CELL BIOLOGY

Father of Cytology – Robert Hook
Rudolf Virchow proposed – Theory of cell lineage, Omnis cellula–e cellulae (all cells living cells are arises from pre-existing cells)
The cell is the structural, functional, and fundamental unit of an organism.

Cell theory – A cell is a basic unit of life that arises from pre-existing cells.
· Viruses are the exception to the cell theory.

Cellular totipotency
· Cellular totipotency given by Haberlandts and Experimentally proved by Stewart et all.
· Ability of a vegetative cell to grow into a new plant.

Types of Cell

A. Prokaryotic cell: (Pro – Primitive, Karyon: nucleus)
· True nucleus is absent.
· Nucleus is an incipient type called Nucleoid. i.e, that lacks Nuclear membrane, Nuceloplasm and Nucleolus.
Note: Chondroid – primitive mitochondria, Viroid – primitive virus. (Viroids are the smallest infectious agent made up of nucleic acid only)
· DNA is single circular, double-stranded, non-repetitive (Absence of non-sense codon) and without histone protein.
· Membrane bounded cell organelles as mitochondria, chloroplast, nucleus etc. are absent.
· Ribosome is 70S (50S + 30S) type.
· Cell division is amitotic type (mitosis and meiosis absent).
· Respiratory and Photosynthetic enzymes are present in the plasma membrane
· Cilia and flagella are present but they lack 9 + 2 arrangement and tubulin protein, but flagellin is present instead of tubulin.
E.g. Bacteria and Blue-green algae (Cyanobacteria)


B. Eukaryotic Cell

· True nucleus is present which contains nuclear membrane, nucleolus, and nucleoplasm.
· DNA is linear, double-stranded, repetitive (Presence of non-sense codon) with his tone protein
· Membrane bounded cell organelles like mitochondria; chloroplast, lysosome etc. are present.
NOTE: Ribosome is 70S (50S + 30S) type in mitochondria and chloroplast and 80S (60S + 40 S) type in the nucleus, 
80S = cytoplasmic Ribosomes, 70S = organelles.
· Cell division – Mitosis and Meiosis type
· Respiratory and photosynthetic enzymes are located in mitochondria and chloroplast respectively.

C. Mesokaryotic Cell
· Behaving like both Prokaryotic and Eukaryotic cells.
· Nuclear membrane is present. (Eukaryotic)
· DNA is without Histone. (Prokaryotic) 
E.g. class Dinophyceae of algae.


Difference Between Prokaryotic and Eukaryotic cell

Characteristic Prokaryotic Cells Eukaryotic Cells
Nucleus No Yes
Membrane-Bound Organelles No Yes
Size of Ribosomes 70s 80s
Cell wall composition Peptidoglycan is present No peptidoglycan
Mitotic division No Yes
DNA associated with histones No Yes
Number of chromosomes One More than one
Cell membrane composition No sterols (except in mycoplasmas) Sterols present
Number of cells Usually unicellular Usually multicellular
Size of cell Smaller (1-5 µm) Larger (10 - 100 µm)


Differences between Plant and Animal cells
Animal cell Plant cell
1. Cell wall absent. Cellulose in any form is also absent. Cellulose cell wall is present in plant cells.
2. Cytoplasm is denser, more granular and occupies most of the space in the cell. The cytoplasm is pushed to the periphery and forms a thin lining against the cell wall.
3. Vacuoles absent. If present, they are small, temporary and concerned with excretion or secretion. Vacuoles are large and prominent. Maybe one or more.
4. Plastids are absent. Plastids are generally present.
5. Centrosome is present with one or two centrioles. The centrosome is absent but two small clear areas called polar caps are present. These participate in cell division.
6. Prominent and highly complex Golgi bodies present near the nucleus. Several subunits of the Golgi apparatus called dictyosomes are present.
7. Reserve food stored in the form of glycogen. Reserve food stored in the form of starch.


Fig: Showing plant cell structure


1. Cell Wall

· Common in plants, bacteria, cyanobacteria and fungi
· Is dead and permeable – 1st observed by Robert Hooke (1665)
· In bacteria and cyanobacteria, the cell wall is made up of Peptidoglycan or Murein or Mucopeptide or Muramic acid
· Bacteria is the smallest organism with cell wall
· In fungi, it is made up of chitin or fungal cellulose
· In higher plants, it primarily contains cellulose and pectin suberin, lignin, etc. are other components

In higher plants, the cell wall consists of
I. Middle lamellae.
· Common wall between adjacent cells.
· Made up of calcium pectate (mainly) and magnesium pectate
· It holds the neighbouring cell walls together.

II. Primary cell wall
· Composition: cellulose and hemicellulose. 
· lies inside middle lamellae
· Meristematic and parenchymatous cells have this type of wall only

III Secondary cell wall:

· It builds up by the addition of extra cellulose layers on the inside surface of the primary cell wall.
· Made up of cellulose and hemicelluloses


Fig: Cell wall
IV. Plasmodesmata
· Reported by Strasburger (1901 A.D)
· Intercom of cell
· Protoplasmic connection or cytoplasmic bridge between two adjacent cells.
· Exchange of materials takes place through it.
· Modification of cell membrane
Note: Dead cells are interconnected by pits.

Chemical composition of Cell Wall:
a. Cellulose – Polymer of β - glucose.
· Cellulose – most abundant carbohydrate in the biosphere
· Rubisco – the most abundant protein in the biosphere
· Structural unit of the cell wall is cellulose
b. Hemicellulose – Binding material in cell wall
c. Pectin – in collenchyma
d. Cuticle
· Found in the exposed part of the plants
· Thick layered in Xerophytes
· Thin layer or absence in Hydrophytes
e. Suberin – in endodermal cells and in cork cell
f. Lignin
· Provided tensile strength
· Found in Sclerenchymatous cells, Tracheids, and vessels.
g. Silica – Leaf cell of grasses
· Nature of cell wall is fully permeable except cork cell wall, which is impermeable due to the deposition of suberin.
Function – Gives mechanical support i.e. exoskeleton of the cell.


2. Cell membrane / Plasma membrane

· Forms outermost boundary of animal cells
· Made up of phospholipids and proteins.
· Often called a biological membrane.
Different models proposed are
1.  Unit Membrane model- By Robertson
2. Trilaminar or Sandwich model
- By Danillae and Davson
3. Fluid mosaic model - By Singer and Nicholson
· Cell membrane is modified into Mesosome in bacteria
· Mesosome – A finger like the one project found in bacteria that helps in respiration
· Semi-permeable in nature
· Size of the molecule that can pass through the membrane is 1 – 15 Å
· It shows Bulk transport

Bulk transport
(a) Exocytosis
· Removal of materials from cell to outside
· It is also called the cell-vomiting phenomenon

(b) Endocytosis
Entry of materials from outside the cell
(i) Phagocytosis
· Entry of solid food
· It is also called the cell-eating process
(ii) Pinocytosis
· Entry of liquids
· It is also called the cell-drinking process


3. Protoplasm

· Protoplasm = Cytoplasm + Nucleoplasm
· Dujardian used the term – Sarcode
· Term protoplasm – by Purkinje
· Huxley defined it as the Physical basis of life
· It mainly contains water (75 – 85%)
· It coagulates above 60 – 70°C.
· Cytoplasm = matrix + cell organelles
· Cytoplasm without cell organelles is Hyaloplasm or Cytosol
· Cytoplasm is the site for glycolysis, synthesis of fatty acids, nucleotide

Largest Component of the Cell – Nucleus
Largest Cell organelles in Plants – Chloroplast
Largest Cell organelles in Animals – Mitochondria
Smallest cell organelles – Ribosome


Membranes present in the Cell organelles:
A. The membrane is absent in
· Ribosomes
· Centrioles (Microtubules + Microfilament)
· Nucleolus
B. Double membrane is found in
1. Mitochondria
2. Chloroplast
3. Nucleolus
C. Single membrane-bounded organelles
a. Endoplasmic Reticulum (ER)
b. Golgi body
c. Lysosome
d. Peroxisomes
e. Glyoxisome
f. Sphaerosomes, etc
The vacuole is also a single membrane-bounded but sometimes it is not considered a cell organelle.


Cell organelles

1. ER (Endoplasmic Reticulum):
· Cytoskeleton or Endoskeleton of a cell
· 1st observed by Porter
Consists of
(a) Cisternae – are not parallel to each other
(b) Tubules
(c) Vesicles
· It is a continuous structure between Nuclear membrane and Cell membrane
· ER continue from one cell to another cell by means of a pore called Desmotubules

Types of ER
(i) Rough ER (RER)
· Ribosome is attached to ER
· Helps in protein synthesis

(ii) Smooth ER (SER)

· Ribosome is absent.
· Helps in the synthesis of fat and lipids

Functions of ER:

· Forms intracellular transport system
· Protein synthesis (RER), lipid synthesis (SER)
· Formation of Golgi body

Fig: Showing endoplasmic reticulum

2. Golgi Bodies

· Also known as Dictyosome, the Traffic police of cell.
· Discovered by - Camilo Golgi
· Is absent in prokaryotes, RBCs, Sieve tube.
Consists of
(a) Cisternae
(b) Tubules
(c) Vesicles
· Vesicles of SER form Golgi body.
· It helps in, the formation of cell plates (Especially vesicles)
· During cytokinesis, spindle protein + vesicles of Golgi body forms phragmoplast, which later changes into cell plate
· Helps in acrosome formation in sperm.
· Secretion of mucilage in root tip region that acts as a lubricant for root penetration.
· Formation of Nissle's granules (In the cyton of a nerve cell)

Fig: Golgi apparatus


3. Lysosomes
· Discovered by C. De Duve
· Formed from Golgi body
· Also called suicidal bag of the cell
· Rich in hydrolytic enzyme, contains about 50 types of hydrolytic enzymes.
· Hydrolytic enzyme helps in hydrolysis i.e., digestion
· Common in animals
- Primary lysosome – formed from Golgi body.
- Secondary lysosome – primary lysosome + food vacuole
- Autophagic vacuole – when a lysosome contains a part of its own cell (generally unwanted structure) and digests it, it is called the Autophagic vacuole
- Residual body – Lysosome with indigestible materials which, helps in exo-cytosis.

Function of Lysosomes:
· Extracellular and intracellular digestion.
· Digestion of harmful substances.
· Involves in Autolysis
Autolysis: self-destruction during starvation


4. Peroxisome/Microbodies
Contains enzyme Peroxidase and Catalase
Peroxidase: Helps in H2O2 formation.
Catalase: Helps in breakdown of H2O2
· Catalases are the fastest acting enzyme.
· In plants peroxisomes are involved in photorespiration.
· In animals, they involve in β - oxidation of fatty acid.
Note: β- oxidation of fatty acid means conversion of fatty acids into acetyl COA.


5. Glyoxisome

· Contains enzyme of Glyoxalate cycle
· Glyoxalate cycle: Conversion of fatty acids into carbohydrates in plants.
· Glyoxalate cycle is an example of Glyconeogenesis
· Gluconeogenesis (formation of glucose from Non-carbohydrate source)
· They are found in only germinating seeds


6. Sphaerosome
· Commonly called plant lysosome.
· Helps in synthesis and storage of lipids.
· They are common in cotyledons and endosperm of oily seeds.


7. Vacuole
· common in plants.
· Vacuolar membrane is called Tonoplast (Semipermiable)
· Vacuolar sap, present in the vacuole, is called cell sap
Cell sap contains
(a) Water and minerals
(b) Pigments
· Anthocyanin is found in vacuole which is water-soluble
· Anthocyanin pigment is also called a vegetable chameleon.
· Different colours (Red, Blue, and Purple) of flower are due to Anthocyanin pigment)
· Red colour of Beetroot (Beta vulgaris) is due to anthocyanin
· It helps in Osmoregulation (water regulation) in protozoans. It contains the waste product or acts as food reserves


8. Plastid
· Discovered by Haeckel
· Found in plant cell only
· Is the semi-autonomous unit
· Develops from pro plastid found in meristematic regions.

A. Chloroplast
Common in almost all photosynthetic organisms except blue-green algae and photosynthetic bacteria.
In the case of photosynthetic bacteria and cyanobacteria, pigments are found in chromatophores or photosynthetic lamella.
· Chloroplast contains chlorophyll and carotenoid pigments.

Various Shape of Chloroplast
a. Cup-shaped – in Chlamydomonas
b. Spiral (Ribbon shaped) – in Spirogyra
c. Reticulate (Net shaped) – in Oedogonium
d. Star-shaped – in Zygnema
e. Girdle shaped or Horseshoe-shaped – in Ulothrix
f. Spherical chloroplast – in Chlorella

Number of Chloroplast Present:
  • Single in – Chlamydomonas
  • Maximum in – mesophyll cell of the leaf
· Double-walled structure, the chamber between the outer two layers are called the outer chamber. Inner membrane encloses inner chamber.
· Internal structure consists of grana, stroma, inter-granal lamellae/stroma lamellae etc.

b. Stroma
· All enzymes of dark reactions are found in stroma.
· It is the ground substance of chloroplast
· It can synthesize a few proteins
· It contains DNA, RNA and ribosome (70S). This DNA helps in the division of chloroplast to some extent but not completely, so it is called a semi-autonomous unit.

c. Grana

· Unit structure of grana is thylakoid
· Thylakoids contains no of Quantasomes or photosynthetic unit.
· Thylakoid is the structural unit of the chloroplast.
· It is the site for light reaction in photosynthesis
· Quntasomes is the functional unit of the chloroplast.
· Each Quatasome contains 280 photosynthetic pigments
· Grana are interconnected by means of stroma lamella or fret membrane.
280 photosynthetic pigments include:
a) 50 carotenoids
b) 230 chlorophylls
i) 160 Chl. a
ii) 70 Chl. b

Various Pigments in Details:
1. Chlorophyll a
· C55H72O5N4Mg
· Functional group – CH3
· Molecular weight – 893
· Main or universal photosynthetic pigment
· Present in all except Bacteria.

2. Chlorophyll b
· C55H70O6N4Mg
· Molecular weight – 907
· Functional group – CHO

3. Carotenoids
a. Carotene – orange and red color
C40H56
E.g. Lycopene, Capsanthin

b. Xanthophylls
- yellow coloured
C40H56O2

B. Chromoplast
· Colored plastid
· Formed either from chloroplast or from leucoplast.
· Conversion of chloroplast into chloroplast e.g., conversion of green tomatoes to red and green chillies to red
· Conversion of leucoplast into chromoplast. e.g. carrot
· Green chillies changed into red due to pigment capsanthin
· Green tomato changes into the red due to Pigment lycopene.
Other coloured plastids are
a. Phaeoplast- found in brown algae, main pigment phaeoxanthin
b. Rhodoplast – Red plastid found in Red algae. The main pigment is phycoerythrin.
c. Phycocyanin- Blue-green pigment in cyanobacteria (BGA)

C. Leucoplast
· Colorless plastid
· Stores reserve food materials.
· Usually found in underground parts of plants.
Types of Leucoplast:
i) Amyloplast- stores carbohydrates or starch egg. i.e. Rice, wheat, potato etc.
ii) Protinoplast or Aleuronoplast - stores protein - e.g. maize grains
iii) Elaioplast (or oleosomes): stores lipids coil) as in cells of endosperm in caster.

Fig: Showing plastid


9. Mitochondria
· Powerhouse of the cell
· Semi-Autonomous unit
· Autonomous unit – Nucleus
· Semi-Autonomous unit – Mitochondria and Chloroplast
· Altman named them as Biplasts.
· Discovered by Kolliker
· Benda gave the name mitochondria.
· DNA is circular and double-stranded & Ribosomes are 70s type
· Found in all except RBCS and prokaryotes.
· Double membranous structure consists of two chambers i.e., outer chamber, and inner chamber
· Inner membrane projected into the central space in the form of a finger-like projection called cristae. This membrane contains enzymes of the electron transport chain.
· Each crista contains elementary particles or oxysomes.
· Oxisomes are called the functional unit of mitochondria
· Oxisomes contains
a) Head
b) stalk
c) Base
a. Head:
· Contains F1 particles site for ETS (electron transport system)
· Inner membrane encloses inner chamber, which is called matrix. Which is filled with DNA, ribosomes (70S) most of the catabolic enzymes controlling the Krebs cycle and fatty acid oxidation.
· Matrix is the site for Krebs cycle.
· About 70% of the total cell enzymes are found in mitochondria.
· Metabolically most active cell organelle is mitochondria.
· Mitochondria involves in cellular respiration
· All the mitochondria present in the cell are collectively called Chondriome
Functions of Mitochondria:
· Site for Krebs cycle in respiration contains all the enzymes, which brings about oxidative phosphorylation (formation of ATP)
· Forms middle piece of sperm (the engine of sperm)

Fig: Mitochondria


10. Ribosomes:
· Protein factory of the cell.
· Smallest non-membranous cytoplasmic organelle
· Made up of – rRNA and protein.
· They are formed inside Nucleolus.
· rRNA synthesized in the nucleus.
· They are isolated from other cell organelles by the process of fractionation and ultracentrifugation.
· Measuring unit of the ribosome – Svedberg unit (s) to measure the sedimentation speed.
Various Type of Ribosomes:
A. 70s (50s + 30s)
· Found in prokaryotes mitochondria and chloroplast of eukaryotes

B. 80s (60s + 40s )
· Found in Eukaryotic Nucleus
· Two ribosomal subunits are joined by Mg+2
· During protein synthesis a number of ribosomes are found to attach within RNA called polysome.
· Ribosome provides the site for protein synthesis.

Fig: Showing Ribosome


11. Centrioles:
· Common in animals and lower plants but absent in higher plants.
· Always found in pair called Diplosome.
· Centrospheres and centrioles (perpendicular to each other) form centrosomes.
· They are made up of protein tubulin
· Centriole helps in spindle fibre and astral rays' formation during cell division.
· The cell of higher plants do not have centrioles but still form spindle fibres (microtubules are responsible for spindle fibre formation).
· Arrangement of microtubules in centriole and structure of the basal body is (9+0) arrangement
· Centrioles give rise to basal bodies, which give rise to cilia and flagella.


12. Cilia and Flagella:
· Eukaryotic cilia and flagella are made up of protein tubulin.
· Arrangement of microtubule: 9+2 in eukaryotes but in prokaryotes, they are made up of protein-flagellin so there are no microtubules and 9+2 arrangement.


13. Nucleus:
· Controlling the centre of the cell.
· Discovered by Robert Brown.
· In prokaryotes, the nucleus is without a distinct nuclear membrane.
· In eukaryotic cells except for mature phloem sieve tube elements and mature RBC of mammals.

Made up of
(i) Nuclear membrane
· Often called Nuclear envelop or Karyotheca.
· It is a perforated double membranous structure that encloses nuclear content.

(ii) Nucleoplasm
· Ground substance of Nucleus.

(iii) Nucleolus
· Membrane less structure.
· Also called ribosomal factory of the cell. however, protein synthesis occurs inside the nucleus.
· Made up of DNA, RNA and protein.
· Formed spindle fibre during the cell division.

(iv) Chromatin
· Composed of coils of DNA bound to a basic protein called his tones.
· During cell division, Chromatin Network condenses into a thread or a rod-like structure called a chromosome.
· Waldeyer coined the term chromosome Johnson coined the term gene.
· Morgan: Discovered that genes are Linearly arranged in the chromosome
· Best stage for the study of chromosome – Metaphase.
· Interphase – is the best stage for the nuclear study.
· In ultrastructure – chromatin appears like beads on a string.
· Beaded structure is called Nucleosome. (DNA + histone)
· chromatin is the most stainable part of the nucleus, which is stained by Acetocarmine

Types of Chromosomes:
(i) HeterochromatinDark stained chromatin.
· That contains genetically inactive genes.

(ii) Euchromatin – lightly stained region of the chromosome which contains genetically active genes

· Each chromosome consists of an Arm and Centromere.
· Arm + Centromere = chromatid.
· Prophase and Metaphase stage of cell division, consists of two chromatids.
· At anaphase, each chromosome consists of one chromatid.
· Shape of the chromosome is studied during anaphase
· Number of chromosomes is studied during metaphase.
· Characteristics of chromosome (no, shape, the position of Centromere, size of arm) of an organism is karyotype.
· Diagrammatic representation of Karyotype in a sequence is known as idiogram.

Structure of Chromosome:
1. Telomere – terminal non–sticky end of the chromosome.
2. Pellicle – the sheath of the chromosome.
3. Centromere - primary constriction.
· It is an unstained region of chromosome
· Has plate-like structure – kinetochore
· Spindle fibres are attached to the kinetochore.
4. Secondary constriction:
· Nucleolus is developed at this point.
· It is often called NOR (Nucleolar organizer Region)
5. Satellite:
· It is the knob like structure near secondary constriction.
6. Chromonemata – DNA fibre inside the chromosome.
· Genome – the total no. of chromosomes present in the haploid cell or haploid set of chromosomes.

Type of chromosome based on the position of Centromere:
(a) Telocentric.
· Centromere is terminal
· Chromosome with a single arm.
· "I" shaped or rod-shaped.
(b) Acrocentric
· Centromere is near the terminal end.
· 'j' shaped
(c) Sub metacentric
· Centromere near the centre
· 'L' shaped.
(d) Metacentric
· Centromere at the centre.
· Arms are equal-sized.
(e) Acentric
· Chromosome without centromere.

· Autosome carries genes for the determination of somatic characters.
· 22 pairs of Autosomes in human

Sex chromosome or Heterosome or Allosome
· Takes part in sex determination.
· 1 pair of sex chromosomes in man.
· XX in female XY in male.
· Sex of the foetus is determined by 'Y' of sperm

· Holoandric gene: genes present in the 'Y' chromosome of male
E.g. gene of hypertrichosis (hair in-ear)


Also, Read Notes of other Lessons in Botany:
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