a) General organization of prokaryotic and eukaryotic cell (Typical animal cell).
All
the cells are membrane bounded units of living matter; these are of two basic
types, viz., prokaryotic and eukaryotic, differing considerably in their
structural organization. The prokaryotic cells are very small and structurally
simple, primitive type of cells having no distinct nucleus. These mostly exist
on earth as free, unicellular organisms and include mycoplasmas, bacteria and
blue green algae (cycanobacteria). In other hand eukaryotic cells are larger,
structurally complex and nucleated cells.
General organization of Prokaryotic cell
Morphological point of view prokaryotic
cells are the most primitive cells (Pro = Primitive, Karyon = nucleus). They
occur in bacteria and blue green algae or cyanobacteria. It consists of central
nuclear components viz., DNA molecule, RNA molecules and nucleoproteins
surrounded by cytoplasmic ground substances with the whole enveloped by a
plasma membrane. The cytoplasm of prokaryotic cell lacks in well-defined
cytoplasmic organelles such as endoplasmic reticulum, Golgi bodies,
mitochondria, centrioles, lysososmes etc.
Structure of Prokaryotic cell (E. coli)
Escherichia coli (Fig. 2.1) is a gram negative bacillus,
which is non-pathogenic and found in the mammalian digestive tract. It measured
about 2.0 micron long and 0.8 to 1.0 micron thick. It has outer cell wall of
100 A0 thicknesses. Its cell wall and plasma membrane have typical
structure of the bacterial cells. The cytoplasm is colloidal containing roughly
5000 chemical compounds ranging from water to DNA molecule. It contains 20,000
to 30,000 ribosome (70 S types) as main cellular organelle. All genetical
information’s are carried by a single, double stranded circular chromosome or
DNA molecule. DNA of E. coli does not
remain wrapped in histone proteins like the DNA of eukaryotic cells. Nuclear region lacks any
membrane in cytoplasm, therefore, called nucleoid. They reproduce amitotically
or asexually i.e. by binary fission.
Fig. 2.1: Prokaryotic
cell (E. coli)
Eukaryotic cells
Eukaryotic
cell (eu = true, Karyotic =
nucleated), these types of cell essentially have two envelope systems and they
are very much larger than prokaryotic cells. Eukaryotic cells are the true
cells which occur in the plants (from algae to angiosperms) and in animals
(from protozoa to mammalian). Though the eukaryotic cells have different
shapes, size and physiology but all the cells are typically composed of plasma
membrane, cytoplasm and its organelles viz., mitochondria, endoplasmic
reticulum, ribosome’s, lysososmes, golgi complex etc and a true nucleus. Here
nuclear contents such as DNA, RNA, nucleoproteins and nucleus remain separated
from the cytoplasm by the thin perforated nuclear membranes. The general
features of different types of eukaryotic cells are as follows;
i)
Cell shape: The basic shape of this cell is
spherical; however the shape is ultimately determined by specific function of
the cell. Thus the shape of cell may be variable or fixed. The variable or
irregular shape is occurring in Amoeba
and white blood cells whereas fixed shape occurs in almost all protists, plants
and animals. In unicellular organism the cell shape is maintained by tough
plasma membrane and exoskeleton. In multicellular organisms, the shape of the
cell depends mainly on its functional adaptations and partly on the surface
tension, viscosity of protoplasm, cytoskeleton of microtubules, microfilaments
and intermediate filaments, the mechanical action of adjacent cells and
rigidity of plasma membrane. The shape of cell may vary from animal to animal
and from organ to organ. Even the cells of same organ may display variation in
shape. Thus cell may have diverse shape such as polyhedral or squamous,
flattened, columnar, discoidal, spherical; spindle shaped, elongated and branched.
Among plants the cell shape also depends upon the function of the cell.
ii)
Cell size: The eukaryotic cells are typically
larger (measured about 10 to 100 µm) than prokaryotic cells (1 to 10 µm). Size
of cells of unicellular organisms is larger than the typical multicelluar
organism’s cells e.g. Amoeba proteus
is biggest among the unicellular organisms; its length being 1000 µm (1 mm).
one species of Euglena is found up to
0.5 mm, Paramecium caudatum is
150-300 µm, diatoms have length of 200 µm or more. The single celled alga,
Acetabularia which consist of a stalk and a cap is exceptionally large sized
and measured up to 10 cm in length. The size of multicellular organism ranges
between 20 to 30 µm.
iii)
Cell volume: The volume of a cell is fairly
constant for a particular cell type and is independent of the size of the
organism. For example. Kidney or liver cells are about same size in the bull,
horse and mouse. The difference in total mass of organ or organism depends on
the number, not one volume of the cells. Thus, the cells of an elephant are not
necessarily larger than those of tiny mammals or plants. The size of elephant
is large due to large number of cells present in its body.
iv)
Cell number: The number of cells present in the
organism is varies from a single cell in unicellular organisms to many cells in
the multicellular organisms. The number of cells in multicellular organism is
remaining correlated with size of the organism and therefore small sized
organism has less number of cells in comparison to large sized organisms. For
example, a human being weighing about 80 kg may contain about 60 thousand
billion cells in his body. This number would be more in certain other
multicellular animals.
General organization of eukaryotic cell
With
few differentiations in eukaryotic cell of plant and animal, the general
organizations of this cell are similar. The general organization of typical
animals (fig.2.2) is consists of three main components i.e. Plasma membrane,
cytoplasm and nucleus.
A) Plasma membrane
Most of animal’s cells have an
external covering known as plasma membrane, cell membrane, or plasmalemma. It is
ultra-thin, delicate, selectively permeable, porous, lipoproteinous living
membrane. Primarily it provides mechanical support and external form to the
inner cytoplasm and nucleus. It shows both exocytosis and endocytosis and also
allows the passage for solvent as well as solute through it.
B) Cytoplasm
The space
lies between the plasma membrane and the nucleus is filled by an amorphous,
translucent, homogenous colloidal liquid known as cytoplasmic matrix. This
matrix is non-living, composed of inorganic as well as organic molecules. The
main living and membrane bounded organelles are as follows,
i)
Endoplasmic reticulum (ER): It is a complex network of hollow,
membrane bounded, fluid filled channels in the cytoplasm. It is connected to
plasma membrane on one side and to nuclear membrane on the other side. The
surface of ER is smooth or rough due to ribosome on it. Functionally it
provides a structural network to the cell and giving mechanical support,
synthesis and storage etc.
Fig. 2.2 Typical eukaryotic cell of animal
ii)
Golgi bodies: Also called Golgi complex. They have
cup or disc shaped structure, attached to ER on one side. It is packaging and
forwarding centre for many secretary products. It is believed that, vesicles of
Golgi bodies are associated with lysosome formations.
iii)
Lysosome: These are many in numbers and freely
distributed in the cytoplasm. These contain about 50 different types of
hydrolytic enzymes and acid phosphatase. Functionally, lysosome concern with
intracellular digestion of food. Lysosome plays role in degeneration of tail
(metamorphosis in frog due to their autolysis nature, they are termed as ‘suicide bag’ of cells.
iv)
Mitochondria: Mitochondria are ‘power house’ of the cells as they are concern with generation of
ATP through electron transport and oxidative phosphorylation. The number of
mitochondria in a cell is variable according to functional status of the cell.
The shapes also variable, but generally spherical shaped. It has double
membrane, encloses its own DNA ring and
70 S types of ribosome and the enzymes of respiration.
v)
Ribosome: These are freely distributed in
cytoplasm or attached to the ER and outer nuclear membrane. Ribosomes are
minute, non-membranous particles, made up of two subunits. Ribosome’s are site
where proteins are synthesized, therefore, termed ‘factories of protein syntheses.
vi)
Centrosomes: Found located near the nucleus. It
consists of pair of centrioles, plays role in formation of spindle apparatus
and basal bodies for cilia and flagella.
vii) Vacuole: These are primarily found in few lower
animals. They are bounded by single selectively permeable membrane called
tonoplast. They maintain turgor of cells. Contractile vacuole in animal are
concerned with osmoregulation.
viii) Microtubules
and microfilaments: The
cytoplasm is transversed by numerous ultrafine tubules of tubulin protein
called microtubules. The function of microtubules is the transportation of
water, ions or small molecules and formation of spindle fibers of the mitotic
or meiotic cell division. Moreover, they form the structural units of the
centrioles, basal granules, cilia and flagella.
The cytoplasm of most animal’s cell contains ultrafine proteinous solid
material called microfilaments. They maintain the structure of cell and form
contractile components of muscle cells.
ix)
Cilia and flagella: The cells of many unicellular
organisms and ciliated epithelium of multicellular organisms consist of some
hair-like cytoplasmic projections outside the surface of the cell, called cilia
or flagella. They help in locomotion of the cell. It consists of nine outer
fibrils around the two large central fibrils.
C)
Nucleus
Nucleus is a centrally located and spherical cellular
component which controls all the vital activities of the cytoplasm and carries
the hereditary material the DNA in it.
Typically nucleus consisting of nuclear membrane, nucleolus, nucleoplasm
and chromatin network
i. e. chromosomes. Outer nuclear
membrane is connected with ER where as inner connected with chromatin fibers.
The interrupted region is called nuclear pore thorough which mRNA passes out
during transcription of DNA for protein synthesis.
b)
Structure of plasma membrane
The plasma membrane is also called
cytoplasmic membrane or cell membrane or plasmalemma. The term cell membrane
was coined by C. Nageli and C. Cramer
in 1855 and the term plasmalemma have been given by J. Q. Plowe in 1931. It
encloses every type of cell, both prokaryotic and eukaryotic cells. It
physically separates the cytoplasm from the surrounding cellular environment.
Plasma membrane is ultrathin, elastic, living, dynamic and selective transport
barrier. It is a fluid mosaic assembly of molecules of lipid or phospholipids,
proteins and carbohydrates. Plasma membrane controls the entry of nutrients and
exit of waste products, and generates differences in ion concentration between
the interior and exterior of the cell. It is also acts as sensor of external
signals and allows the cell to react or change in response to environmental
signals.
The plasma
membrane is so thin that it cannot be observed by the light microscope.
Structure of the plasma membrane of various cells has been studied by their
isolation from the living systems and also by their artificial synthesis by
using their constituent molecules. The isolated membrane is then studied by
biochemical and biophysical methods. A variety of cells such as mammalian red blood cell, medullated nerve fibres, liver
cell, striated muscle, amoeba, sea urchin egg and bacteria have been used in
studying the ultra structure of the plasma membrane. For plasma membrane the red blood cell is suitable
material and easily isolated when blood cell subjected to haemolysis. The cells
are treated with hypotonic solutions that due to endosmosis produce swelling
and then loss of the hemoglobin content or haemolysis; the resulting membrane
is called a red cell ghost. The red cell ghost is used further for the
study of physiological as well as biochemical properties.
The existence of the plasma membrane of
the cell was difficult to prove by direct examination before 1930’s because of
technological limitations. Later on with advent of electron microscope,
following model describe the structure of plasma membrane.
i.
Bilayer model of Danielli and Davson
The bilayer or sandwich model of
Danielli and Davson proposed on the basis of comparative datas of surface
tensions of lipid layers and cell surfaces, collected by H. Davson during early
1930s and J. Danielli (1935) concluded
that the plasma membrane contains a good amount of proteins in addition to the
“lipid bilayer”. They hypothesized that;
a)
The
polar (hydrophilic) heads of inner lipid layer are embedded in the surface of
the aqueous cytosol, whereas the heads of outer lipid layer are embedded in the
surface of aqueous extracellular medium.
b)
The
non-polar (hydrophobic) fatty acid tails of the lipid molecules of both layers
are directed away from the respective aqueous phases. Hence these face each
other and form a middle core region of
the plasma membrane, and
c)
The
polar heads of both lipid layers are coated or covered by hydrophilic proteins
which form almost continuous layers.
Fig.2.3 A: Sandwich model of Danielli Fig. 2.3 B: Pore structure in model of
Danielli
and Davson and Davson
Thus, Davson and Danielli
envisaged a “Sandwich” model of membrane structure with a “bimolecular lipid
layer” being sandwiched between two protein layers (fig 2.3 A). Later (1954),
Danielli further proposed that at places the lipid bilayer is perforated by
pores which are lined by protein molecules (fig. 2.3 B) and which serve as
diffusion channels for polar molecules and ions.
ii. Unit membrane model of
Robertson
The plasma membrane was actually
observed for the first time with the help of electron microscope during early
1950s. It was found to be a
three-layered (trilaminar) structure showing two dark staining lines separated
by a middle light staining line in a dark -light-dark ‘railway-track’ pattern.
Each dark line represents a continuous sheet
of mostly small and globular hydrophilic proteins together with the
underlying polar heads of lipid molecules, whereas middle light lines
represents a bilayer of non-polar fatty acid tails of lipid molecules of both
layers. It was also found that the proteins of external and internal layers are
dissimilar (fig. 2.4) so that the trilaminar structure is asymmetrical. Since
certain cell organelles like nucleus, mitochondria and chloroplast etc are
surrounded by two concentric membranes, each having this trilaminar structure.
J. D. Robertson (1957) proposed that the trilaminar structure represents a “Unit membrane”. This concept confirms
that the plasma membrane is a water-insoluble barrier so that only
lipid-soluble substances can pass directly through it.
Fig. 2.4 : Robertson’s unit memberane model
iii.
Fluid mosaic model.
S.
J. Singer and G. L. Nicolson (1972) proposed and widely accepted fluid mosaic
model of biological membranes. According to this model (fig 2.5), the plasma
membrane contains a bimolecular lipid layer, both surfaces of which are
interrupted by protein molecules. Proteins occur in the form of globular
molecules and they are dotted about here and there in a mosaic pattern. Some
proteins are attached at the polar surface of the lipid (i.e. the extrinsic or
peripheral proteins) while other at the non-polar surface (i.e. integral
proteins) either partially penetrate the bilayer or span the membrane entirely
to stick out on both sides (transmembrane protein). Further, the peripheral
proteins and those parts of the integral proteins that stick on the outer
surface frequently contain chain of sugar or glycoprotein’s. Likewise some
lipids of outer surface are glycolipids. The fluid mosaic model is found to be
applied to all biological membranes in general and it is seen as a dynamic,
ever-changing structure. The proteins are present not to give it strength, but
to serve as enzymes catalyzing chemical reactions within the membrane and as
pumps moving things across it.
Fig. 2.5 : Fluid mosaic
model of Singer and Nicolson
c)
Functions of plasma membrane.
Being
the delineating covering of the cells, plasma membrane is a multifunctional structure. It various functions can be grouped
into five main categories as described bellows;
1. Delineating
the cells
As the delineating boundary of
cells, the plasma membrane plays following roles;
i.
It
provides mechanical support to a cell and maintains its shape and volume.
ii.
It
prevents a free mixing of the intracellular living material and the
extracellular non-living material, maintaining the structural and functional
integrity of a cell as an “autonomous
unit of life”.
iii.
It
protects the cellular contents from invasion of pathogenic microbes and ill
effects of antigens or poisonous substances
and changes in the physic-chemical environmental conditions, such as
temperature, pH, pressure, chemical composition etc.
iv.
Being
a dynamic structure, it is quite flexible. Hence, it does not restrict growth ,
movements and changes in the shape of cell.
v.
Being
highly regenerative, it quickly helps up if injured, preventing disintegration
of a cell.
2. Regulating
chemical exchange between a cell and extracellular medium
Plasma membrane as an autonomous unit
of life, every cell is analogous to a “miniature chemical factory”, because
molecular are continuously metabolized in it by thousands metabolic reactions.
Obviously, the cell continuously obtain useful raw material from its
surrounding and disposes off its waste products in returns. This chemical
exchange occurs through plasma membrane which, therefore, functions as a selectively permeable barrier. The
membrane mediates the traffic material in and out of the cells by several
mechanisms which can be grouped into three main categories, viz., passive
transport, active transport and bulk transport.
I.
Passive transport: It is by diffusion including
diffusion of water or osmosis. Ions and molecules diffuses across the membrane
“downhill” in accordance with their
concentration gradients i.e. from the side of their higher concentration
towards the side of their lower concentration till an equilibrium is attained.
This diffusion may be simple or facilitated. In simple diffusion, small and
uncharged molecules like O2, CO2 and H2O etc
simply slip between lipid molecules. Facilitated diffusion is however, mediated
by certain membrane proteins called transport proteins, which transport inorganic ions (K+,
Na+, Cl-, H+, Ca++, HCO3-
etc) and large polar organic molecules (sugars, amino acids, nitrogenous bases,
nucleosides etc.) across the membrane without any expenditure of energy.
II.
Active transport: It is a special category of
transmembrane integral proteins transport certain materials particularly ions,
across the membrane “Uphill” against
their concentration gradients i.e. from the sides of their lower towards higher
concentrations, obviously with great expenditure of energy.
III.
Bulk transport: In this process, plasma membrane
mediates ingestion or engulfing and egestion or extrusion of large molecules
(macromolecules), liquid droplets and solid particles by the cells. Bulk
transport is and out of the cell are respectively called endocytosis and exocytosis.
In endocytosis, the plasma membrane bulges towards, forming a cup-like
invagination which contains the extracellular material to be ingested.
Eventually, the invagination closes, becoming a membrane bound endocytic
vesicle which pinches off form plasma membrane and moves into the cytosol.
Thus, it is internalizatoin of external material. The process is called pinocytosis (cell drinking) or phagocytosis (cell eating) if the internalized material is
respectively fluid or solid. In exocytosis,
membrane bound vesicle formed intracellularly, move to the plasma membrane and
fuse with it throwing out their contents in the process. These vesicles
generally contain secretary material and hence called secretary vesicles.
3. Receiving
external signals and mediating cellular responses
Any physic-chemical change in
extracellular medium which affects a cell is called a stimulus (extrinsic
signal or information). Becoming aware of stimuli is the sensory function of
cells, which is obviously mediated by plasma membrane with the help of membrane
proteins called receptor proteins. Thus the membrane serves as the “sensory surface” of cells. Stimuli
threaten the steady state or functional equilibrium of cells. Hence the cell
reacts or responds to the stimuli by modifying their metabolism to counteract
the effects of stimuli and thus, maintain their steady state. This capacity of
cell is called homeostasis.
Metabolic changes for homeostasis are mostly mediated by enzymes present in
plasma membrane. Besides mediating cellular responses and homeostasis, the
receptor proteins also helps in cell-to –cell communication and signaling
through hormones, growth factors, neurotransmitters etc.
4. Mediating
cell-cell interactions
In 1970, H. Wilson
dissociated the cells of a marine sponge by pressing the sponge through a
muslin cloth is sea water. Later the segregated cells reaggregated into a
similar sponge. Afterwards, it was discovered that cells can recognize other
cells, similar or dissimilar by means of the oligosaccharide molecules of their
glycocalyx, serving as tagmarks or markers, like the destination tags of postal
bags or railway parcels, these oligosaccharide molecules enable cells to recognize
other cells and hence, mediate cell-cell interactions like cell-cell adhesion
for tissue formation and immune reactions, formation of cell-cell junctions for
intercellular communication, and exchange of ions and small molecules and so
on.
5. Facilitating
increase in surface area and cell movements and migration
Many types of cell like absorptive cells of
intestinal epithelium may form minute, finger like extension, called microvilli
which increase their surface area. Likewise many types of cells bear locomotory
appendages such as pseudopodia, flagella or cilia. These structures involve
extension of plasma membrane. Certain cells even crawl on solid substrata by
forming and withdrawing peg-like extensions of plasma membrane.
d)
Study of cell organelles with reference
to ultra structure and functions of :
The cellular components are called cell organelles. These cell
organelles include both membrane and non-membrane bound organelles, present
within the cells and are distinct in their structures and functions. They
coordinate and function efficiently for the normal functioning of the cell. A
few of them function by providing shape and support, whereas some are involved
in the locomotion and reproduction of a cell. There are various organelles
present within the cell and are classified into three categories based on the presence or absence of
membrane.
a.
Organelles without membrane: The Cell wall,
Ribosomes, and Cytoskeleton are non-membrane-bound cell organelles. They are
present both in the prokaryotic cell and the eukaryotic cell.
b.
Single membrane-bound organelles: Vacuole, Lysosome,
Golgi Apparatus, Endoplasmic Reticulum are single membrane-bound organelles
present only in a eukaryotic cell.
c.
Double membrane-bound organelles: Nucleus,
mitochondria and chloroplast are double membrane-bound organelles present only
in a eukaryotic cell.
Let us learn more in detail about the different cell organelles in brief.
i.
Ultra structure and functions of Nucleus
The nucleus
(Greek root, Karyon) is the heart of the cell. It contain almost all the cell’s
DNA, therefore it controls different metabolic as well as hereditary activities
of the cell. It serves as the main distinguishing feature of eukaryotic cells. It was first discovered and named by Robert
Brown (1833) in plant cells and were quickly recognized as a constant
feature of all animal and plant cells. Nucleoli were described by M. J. Schleiden in 1838, although first noted by Fontana
(1781). The term nucleolus was coined by Bowman in 1840 whereas W.
Flemming (1879) coined the term chromatin for chromosomal meshwork.
Ultra structure of nucleus
The
nucleus is composed of following structure (fig 2.6);
i. The Nuclear envelope or nuclear membrane or karyotheca
ii. The nuclear sap or nucleoplasm
iii. The chromatin fibres and
iv.
The
nucleolus.
i.
The
Nuclear envelope
It forms an envelope like structure
around the nuclear contents therefore commonly called nuclear envelope. It
separates the nucleoplasm from cytoplasm of the cell. The electron microscopic
structure of nuclear envelope has shown that it is composed of two unit
membranes, an outer and inner membrane. Each membrane is about 75 to 90 A0
thick and lipoproteinous in nature. The outer and inner nuclear membrane
remains separated by space of 100 to 150 A0. This inter membrane
space is known as perinuclear space. The outer nuclear membrane remains rough
due to attached ribosomes with it. Sometimes it remains continuous with the
membrane of endoplasmic reticulum, Golgi complex and mitochondria etc. The
inner nuclear membrane remains associated with the chromatin. Further, the
nuclear membrane is followed by a supporting proteinous membrane of uniform
thickness known as inner lamina or fibrous lamina which is about 300A0 in
thickness.
Nuclear pores: The nuclear membranes are not
continuous but at several places they are broken by the nuclear openings or
pores. The number of the pores for a particular nucleus is variable and often
depends on the species and the type of cell. The nuclear pores are octagonal in
shape and have diameter of 600 A0. These are enclosed by a circular
structure known as annuli. The pores and annuli are collectively known to form
the pore complex. The pore complex remains arranged hexagonally on the surface
of the nuclear envelope. The annuli have an outer diameter of 1200A0
and an inner diameter between 0 to 400 A0.
ii.
Nucleoplasm
The space between the nuclear envelope and the
nucleolus is filled by a transparent, semisolid, granular and slightly acidic
substance called nuclear sap or nucleoplasm. The nuclear components like
chromatin threads and the nucleolus remains suspended in the nucleoplasm. It
has complex chemical composition, mainly composed of nucleoprotein as well as
other organic and inorganic matter like nucleic acids, proteins, enzymes and
minerals.
a) Nucleic acids: The most common nucleic acids are DNA
and RNA. Both may occur in macromolecular state or in the form of their monomer
nucleotides.
b) Proteins: The nucleoplasm contains mainly two
types of proteins viz., acidic or non-histone proteins and basic or histone proteins.
The acidic proteins occur in chromatin fibres e. g. The proteins of euchromatin are the
phosphoproteins. The basic proteins, which take basic, stain. The basic
proteins of nucleus are nucleoprotamines and the nucleohistamines.
c) Enzymes: The enzymes of nucleoplasm are mainly
for synthesis of the DNA and RNA; these are DNA polymerase, RNA polymerase, NAD
synthetase, nucleoside triphosphate,
adenosine diaminase, nucleoside phosphorylase, guanase, aldolase and pyruvate
kinase etc.
d)
Minerals: the nucleoplasm contains the
minerals like phosphorus, potassium,
sodium, calcium and magnesium etc
iii. Chromatin fibres
The nucleoplasm contains much thread
like, coiled and much elongated structures called chromatin fibres; which takes
basic stain such as the basic fichsin. These are occurring mainly in the
interphase nucleus whereas during cell division, these fibres become thick
ribbon like structure called chromosomes. The fibres are twisted, fine
anastomose and uniformly distributed.
There are two types of fibres viz.,
a) Heterochromatin: These are darkly stained, condensed
region of the chromatin. It occurs around the nucleolus and at the periphery.
It is supposed to be metabolically and genetically inert because it contains
comparatively small amount of the DNA and large amount of the RNA.
b)
Euchromatin: It is light stained and diffused
region, it contains large amount of genetically active substance the DNA.
iv. Nucleolus
Most cells contain in their one or more
prominent spherical colloidal acidophilic bodies called nucleoli. However, cell
of the bacteria and yeast lack nucleolus. The size of the nucleolus is found to
be related with the synthetic activity of the cell. Therefore, the cells with
little or no synthesize activities e. g. sperm cells, blastomeres, muscle cell etc are found to contain smaller
or no nucleoli. While the oocytes, neurons and secretary cell which synthesize
the proteins and other material contain comparatively large sized nucleoli. The
number of the nucleoli in the nucleus depends on the species and the number of
the chromosomes. The number of the nucleoli in the cell may be one, two or
four. The position of the nucleolus in the nucleus is eccentric.
Fig. 2.6 : Ultrastructure of Nucleus
Functions of
nucleus
Nucleus is the controlling centre of
the cell, it perform following important functions;
i.
It
controls all the metabolic activities of
the cell by controlling the synthesis of required enzymes.
ii.
The
nuclear membrane allows exchange of ions between the nucleus and the cytoplasm.
iii.
The
nuclear envelope also acts as barrier for diffusion of substances and ions like
K+, Na+ and Cl-.
iv.
The
nuclear pores are acts as pathway for the exchange of macromolecules like mRNA.
v.
The
nucleus controls the inheritance of characters from parents to offspring’s.
vi.
The
nucleolus performs a function of biogenesis of ribosomes.
ii.
Ultra structure and functions of E.
R.
The
cytoplasmic matrix of the cell is
transverse by a complex network of interconnecting membrane bound vesicles,
called endoplasmic reticulum; these are often remain concentrated in the
endoplasmic region of cytoplasm. The
name derived from the fact that in the light microscopy it looks like a ‘net in
the cytoplasm’. It was first observed by Porter (1945) under electron microscope from liver
cell and gave the name ER in 1953. Later on Fawcett, Ito and Thiery (1958) and Rose and Pomerat
(1960) have made various important contribution to the endoplasmic reticulum.
Morphologically,
the ER may occur in the following three forms (fig 2.7), each bounded by unit
membrane made up of protein and lipid molecules.
i.
Cisternae:
These are long, flattened, sac-like,
unbranched tubules having the diameter of 40 to 50 micron. They remain arranged
parallel in bundles or stakes. RER usually exists as cisternae which occur in
those cells which have synthetic roles as the cells of pancrease, liver
and brain.
ii.
Vesicles:
These are oval, membrane bound vacuolar
structure having the diameter ranging from 25 to 500 micron. They often remain
isolated in the cytoplasm and occur in most cells especially abundant in the
SER.
iii.
Tubules: These are branched structure forming
the reticular system along with the cisternae and vesicles. They usually have
the diameter form 50 to 190 micron and occur in almost all cells.
Types of Endoplasmic Reticulum
Two types of ER have been
observed in same or different types of cells which are as follows;
a) Agranular or Smooth Endoplasmic
reticulum (SER)
This type of ER
possesses smooth walls because the ribosomes are not attached with its
membranes. This type of ER occurs mostly in those cells which are involved in
the metabolism of lipids and glycogen e.g. adipose cells, interstitial cells,
glycogen storing cells of the liver, conduction fibres of heart, spermatocytes
and leucocytes. Although the SER forms a continuous system with RER.
b)
Granular
or Rough Endoplasmic reticulum (RER)
This type of ER possesses rough walls
because the ribosomes remain attached with its membranes. Ribosomes play a
vital role in the process of protein synthesis. This type o ER abundantly found
in those cells which are active in protein synthesis e.g. Pancreatic cells, plasma cells, goblet cells
and liver cells.
Functions of ER
The ER acts as secretary,
storage, circulatory and nervous system for the cell. It performs following
important functions;
i.
Mechanical
support: The ER divides
the fluid content of cells into smaller compartments and thus provides
mechanical support for the colloidal cytoplasm.
ii.
Protein
synthesis: The
RER is the site of protein synthesis. The proteins are synthesized on the
ribosomes and enter the ER cisternae thorough channels in the membrane i.e.
secretory proteins leave the ER an enter the Golgi complex, from where they are
secreted outside the cell.
iii.
Glycogen
synthesis and storage: In
fasted animals, the residual glycogen has been found associated with the ER.
iv.
Lipid
Synthesis and storage: The
ER has been associated with synthesis of triglyceride as well as intracellular
transport and storage of lipids.
v.
Synthesis
of cholesterol and steroid hormones: ER
is the major site for cholesterol synthesis. SER is believed to be concerned
with both the synthesis and storage of cholesterol. The testis, ovary and adrenal cortex, the SER
has a role in the synthesis of steroid hormone. Cholesterol is an important
precursor of steroid hormone.
vi.
Detoxification: The SER contain an enzyme system
concern with detoxification properties.
vii.
Circulation
and exchange: The
ER acts as an intracellular transport system for various substances. Watson
(1959) suggested that the exchange between the nucleus and the cytoplasm takes
place thorough the nuclear openings which communicate with the ER. Substance may
pass across the ER membrane by diffusion as well as by active transport.
viii.
ix.
Other synthetic funtions: The SER synthesizes the steroid compounds like glycerides,
hormones, testosterone and progestin. It also synthesizes certain visual
pigments from vitamin A.
iii. Ultra
structure and functions of Golgi
bodies
It was in 1890, an Italian
neurologist Camilo Golgi described the internal reticular
apparatus in the nerve cells of the barn owl. The Golgi complex or Golgi
apparatus is a complex cytoplasmic organelle of animal and plant cells. It is
for the performance of certain important cellular functions such as biosynthesis
of polysaccharides, packaging of cellular synthetic products like proteins,
production of secretary vesicle and differentiation of cellular membrane. Like
ER, the Golgi complex is a canalicular system with sacs, but it has parallel
arranged flattened membrane bounded vesicles which lacks ribosomes.
Morphology
of Golgi complex
Morphologically the Golgi complex of
plant and animal cells has basic similarity. The Golgi bodies of plant cell
measure about 1- 3 micron in length and about 0.5 micron width. The organelle
is disc shaped (Fig 2.8) and consists of central flattened plate like
compartments called cisternae, peripheral network of interconnecting tubules
and peripherally occurring vesicles and Golgian vacuoles.
i.
Cisternae:
A Golgi cisternae is a sac like
structure filled with fluid. A Golgi complex usually consist of variable number
(3 to 12) flat, tubular or filamentous cisternae which are closely packed in
parallel bundles or stack one above the other. All the cisternae are slightly
curved so as to give a concave and convex face to the whole stack. In the
stack, the cisternae are separated from each other by an inter-cisternal space
of about 100-150 A0 width.
The cisternae have some definite polarity. One pole of each of them is
associated with nuclear envelope or with endoplasmic reticulum; this is called
proximal pole or forming face. Toward the opposite pole or distal pole, called
maturing face, the cisternal membranes become progressively more like plasma
membrane. The secretary vesicles are formed from this distal pole of cisternae.
ii.
Tubules:
Tubules are arises from the peripheral
area of cisternae. These are complex, anatomizing flat network having 300 – 500
A0 diameter.
iii.
Vesicles:
These are small; droplet like sacs
which remain attached to tubules at the periphery of cisternae. There are
either smooth or coated vesicles.
iv.
Golgian
vacuole: These are large,
spacious, rounded sac like structure lies at distal ends of cisternae. These
are produced by vesiculation of cisternae.
Functions of
Golgi complex
Golgi complex is actively
involved in the metabolism of cells. It performs following important functions;
1.
The
primary function of Golgi complex is formation of secretary vesicles or primary
lysosome.
2.
It
acts as packaging and forwarding centre of cell because it works as
condensation unit of cell. At this region the molecules of lipids, enzymes,
hormones etc get condensed.
3.
It
synthesizes complex carbohydrate from simple sugars. Afterwards, these get
combined with proteins to forms glycoproteins.
4.
It
manufactures acrosomal part of sperm in testes of most vertebrate. Acrosome is
a cap like structure lies at head of the spermatozoa. It contains various enzymes,
required for the penetration of vitelline membrane of egg.
5.
In
the cells of endocrine glands, Golgi complex is actively concerned with
synthesis of various hormones.
6.
In
protozoan cells, Golgi complex forms and functioning of contractile vacuoles.
7.
In
plant cells, it is responsible for synthesis of cell wall.
8.
Golgi
complex plays active role in the formation of conjugated compounds like
phospholipids and lipo-proteins that are most essential for the synthesis of
plasma membrane.
iv. Ultra
structure and functions of Lysosomes
The
lysosome (Gr. Lyso = digestive, soma = body) are tiny membrane bounded vesicle
involved in intracellular digestion. They contain a variety of hydrolytic
enzymes that remain active under acidic conditions at the pH around 5. The
lysosome particles were observed in liver cells as rounded, dense bodies and
were first called pericanalicular dense bodies. They were observed and named by
Christian de Duve in 1955 as cytoplasmic dense bodies and termed
as ‘suicide bags’ in 1959. Thus, lysosomes are remarkable cytoplasmic
organelles are primarily meant for the digestion of a variety of biological
materials and secondarily cause aging and death of animal cells and also a variety of human disease such as cancer,
gout, Pompe’s disease and silicosis etc.
Structure of
lysosome
The lysosomes are difficult to
identify under the electron microscope because they have irregular shape or
structure. They are generally spherical or globular shaped, but their shape and
density vary greatly. Lysosomes are vary from
cell to cell and time to time i.e. they are polymorphic, they are 0.2 to
0.8 micron in size but may be exceptionally large i.e. measure up to 5.0 micron in the cells of mammalian kidney (fig 2.9) or
even more in phagocytes and leucocytes. The lysosomes are remains filled with
dense material or enzymes and are bounded by single unit membrane. If the enzymes are released they can digest
the cell, and hence lysosomes are sometimes called ‘Suicide bags’.
Functions of lysosomes
The lysosomes perform following
important functions;
i.
Digestion
of large extracellular particles: The
lysosomes digest the food contents of the phagosomes or pinosomes. The
lysosomes of leucocytes are able to digest the foreign germ particles like bacteria,
viruses etc.
ii.
Digestion
of intracellular substances:
During the starvation, the lysosomes digest the stored reserved food contents
viz., proteins, lipids and carbohydrates or glycogen of the cytoplasm and
supply necessary amount of energy to the cell.
iii.
Autolysis: In certain pathological conditions,
the lysosomes start to digest the various organelles of the own cell, this
process is called autolysis or cellular autophagy. When a cell dies, the
lysosome membrane ruptures and enzymes are liberated. These enzymes digest the
dead cells. in the process of metamorphosis of amphibians and others many embryonic tissues like gills,
fins, tail etc are digested by the lysosomes.
iv.
Extracellular
digestion: The lysosome of
certain cells such as sperms discharges their enzymes outside the cell during
the process of fertilization. The Lysosomal enzymes digest the limiting
membranes of the ovum and form penetration path in ovum for the sperms. The
chondrioblast cells and osteoblast cells digest the cartilage and bone
respectively by the process of extracellular digestion.
v.
The
lysosomes are supposed to initiate the mitosis in cells.
vi.
Some
researchers suggested that the lysosomes release the enzymes may be involved in
carcinogenesis.
v.
Ultra structure and functions of
Mitochondria.
The mitochondria are filamentous or granular cytoplasmic
organelles of higher plants and animals cells and also of certain micro
organisms like algae, protozoa and fungi.
These are absent in prokaryotic cells.
It has lipoproteinous frame work which contains many enzymes and
coenzymes required for energy metabolism. They have its own DNA for the
cytoplasmic inheritance and 70 S ribosomes for the protein synthesis.
Structure
of mitochondria
Microscopic
structure of mitochondria (fig 2.10) consist of a fluid filled cavity
surrounded by two trilaminar unit membranes an outer limiting membrane and
somewhat thicker inner membrane having 60-70 A0 thickness. The outer membrane is smooth and straight. Both membranes are separated by an
inter-membrane space called perimitochondrial space of 60-80 A0
width. The inner membrane is infolded into the cavity, forming a number of
simple or branched plates like septa or lamellae called cristae. The cristae
bear many regularly spaced, minute club shaped particles projecting into
mitochondrial cavity or mitochondrial matrix. These particles are called
elementary or F1 particles or oxysomes. Each particle has three
distinct parts- a base piece embedded in the membrane, a projecting stalk or
pedicel and a knob like head at the tip of the stalk. The matrix contains lipids, proteins,
circular DNA molecules, 70 S ribosome and certain granules. The granules are
prominent in the mitochondria of cell concerned with the transport of ions and
water.
Function
of mitochondria
The structure
and enzymatic system of mitochondria are fully adapted for different function
such as follows;
i.
Mitochondria
are the actual respiratory organs of the cell where foodstuffs i.e.
carbohydrates and fats are completely oxidized into CO2 and H2O.
ii.
During
biological oxidation of foodstuffs large amount of energy is released which is
utilized by the mitochondria for synthesis of the energy rich compound known as
adenosine triphosphate (ATP); therefore it is known as ‘power house of the
cell’.
iii.
Mitochondria
acts as site for oxidative phosphorylation
iv.
Mitochondria
provide site for Krebs cycle.
v.
It
provide site for dehydration.
vi.
It
provide site for glycolysis
vii.
It
acts as classical example of cytoplasmic inheritance.
viii.
Occasionally
mitochondria function as store organs e. g. mitochondria of ovum store yolk
protein and ultimately get converted into yolk bodies of egg.
VI Ultra-structure and functions of Ribosomes
A ribosome is a
complex molecular machine found inside the living cells that produce proteins
from amino acids during a process called protein synthesis or translation. The
process of protein synthesis is a primary function, which is performed by all
living cells.
Ribosomes are
specialized cell organelles and are found in both prokaryotic and eukaryotic cells.
Every living cell requires ribosomes for the production of proteins.
This cell
organelle also functions by binding to a messenger ribonucleic acid (mRNA) and
decoding the information carried by the nucleotide sequence of the mRNA. They transfer
RNAs (tRNAs) comprising amino acids and enter into the ribosome at the acceptor
site. Once it gets bound up, it adds amino acid to the growing protein chain on
tRNA.
A ribosome is a
complex of RNA and protein and is, therefore, known as a ribonucleoprotein. It
is composed of two subunits – smaller and larger.
The smaller
subunit is where the mRNA binds and is decoded, and in the larger subunit, the
amino acids get added. Both of the subunits contain both protein and
ribonucleic acid components.
The two subunits
are joined to each other by interactions between the rRNAs in one subunit and
proteins in the other subunit.
Ribosomes are
located inside the cytosol found in the plant cell and animal cells.
The ribosome
structure includes the following:
v It is located in two areas of
cytoplasm.
v Scattered in the cytoplasm.
v Prokaryotes have 70S ribosomes while
eukaryotes have 80S ribosomes.
v Around 62% of ribosomes are comprised
of RNA, while the rest is proteins.
v The structure of free and bound
ribosomes is similar and is associated with protein synthesis.
Ribosomes
Function
The important
ribosome function includes:
v
It
assembles amino acids to form proteins that are essential to carry out cellular
functions.
v The DNA produces mRNA by the process of
DNA transcription.
v The mRNA is synthesized in the nucleus
and transported to the cytoplasm for the process of protein synthesis.
v The ribosomal subunits in the cytoplasm
are bound around mRNA polymers. The tRNA then synthesizes proteins.
v Ribosomes are the site of protein
synthesis.
v
The
proteins synthesized in the cytoplasm are utilized in the cytoplasm itself, the
proteins synthesized by bound ribosomes are transported outside the cell.
Exercise
Q.
1 A) Multiple choice questions (1 mark each)
1.
E.
R. is acts as secretary, storage, circulatory and ……. System for the cell.
a.
Digestive b. Reproductive c.
Nervous d.
Respiratory
2.
……..
is called autonomous unit of life
a. Mitochondria b. Plasma membrane c. Nucleus d.
Golgi complex
3.
Morphological
point of view ….. cell is the most primitive cell
a.
Prokaryotic b. Eukaryotic c. Plant d.
Animal
4.
Plasma
membrane performs the functions like ….. ..
a. Exocytosis b.
Endocytosis c. Both a and b d.
None
5.
……
is commonly called suicide bag of cell
a.
Lysosome b. Mitochondria c. Golgi complex d. ER
6.
……
is commonly called power house of cell
a.
Lysosome b. Mitochondria c. Golgi complex d.
ER
7.
The
term plasma lemma has been given by ……
a. C. Nageli b.
C. Cramer c. J. Q. Plowe d.
R. Brwon
8.
Sandwich
model of plasma membrane was proposed by …..
a. Danielli and Davson b.
Singer and Nicolson
c.
Robertson d. None
9.
In
1833, …….. was first discovered and named by Robert Brown
a. Ribosome b. Nucleus c. Centriole d.
None
10.
The
nucleus performs a function of biogenesis of ……
a.
Mitochondria b. Golgi complex c. Lysosome d. Ribosome
11.
…….
types of E.R. found in cells which are active in protein synthesis.
a.
Rough b. Smooth c. Both a and b d. None
12.
………..
manufactures acrosomal part of sperm
a. Mitochondria b. Golgi complex c. Lysosome d.
Ribosome
13.
Aging
and death of animal cells is concerned with ……..
a. Mitochondria b.
Golgi complex c. Lysosome d.
Ribosome
14.
…….
Has its own DNA for cytoplasmic inheritance
a. Mitochondria b.
Golgi complex c. Lysosome d. Ribosome
Q. 1 B) Answer in one sentences (1 mark
each)
i.
What
do you mean by perinuclear space
ii.
Write
the names of enzymes present in the nucleus
iii.
What
is the different between SER and RER
iv.
What
do you mean by digestive body
v.
What
do you mean by autophagy
vi.
Write
examples of prokaryotic cells
vii.
What
are the characteristic feature of prokaryotic cell
viii.
What
are the characteristic feature of eukaryotic cell
ix.
What
do you mean by plasma lemma
x.
What
is the main function of ribosome
xi.
What are the different components of nucleus
xii.
What
do you mean by sandwich model
xiii.
What
are the main composition of unit membrane
xiv.
What
do you mean by ‘selectively permeable’
xv.
What
do you mean by ‘downhill’
xvi.
What
do you mean by ‘phagocytosis’
Q.2. Define/ Explain/ Comments (2 mark
each)
i. Prokaryotic cell ii. Eukaryotic cell
iii. Passive
transport iv. Active transport
v. Exocytosis vi. Endocytosis
vii. Cell
drinking viii. Cilia and flagella
ix. Power
house of cell x. Suicide bag
xi. Autolysis xii. Extracellular digestion
xiii. Rough ER xiv. Smooth ER
xv Autolysis xvi. Heterochromatin and euchromatin
Q. 3. Attempt the following (3 mark
each)
a. Explain briefly bulk transport
b. Explain the term passive transport
c. Explain briefly structure of plasma
membrane
d. Describe structure of prokaryotic cell
e. Describe in brief nucleoplasm
f. Describe in brief nuclear envelope
g.
Sketch
and label sandwich model of Danielli and Davson
h.
Sketch
and label Pore structure in model of
Danielli and Davson
i.
Sketch
and label Robertson’s unit membrane model
Q. 4. Attempt the following (4 mark
each)
a.
Describe
unit membrane model of Robertson.
b.
Describe
bilayer model of Danielli and Davson
c.
Write
functions of nucleus
d.
Write
functions of mitochondria
e.
Write
functions of Endoplasmic reticulum
f.
Describe
types of Endoplasmic reticulum
g.
Describe
structure of mitochondria
h.
Write
functions of lysosomes
i.
Sketch
and label Prokaryotic cell
j.
Sketch
and label typical eukaryotic cell of
animal
k.
Sketch
and label fluid mosaic model of Singer and Nicolson
l.
Sketch
and label ultra structure of nucleus
m.
Sketch
and label ultra structure of mitochondria
n.
Sketch
and label ultra structure of Golgi Complex
Q. 5. Attempt the following (6 mark each)
1.
Describe in detail account on general organization of
eukaryotic cell of animal.
2.
With
labeled diagram describe fluid mosaic model of Singer and Nicolson.
3.
Describe
briefly various functions of plasma membrane
4.
Describe
general features of eukaryotic cells
5.
Describe
ultra structure and functions of nucleus
6.
Describe
ultra structure and functions of endoplasmic reticulum
7.
Describe
ultra structure and functions of Golgi Complex
8.
Describe
ultra structure and functions of lysosome
9.
Describe
ultra structure and functions of mitochondria
