Biology REVISION OF Cell and Cell Division FOR NDA

CELL THEORY

Cell theory was proposed by Matthias Schleiden
(German Botanist) in 1838 and Theodor Schwann
(German Zoologist) in 1839.

Cell theory now suggests

(i) living organisms arc composed of cells and their products.

(ii) cell is the functional unit of life.

(iii) Later cell theory w<,s extended by Rudolf Virchow in 1855. H c proposed Omnis cellula-e-cellula, i.e. cells
arise only by division of previously existing cell. Viruses arc considered to be the exceptions to cell
theory as they arc acellular.

Types of Cells

All living cells are of two basic types

(i) Prokaryotic Those primitive cells lacking a true nucleus and cell organelles like mitochondria, Golgi body, ER, e.g.
bacterial cells, cyanobacteria (blue-green algae), mycoplasma.

(ii) Eukaryotic Those cells have the nucleus with membrane bound cell organelles, e.g. animals and higher plant's cells.


Eukaryotic cells are further categorised into animal cells
and plant cells.

Plaint and Animal Cells

Plant cells and animal cells have common basic structure with cell membrane, cytoplasm, nucleus and various
cytoplasmic organelles, but they differ in many features.

• Largest cell is egg of ostrich.

• Smallest cell is Mycoplasma gallisepticum.

• Totipotency Animal and plant cells have the capability of totipotency, i.e. they can transform into other types
of cells.

STRUCTURE AND WORK OF CELL

Knoll and Ruska (1932) discovered electron microscope.
This microscope magnifies a cell about 1-2 lakh times.
Structural organisation of a cell viewed by a electron
microscope includes following structures

Cell Wall

• Cells of most fungi, prokaryotes (bacteria and blue-green algae) and plants (:except gametes) are surrounded by a
wall. In animals, cell wall is found to be absent.

• In true bacteria and cyanobacteria, cell wall is of peptidoglycan, while in most fungi, it is of chitin, but in
most of the algae and higher plants, it is of cellulose. In plants, it mainly consists of four layers, i.e.

(i) Middle lamella (composed of pectin compounds)

(ii) Primary wall

(iii) Secondary wall

(iv) Tertiary wa.ll

• The main function of cell wall is to protect cytoplasm and plasma membrane from external shocks.

• In ripened fruits, softening occurs due to dissolution of
pectin.

Plasana Membrane (Cytoplasmic Membrane or Plasmalemma)

• Cell membrane is the selectively permeable (membrane that allow movement of some molecules, which restrict
the others) thin film-like covering present around the cell. Singer and Nicolson (1972) proposed the most accepted
'Fluid m1iaic model' of plasma membrane structure.

• Beside plasma membrane, in eukaryotic cells intracellular membrane is also present, which surrounds the vacuole
and organelles.

• Main function of cell membrane is to regulate and control the flow of materials into and outside the cell and diffusion of `O_2` and `CO_2`. The movement of substances across a membrane occurs by following processes

(i) Diffusion

• It is the spontaneous movement of substances (such as `CO_2` ). From a region of their high concentration to a
region of their low concentration.

• It does not require a semipermeable membrane.

(ii) Osmosis

• It is the passage of water from a region of high water concentration through a semipermeable membrane to a
region of low water concentration.

• Osmosis is a special case of diffusion in which water (solvent) is diffused across the membrane.

• The movement of water in animal cells may take place in different manners

(a) If the water concentration of the solution surrounding the cell is too high, the cell bursts due to endosmosis
(entry of water inside the cell). Such a solution is known as hypotonic solution.

(b) If the water concentration of the solution surrounding the cell is too low, the cell shrinks due to exosmosis,
i.e. outward movement of water. This solution is known as hypertonic solution.

Protoplasm

Protoplasm is an elastic, viscous, complex, colloidal system. It constitutes the living part of a cell.

• In 1861, Max Schultze proposed the 'protoplasm theory. According to this, 'cell is an accumulation of
living substance (or protoplasm), which is limited by an outer membrane and posseses a nucleus'.

• Purkinje (1837) coined the term protoplasm.

• The presence of protoplasm is the most important characteristic of living organism.

• All life activities take place in protoplasm.

• Huxley ( 1 :368) called it as physical basis of life.

• The parts other than protoplasm are cell wall, vacuole, granular waste material, fat droplets, etc. These constitute
the non-living part of cell.

Cytoplasm

Cytoplasm is a part of protoplasm present between plasma membrane and nucleus. It is a jelly-like fluid containing
many biomoleculcs Such as lipids, made of fatty acid and glycerol. Proteins made of amino acids, carbohydrats
made of monosaccharides and polysaccharides, vitamins,
minerals, etc.

CELL ORGANELLES

A cell is consist of certain cell organelles. Presence of
these organelles is an example of division of labour. Some
of the cell organelles are as follows.

Mitochondria

• Altmann ( 1890) found them as granules and named these as bioplast. C Benda ( 1897) coined the term
'mitochondria' for the first time.

• Mitochondria play a significant role as the site of cellular respiration.

• Plant cells have fewer number of mitochondria as compared to animal cell.

• It is usually 1-10 p in size.

•Mitochondria is a d ouble-membrane bound structure.

• The outer membrane is separated from the inner membrane by meals of a space called
perimitochondrial ( 6-10 nm wide).

• Inner membrane is folded and projected into a number of finger-like structures called cristae.

• On the surface of cristae, small particles known as F1-particles or oxy~;omes arc found. These particles are
involved in the symhesis of ATP.

• They are semiauto 1omous organelles. They contain circular DNA, mRNA, ribosomes and able to synthesise their own proteins.

• It is the powerhouse of cell, as oxidation of fuel occurs stepwise in it resulting in the release of ATP.

• It is absent in Nostoc, Clostridium and Gleotricha.

Endoplasmic Reticulum (ER)

• It is a membrane bound intercommunicating system of channels consisting- of cisternae, vesicles and narrow
fluid-filled tubules'' It was named endoplasmic reticulum by Porter in 1953.

• Endoplasmic reticulum is also known as the endoskeleton of cell.

ER is of two types

(i) Smooth Endoplasmic Reticulum (SER) without ribosomes, takes part in lipid synthesis.

(ii) Rough Endoplasmic Reticulum (RER) with

ribosomes, take:; part in protein synthesis.

Ribosomes arc attached to this.

• Mitochondria also provide space for storage of synthetic
product like glycogen. They also help in secretion.

Ribosomes

• Ribosomes were discovered and named by Palade (1955).

• Ribosomes arc minute organelles rhat could be seen through electron microscope only. These are synthesised
in the nucleus.

• Two basic types of ribosomes are 70S and 80S type.

• 70S ribosomes are found in prokaryotic cells, mitochondria and plastids of cukaryotic cell and consist
of 50S (larger) and 30S (smaller) subunits. 80S ribosomes are present in cytoplasm of cukaryotic cell and consist d 60S (larger) and 40S (smaller) subunits.

• It provides space as well as enzyme for the synthesis of protein in a cell, hence called protein factories of cells.

• Venlatraman Ramakrishnan got Nobel Prize in 2009 for his work on structure and function of ribosomes.

Lysosomes

• Lysosomcs were discovered by Christian de Duve in 1955.

• These are commonly called suicidal bags of the cell.

• A lysosome is a singlc-mcmbrancd, spherical, tiny sac-like body.

• It is most common in eukaryotic cells, but abundantly found in animal cells exhibiting phagocytic activity.

• These are reservoirs of hydrolytic enzymes (about 40).

These cnz.ymes can hydrolyse different substances such as proteins, nucleic acids, etc.

• These help in the digestion of material taken in by endocytosis (intcrnalisation of substances).

Golgi Bodies

• Golgi bodies were discovered by Camillo Golgi in 1898. He got Nobel Prize for this.

• It is a complex organisation of net-like tubules or vesicles surrounded by smaller spherical vesicles.

• lt is mainly present in cukaryotic cells, except mammalian RBCs. These arc abundantly found in
secretory cells.

• The Golgi apparatus plays an important role in the formation of lysosome, acrosome of sperms, formation
of yolk and storage of secretion products, formation of pectin in plants, formation of melanin granules, secretion
of hormones. The carbohydrates link with proteins to form glycoprotcins in Golgi bodies.

Centrosome

• Centrosome is a structure found in animal's cell and some lower plants Chlamydomonas. It is composed of
two granule-like structures called centrioles.

• The term 'centrosome' was coined by T Boveri in 1888.

• During cell division, it is divided into two pairs of centrioles, which move towards two opposite poles.

• Each centriole is made up of microtubules with a group of triplet fibres.

Vacuole

• It is surrounded by a single membrane called tonoplast and filled with liquid substances.

• Its size is large in plants, while in animal it is absent or very minute in size.

• It consists of anthocyanin pigment that gives red or purple colour to flowers.

Plastids

Plastids were discovered by Haeckel, but term plastid was given by Schimper. Plastids arc of three types

(i) Lcucoplast [t is colourless plastid found in underground roots, stems and stores food.

(ii) Chromoplast It is coloured plastid and found in coloured parts of flower, leaves, petals and fruits, etc.
It is foumi in the form of lycopenc in tomato, carotene i 1 carrot, etc.

(iii) Chloroplast It is green coloured plastid found in all green plan·:s.

Chloroplast

Chloroplast was discovered by Schimper (1885 ).

• Chloroplast is a cell organelle found in green plants.

• It can be variously shaped, i.e. cup-shaped, e.g. Chlorclla, Ch/amydomonas, girdle-shaped, e.g. Ulothrix,
ribbon-shaped, e.g. Spirogyra, etc.

• Chloroplast consists of pigments such as chlorophyll, carotene and xanthophyll.

• It is composed of a matrix-stroma covered by double-membranes.

• In the stroma, sac-like membranous units, i.e. thylakoids are found. Tb('se thvlakoids arc stacked one above the
other. One such stack is called as granum. In cyanobacteria granalcss chloroplasts, i.e.
chromatophores are present.

• On the outer surface of thylakoids, quantasomes are found. Quantasome is a unit of photosynthesis
consisting of 200-300 chlorophyll molecules.

• Chloroplast possesses 70S ribosomes and its own DNA. Due to this, they arc also called as semiautonomous
organelles.

• Chloroplast ma.y also consist of pyrcnoids with starch. Light and dark reaction of photosynthesis occur in
chloroplast.

Nucleus

• Nucleus was discovered by Robert Brown in 1831.

• Every eukaryotic cell consists of at least one, almost spherical, dense, highly specialised structure called as
nucleus (exceptions-sieve tube clement of mature phloem in plants, RBCs of mammals).

• All the mctabolic activities of a cell are under the control of nucleus.

• Chemical!) nucleus consists of 70% proteins, 20% DNA, 5% RNA and 3% lipids.

• Nucleus is considered to he the vehicle of heredity as it contains the genetic information reproduction.
development, metabolism as well as behaviour of irritability organism. Nucleus is the smallest unit that
show capacity for metabolism, irritability growth and reproduction.

• The nucleus consists of following parts

(i) Nuclear Membrane it is double-layered membrane and surrounds the nucleolus of a eukaryntic cell.

(ii) Nucleoplasm It is clear gel-like, ground matrix having ions, proteins, nucleotide and ribosomes.
Chromatin threads and nucleolus are and embedd in it.

(iii) Nucleolus Nucleolus was discovered by Fontana in 1781. It is non-memhranou:,, sphere-like strunurc,
composed of 10% RNA, 85% protein and 5% DNA. it takes part in the production of ribosomal subsuits.
rRNA and some specific: proteins.

(iv) Chromatin lt is present in the nucloplasmi as thin-threads.

lt is made up of DNA basic proteins (histones) and non-histone proteins. During cell division, chromatin
condenses into chromosomes.

NUCLEIC ACIDS

• Nucliic acids are complex ctrhonic compound:, and most important macromolecules of cell. They are
polymers of smaller units caIIed nuclcotides, joined and to end by phosphodicster bonds to form long chain.

• Nucleotidoes = Sugar + Nitrogenous base + Phosphate

• Nucleosidoes = Sugar + Nitrogenous base

Two nucleic acids abundant in living organisms are

(i) DNA (Deoxyribonucleic acid) DNA is a double belical structure with two nucleotide chains that run
antiparallel to earh other. It consists of pentosc sugar deoxyribose, phosphorie acid and nitrogenous base.
Nitrogcnous bases are of two type

• DNA Carries the genetic infonnatiou of cell and controls the structure and function or cell. ll serves as hereditry
material , so it functions as blueprint for building and runnn1g cellular machinery.

• The. segments of DNA arc genes, which a.re located on chromosome. Gen es code for different proteins. Apart
from genes, non-coding sequences arc also present in DNA.

• DNA is of different types like A-DNA, B-DNA, C-DNA, D-DNA, while T-DNA is present inTi plasmid.

(ii) RNA (Rib()nucleic .1cid) It consist-; of ribose sugar, phosphoric acid joined with four nitrogenous bases.

• ln some virsus RNA is genetic material, e.g. retrovirus (ds). Noln-genetic RNA is of three types, i.e. mRNA,
rRNA and rRNt, of which, tRNA is rhc smallest RNA.

• Transcription is the process by which DNA give's ris w RNA.
Restrictio En.don ucleases special ymcs rhat DNA at specific itc, so called molecular
scisso

CHROMOSOME

• The term chromosorne was wined by waldeyer (1888).

• Sutton and Boveri ( 1902) pro,·ed that clv mnsnme is the physical basis cof heredity, in the chromosomal
theory of inheritance.

• Chromatin materid (made of DNA), which is found in the interphase nucleus , condensed at the time of
division into small and thick threads called chromosome.

• Cromosome number for a specis remains always same. lt is diploid (2n ) in somatic cell and haploid (n) in
gametes. Chromosome has cell or more centromercs (centre:) and arms (chrom.uids).

• Genome is the haploid entity of an organism's hereditary information. It includes both genes and
non-ending sequcnces of DNA/RNA.

Centromere

There are four type of chromosomes based upon the
position of centromere

• Largest chromosome number 1262 has been recorded in a fern plant, i.e. Ophioglossum.

• Karyotype is the number and appearance of chromosomes, in nucleus of a eukaryotic cell.

• Idiogram is a diagrammatic representation of the karyotype, it is useful point of reference of analysing
mutations.

CELL DIVISION

• Division of cell at the time of growth is the character of life. The period from the starting of one cell division to
another is known as cell cycle. During cell cycle, karyokinesis (division of nucleus) and cytokinesis
(divisicn of cytoplasm) occurs.

Cell cycle occurs in two phases

1. lnterphase
During interphase, the cell synthesises all the proteins required in cell division. It also includes replication of DNA.
Interphase is divided into `G_1`, `S` and `G_2` phases.

2. M-phase

• M-phase is the dividing phase, it can be of two types, i.e.
mitosis and meiosis.

Mitosis

•Mitosis was first described by E Strasburger (1875) in plants and by W Flemming (1879) in animals.
' Mitosis term was coined by W Flemming in 1882.

• It usually occurs in somatic cells, hence called somatic

divisio:n. Mitosis mainly occurs for growth.

Mitosi~; occurs into two stages:

(i) Karyetkinesis

Karyokinesis includes four substages

(a) Prophase After interphase, prophase comes, which is first and longest stage of mitosis. Longitudinal
splitting or division of each chromosome into two sister chromatids takes place. Nuclear membrane
breaks down and nucleolus disappears. Spindle is also formed at late prophase.

(b) Metaphase During this stage, spindle formation occurs. Due to attachment of spindle fibres at the
centromere of chromosomes, the chromosome aligned themselves in the centre or equator due to their
active movements.

(c) Anaphase This is the shortest stage. Chromosomes divide at the point of centromere and thus, two sister
chromatids arc formed. These sister chromatids move towards the opposite poles of spindle.

(d) Telophase Chromosomes reach opposite poles and nuclear membrane reappears around each group of
chromosomes, thus forming two daughter nuclei.

(ii) Cytokinesis

Formation of two nuclei is followed by division of cytoplasm, thus forming two cells (daughter cells) by cell
plate (only in plants) or cell furrow method (only in animals).

Meiosis

• Farmer and Moore (1905) proposed the name meiosis (reduction division).

• This type of eel division is found only in reproductive cells. The nucleus divides twice, but chromosome divides
only once.

• The number of chromosomes in the daughter cells is half the number of duomosomes of mother cell.

• Four haploid (n) daughter cells arc formed.

• Best material to study the meiosis is unopend flower buds of onion.

It consists of two divisions

1. Meiosis-1 or First Meiotic Division

It is very important because it reduces the chromosome number to half. It comprises four substages, i.e. prophase-I,
metaphase-I, anaphasc-I and telophase-!.

(i) Prophase-I It is the longest phase and divided into five steps

(a) Leptotene Chromosomes appear as thin, uncoiled thread-like structures. Nuclear membrane starts
disappearing and chromatin condenses to form chromosomes.

(b) Zygotene Pairing of homologous chromosomes (called synapsis) takes place. This results into the
formation of bivalents.

(c) Pachytene Chromosomes split and show tetravalent stage and crossing over takes place
between non-sister chromatids.

(d) Diplotcllie Separation starts from centromere (terminalisation), but it is not complete, but the
homologous chromosomes remain attached at one or more points and these points are called
chiasmata.

(e) Diakinesis Terminalisation is almost complete. Both nucleolu:; and nuclear membrane completely
disappear.

(ii) Metaphase-I Chromosomes arranged at equator and attached to the spindle fibres.

(iii) Anaphase-I The chromosome with its two chromatids moves to the opposite pole.

(iv) Telophase-] Nuclear membrane appears around the group of chromosomes at each pole of the cell. 'The
two daughter nuclei with half the number of chromosomes arc formed. First meiotic division may or
may not be followed by cytokinesis.

2. Meiosis-II or Second Meiotic Division

After the completion of meiosis-1, second meiotic division starts. Mciosis-II is similar to mitosis.

(i) Prophase-Il Chromosomes become thick and short, nuclear membrane disappears.

(ii) Metaphase··Il Chromosomes get arranged on equator and spindle apparatus is formed.

(iii) Anaphase-U Centromere divides into two chromatids which move to the opposite poles.

(iv) Telophase-II Cbronosomcs form a group at the opposite poles. Nuclear membrane and nucleolus
reappear. Thus, twc daughter nuclei arc formed.

Cytokinesis

Later, karyokinesis is followed by cytokinesis. As a result of this four haploid cells arc formed.

Arnitosis

• It was first observed by Rober Remak (a German scientist) in the RBCs of chick embryo.

• It is direct nuclear division, without formation of spindle and recognisable appearance of chromosomes.

• It is a primitive type of division.

• It occurs in prokaryotes, protozoans, yeast, foetal membranes of mamnuls and in degenerative and old
tissues.

4. Proteins

• The term 'protein' was coined by Mulder in 1838.

• They are made up of carbon, hydrogen, oxygen, nitrogen and sulphur. Thus, these are required for protein
formation in body.

• They play a vital role in growth, development and repair of the body. All enzymes are protins except ribozymes.

• Tney are digested in alimentary canal by the action of pepsin and trypsin enzymes.

• They are the building block substances of body.

• Proteins are polymers of amino acids.

Amino acids are of two types

(i) Essential cannot be synthesised in the body and must be taken in diet, e.g. lysine, methionine, valine,
tryptophan, pheny.'.alanine, etc.

(ii ) Non-essential synthesised in the body and do not need to be taken f:om outside, e.g. alanine, arginine,
aspartic acid, glutamin, cysteine, proline, serine, hystidine, tyrosine.

• In stomach proteins ;1re first broken down in amino acids then are digested. Hence, amino acids are the
substances, which are never excreted out through urine under normal circumstances in any healthy individual.

• 10 g of protein may yield 5.65 kcal energy.

• Daily requirement of protein is 70-100 g.

• Excessive thinning of hair in man is due to low protein content.

• They build up various protoplasmic structures including
cell membrane.

• Main sources of protein are groundnuts, soybean, meat,
pulses, fish, egg, milk, etc.

Functions of Proteins

•Essential for growth and repair of body.

•Act as enzymes or biological catalyst in metabolic reactions.

• Antibodies are proteins, which work for the defence of body.

• Collagen protein is present in bone, tendons, cartilage and transports fatty acids and lipids in blood.

• Insulin protein helps to regulate glucose metabolism.

• Keratin is present in skin, nails, hairs, horns, etc.

• Haemoglobin, visual pigments, cytochromes are proteins.

.

5. Minerals

• Metals, non-metals and their salts arc called minerals.

• These are essential parts of enzyme (known as cofactors and prosthetic group) and vitamins.

Sodium (Na) and Potassium (K)

• These are main cations of extracellular and intracellular fluids.

• Sodium helps in absorption of glucose and electrochemical impulse conduction in nerves and
muscles.

• Potassium takes part in muscles and nerves activity, glycogen and protein synthesis.

• Deficiency of sodium and potassium causes cramps and convulsions, respectively.

• Its sources are salt, milk, vegetables, etc.

Chlorine (CI)

• It is the main anion of extracellular fluid.

• It helps in synthesis of HCl in alimentary canal for food digestion and acid-base balance.

• Deficiency of chlorine causes loss of appetite and muscle cramps.

• Its sources are salted food and sea food.


Magnesium (Mg)

• It is enzyme activator.

• It is a component of bones and teeth.

• Its deficiency produces convulsion and irregularity of metabolism.

Its sources are meat and green vegetables.

Sulphur (S)

• It is the main constituent of many proteins, enzymes ane coenzymes.

• Its deficiency disturbs protein metabolism.

• Its sources are dairy products, meat, eggs and broccoli. Cobalt (Co)

• It is a component of vitamin-B12 .

Its deficiency causes pernicious anaemia.

• Its sources are meat, yeast and milk.

Fluorine (F)

• It maintains enamel and checks dental decay or caries.

• In excess, harmful to teeth and hnnes, i.e. caused fluorosis.

• It is prescllt in n1ilk and drinking water.

• Its sources are water supplies, tea, seafood, meat, liver and beans. Calcium (Ca)

• It is a major component of bones and teeth.

• It is required for blood clotting and muscles contraction and heart functioning.

• It is required more in children and pregnant ladies.

• It is present in milk, green vegetables, gram, fish, etc.

• Its dcficien.:y causes rickets, muscles spasms and tetany Iodine (I)

• It is essential for production of thyroxine hormone from thyroid gland.
Its deficiency causes goitre.

• Its sources .ue iodised salt, fish and seafood. Phosphorus (P)

• Along with calcium, it occurs in bones and teeth. It is a component of nucleic acids, phospholipids and
ATP.

• Its deficiency reduces growth, metabolism and causes rickets in children.

• Its sources are milk, cheese, eggs, peanuts and most foods.

Iron (Fe)

• It is an important component of haemoglobin.

• These arc required more in girls (35 mg) as compare to boys (25 mg). Because blood is loss in menstrual cyele
in girls.

• Its deficiency may leads to anaemia.

• Its sources are green leafy vegetables like spinach, Chenopodium, methi, etc.

6. Vitatmins

• These are accessory food factors, required in small quantity for controlling metabolism and body
functioning. They do not provide energy.

• Vitamins were discovered by Funk in 1912. These arc of :wo types; fat soluble (A, D, E and K)
and water soluble (B-complex and C).

• Fat soublc vitamins are stored in liver.

• Eating of raw fish can cause the deficiency of vitamin-B (thiamine).

• Milk is a pool' source of vitamin-C.

7.Roughage

lndigestable fibrous material present in the food is called roughage. It provides bulk to the diet for satisfying appetite.
Bulk amount of roughage helps in expanding gut and also stimulates peristalsis to eliminate the faecal mater.
Thus, food rich in roughage may help to prevent constipation, e.g. cellulose present in the cell walls of plant material such
as vegetables, fruits and bran all contain roughage.




Nutritional Deficiencies and Imbalances

• Protein Energy Malnutrition (PEM) It causes two types of diseases-kwashiorkor and marasmus.

(i) Kwashiorkor commonly affects children. Its symptoms are retarded growth, oedema, slender legs and bulging eyes.

(ii) Marasmus commonly affects infants under one year of age. Its symptoms are mental retardation and weak body,
wrinkled skin, thin limbs, loss of weight.

• Obesity causes by excess of food fat.

• Excess of vitamin-A, D and K causes hypervitaminosis.

• Excess of saturated lipids causes hypercholesterolemia

 
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