<br />
Cell type Size (µ)<br />
1. Mycoplasma (PPLO) 0.10<br />
2. Bacteria 0.20–2.50<br />
3. Small lymphocytes 4.0<br />
4. Erythrocytes 8.0<br />
5. Amoeba proteus 8.0–15.0<br />
6. Liver cells 20.00<br />
7. Euglena 100–200<br />
8. Human oocyte 250<br />
9. Ostrich egg 75 × 10 3<br />
10. Neurons (length) 2 × 10 6
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The Cell<br />
5<br />
Cell shape—Cell shape is controlled by certain physical, chemical<br />
and physiological factors. Majority of cells have characteristic fixed<br />
shape. While certain cells have no fixed shape called variable cells, for<br />
e.g., amoeba and leucocytes. Generally multicellular organisms are made<br />
up of different shaped cells (Table 1.2).<br />
Table 1.2<br />
VARIOUS <strong>CELL</strong> SHAPES AND THEIR EXAMPLES<br />
Cell shape<br />
Example<br />
1. Variable cells Amoeba, Leucocytes, Myxomycetes<br />
2. Fixed cells<br />
(i) Spherical cells<br />
egg of many animals<br />
(ii) Flattened cells<br />
Squamous epithelium, endotheliums, and<br />
upper layer of epidermis<br />
(iii) Cuboidal cells<br />
Thyroid gland follicles<br />
(iv) Columner cells<br />
The cells lining the intestine.<br />
(v) Discoidal cells<br />
Erythrocytes<br />
(vi) Spindle shaped cells Smooth muscle fibres<br />
(vii) Elongated cells<br />
Nerve cells<br />
(viii) Branched cells<br />
Pigment cells of skin<br />
Depending upon their organisation, cells are basically of two types<br />
i.e., prokaryotic cells and eukaryotic cells. Prokaryotic cells are primitive<br />
type of cells. These cells lacks true nucleus and other cell organelle like<br />
mitochondria, chloroplasts, endoplasmic reticulum, golgi complex, etc.<br />
The word prokaryotes is derived from Greek word pro means primitive<br />
or old and karyotes means nucleus or main. Eukaryotic cells have true<br />
nucleus and it also contains cell organelles. The word eukaryotes is<br />
derived from Greek word eu means true and karyotes means nucleus.<br />
The main differences between prokaryotic and eukaryotic cells is<br />
enlisted in Table 1.3.<br />
Table 1.3<br />
DIFFERENCE BETWEEN PROKARYOTIC AND EUKARYOTIC <strong>CELL</strong>S<br />
Prokaryotic cell<br />
Eukaryotic cell<br />
1. A true nucleus is absent. A true nucleus is present.<br />
2. Nuclear membrane is absent. Nuclear membrane is present.<br />
3. Nucleolus is absent. Nucleolus is present.<br />
4. Chromosome is single, circular. Chromosomes are many and linear.<br />
5. Chromosome present freely in Chromosomes are enclosed inside the<br />
the cytoplasm.<br />
nuclear membrane.<br />
(Contd.)
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6 Biotechnology<br />
Prokaryotic cell<br />
Eukaryotic cell<br />
6. Histone proteins are absent in Chromosomes are well organised with<br />
the organisation of chromosome. histone protein.<br />
7. In photosynthetic cells, chloro- In photosynthetic cells, the chlorophyll<br />
phyll pigments are present in pigments are present in the plastids—<br />
cell membrane.<br />
chloroplast.<br />
8. Ribosomes are 70s type. Ribosomes are 80s type.<br />
9. Cell organells are absent. Cell organells are present.<br />
10. Streaming or amoeboid move- Streaming or amoeboid movement of<br />
ment of cytoplasm absent. cytoplasm present.<br />
11. Flagella if present, do not have Flagella shows typical 9 + 2<br />
typical 9 + 2 arrangement. arrangement.<br />
12. Enzymes necessary for respi- Enzymes necessary for respiration are<br />
ration are present in plasma- present in mitochondria.<br />
membrane.<br />
13. Cell division by the process of Cell division by the process of mitosis<br />
amitosis i.e., mitosis and meiosis and meiosis.<br />
is absent.<br />
14. mRNA will not have the 5' mRNA having 5' methyl cap and 3'<br />
methyl cap and 3' poly A tail. poly A tail.<br />
15. Transcription and translation are Transcription and translation are not<br />
combined process, both takes combined process. Transcription takes<br />
place in cytoplasm.<br />
place in nuclers, while translation<br />
occurs in cytoplasm.<br />
16. In genes non-coding regions are In genes non-coding region (i.e.,<br />
absent.<br />
introns) are present.<br />
17. After transcription there is no After transcription, post transcriptional<br />
post transcriptional modification modifications occurs in mRNA.<br />
in mRNA.<br />
18. Polycistronic mRNA. Mono cistronic mRNA.<br />
19. The cell wall contains amino When cell wall present, it doesnot consugars<br />
and muramic acid. tain amino sugar and muramic acid.<br />
Ultrastructure of Eukaryotic cell—Eukaryotic cells are highly<br />
evolved cells and show striking characters in contrast to prokaryotes. It<br />
contains specialised membrane bound cell organelles. The most<br />
important cell organelle is nucleus, which is membrane bound and<br />
includes filamentous chromosomes. Other membrane bound organelles<br />
include chloroplants, mitochondria, lysosomes, golgi complex,<br />
endoplasmic reticulum, etc. The cell organelles provides micro<br />
environments to subcellular metabolic units.<br />
The eukaryotic organisms are very diverse, few are unicellular,<br />
where all the functions are carried out by same cell, while many are<br />
multicellular organisms. Multicellular eukaryotes contains variety of cell
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The Cell<br />
7<br />
types originate as a result of cell differentiation and it’s organised<br />
aggregates, forming tissue systems. Although there is a basic pattern of<br />
organisation, several functional specializations are acquired by cells.<br />
Eukaryotes are grouped into phototrophs (e.g., plants) and<br />
chemotrops (e.g., animals). The phototrophs have a photosynthetic<br />
apparatus and a rigid cell wall of cellulose.<br />
Photosynthetic Eukaryotes—The eukaryotes, which are able to<br />
synthesize their own food in presence of the sunlight is called—<br />
Photosynthetic Eukaryotes. As an example of this we can consider a<br />
typical plant cell. A plant cell has a thick outer most coating called cell<br />
wall, which protect the cell from various osmatic stresses. Cell wall is<br />
made up of cellulose. Inside the cell wall, there is a plasma membrane,<br />
which encloses the cytoplasm. At several locations, two adjacent cells<br />
have intercellular channels called plasmodesmata, which help cellular<br />
communication.<br />
The cytoplasm of plant cell is semifluid, in which many cell<br />
organelles micro and macroneutrients and enzymes are suspended. All<br />
cell organelles are bound by double membrane. Different cell organelles<br />
which are present in plant cell are as follows (Fig. 1.1)<br />
(a) Nucleus—Nucleus is bounded by double layered nuclear<br />
membrane. It contains filamentous chromosomes, which contains genetic<br />
information.<br />
(b) Endoplasmic reticulum—The endoplasmic reticulum forms a<br />
network of tubules whose one end is connected with nuclear membrane.<br />
There are two types of endoplasmic reticulum—smooth endoplasmic<br />
reticulum and rough or granular endoplasmic reticulum. The main<br />
function of endoplasmic reticulam is synthesis and storage of protein/<br />
enzymes.<br />
(c) Golgi complex—It is made up of cisternae and vesicles. It’s main<br />
function is storage.<br />
(d) Mitochondria—Mitochondria synthesize ATP molecule (energy<br />
curency of cell) by oxidative phosphorylation in the absence of light by<br />
breaking down organic molecules.<br />
(e) plastids—Plastids are characteristic of plant cells only. Plastids<br />
are of two types—leucoplasts (with out pigment) and chromoplasts (with<br />
pigments). The most important plastid is with green chlorophyll pigment<br />
called chloroplasts. Chloroplasts participate in the photosynthetic activity.<br />
(f) Lysosomes—It contains hydrolytic enzymes for digestion.<br />
(g) Peroxysomes—It neutralise metabolic peroxides in the cell.<br />
(h) Glyoxysomes—It is involved in glyoxylate metabolism.
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8 Biotechnology<br />
(i) Ribosomes—Plant cell contains 80s types ribosomes. Ribosomes<br />
participates in protein synthesis.<br />
(j) Vacuoles—The plant cell contains a well defined, large vacuoles.<br />
It is usually filled with liquid material. Unlike other cell organelles,<br />
vacuoles are bound by single membrane called tonoplast. In plant cell,<br />
large volume is made up of vacuole generally central and cytoplasm and<br />
other cell organells occupy little space, generally peripheral.<br />
Mitochondrion<br />
Cellulose cell<br />
wall<br />
Chloroplast<br />
Cell membrane<br />
(plasmatemma)<br />
Central vacuole<br />
Leucoplast<br />
containing<br />
starch grain<br />
Ground substance<br />
of cytoplasm<br />
with granular<br />
inclusions<br />
Plasmodesma<br />
Small vacuole<br />
Golgicomplex<br />
(dictyosome)<br />
Nucleolus<br />
Nucleoplasm<br />
containing<br />
chromatin<br />
Endoplasmic<br />
reticulum<br />
with attached<br />
ribosomes<br />
Fig. 1.1 A typical plant cell.<br />
Chemosynthetic Eukaryotes—The eukaryotes which are not able<br />
to synthesise their food in the presence of sunlight are called<br />
chemosynthetic eukaryotes. e.g.. Animal cell. Animal cells are devoid of<br />
a cell wall, and the plasma membrane is the outermost cell boundary,<br />
which carries out catalytic as well as osmotic protection functions. The<br />
animal cell contains all membrane-bound cell organelles in cytoplasm<br />
similar to those of plant cells, with the exception of plastids. Other than
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The Cell<br />
9<br />
cell organelles, animal cells have centrosomes, locomotory organelles<br />
and an elaborate cytoskeleton (Fig. 1.2).<br />
Region containing microtubules and microfibrils<br />
Pinocytic vesicles<br />
Golgi<br />
apparatus<br />
Secretion<br />
vacuole<br />
Phospholipid<br />
storage granule<br />
Tysosome<br />
Centrosome<br />
Nucleus<br />
Nucleolus<br />
Ribosomes<br />
attached<br />
to endoplasmic<br />
reticulum<br />
Nuclear<br />
membrane<br />
Neutral lipid<br />
storage<br />
granule<br />
Cytoplasmic<br />
matrix<br />
Mitochondrion<br />
Plasma membrane<br />
Fig. 1.2 A typical animal cell.<br />
Centrosomes are usually one per cell, which is located near the<br />
nuclear surface. It is a complex structure with a dense outer granular<br />
mantle that encloses a less granular material around two pairs of<br />
cylindrical bodies called centrioles. The centrioles participate in mitosis<br />
and also anchor the flagellum in sperm cell. The cytoplasmic matrix<br />
consists of a complex network of microtubules and microfilaments,<br />
forming the cytoskeleton. The cytoskeleton helps in the movement of<br />
cell organelles, change in cell shape, amoeboid locomotion, cytokinesis,<br />
etc.
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10 Biotechnology<br />
Summary<br />
The cell is the smallest structural and functional unit of the all living<br />
organisms. Cell biology is the study of cell. In 1938, M.J. Schleiden and<br />
T.S. Schwann proposed the cell theory. There is no generalisation in cell<br />
size, shape and structure. Basically cells are of two types namely<br />
prokaryotic cell and eukaryotic cell. Prokaryotic cell is more primitive<br />
cell. Eukaryotic cells depending upon its capacity to produce their own<br />
food in presence of sunlight is divided into photosynthetic eukaryotes<br />
(e.g. plant cell) and chemosynthetic eukaryotes (e.g. animal cell). The<br />
main characteristic of plant cells is it contains chloroplasts and large<br />
vacuoles. The main characteristic of animal cells is it contains<br />
centrosomes, locomotory organelles, cytoskeleton. The remaining cell<br />
organelles commonly present in both plant and animal cells are nucleus,<br />
mitochondria, golgi complex, 80s ribosome, endoplasmic reticulum,<br />
lysosomes, peroxysomes.<br />
EXERCISE<br />
I. 1. What is cell? Describe cell theory.<br />
2. Explain broad classification of cell.<br />
II. Differentiate between the following:<br />
1. Eukaryotic cells and Prokaryotic cells.<br />
2. Plant cells and animal cells.