BASIC KNOWLEDGE OF CELL&CELL BIOLOGY

1. ASHIKUZZAMAN ANTOR CELL BIOLOGY
BT-3103(CELL&DEVELOPMENTAL BIOLOGY)
ISLAMIC UNIVERSITY BANGLADESH
SESSION: 2017-18

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DEFINITION OF CELL BIOLOGY
The biological science which deals with the study of structure, function, molecular organization, growth,
reproduction and genetics of the cells, is called cytology (Gr., kytos = hollow vessel or cell; logous = to discourse)
or cell biology.
Much of the cell biology is devoted to the study of structures and functions of specialized cells.
HISTORY OF CELL BIOLOGY
ORIGIN
The origin of cells has to do with the origin of life, which began the history of life on Earth.
Origin of the first cell
Stromatolites are left behind by cyanobacteria, also called blue-green algae. They are the oldest known fossils of
life on Earth. This one-billion-year-old fossil is from Glacier National Park in the United States.
Further information: Abiogenesis and Evolution of cells
There are several theories about the origin of small molecules that led to life on the early Earth. They may have
been carried to Earth on meteorites (see Murchison meteorite), created at deep-sea vents, or synthesized by
lightning in a reducing atmosphere (see Miller–Urey experiment). There is little experimental data defining what
the first self-replicating forms were. RNA is thought to be the earliest self-replicating molecule, as it is capable of
both storing genetic information and catalyzing chemical reactions, but some other entity with the potential to self-
replicate could have preceded RNA, such as clay or peptide nucleic acid.
Cells emerged at least 3.5 billion years ago. The current belief is that these cells were heterotrophs. The early cell
membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per
lipid. Lipids are known to spontaneously form bilayered vesicles in water, and could have preceded RNA, but the

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first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins
before they could form.
Origin of eukaryotic cells
The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearing
organelles like the mitochondria and the chloroplasts are descended from ancient symbiotic oxygen-
breathing proteobacteria and cyanobacteria, respectively, which were endosymbiosed by an
ancestral archaean prokaryote.
There is still considerable debate about whether organelles like the hydrogenosome predated the origin
of mitochondria, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells.
CELL THEORY
In biology, cell theory is a scientific theory first formulated in the mid-nineteenth century, that living organisms are
made up of cells, that they are the basic structural/organizational unit of all organisms, and that all cells come from
pre-existing cells. Cells are the basic unit of structure in all organisms and also the basic unit of reproduction.
The three tenets to the cell theory are as described below:
1. All living organisms are composed of one or more cells.
2. The cell is the basic unit of structure and organization in organisms.
3. Cells arise from pre-existing cells.
The theory was once universally accepted, but now some biologists consider non-cellular entities such
as viruses living organisms, and thus disagree with the first tenet. As of 2021: "expert opinion remains divided
roughly a third each between yes, no and don’t know". As there is no universally accepted definition of life,
discussion will continue.
TYPES OF CELL
Cells are of two types: eukaryotic, which contain a nucleus, and prokaryotic cells, which do not have a nucleus, but
a nucleoid region is still present. Prokaryotes are single-celled organisms, while eukaryotes can be either single-
celled or multicellular.
Prokaryotic cells:
Prokaryotes include bacteria and archaea, two of the three domains of life. Prokaryotic cells were the first form
of life on Earth, characterized by having vital biological processes including cell signaling. They are simpler and
smaller than eukaryotic cells, and lack a nucleus, and other membrane-bound organelles. The DNA of a
prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. The nuclear
region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5
to 2.0 μm in diameter.
A prokaryotic cell has three regions:
 Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell
wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most
prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria)
and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the
cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall

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consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents
the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some
eukaryotic cells (plant cells and fungal cells) also have a cell wall.
 Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of
inclusions. The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal
DNA elements called plasmids, which are usually circular. Linear bacterial plasmids have been identified in
several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi,
which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids
encode additional genes, such as antibiotic resistance genes.
 On the outside, flagella and pili project from the cell's surface. These are structures (not present in all
prokaryotes) made of proteins that facilitate movement and communication between cells.
Eukaryotic cells
Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times
wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing
feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-
bound organelles (compartments) in which specific activities take place. Most important among these is a cell
nucleus, an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means "true
kernel (nucleus)". Some of the other differences are:
 The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell
walls may or may not be present.
 The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated
with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a
membrane. Some eukaryotic organelles such as mitochondria also contain some DNA.
 Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in
chemosensation, mechanosensation, and thermosensation. Each cilium may thus be "viewed as a sensory
cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the
signaling to ciliary motility or alternatively to cell division and differentiation."
 Motile eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering
plants. Eukaryotic flagella are more complex than those of prokaryotes.

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CELL MEMBRANE
Cell (Plasma) Membrane Definition
 Membranes are lipid structures that separate the contents of the compartment they surround from its
environment.
 Plasma membranes separate the cell from its environment while other membranes define the boundaries of
organelles and provide a matrix upon which complex chemical reactions can occur.
 The plasma membrane, also known as the cell surface membrane or plasmalemma, defines the boundary of the
cell.
 It is a phospholipid bilayer with embedded proteins that encloses every living cell.
 It regulates the movement of materials into and out of the cell and facilitates electrical signaling between them.
 It is said to be semi-permeable because it allows certain molecules but not others to enter into the cell.
 It serves some specific functions such as controlling the flow of nutrients and ions into and out of the cells,
mediating the response of a cell to external stimuli (a process called signal transduction), and interacting with
bordering cells.
Figure: Diagram of Cell (Plasma) Membrane
Structure and Composition
All biological membranes are constructed according to a standard pattern. They consist of a continuous bilayer of
amphipathic lipids approximately 5 nm thick, into which proteins are embedded. In addition, some membranes
also carry carbohydrates (mono- and oligosaccharides) on their exterior, which are bound to lipids and proteins.
The proportions of lipids, proteins, and carbohydrates differ markedly depending on the type of cell and membrane.
 The plasma membrane consists of a lipid bilayer containing embedded and peripheral proteins. The major
component of membranes is lipids.
 The lipids in the plasma membrane are in the form of phospholipids, which contain a polar head group
attached to two hydrophobic fatty acid tails; the head group faces the aqueous environment, the fatty acid tails
the interior of the bilayer.
1. Glycerol-based lipids contain a glycerol backbone, and consist of phosphatidic acid (PA),
phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylglycerol
(PG), phosphatidylinositol (PI), and cardiolipin (CL).
2. The one sphingosine-based lipid is sphingomyelin (SM).
3. Cholesterol is present in eukaryotic membranes and maintains membrane fluidity at a variety of temperatures.
Fluidity is also determined by the content of unsaturated fatty acids in the membrane, which are liquids at
room temperature, and the chain length of the fatty acids (shorter chains are more fluid than longer chains).

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 The embedded proteins in the plasma membrane function as either channels or transporters for the movement
of compounds across the membrane, as receptors for the binding of hormones and neurotransmitters, or as
structural proteins.
 The peripheral membrane proteins provide mechanical support to the membrane through the inner membrane
skeleton or the cortical skeleton. An example of this is spectrin in the red blood cell membrane. These can be
removed from the membrane by ionic agents.
 The third type of membrane proteins is the glycophosphatidylinositol (GPI) glycan-anchored proteins. One
example of a GPI-anchored protein is the prion protein, present in neuronal membranes.
 The plasma membrane glycocalyx consists of short chains of carbohydrates attached to proteins and lipids
which extend in the aqueous media and both protects the cell from digestion and restricts the uptake of
hydrophobic molecules.
Note:
 Membrane lipids are strongly amphipathic molecules with a polar hydrophilic “head group” and a polar
hydrophobic “tail.” In membranes, they are primarily held together by the hydrophobic effect and weak Van
der Waals forces and are therefore mobile relative to each other. This gives membranes a more or less fluid
quality.
 Lipids and proteins are mobile within the membrane. If they are not fixed in place by special mechanisms,
they float within the lipid layer as if in a two-dimensional liquid; biological membranes are therefore also
described as being a “fluid mosaic”.
Functions of Membranes (Cell/Plasma Membrane and Biological Membranes)
 The most important membranes in animal cells are the plasma membrane, the inner and outer nuclear
membranes, the membranes of the endoplasmic reticulum (ER) and the Golgi apparatus, and the inner and
outer mitochondrial membranes. Lysosomes, peroxisomes, and various vesicles are also separated from the
cytoplasm by membranes.
 In plants, additional membranes are seen in the plastids and vacuoles.
Membranes and their components have the following functions:
1. Enclosure and insulation of cells and organelles.
 The enclosure provided by the plasma membrane protects cells from their environment both mechanically and
chemically.
 The plasma membrane is essential for maintaining differences in the concentration of many substances
between the intracellular and extracellular compartments.
2. Regulated transport of substances
 This determines the internal milieu and is a precondition for homeostasis—i. e., the maintenance of constant
concentrations of substances and physiological parameters.
 Regulated and selective transport of substances through pores, channels, and transporters is necessary because
the cells and organelles are enclosed by membrane systems.
3. Signal Transduction
 Reception of extracellular signals and transfer of these signals to the inside of the cell as well as the production
of signals.
4. Enzymatic catalysis of reactions.
 Important enzymes are located in membranes at the interface between the lipid and aqueous phases. This is
where reactions with apolar substrates occur.
 Examples include lipid biosynthesis and the metabolism of apolar xenobiotics. The most important reactions
in energy conversion—i. e., oxidative phosphorylation and photosynthesis also occur in membranes.
5. Interactions with other cells
 For the purposes of cell fusion and tissue formation, as well as communication with the extracellular matrix.
6. Anchoring of the cytoskeleton
 To maintain the shape of cells and organelles and to provide the basis for movement processes.

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NUCLEUS
Nucleus Definition
 The cell nucleus is a membrane-bound structure that contains the cell’s hereditary information and controls the
cell’s growth and reproduction.
 It is the command center of a eukaryotic cell and is commonly the most prominent organelle in a cell
accounting for about 10 percent of the cell’s volume.
 In general, a eukaryotic cell has only one nucleus. However, some eukaryotic cells are enucleated cells
(without a nucleus), for example, red blood cells (RBCs); whereas, some are multinucleate (consists of two or
more nuclei), for example, slime molds.
 The nucleus is separated from the rest of the cell or the cytoplasm by a nuclear membrane.
 As the nucleus regulates the integrity of genes and gene expression, it is also referred to as the control center
of a cell.
Structure of Nucleus
The structure of a nucleus encompasses the nuclear membrane, nucleoplasm, chromosomes, and nucleolus.
Nuclear Membrane
 The nuclear membrane is a double-layered structure that encloses the contents of the nucleus. The outer layer
of the membrane is connected to the endoplasmic reticulum.
 Like the cell membrane, the nuclear envelope consists of phospholipids that form a lipid bilayer.
 The envelope helps to maintain the shape of the nucleus and assists in regulating the flow of molecules into
and out of the nucleus through nuclear pores. The nucleus communicates with the remaining of the cell or the
cytoplasm through several openings called nuclear pores.
 Such nuclear pores are the sites for the exchange of large molecules (proteins and RNA) between the nucleus
and cytoplasm.
 A fluid-filled space or perinuclear space is present between the two layers of a nuclear membrane.
Nucleoplasm
 Nucleoplasm is the gelatinous substance within the nuclear envelope.
 Also called karyoplasm, this semi-aqueous material is similar to the cytoplasm and is composed mainly of
water with dissolved salts, enzymes, and organic molecules suspended within.
 The nucleolus and chromosomes are surrounded by nucleoplasm, which functions to cushion and protect the
contents of the nucleus.

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 Nucleoplasm also supports the nucleus by helping to maintain its shape. Additionally, nucleoplasm provides a
medium by which materials, such as enzymes and nucleotides (DNA and RNA subunits), can be transported
throughout the nucleus. Substances are exchanged between the cytoplasm and nucleoplasm through nuclear
pores.
Nucleolus
 Contained within the nucleus is a dense, membrane-less structure composed of RNA and proteins called
the nucleolus.
 Some of the eukaryotic organisms have a nucleus that contains up to four nucleoli.
 The nucleolus contains nucleolar organizers, which are parts of chromosomes with the genes for ribosome
synthesis on them. The nucleolus helps to synthesize ribosomes by transcribing and assembling ribosomal
RNA subunits. These subunits join together to form a ribosome during protein synthesis.
 The nucleolus disappears when a cell undergoes division and is reformed after the completion of cell division.
Chromosomes
 The nucleus is the organelle that houses chromosomes.
 Chromosomes consist of DNA, which contains heredity information and instructions for cell growth,
development, and reproduction.
 Chromosomes are present in the form of strings of DNA and histones (protein molecules) called chromatin.
 When a cell is “resting” i.e. not dividing, the chromosomes are organized into long entangled structures
called chromatin.
 The chromatin is further classified into heterochromatin and euchromatin based on the functions. The former
type is a highly condensed, transcriptionally inactive form, mostly present adjacent to the nuclear membrane.
On the other hand, euchromatin is a delicate, less condensed organization of chromatin, which is found
abundantly in a transcribing cell.
Besides the nucleolus, the nucleus contains a number of other non-membrane-delineated bodies. These include
Cajal bodies, Gemini of coiled bodies, polymorphic interphase karyosome association (PIKA), promyelocytic
leukemia (PML) bodies, paraspeckles, and splicing speckles.
Functions of Nucleus
The nucleus provides a site for genetic transcription that is segregated from the location of translation in the
cytoplasm, allowing levels of gene regulation that are not available to prokaryotes. The main function of the cell
nucleus is to control gene expression and mediate the replication of DNA during the cell cycle.
 It controls the hereditary characteristics of an organism.
 The organelle is also responsible for protein synthesis, cell division, growth, and differentiation.
 Storage of hereditary material, the genes in the form of long and thin DNA (deoxyribonucleic acid) strands,
referred to as chromatin.
 Storage of proteins and RNA (ribonucleic acid) in the nucleolus.
 The nucleus is a site for transcription in which messenger RNA (mRNA) are produced for protein synthesis.
 During the cell division, chromatins are arranged into chromosomes in the nucleus.
 Production of ribosomes (protein factories) in the nucleolus.
 Selective transportation of regulatory factors and energy molecules through nuclear pores.

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The Nucleus of the Plant Cell:
Plant cells are eukaryotic cells that are found in the organism within the plant kingdom. Eukaryotic cells contain
nucleus Plant cells differ from other eukaryotic cells because the organelles existing are different. Organelles are a
major part of the cell.
In the Plant Cell there are Different Types of the Nucleus-
 Uninucleate cell: It is also referred to as monokaryotic cells, mostly plant cells which contain a
single nucleus.
 Bi-nucleate cell: It is also called a dikaryotic cell. It contains two nuclei at a time. The examples are one
paramecium (have mega and micronucleus), balantidium, and liver cells and cartilage cells.
 Multinucleate cells: It is also known as the polynucleated cell which contains more than 2 nuclei at a time.
For example, plants latex cells and latex vessels. In animals, striated muscle cells and bone marrow cells.
 Enucleate cells: Cells without a nucleus are called enucleate cells. However, some living cells like mature
sieve tubes of phloem and RBC’s of mature mammals lack nuclei.
The Plant Cell has Four Parts of the Nucleus:
 Nuclear membrane or envelope or karyotheca
 Chromatin threads or nuclear reticulum
 Nuclear sap or nucleoplasm or karyolymph
 Nucleolus.
1. Nuclear Membrane:
The nuclear membrane is made up of the outer and inner membrane, made up of lipoproteins, perinuclear space,
pores, annuli material, and inner dense lamella. The outer membrane is continuous with the endoplasmic reticulum.
The exchange of different substances between nucleus and cytoplasm takes place through minute pores already
present in the nuclear membrane.
2. Chromatin Threads:
The term chromatin thread was proposed by W. Flemming. Chromatin threads are associated with one another and
form a network called chromatin reticulum. At the time of cell division, the chromatin threads isolated from one
another become thicker or massive and smaller and are now termed as chromosomes.
It is primarily nucleoprotein, made up of nucleic acid and basic protein histone. Nucleic acid contains sugar,
nitrogenous bases, phosphate, and is a very complex organic acid.
Nucleic Acids are of Two Types:
DNA (Deoxyribonucleic acid) especially found in the cytoplasm in soluble form and is called soluble RNA. It is also
present in some amounts in the ribosomes of nucleus, chromatin, and nucleolus. It is synthesized from DNA and is
piled up in the nucleolus. It travels to the cytoplasm and gets attached to the ribosome.
Chromatin is basophilic in type and most of the chromatin material is transferred into the specific number of
chromosomes during cell division. The chromatin material may be heterochromatin, sex chromatin, and
euchromatin.
1. Nuclear Sap:
The nuclear membrane encloses the clear, homogeneous, transparent, colloidal liquid of variable consistency. It is
chiefly organized of nucleoproteins, a small amount of inorganic and organic substances like nucleic acids, proteins
dissolved phosphorus, ribose sugars, minerals, enzymes, and nucleotides.

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1. Nucleolus:
It was observed by Wagner and the term was proposed by Browman, subsequently described by Fontana
Characteristics of Nucleolus:
 The one or more nucleoli may be present within a nucleus. Four nucleoli are found in each nucleus in an
onion.
 Nucleolus disappears in the late prophase stage.
 Reappears in the telophase stage
 It is storehouse of RNA.
CHROMOSOME
DEFINITION
 In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes.
 Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support
its structure.
 Chromosomes were first described by Strasburger (1815), and the term ‘chromosome’ was first used by
Waldeyer in 1888.
 They appear as rod shaped dark stained bodies during the metaphase stage of mitosis when cells are stained
with a suitable basic dye and viewed under a light microscope.
Structure of Chromosome
 In eukaryotes the chromosomes are multiple large, linear and are present in the nucleus of the cell.
 Each chromosome typically has one centromere and one or two arms that project from the centromere.
 Structurally, each chromosome is differentiated into three parts—
1. Pellicle
2. Matrix
3. Chromonemata
Pellicle
 It is the outer envelope around the substance of chromosome.
 It is very thin and is formed of achromatic substances.
Matrix
 It is the ground substance of chromosome which contains the chromonemata.
 It is also formed of non-genic materials.
Chromonemata
 Embedded in the matrix of each chromosome are two identical, spirally coiled threads, the chromonemata.
 The two chromonemata are also tightly coiled together that they appear as single thread of about 800A
thickness.

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 Each chromonemata consists of about 8 microfibrils, each of which is formed of a double helix of DNA.
In mitotic metaphase chromosomes, the following structural feature (except chromomere) can be seen under the
light microscope:
(1) Chromatid,
(2) Chromonema,
(3) Chromomeres,
(4) Centromere,
(5) Secondary constriction or Nucleolar organizer,
(6) Telomere and
(7) Satellite.
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Centromere
 A small structure in the chromonema, marked by a constriction which is recognised as permanent structure in
the chromosome is termed as the centromere.
 At this point the two chromonemata are joined together.
 It is known as centromere or kinetochore or primary constriction.
 It divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p
arm.” The long arm of the chromosome is labeled the “q arm.”
 Its position is constant for a given type of chromosome and forms a feature of identification.
 In thin electron microscopic sections, the kinetochore shows a trilaminar structure, i.e., a 10 nm thick dense
outer protein aceous layer, a middle layer of low density and a dense inner layer tightly bound to the
centromere.
 The chromosomes are attached to spindle fibres at this region during cell division.
Secondary Constriction or Nucleolar Organiser
 The chromosome besides having the primary constriction or the centromere possesses secondary constriction
at any point of the chromosome.
 Constant in their position and extent, these constrictions are useful in identifying particular chromosomes in a
set.
 The chromosome region distal to the secondary constriction i.e., the region between the secondary constriction
and the nearest telomere is known as satellite.
 Therefore, chromosomes having secondary constrictions are called satellite chromosomes or sat-
chromosomes.
 Nucleolus is always associated with the secondary constriction of sat-chromosomes. Therefore, secondary
constrictions are also called nucleolus organiser region (NOR) and sat-chromosomes are often referred to as
nucleolus organiser chromosomes.
Telomeres
 These are specialized ends of a chromosome which exhibits physiological differentiation and polarity.
 Each extremity of the chromosome due to its polarity prevents other chromosomal segments to be fused with
it. The chromosomal ends are known as the telomeres.
 If a chromosome breaks, the broken ends can fuse with each other due to lack of telomere.
Types of Chromosomes
A. Autosomes and Sex Chromosomes
 Human chromosomes are of two types autosomes and sex chromosomes.

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 Genetic traits that are linked to the sex of the person are passed on through the sex chromosomes. The rest of
the genetic information is present in the autosomes.
 Humans have 23 pairs of chromosomes in their cells, of which 22 pairs are autosomes and one pair of
sex chromosomes, making a total of 46 chromosomes in each cell.
B. On the Basis of Number of Centromeres
1. Monocentric with one centromere.
2. Dicentric with two centromeres.
3. Polycentric with more than two centromeres
4. Acentric without centromere. Such chromosomes represent freshly broken segments of chromosomes which
do not survive for long.
5. Diffused or non-located with indistinct centromere diffused throughout the length of chromosome.
C. On the Basis of Location of Centromere
1. Telocentric are rod-shaped chromosomes with centromere occupying the terminal position, so that the
chromosome has just one arm.
2. Acrocentric are also rod-shaped chromosomes with centromere occupying a sub-terminal position. One arm is
very long and the other is very short.
3. Sub-metacentric chromosomes are with centromere slightly away from the mid-point so that the two arms are
unequal.
4. Metacentric are V-shaped chromosomes in which centromere lies in the middle of chromosome so that the
two arms are almost equal.
Function and Significance of Chromosomes
 The number of the chromosomes is constant for a particular species. Therefore, these are of great importance
in the determination of the phylogeny and taxonomy of the species.
 Genetic Code Storage: Chromosome contains the genetic material that is required by the organism to develop
and grow. DNA molecules are made of chain of units called genes. Genes are those sections of the DNA
which code for specific proteins required by the cell for its proper functioning.
 Sex Determination: Humans have 23 pairs of chromosomes out of which one pair is the sex chromosome.
Females have two X chromosomes and males have one X and one Y chromosome. The sex of the child is
determined by the chromosome passed down by the male. If X chromosome is passed out of XY chromosome,
the child will be a female and if a Y chromosome is passed, a male child develops.
 Control of Cell Division: Chromosomes check successful division of cells during the process of mitosis. The
chromosomes of the parent cells insure that the correct information is passed on to the daughter cells required
by the cell to grow and develop correctly.
 Formation of Proteins and Storage: The chromosomes direct the sequences of proteins formed in our body
and also maintain the order of DNA. The proteins are also stored in the coiled structure of the chromosomes.
These proteins bound to the DNA help in proper packaging of the DNA.


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MITOCHONDRIA
Mitochondria Definition
 Mitochondria are oxygen-consuming ribbon-shaped cellular organelles of immense importance floating free
throughout the cell.
 They are known as the “powerhouse of the cell” since these organelles supply all the necessary biological
energy to the cell by oxidizing the substrates available.
 The enzymatic oxidation of chemical compounds in the mitochondria releases energy.
 Since mitochondria act as the power-houses, they are abundantly found on those sites where energy is
earnestly required such as sperm tail, muscle cell, liver cell (up to 1600 mitochondria), microvilli, oocyte
(more than 300,000 mitochondria), etc.
 Typically, there are about 2000 mitochondria per cell, representing around 25% of the cell volume.
 In 1890, mitochondria were first described by Richard Altmann and he called them bioblasts. Benda in the
year 1897 coined the term ‘mitochondrion’.
Mitochondria Properties
 Mitochondria is the cell organelle which is filamentous and granular structure.
 It is present in higher plants, animals, and some microorganisms.
 It is absent in bacteria but is found in algae, protozoa, and fungi.
 To carry out the energy metabolism, mitochondria have got the lipoprotein framework.
 It consists of the different enzymes and coenzymes.
 Mitochondria also consist of specific DNA and ribosomes.
 Ribosomes are involved in protein synthesis whereas specific DNA is involved in the cytoplasmic inheritance.
 Mitochondria were first observed in 1850 by Kolliker.
 In the striated muscles, it was observed as the granular structure.
 Later in 1888, he isolated them from the insect muscles.
 It was named fila by Flemming in 1882.
 In the early days, mitochondria were given the different names such as
 Fuchsinophilic granules
 parabasal bodies
 plasmosomes
 plastosomes
 fila
 vermicules
 bioblasts
 chondriosomes
 In the cytoplasm, mitochondria are uniformly distributed.
 But it is also found that their distribution is restricted in many cells.
 Depending on the function of the cell, the mitochondria are distributed accordingly.
 The distribution of mitochondria varies according to species and cell type.
 But some cell may contain a large number of mitochondria as:
 50,000 in Chaos chaos
 140,000 to 150,000 in eggs of sea urchin
 300,000 in oocytes of amphibians.
 It is found that only 500 to 1600 mitochondria are present in the liver cells of the rat.
 As compared to the animal cell, the number of mitochondria is less in green plants.
 It is because in the green plants there is the presence of chloroplast which carries out the function as of
mitochondria.

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 In the animal cell, sarcosomes are the mitochondria present in the myocardial muscle cell. They are numerous
and large.
 The shape of the mitochondria can be filamentous or granular.
 Depending upon the physiological condition of the cell, their shape may change from one form into another.
 So the shapes can be of club, racket, vesicular, ring or round-shape.
 In the rat or primary spermatocyte, mitochondria are granular.
 In the liver cell mitochondria are club-shaped.
 Mitochondria may fuse and separate the cell which causes the changes in shape.
 During the day the mitochondria may fuse into the reticulate structure in certain cells of euglenoid.
 Then it separates or dissociates during the darkness.
 In the case of yeast depending on the different cultural condition changes occurs in it.
 The size of the mitochondria is 0.5 to 2.0 µm, so it cannot be seen clearly under the light microscope.
 It is also found sometimes the length reaches 7 µm.
Structure of mitochondria
Mitochondria consists of mitochondrial membrane and mitochondrial chamber.
1. Mitochondrial membrane
It consists of two membranes. They are:
a. Outer membrane
 It is a smooth membrane.
 It is made up of 40% lipids and 60% proteins.
 Due to the presence of the pores or porins, it is permeable.
b. Inner membrane
 It is made up of 20% lipids and 80% proteins.
 It is the selectively permeable membrane.
 Since the membrane is folded inwards and there is the presence of cristae, it is said as the rough membrane.
 Cristae are the numerous and finger-like projections.
 Tennis racket-like particles are present in each crista.
 The particles were previously named as the inner membrane subunits, F0-F1 particles, elementary particles, or
oxysomes.
 It was called the electron transport particles (ETP)by Parsons in 1963.
 In each mitochondrion about 104-105 particles are present.
 Base, stalk, and head are present in each elementary particle.
 Base and head are also called the F0 and F1 particles respectively.
 Stalk acts as the link which connects the base and head.
 The base is made up of hydrophobic proteins and is embedded in the lipid molecules of the membrane.
 The Head is made up of five types of the polypeptide.
 ATPase or ATP synthetase is the enzyme present in it.
 The ADP aids in the formation of ATP.
 Similarly, inorganic phosphate is also formed. It is due to oxidative phosphorylation.

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 The stalk also consists of the coupling factors which connect the respiratory chain with the elementary
particle.
2. Mitochondrial chambers
Two chambers are present in the mitochondria i.e Outer and inner chamber.
a. Outer chamber (peri-mitochondrion space)
 Between the outer and the inner mitochondrial membrane, a space is present in between them which is known
as the peri-mitochondrion space.
 Few enzymes are also present in the fluid present in it.
b. Inner chamber
 It is present in the inner part of the inner membrane.
 A semi-fluid matrix is present in it which consists of:
 Water
 Minerals
 Protein particles
 70s ribosomes
 RNA
 Circular DNA
 Enzymes
Inter-membrane Space
 It is the space between the outer and inner membrane of the mitochondria, it has the same composition as that
of the cell’s cytoplasm.
 There is a difference in the protein content in the intermembrane space.
Mitochondrial Matrix
 The mitochondrial matrix which is the liquid (colloidal) area encircled by the inner membrane, contains the
soluble enzymes of the Krebs cycle which completely oxidize the acetyl-CoA to produce CO2, H2O and
hydrogen ions. Hydrogen ions reduce the molecules of NAD and FAD, both of which pass on hydrogen ions
to respiratory or electron transport chain where oxidative phosphorylation takes place to generate energy-rich
ATP molecules.
 Mitochondria also contain in their matrix single or double circular and double-stranded DNA molecules called
mt DNA and also the 55S ribosomes, called mitoribosomes. Since mitochondria can synthesize 10 percent of
their proteins in their own protein-synthetic machinery, they are considered as semi-autonomous organelles.
Functions of mitochondria
 Mitochondria stores and releases energy in the form of ATP ( Adenosine triphosphate ). It occurs by the
oxidation of carbohydrates, proteins, and fats. It will be further utilized in the different metabolic activities.
So, mitochondria are known as the powerhouse of the cell or storage batteries of the cell.
 Mitochondria help in the formation of the heme of hemoglobin.
 During cellular respiration, mitochondria form the different intermediate products. They are utilized for the
synthesis of cytochromes, chlorophyll, ferredoxin, steroids, alkaloids, pyrimidines, etc.
 Calcium can be stored and released by the mitochondria.

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 It helps in the formation of amino acids.
 In the matrix of the mitochondria, several fatty acids can be synthesized.
 During the process of oogenesis, they help in the formation of the yolk.
 During the process of spermatogenesis, they help in the formation of the middle part of the sperms.
 By the process of maternal inheritance, traits are directly transferred by mitochondria from the mothers to the
offsprings.
 Mitochondria are also present in the liver cell. They help in the detoxification of ammonia using their
enzymes.
 Mitochondria are the site of heat generation which is known as thermogenesis.
 Sometimes there can be the abnormal death of the cell. It might be due to the dysfunctioning of the
mitochondria. It can affect the function of the organ.
 It helps in the formation of some parts of the hormone of testosterone and estrogen.
RIBOSOME
Ribosomes Definition
 The ribosome word is derived – ‘ribo’ from ribonucleic acid and ‘somes’ from the Greek word ‘soma’ which
means ‘body’.
 Ribosomes are tiny spheroidal dense particles (of 150 to 200 A0
diameters) that are primarily found in most
prokaryotic and eukaryotic.
 They are sites of protein synthesis.
 They are structures containing approximately equal amounts of RNA and proteins and serve as a scaffold for
the ordered interaction of the numerous molecules involved in protein synthesis.
 The ribosomes occur in cells, both prokaryotic and eukaryotic cells.
 In prokaryotic cells, the ribosomes often occur freely in the cytoplasm.
 In eukaryotic cells, the ribosomes either occur freely in the cytoplasm or remain attached to the outer surface
of the membrane of the endoplasmic reticulum.
 The location of the ribosomes in a cell determines what kind of protein it makes.
 If the ribosomes are floating freely throughout the cell, it will make proteins that will be utilized within the cell
itself.
 When ribosomes are attached to the endoplasmic reticulum, it is referred to as rough endoplasmic reticulum or
rough ER.
 Proteins made on the rough ER are used for usage inside the cell or outside the cell.
 The number of ribosomes in a cell depends on the activity of the cell.
 On average in a mammalian cell, there can be about 10 million ribosomes.
Figure: Diagram of Ribosomes
Nearly 20% of the World Could Live in Sahara-Like Heat by 2070

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Structure of Ribosomes
 A ribosome is made from complexes of RNAs and proteins and is, therefore, a ribonucleoprotein.
 Around 37 to 62% of RNA is comprised of RNA and the rest is proteins.
 Each ribosome is divided into two subunits:
1. A smaller subunit which binds to a larger subunit and the mRNA pattern, and
2. A larger subunit which binds to the tRNA, the amino acids, and the smaller subunit.
 Prokaryotes have 70S ribosomes respectively subunits comprising the little subunit of 30S and the bigger
subunit of 50S.
 Their small subunit has a 16S RNA subunit (consisting of 1540 nucleotides) bound to 21 proteins.
 The large subunit is composed of a 5S RNA subunit (120 nucleotides), a 23S RNA subunit (2900 nucleotides)
and 31 proteins.
 Eukaryotes have 80S ribosomes respectively comprising of little (40S) and substantial (60S) subunits.
 The smaller 40S ribosomal subunit is prolate ellipsoid in shape and consists of one molecule of 18S ribosomal
RNA (or rRNA) and 30 proteins (named as S1, S2, S3, and so on).
 The larger 60S ribosomal subunit is round in shape and contains a channel through which growing polypeptide
chain makes its exit.
 It consists of three types of rRNA molecules, i.e., 28S rRNA, 5.8 rRNA and 5S rRNA, and 40 proteins (named
as L1, L2, L3 and so on).
 The differences between the ribosomes of bacterial and eukaryotic are used to create antibiotics that can
destroy bacterial infection without harming human cells.
 The ribosomes seen in the chloroplasts of mitochondria of eukaryotes are comprised of big and little subunits
composed of proteins inside a 70S particle.
 The ribosomes share a core structure that is similar to all ribosomes despite differences in its size.
 The two subunits fit together and work as one to translate the mRNA into a polypeptide chain during protein
synthesis.
 Because they are formed from two subunits of non-equal size, they are slightly longer in the axis than in
diameter.
 During protein synthesis, when multiple ribosomes are attached to the same mRNA strand, this structure is
known as polysome.
 The existence of ribosomes is temporary, after the synthesis of polypeptide the two sub-units separate and are
reused or broken up.
Functions of Ribosomes
 The ribosome is a complex molecular machine, found within all living cells, that serves as the site
of biological protein synthesis (translation).
 Ribosomes link amino acids together in the order specified by messenger RNA (mRNA) molecules.
 Ribosomes act as catalysts in two extremely important biological processes called peptidyl transfer and
peptidyl hydrolysis.
PLASTID
Plastids Definition
 Plastid is a double membrane-bound organelle involved in the synthesis and storage of food, commonly found
within the cells of photosynthetic plants.

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 Plastids were discovered and named by Ernst Haeckel, but A. F. W. Schimper was the first to provide a clear
definition.
 They are necessary for essential life processes, like photosynthesis and food storage.
 A plastid containing green pigment (chlorophyll) is called chloroplast whereas a plastid containing pigments
apart from green is called a chromoplast. A plastid that lacks pigments is called a leucoplast and is involved
mainly in food storage.
Figure: Diagram of Plastids.
ly 20% of the World Could Live in Sahara-Like Heat by 2070
Types of Plastids
An undifferentiated plastid is called a proplastid. It may develop later into any of the other plastids.
A. Chloroplasts
 The chloroplasts are probably the most-known of the plastids.
 These are responsible for photosynthesis.
 The chloroplast is filled with thylakoids, which is where photosynthesis occurs, and chlorophyll remains.
B. Chromoplasts
 Chromoplasts are units where pigments are stored and synthesized in the plant.
 These are found in flowering plants, fruits, and aging leaves.
 The chloroplasts actually convert over to chromoplasts.
 The carotenoid pigments allow for the different colors seen in fruits and the fall leaves. One of the main
reasons for these structures and the colors is to attract pollinators.
C. Leucoplasts
 Leucoplasts are the non-pigmented organelles.
 They are found in the non-photosynthetic parts of the plant, such as the roots.
 Depending on what the plant needs, they may become essentially just storage sheds for starches, lipids, and
proteins.
 They are more readily used for synthesizing amino acids and fatty acids.
 A leucoplast may be an amyloplast that stores starch, an elaioplast that stores fat, or a proteinoplast that
stores proteins.
D. Gerontoplasts
 Gerontoplasts are basically chloroplasts that are going through the aging process.

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 These are chloroplasts of the leaves that are beginning to convert into different organelles or are being re-
purposed since the leaf is no longer utilizing photosynthesis (such as in the fall months).
Depending on their morphology and function, plastids have the ability to differentiate or redifferentiate, between
these and other forms.
Structure of Plastids
 Chloroplasts may be spherical, ovoid, or discoid in higher plants and stellate, cup-shaped, or spiral as in some
algae.
 They are usually 4-6 µm in diameter and 20 to 40 in number in each cell of higher plants, evenly distributed
throughout the cytoplasm.
 The chloroplast is bounded by two lipoprotein membranes, an outer and an inner membrane, with an
intermembrane space between them.
 The inner membrane encloses a matrix, the stroma which contains small cylindrical structures called grana.
Most chloroplasts contain 10-100 grana.
The Grana and Thylakoids
 Each granum has a number of disc-shaped membranous sacs called grana lamellae or thylakoids (80-120Å
across) piled one over the other.
 The grana are interconnected by a network of anastomosing tubules called inter-grana or stroma lamellae.
 Single thylakoids, called stroma thylakoids, are also found in chloroplasts.
 Electron dense bodies, osmophilic granules along with ribosomes (70S), circular DNA, RNA and soluble
enzymes of Calvin cycles are also present in the matrix of the stroma.
 Chloroplasts thus have three different membranes, the outer, the inner and the thylakoid membrane.
 The thylakoid membrane consists of lipoprotein with a greater amount of lipids which are galactolipids,
sulpholipids, phospholipids.
 The inner surface of the thylakoid membrane is granular in the organization due to small spheroidal
quantosomes.
 The quantosomes are the photosynthetic units, and consist of two structurally distinct photosystems, PS I and
PS II, containing about 250 chlorophyll molecules. Each photosystem has antenna chlorophyll complexes and
one reaction center in which energy conversion takes place. In higher plants, the pigments present are
chlorophyll-a, chlorophyll-b, carotene, and xanthophyll.
 The two photosystems and the components of the electron transport chain are asymmetrically distributed
across the thylakoid membrane. Electron acceptors of both PS I and PS II are on the outer (stroma) surface of
the thylakoid membrane. Electron donors of PS I are on the inner (thylakoid space) surface.
Functions
All plant cells contain plastids in some shape or form. This roll-call indicates their functional diversity and
demonstrates that plastids lie at the very core of plant cellular function.

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 Plastids are the site of manufacture and storage of important chemical compounds used by the cells
of autotrophic eukaryotes.
 The thylakoid membrane contains all the enzymatic components required for photosynthesis. Interaction
between chlorophyll, electron carriers, coupling factors, and other components takes place within the thylakoid
membrane. Thus the thylakoid membrane is a specialized structure that plays a key role in the capture of light
and electron transport.
 Thus, chloroplasts are the centers of synthesis and metabolism of carbohydrates.
 They are not only of crucial importance in photosynthesis but also in the storage of primary foodstuffs,
particularly starch.
 Its function largely depends on the presence of pigments. A plastid involved in food synthesis typically
contains pigments, which are also the ones responsible for the color of a plant structure (e.g. green leaf,
red flower, yellow fruit, etc.).
 Like mitochondria, plastids have their own DNA and ribosomes. Hence, they may be used in phylogenetic
studies.
Cell Definition
“A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes.”
Cells are the structural, functional, and biological units of all living beings. A cell can replicate itself
independently. Hence, they are known as the building blocks of life.
Each cell contains a fluid called the cytoplasm, which is enclosed by a membrane. Also present in the cytoplasm
are several biomolecules like proteins, nucleic acids and lipids. Moreover, cellular structures called cell organelles
are suspended in the cytoplasm.
What is a Cell?
A cell is the structural and fundamental unit of life. The study of cells from its basic structure to the functions of
every cell organelle is called Cell Biology. Robert Hooke was the first Biologist who discovered cells.
All organisms are made up of cells. They may be made up of a single cell (unicellular), or many cells
(multicellular). Mycoplasmas are the smallest known cells. Cells are the building blocks of all living beings. They
provide structure to the body and convert the nutrients taken from the food into energy.
Cells are complex and their components perform various functions in an organism. They are of different shapes and
sizes, pretty much like bricks of the buildings. Our body is made up of cells of different shapes and sizes.
Cells are the lowest level of organisation in every life form. From organism to organism, the count of cells may
vary. Humans have the number of cells compared to that of bacteria.
Cells comprise several cell organelles that perform specialised functions to carry out life processes. Every organelle
has a specific structure. The hereditary material of the organisms is also present in the cells.
Discovery of Cells
Discovery of cells is one of the remarkable advancements in the field of science. It helps us know that all the
organisms are made up of cells, and these cells help in carrying out various life processes. The structure and
functions of cells helped us to understand life in a better way.
Who discovered cells?
Robert Hooke discovered the cell in 1665. Robert Hooke observed a piece of bottle cork under a compound
microscope and noticed minuscule structures that reminded him of small rooms. Consequently, he named these

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“rooms” as cells. However, his compound microscope had limited magnification, and hence, he could not see any
details in the structure. Owing to this limitation, Hooke concluded that these were non-living entities.
Later Anton Van Leeuwenhoek observed cells under another compound microscope with higher magnification.
This time, he had noted that the cells exhibited some form of movement (motility). As a result, Leeuwenhoek
concluded that these microscopic entities were “alive.” Eventually, after a host of other observations, these entities
were named as animalcules.
In 1883, Robert Brown, a Scottish botanist, provided the very first insights into the cell structure. He was able to
describe the nucleus present in the cells of orchids.
Types of Cells
Cells are similar to factories with different labourers and departments that work towards a common objective.
Various types of cells perform different functions. Based on cellular structure, there are two types of cells:
 Prokaryotes
 Eukaryotes
Prokaryotic Cells
1. Prokaryotic cells have no nucleus. Instead, some prokaryotes such as bacteria have a region within the cell
where the genetic material is freely suspended. This region is called the nucleoid.
2. They all are single-celled microorganisms. Examples include archaea, bacteria, and cyanobacteria.
3. The cell size ranges from 0.1 to 0.5 µm in diameter.
4. The hereditary material can either be DNA or RNA.
5. Prokaryotes generally reproduce by binary fission, a form of asexual reproduction. They are also known to
use conjugation – which is often seen as the prokaryotic equivalent to sexual reproduction (however, it is
NOT sexual reproduction).
Eukaryotic Cells
1. Eukaryotic cells are characterised by a true nucleus.
2. The size of the cells ranges between 10–100 µm in diameter.
3. This broad category involves plants, fungi, protozoans, and animals.
4. The plasma membrane is responsible for monitoring the transport of nutrients and electrolytes in and out of
the cells. It is also responsible for cell to cell communication.
5. They reproduce sexually as well as asexually.
6. There are some contrasting features between plant and animal cells. For eg., the plant cell contains
chloroplast, central vacuoles, and other plastids, whereas the animal cells do not.
Cell Structure
The cell structure comprises individual components with specific functions essential to carry out life’s processes.
These components include- cell wall, cell membrane, cytoplasm, nucleus, and cell organelles. Read on to explore
more insights on cell structure and function.

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Cell Organelles
Cells are composed of various cell organelles that perform certain specific functions to carry out life’s processes.
The different cell organelles, along with its principal functions, are as follows:
Cell Organelle and its Functions
Nucleolus
The nucleolus is the site of ribosome synthesis. Also, it is involved in controlling cellular activities and cellular
reproduction
Nuclear membrane
The nuclear membrane protects the nucleus by forming a boundary between the nucleus and other cell organelles.
Chromosomes
Chromosomes play a crucial role in determining the sex of an individual. Each human cells contain 23 pairs of
chromosomes
Endoplasmic reticulum
The endoplasmic reticulum is involved in the transportation of substances throughout the cell. It plays a primary
role in the metabolism of carbohydrates, synthesis of lipids, steroids and proteins.
Golgi Bodies
Golgi bodies are called the cell’s post office as it is involved in the transportation of materials within the cell
Ribosome

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Ribosomes are the protein synthesisers of the cell
Mitochondria
The mitochondrion is called “the powerhouse of the cell.” It is called so because it produces ATP – the cell’s
energy currency
Lysosomes
Lysosomes protect the cell by engulfing the foreign bodies entering the cell and helps in cell renewal. Therefore,
it is known as the cell’s suicide bags
Chloroplast
Chloroplasts are the primary organelles for photosynthesis. It contains the pigment chlorophyll
Vacuoles
Vacuoles stores food, water, and other waste materials in the cell
Cell Theory
Cell Theory was proposed by the German scientists, Theodor Schwann, Matthias Schleiden, and Rudolf Virchow.
The cell theory states that:
 All living species on Earth are composed of cells.
 A cell is the basic unit of life.
 All cells arise from pre-existing cells.
A modern version of the cell theory was eventually formulated, and it contains the following postulates:
 Energy flows within the cells.
 Genetic information is passed on from one cell to the other.
 The chemical composition of all the cells is the same.
Functions of Cell
A cell performs these major functions essential for the growth and development of an organism. Important
functions of cell are as follows:
Provides Support and Structure
All the organisms are made up of cells. They form the structural basis of all the organisms. The cell wall and the
cell membrane are the main components that function to provide support and structure to the organism. For eg., the

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skin is made up of a large number of cells. Xylem present in the vascular plants is made of cells that provide
structural support to the plants.
Facilitate Growth Mitosis
In the process of mitosis, the parent cell divides into the daughter cells. Thus, the cells multiply and facilitate the
growth in an organism.
Allows Transport of Substances
Various nutrients are imported by the cells to carry out various chemical processes going on inside the cells. The
waste produced by the chemical processes is eliminated from the cells by active and passive transport. Small
molecules such as oxygen, carbon dioxide, and ethanol diffuse across the cell membrane along the concentration
gradient. This is known as passive transport. The larger molecules diffuse across the cell membrane through active
transport where the cells require a lot of energy to transport the substances.
Energy Production
Cells require energy to carry out various chemical processes. This energy is produced by the cells through a process
called photosynthesis in plants and respiration in animals.
Aids in Reproduction
A cell aids in reproduction through the processes called mitosis and meiosis. Mitosis is termed as the asexual
reproduction where the parent cell divides to form daughter cells. Meiosis causes the daughter cells to be
genetically different from the parent cells. Thus, we can understand why cells are known as the structural and
functional unit of life. This is because they are responsible for providing structure to the organisms and performs
several functions necessary for carrying out life’s processes.
Differences Between Plant Cell and Animal Cell
Cells are the basic unit of a living organism and where all life processes are carried out. Animal cells and plant
cells share the common components of a nucleus, cytoplasm, mitochondria and a cell membrane. Plant cells have
three extra components, a vacuole, chloroplast and a cell wall. Both plant and animal cells share a few common
cell organelles, as both are eukaryotes. The function of all these organelles is said to be very much similar.
However, the major differences between the plant and animal cells, which significantly reflect the difference in the
functions of each cell.
The major differences between the plant cell and animal cell are mentioned below:
Plant Cell Animal Cell
Cell Shape
Square or rectangular in shape Irregular or round in shape
Cell Wall

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Present Absent
Plasma/Cell Membrane
Present Present
Endoplasmic Reticulum
Present Present
Nucleus
Present and lies on one side of the cell Present and lies in the centre of the cell
Lysosomes
Present but are very rare Present
Centrosomes
Absent Present
Golgi Apparatus
Present Present
Cytoplasm
Present Present
Ribosomes
Present Present

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Plastids
Present Absent
Vacuoles
Few large or a single, centrally positioned vacuole Usually small and numerous
Cilia
Absent Present in most of the animal cells
Mitochondria
Present but fewer in number Present and are numerous
Mode of Nutrition
Primarily autotrophic Heterotrophic
Cell Cycle Definition
The cell cycle is the sequence of events occurring in an ordered fashion which results in cell growth and cell
division.
 The cycle begins at the end of each nuclear division and ends with the beginning of the next.
 A cell cycle acts as a unit of biological time that defines the life history of the cell.
 The cell cycle is a continuous process that includes all significant events of the cell, ranging from duplication
of DNA and cell organelles to subsequent partitioning of the cytoplasm.
 In addition, the process of cell growth where the cell absorbs nutrients and prepares for its cell division is also
a part of the cell cycle.
 The process of the cell cycle occurs in various phases, all of which are specialized for a particular stage of the
cell.
 The overall process and steps of the cell cycle might differ in eukaryotic and prokaryotic organisms as a result
of the differences in their cell complexity.
 Three main cycles are involved in the cell cycle; chromosome cycle, cytoplasmic cycle, and centrosome cycle.
 The chromosome cycle involves DNA synthesis that alternates with mitosis. During this cycle, the double-
helical DNA of the cell replicates to form two identical daughter DNA molecules. This is followed by mitosis
to separate the cell into two daughter cells.
 The cytoplasmic cycle involves cell growth that alternates with cytokinesis. During growth, the cell
accumulates nutrients and growth factors and doubles the contents of the cytoplasm. Eventually, the
cytoplasm divides via cytokinesis to equally divide the cytoplasmic contents into two cells.

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 The final cycle is the centrosome cycle where the centrosome is divided so that it can be inherited reliably and
duplicated accordingly to form two poles of the mitotic spindle fibers.
 The cell cycle is regulated by various stimulatory and inhibitory factors that decide whether the cell needs to
divide or grow.
 The cell cycle is divided into different phases (according to Howard and Pelc), each of which is defined by
various processes.
0% of the World Could
Live in Sahara-Like Heat by 2070
Phases of the Cell Cycle
1. Gap 0 Phase (G0)
 Gap 0 phase or G0 phase of the cell cycle is a period of time where the cell is present in a quiescent stage or
resting phase, as it neither divides nor grows.
 The G0 phase can be considered either an extended G1 phase or a separate phase-out of the cell cycle.
 Usually, cells enter the G0 phase when they reach maturity like in the case of muscle cells and nerve cells, but
the cells continue to perform their function throughout their life.
 In some cases, however, cells might enter the G0 phase from the checkpoint in the G1 phase due to the lack of
growth factors or nutrients.
 In the G0 phase, the cell cycle machinery of the cell is dismantled, and the cell continues to remain in the G0
phase until there is a reason for the cell to divide.
 There are some cells like the parenchymal cells of the liver and kidneys that enter the G0 phase semi-
permanently and can be induced to divide.
 Even though the G0 phase is often associated as senescence, the G0 phase is a reversible stage where a cell
can enter the cell cycle again to divide.
 The cells in the G0 phase have different regulators that ensure the proper functioning of the cell.
2. Gap 1 Phase(G1)
 The G1 phase of the cell cycle is a part of the interphase where the cell begins to prepare for cell division.
 A cell enters the G1 phase after the M phase of the previous cycle, and thus, it is termed as the first gap phase
of the first growth phase.
 In this phase, no DNA synthesis takes place, but RNA synthesis occurs in order to produce proteins required
for proper cell growth.
 G1 phase is considered a time of resumption where the cell finally picks up normal cell metabolism that had
slowed down during the M phase of the previous cycle.
 The process and steps of the G1 phase are highly variable, even within the cells of the same species.
 The most important event of the G1 phase, however, is the transcription of all three types of RNAs which then
undergo translation to form proteins and enzymes necessary for other events in the cell cycle.
 The duration of the G1 phase is also highly variable among cells. In some cells, it occupies about 50% of the
total cell cycle time, whereas, in rapidly dividing cells, the phase is entirely omitted.
 An important in the G1 phase is the G1/S checkpoint that determines if the cell is ready enough to proceed into
the division phase.
 At this point, events like the detection of DNA damage and nutrient concentration are performed to ensure that
the cell has enough machinery to undergo cell division.

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3. Synthesis Phase (S)
 The S phase or synthesis phase of the cell cycle is a part of the interphase where important events like DNA
replication and formation of histone proteins take place.
 The processes of the S phase are tightly regulated as the synthesis of proteins and replication of DNA require
utmost precision.
 The production of histone proteins and other proteins are crucial in this phase as the newly replicated DNA
molecules require histone proteins to form nucleosomes.
 The entry into the S phase is regulated by the G1/S checkpoint that only allows cells with enough nutrients and
healthy DNA to enter the next phase.
 The phase is moderately long, occupying about 30% of the total cell cycle time.
 During this phase, the content of DNA doubles in the cell, but the number of chromosomes remain the same as
the division of chromosome doesn’t take place just yet.
 The regulatory mechanism of the S phase also ensures that the process of DNA synthesis takes place before
the M phase and with precision.
 In order to preserve the epigenetic information, different regions of the DNA are replicated at different times.
 Similarly, actively expressed genes tend to replicate during the first half of the S phase, whereas inactive genes
and structural DNA tend to replicate during the latter half.
 Therefore, at the end of the S phase, each chromosome of the cell has double the amount of DNA with a
double set of genes.
4. Gap 2 Phase (G2)
 The G2 phase or Gap Phase 2 or Growth Phase 2 is a phase of the cell cycle where the cell collects nutrients
and releases proteins in order to prepare the cell for the M phase.
 The G2 phase is also a part of the interphase when the cell is still in the resting phase while preparing for cell
division.
 The G2 phase is also important as it checks for DNA damage (during replication) to ensure that the cell is in
proper condition to undergo division.
 The phase might be skipped in some rapidly dividing cells that directly enter the mitotic phase after DNA
replication.
 It is, however, an essential phase that checks for mutations and DNA damage to prevent excessive cell
proliferation.
 Even though information on the regulation and working of the G2 phase has been studied, its role in cancer
initiation and development is yet to be determined.
 DNA repair is a crucial step in the G2 phase as it repairs breaks that might be present in the DNA strand after
replication.
 The entry of the cell from the G2 phase to the M phase is regulated by the G2 checkpoint, where different
proteins and complexes are involved.
 In the case of DNA damage or insufficient nutrients, the cell remains in the G2 phase and is not passed for cell
division.
5. Mitosis Phase (M)

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 The M phase or Mitotic phase of the cell cycle is the most crucial and dramatic phase of the entire cycle where
the cell divides to form identical daughter cells.
 The most important event of this phase is the karyokinesis (nuclear division) where the chromosomes separate
into form two distinct cells.
 The process of mitosis might differ from one organism to another and even from one cell to another.
 Mitosis begins with the condensation of chromosomes which then separate and move towards opposite poles.
 A cell entering the M phase has a 4N concentration of genetic material and ends with two cells, each
containing a 2N concentration of DNA.
 Mitotic cell division occurs via four distinct steps; prophase, metaphase, anaphase, and telophase.
 Prophase is the first stage of mitosis where the chromosome of the cell divides into two chromatids held
together by a unique DNA region called the centromere. As the prophase progresses, the chromatids become
shorter and thicker. Prophase also includes the division of centriole that move toward the two opposite ends of
the cell.
 Metaphase is the second and the longest stage of cell division where the chromatids are lined up on the
metaphase plate. The chromatids are shorter and thicker and are still held together by a centromere.
 Anaphase is the next stage of mitosis involving the splitting of each chromosome into sister chromatids to
form daughter chromosomes. After splitting, the chromatids are moved towards the pole due to the shortening
of the microtubules.
 Telophase is the final stage of mitosis which involves the reorganization of two nuclei and the entry of the cell
into the next phase. During this phase, a nuclear envelope is formed around the chromosomes to form two
distinct daughter nuclei.
 Telophase indicates the end of the M phase, which initiates the division of cell organelles and separation of
cytoplasm into two cells (cytokinesis).
6. Cytokinesis
 Cytokinesis is the division of cytoplasm into two halves, indicating the end of cell division.
 Cytokinesis occurs immediately after the M phase to separate the nucleus, cell membrane and the rest of the
cytoplasm into two halves to form two distinct and complete cells.
 The phase begins with the constriction of the cell membrane, which ultimately leads to cleavage and division.
 The constriction is first observed during anaphase, which continues to grow deeper to finally cause cleavage.
 The process and mechanism of cytokinesis might be different in different cells.
 In some cases, cytokinesis is often considered to be a part of the M phase, but in the case of animal cells,
cytokinesis and mitosis might occur independently.
 The contraction of the cell membrane during cytokinesis is brought about by the contraction of actin fibres that
form a bundle, called a contractile ring.
 In the case of a plant cell, however, a distinct cell plate is formed at the middle of the dividing cell which
separates the cytoplasm and cell organelles into equal halves.
 Cytokinesis, like the rest of the cell cycle, is also regulated by several factors that are responsible for the
initiation of division as well as the termination.
The importance of cell division can be appreciated by realizing the following facts:
1. Cell division is a pre-requisite for the continuity of life and forms the basis of evolution to various life forms.
2. In unicellular organisms, cell division is the means of asexual reproduction, which produces two or more new
individuals from the mother cell. The group of such identical individuals is known as clone.
3. In multi-cellular organisms, life starts from a single cell called zygote (fertilized egg). The zygote transforms
into an adult that is composed of millions of cells formed by successive divisions.
4. Cell division is the basis of repair and regeneration of old and worn out tissues.

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Cell Division: Cell division, cell reproduction or cell multiplication is the process of formation of new or daughter
cells from the pre-existing or parent cells.
It occurs in three ways: Amitosis or Direct cell division, Mitosis or Indirect cell division, Meiosis or Reductional
cell division
Definition of Mitosis:
Mitosis is a type of cell division in which chromosomes are equally distributed resulting in two genetically
identical daughter cells.
History: Mitosis was first discovered in plant cells by Strasburger (1875). Later on, W. Flemming (1879)
discovered it in animal cells. The term mitosis was coined by Flemming (1882).
Occurrence: The cells undergoing mitosis are called mitocytes. In plants, the mitocytes are mostly meristematic
cells. In animals, the mitocytes are stem cells, germinal epithelium & embryonic cells. It also occurs during
regeneration. Root tip is the best material to study mitosis.
Duration: It varies from 30 minutes to 3 hours.
Steps of Mitosis:
Mitosis is a continuous process and for better understanding the whole process is divided into following six
stages:
1. Prophase:
i. Nucleus becomes spherical and cytoplasm becomes more viscous.
ii. The chromatin slowly condenses into well-defined chromosomes.
iii. Each chromosome appears as two sister chromatids joined at the centromere.
iv. The spindle (microtubules) begins to form outside nucleus. In plants the spindle apparatus or mitotic spindle is
anastral. In animals and brown algae the mitotic spindle is amphiastral which include two asters in opposite
poles of the spindle. Each aster consists of two centrioles surrounded by astral rays.
2. Prometaphase:
i. Nuclear envelop breaks down into membrane vesicles and the chromosomes set free into the cytoplasm.

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ii. Chromosomes are attached to spindle microtubules through kinetochores. Specialized protein complexes that
mature on each centromere are called Kinetochores.
iii. Nucleolus disappears.
3. Metaphase:
i. Kinetochore microtubules align the chromosomes in one plane to form metaphasic plate or equatorial plate.
The process of formation of metaphasic plate is called congression.
ii. Centromeres lie on the equatorial plane while the chromosome arms are directed away from t he equator called
auto orientation.
iii. Smaller chromosomes remain towards the centre while larger ones occupy the periphery.
4. Anaphase:
i. Chromosomes split simultaneously at the centromeres so that the sister chromatids separate. They are now
called daughter chromosomes. Where each one consists of single chromatid.
ii. The separated sister chromatids move towards opposite poles at the speed of 1µm per minute.
iii. Pole-ward movement of daughter chromosomes occurs due to shortening of kinetochore microtubules;
appearance and elongation of inter-zonal fibers.
iv. Daughter chromosomes appear V-shaped (metacentric), L-shaped (sub-metacentric), I-shaped (acrocentric) and
I-shaped (telocentric).
v. It is the shortest of all stages of mitosis.
5. Telophase:
i. Daughter chromosomes arrive at the poles.
ii. Kinetochore microtubules disappear.
iii. Nuclear envelope reforms around each chromosome cluster of each pole.
iv. Chromosomes uncoil into chromatin.
v. Nucleolus re-appears.
vi. It is considered as the reverse of prophase.
Significance of-Mitosis:
1. Genetic Stability:
Mitosis maintains constant chromosome number and genetic stability in all somatic or vegetative cells of the body.
2. Growth:
Mitosis increases cell number so that a zygote transforms into a multicellular adult.
3. Surface-Volume ratio:
As the size (volume) of a cell increases, the surface area decreases accordingly. By mitosis, the cell becomes
smaller in size and the surface volume ratio is restored.

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5. Nucleo-plasmic ratio:
When a cell grows in size, nucleocytoplasmic ratio decreases. 11 are restored by mitosis.
6. Mitosis is a method of asexual reproduction and vegetative propagation.
7. Mitosis provides new cells for repair, regeneration and wound healing.
8. DNA content is reduced to half from parent cell to daughter cell.
Meiosis (Reductional Cell Division):
(Gr. meioum = to reduce, osis = state)
Definition: Meiosis is a double division in which a diploid cell divides twice to form four haploid daughter cells.
History: Vanzeneden (1883)-First reported meiosis Farmer & Moore (1905) Coined the term meiosis
Occurrence: The cells undergoing meiosis are called meiocytes. In plants, the meiocytes are microsporocytes
(Pollen mother cell) of anthers and megasporocytes (megaspore mother cell) of ovules. In animals, the meiocytes
are primary spermatocytes in testes and primary oocytes in ovaries.
lnterphase I:
i. Physiologically most active stage
ii. Nuclear envelop remains intact
iii. Nucleoli is prominent
iv. Chromosomes appear in form of chromatin reticulum.
Prophase I:
It is typically longer and more complex phases… On the basis of chromosomal behaviour, it is divided into 5 sub-
stages: Ieptotene, zygotene, pachytene, diplotene and diakinesis.
1. Leptotene (= Leptonema-thin thread)
2. Zygotene (=Zygonema-pairedpairing)

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3. Pachytene (= Pachynema)
4. Diplotene (= Diplonema)
5. Diakinesis
Metaphase I:
i. Bivalents arrange on two equatorial plates.
ii. The centromeres are directed towards poles and the arms of chromosomes face the equatorial plate called co-
orientation.
iii. The microtubules (chromosomal fibers) from opposite poles of the spindle attach to the bivalents.
Anaphase I:
i. Half of the homologues chromosome separate and move to opposite pole. This process is known as disjunction.
ii. Chromosomal fibres contract causing attraction while interzonal spindle fibres elongate causing repulsion.
Telophase I:
i. Each pole possesses a group of dyad chromosomes.
ii. Spindle fibers disappear
iii. Nucleoli reappear and nuclear envelope reformed.
iv. In Trillium telophase I is absent. The chromosomes pass from anaphase I lo prophase
Interkinesis:
i. It is a very brief interphase between meiosis I and meiosis II.
ii. There is no DNA replication i.e. S-phase absent.
Meiosis II (Equational or Homotypic D. vision):
The meiosis II is similar to mitosis in which chromosomes number remains constant.
Prophase II:
i. Chromosomes shorten and thicken
ii. Nucleoli disappear.
iii. Nuclear envelope breaks down
iv. The spindle fibres appear at right angles to the spindle of meiosis-I.
v. In animal cells, centrosome divides and moves to opposite poles.
Metaphase-II:
i. Chromosomes aligned in one equatorial plate.
ii. Spindle fibres attached to kinetochores of sister chromatids.
iii. Centromeres remain on the metaphasic plate while the chromatids are extended towards the poles.

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Anaphase-II:
i. The centromere divides and the two chromatids of each chromosome separate and pulled towards opposite
poles.
ii. The separated chromatids are now called as daughter chromosomes.
Telophase II:
i. The daughter chromosomes reach at the opposite poles.
ii. The chromosomes uncoiled to form chromatin.
iii. Nucleoli and nuclear envelope reappear.
iv. Spindle fibers dissapear.
 Cytokinesis:
In meiosis, 2 types of cytokinesis can be seen.
a. Successive type: In this case, cytokinesis occurs after both meiosis I and meiosis 11. As a result four haploid
cells are formed. In plants the cells are arranged in form of isobilateral tetrad or in a linear manner.
b. Simultaneous type: In this case, cytokinesis occurs twice only after meiosis II. The four haploid cells arranged
in form of a tetrahedral tetrad. In plant cells, cytokinesis takes place by cell plate method, while in animal cells
cleavage or furrowing method generally occurs.
Significance of Meiosis:
1. Meiosis essentially maintains constancy in chromosomes from generation to generation.
2. Crossing over and disjunction bring genetic variation within the species. The variations are important raw
materials for evolution and also help in improvement of races.
3. Meiosis causes segregation and random assortment of genes.
4. Meiosis causes conversion from sporophytic generation to gametophytic generation in plants.
5. It leads to the formation of haploid gametes (n) which is an essential process in sexually reproducing
organisms. Fertilization restores the normal somatic (2n) chromosome number.
Types of Meiosis:
There are three types of meiosis based on the variations in time and place of the division in the life- cycle of the
plant.
1. Zygotic or Initial Meiosis (Haplontoic Pattern):
During the process of fertilization, the two gametes fuse to form zygote which represents the only diploid stage in
the life-cycle. The zygote undergoes meiosis and forms four haploid cells which later on develop into haploid
individuals, e.g., Thallophyta.

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2. Gametic or Terminal Meiosis (Diplontoic Pattern):
This type of meiosis can be seen in animals and some lower plants. Here the meiotic division takes place
immediately before gamete formation and the haploid cells thus formed are transformed into sperm (male gamete)
and egg (female gamete).
3. Sporic or Intermediate Meiosis (Diplo- haplontoic Pattern):
We come across this type of meiosis in higher plants and in some thallophyta but not in animals. The life-cycles of
these organisms are characterised by alternation of haploid and diploid generations (i.e., gametophytic and
sporophytic generations).
Meiosis occurs in the sporogenous cells (micro-and megaspore mother cells) of the sporophyte producing haploid
spores. The spores on germination form gametophytes (male and female). Cells of the gametophyte form gametes.
Fusion of these gametes again leads to diploid or sporophytic generation, and in this way alternation between
gametophytic and sporophytic generations keeps on going.
 Developmental Biology
Developmental biology is a branch of natural science that studies a variety of interactions involved in the formation
of the heterogeneous shape, structure, and size of different organisms that occur during the development of an
embryo into an adult.
Significance of Developmental Biology Studies
 It helps to explain how a variety of interacting processes generate an organism’s heterogeneous shapes, size,
and structural features that arise on the trajectory from embryo to adult, or more generally throughout a life
cycle.
 It helps to understand the molecular, genetic, cellular, and integrative aspects of building an organism.
 Knowledge of normal developmental processes can aid in the understanding of developmental abnormalities
and other conditions such as cancer.
 Philosophers of biology have shown renewed interest in developmental biology due to the potential relevance
of development for understanding evolution and the theme of reductionism in genetic explanations.
Applications
Developmental Biology enquires about the fundamental processes that underpin the fertilization of an egg cell and
its step-by-step transformation into the fascinating complexity of a whole organism which renders many
applications in different fields.
In Fertility Clinics:
 Studying the underlying processes of fertilization has provided the foundations for much of what fertility
clinics can do these days.
 Developmental biology investigates how fertilized egg cells divide in regulated manners to grow into full-size
bodies, how the cells formed in this process communicate in meaningful ways to become different from each
other, migrate, change shape and attach to each other, thus assembling into tissues and complex organs.

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In Tissue Engineering:
 ‘Tissue engineering’, which aims to grow replacement tissues in a plastic dish, is essentially guided by DB
research.
 In cancer, cells lose their identity, divide excessively, detach from their local environments and migrate to
form metastases.
 Much of this understanding that can instruct cures to contain these aberrant cells, comes from DB research.
 Tissues keep so-called stem cells which can be re-activated in orderly manners to divide and grow replacement
tissues.
 There are high hopes from stem cell research, for example, to replace cartilage in arthritis or damaged discs,
or brain cells in dementia, much of which is guided by the vast knowledge gained through developmental
biology.
In Breeding Programs:
 For example, understanding plant development provides a means to speed up breeding processes, such as
optimizing root systems, plant size or flowering time, thus contributing to the efforts of achieving sustainable
food security in times of over-population.
In understanding factors influencing development:
 Understanding environmental influences on development, such as temperature-dependent sex determination in
turtles, has enormous importance for conservation biology, especially in times of increasing pollution and
global warming.
Number of Cells
Cells are the most minimal degree of association in each living thing. The count of the cells may vary from organism
to organism. Humans have more cells than bacteria. If an organism is formed from one cell, it's called a unicellular
organism (uni: one; cellular: cell). Whereas, the organisms which are made from more than one cell are called
multicellular organisms (multi: many; cellular: cell). Among the multicellular organisms, the count of the cell varies.
Some may have billions of cells while other organisms may have trillions (like humans). But every organism starts
its life from one cell which further divides into thousands and millions.
Unicellular/Monocellular v/s Multicellular
Unicellular/monocellular organisms are made up of only one cell that carries out all of the functions needed by the
organism, while multicellular organisms use many different cells to function. Unicellular organisms include bacteria,
protists, and yeast.
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. In
complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions.
As the size of the living being gradually increases, so does the quantity of cells that they have. Notwithstanding, this
check won't decide the proficiency of a creature i.e., capacity and effectiveness of a cell in a unicellular life form and
multicellular organic entity will be something similar.
Living organisms are made from differing types of cells, of different shapes and sizes. A unicellular organic entity
varies fit as a fiddle from another unicellular creature. Within a multicellular organism, there is a spread of cells.
Some are short while others are long and slender; some are roundabout while some are oval.
Size and Shape of Cell
The shape and size vary from cell to cell consistent with their functions and composition. For example, a neuron is
long and branched, meant for the transmission of signals throughout our body while a muscle fibre is little and
spindle-shaped which helps in movement.

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Considering an animal cell, we can generalize the form of a cell as round (spherical) or irregular. Plant cells are far
more rigid and rectangular in shape. The size of a cell is often as small as 0.0001 mm (Mycoplasma) and as large as
six to 12 inches (Caulerpa taxifolia). For the most part, the unicellular creatures are minuscule, similar to bacteria.
But one cell like an egg is large enough to the touch. Regardless of their shape, they all comprise similar organelles
and assist us with playing out the everyday exercises proficiently.
Living organisms are made up of different types of cells, of different shapes and sizes. A unicellular organism differs in
shape from another unicellular organism. Within a multicellular organism, there are a variety of cells. Some are long while
others are short; some are circular while some are oval. Shape and size vary from cell to cell according to their functions
and composition. For example, a nerve cell is long and branched, meant for the transmission of signals throughout our
body while a muscle cell is small and spindle-shaped which helps in movement.
Considering an animal cell, we can generalize the shape of a cell as round (spherical) or irregular. Plant cells are much
more rigid and rectangular in shape. The size of a cell can be as small as 0.0001 mm (mycoplasma) and as large as six to
twelve inches (Caulerpa taxifolia). Generally, the unicellular organisms are microscopic, like bacteria. But a single cell
like an egg is large enough to touch. Whether regular or irregular in shape, they all consist of the same organelles and help
us to perform the daily activities efficiently.
 DESCRIBE THE PATTERN OF VIVIPAROUS, OVIPARAOUS AND OVOVIVIPAROUS IN
ANIMAL CELL DEVELOPMENT.
Viviparous Animals: Animals that give birth to offspring are called viviparous. In viviparous animals, both
fertilization, as well as the development of the embryo, takes place inside the female reproductive system. Once the
fetus development is complete, the mother delivers the baby. This condition is referred to as matrotrophy where the
embryo obtains the nutrients directly from the mother and not the yolk.
Examples of Viviparous Animals: Human beings, dogs, cats, elephants, etc are few examples of viviparous
animals.
Oviparous Animals: Animals that lay eggs are called oviparous. In oviparous animals, fertilization takes place
internally but embryo development takes place externally.
The eggs of birds such as hen and duck carry immature embryo in them. The hard shells of eggs protect them
from damage. Once the fetus is matured, the egg hatches. The trait of egg-laying animals is known as oviparity.
Examples of Oviparous Animals: All birds lay eggs with a typical hard calcium shell. Frogs are egg-laying
amphibians which have soft gelatinous eggs requiring constant hydration. Almost all fishes are oviparous. Except
for some species of snakes, all other reptiles are oviparous. In mammals, Echidna and platypus are egg-laying.

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Metamorphosis in Viviparous Animals: Viviparous animals give birth to young ones. All organisms mature,
grow, and eventually become adults. But the process of “growing up” varies. Insects and most other invertebrates
undergo a sequential transformation from young ones to adult. This process of a drastic change of a larva into an
adult is called metamorphosis. This type of growth stages can be observed in many insects like butterflies,
silkworms, cockroach, etc.
The only animals with backbones that can undergo metamorphosis are amphibians. For example, in frogs, there are
three stages. Their appearance at each stage differs. They begin as an egg, then become a larva (tadpole) and later
become an adult frog.
Ovoviviparity In Ovoviviparous Animals: Ovoviviparous animals lay eggs and develop the eggs inside the
mother’s body. The eggs are hatched inside the mother. Once the egg hatches, it remains inside the mother for a
period of time and is nurtured from within but not via a placental appendage. Ovoviviparous animals are born live.
Some examples of ovoviviparous animals are- sharks, rays, snakes, fishes, and insects. Oviparity is different
from ovoviviparity in a way that the eggs in oviparity may or may not undergo internal fertilization but are laid and
depend on the yolk sac to get nourished till the time they hatch.
Ovoviviparity shows internal fertilization of eggs typically via copulation. For instance, a male shark penetrates
his clasper into the female to release sperms. Fertilization of eggs takes place when they are in the oviducts and
sustain to develop here, and are supplied by the egg yolk in their egg. The female counterpart of guppies
accumulates extra sperms which they use to fertilize their eggs for a period up to eight months. The younger ones
remain in the oviducts when the eggs hatch and last there to grow and develop till they mature to be given birth and
sustain life.
These animals show no umbilical cord which is typically their physical attachment to the mother for nutrient
requirements and gas exchange. In such cases, nourishment is obtained from the yolk of the egg. When this yolk is
depleted, the mother provides additional nutrition in the form of unfertilized eggs and uterine secretions.
One of the advantages ovoviviparous animals is that, after birth, the young are competent enough to feed and
defend on their own. This means that they can fend for themselves in the wild and are capable of living without the
need for their mother’s protection. For instance, rattlesnakes are ovoviviparous and right after birth, they have fully
developed venom glands that are as potent as the adult rattlesnakes.
 WHY WE STYDY CELL BIOLOGY IN BIOTECHNOLOGY?
"Cell Biology for Biotechnologists" enumerates the basic structure of prokaryotic and eukaryotic cells and the
exceptions for cell theory and explains the mechanisms of transport within and out of the cell, the receptors and
their role in signal transduction and cell culture. Biotechnology uses techniques and information from cell biology
to genetically modify crops to produce alternative characteristics; to clone plants and animals; to produce and
ensure high quality food is available at lower costs; to produce purer medicines and in time organs for the many
people who need transplants.
Biotechnology is a broad area of biology, involving the use of living systems and organisms to develop or make
products. Depending on the tools and applications, it often overlaps with related scientific fields. In the late 20th

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and early 21st centuries, biotechnology has expanded to include new and diverse sciences, such as genomics,
recombinant gene techniques, applied immunology and so on.
With advanced biotechnical platforms, Leading Biology provides a wide range of solutions including recombinant
antibody production, protein expression, phage display to support the development of pharmaceutical therapies and
diagnostic tests.
1. Studies of the cell structure, a very integral part of Cell Biology, is essential in Biotechnology research as the
latter involves knowledge of cell structure of living cells in order to carry out cell therapeutics and related genetic
studies
2. Cell and tissue culturing, an essential unit of cell biology inculcates knowledge and practice of the fundamental
techniques involved in the growth of the cell type of interest. This is applied in biotechnology to nature cells of
interest in preparation for Genetic studies.
3. Cell division in Cell Biology, is crucial in Biotechnological studies, when monitoring growth of Cancer cells
for therapeutic purposes.
4. Cell physiology, studied in Cell Biology, helps Biotechnologists to understand the concept of Cell transport
which they apply in Mutation studies to confirm how wild strains and mutants behave physiologically.
5. Biotechnologists apply the concept of Cell Death (a unit in Cell Biology) to study the effects of external and
internal forces influencing the cell’s life-maintaining signals, this therefore helps them know and appreciate the
concept of cell apoptosis (programmed cell death).