Cell Is The Unit Of Life

About cell

The term “Cell” is derived from latin word Cella means store room or chamber. Robert Hooke first saw some structures of cork under his own microscope and termed them ‘cell’
Cell is a structural and functional unit of life.
Schleiden and Schwann proposed first cell theory.
Exception of cell theory is Virus.
Prokaryote and Eukaryote terms were coined by Dougherty (1957).
Prokaryotes  lack membranous organelles such  as plastids, mitochondria, golgi apparatus, endoplasmic reticulum, nucleus etc.
In prokaryotes instead of true nucleus a fibrilar area is present which is known as nucleoid.
Some dinoflagellates are called as mesokaryotes as they have a nuclear membrane but DNA is not associated with histone proteins.
Prokaryotes are haploid and nonsexual organisms.
Cell wall was prevents busting of cell in hypotonic solution.
Cells of all organisms have close similarity in origin, structure, molecular organization and biological activities.
Exception of cell theory are virus, viroids and prions.
Viruses are acellular structure made up of only nucleic acid surrounded by protein. These do not possess any protoplasm or organelle etc. and can reproduce only in the living cell. They are also called as ‘living chemical”.
Cell may be different shape e.g., irregular (Amoeba)’, flat, cuboid, columnar, discoide shaped (Bucillus), coma shaped (Vibrio), spiral (Spirillum), pear shape (Chlamydomonas) etc.
Acetabularia (a unicellular marine green alga) has a base, stalk and cap.
Largest unicellular plants is Acetabularia (10 cm) and animal is Amoeba (1 mm).
Smallest living cell is of PPLO or Mycoplasma gallisepticum measuring about 0.1-0.3 μm.
Number of cells in the organisms may be different. Unicellular organisms have one cell only. Some of the algae have definite number of cells e.g., Eudorina (16, 32, 64), Pandorina (8 - 16 or 72) and Tetraspora (4 cells) etc. Multicellular organisms have indefinite number of cells
Multicellular organisms have advantage of increased uninterrupted activity, division of labour. Dead cells may also be important in some cases (xylem, cork), differentiated cells may be dedifferentiated.
Cells may be undifferentiated (merismatic), differentiated (RBCs, muscle cell, gland cell etc.); dedifferentiated (wound healing, regeneration, secondary growth) etc.
Cell in multicellular organisms has dual existence as an individual, and as a part of community.
Cells may suspended or suffer irreversible loss of their basic activity e.g., mature human RBCs lose nucleus and capability of aerobic respiration.
Mature human nerve cell is not capable of division.
Liver and muscle cells divide generally by endomitosis.
Each cell functions as a self contained unit with considerable autonomy.
Isolated living cell of multicellular organisms can be cultured in nutrient media
In multicellular organisms distinct types of cells differentiate and perform different functions.
Exocytosis is reverse pinocytosis or emiocytosis.
Sea quills and Penguins secrets NaCl through their nasal glands having Na-K pump.
Plasmodesmata were discovered by Tangel but named by Strassburger.
In fungus the cell wall is made up of fungal cellulose or chitin.
The living matter of the cell is called as protoplast by Hanstein.
Golgi bodies are also called as lipochondria and are generally present in animal cells
Science of biological functions and mechanism is called Bionics.
Mammalian mitochondria has 55S ribosomes.
Vesicle like membranous bodies present between fungal cell wall and plasma membrane are called as Lomasomes
Avery, Mc Leod and Mc Carty proved that DNA is the genetic material.
The set of chromosomes inherited as a unit is called as Genome.
Protamines are basic proteins rich in arginine and associated with DNA of sperm.
Microfilaments were discovered by Paleviz et al. (1974).
Sphaeraphides are star like crystals of Ca-oxalate.
A cell with raphides is called idioblast.
Oocytes of egg of Xenopus has 1600 nucleoli
Plant cells without cell wall are motile spores (zoospores) and gametes of Algae and Fungi,
Cell organelles without membrane are e.g., Ribosome, Centrosome, Centriole, Nucleolus, cytoskeletal structures.
Cell organelles with single membrane are e.g., ER, Golgi complex, Vacuole, Lysosome, Sphaerosome, Peroxisome, Glyoxysome, Thylakoid etc.
Cell organelles with double membrane are e.g., Plastids, mitochondria, nucleus etc.
Cell organelles with triple membrane is Transosome - occurs in egg of birds.

Structure Of Cell

Prokaryotic Vs Eukaryotic cells


  Prokaryotic cells   

 Eukaryotic cells


         0.1– 5 μ    3 – 30 μ

2.Nuclear membrane


Transcription is absent.

3.DNA: RNA ratio


Euchromatin   is narrower, 30 – 80 Aº  in


Heterochromatin is thicker, 250 Aº or more.

4.[A] + [T]/[G] + [C] ratio

 0.88 (as [G]: [C] is more)

    1.52 (as [G] : [C] is less)


 Composed entirely of nuclei acids

Includes nucleic acids and proteins





Present (single loop)

Present double helix

8.Membrane-bound organelles




Rotating and lacking 9 + 2

Fixed with 9 + 2 structure


Not in chloroplasts

In chloroplasts

11.Cell wall and sugars (when wall is

Contain amino acids and

Does not contain amino acids

sugars. Usually peptidoglycan.


    Present    Absent

13.Endoplasmic reticulum

Absent  Present



Present (Smaller)

Ribosomal RNA (rRNA) about 65%


Ribosomal RNA (rRNA) about 45%














Bacteria, Cyanobacteria,

Archaebacteria, Mycoplasma

(MLB/PPLO) and Rickettssiae.

Cells of all the higher

Mycoplasma organisms

Layers of cell wall

Plants, cells are joined together like the rooms of a building. Such cells can have upto three parts:

1.Middle Lamella

2.Primary wall

3.Secondary wall

  • Middle lamella: It is thin amorphous intercellular matrix between two adjacent plant cells that functions as cementing layer between them. It is considered to be outermost layer of wall. It is chiefly made up of calcium pectate.


Primary wall

Secondary wall


Primary wall is the first formed wall of the cell.

Secondary wall is later formed wall of cell.


It is laid inner to middle lamella when the latter is present.

It is laid inner to primary wall.


Primary wall is single layered.

Secondary wall is made of three or more layers.


It occurs in all types of plant cells.

Secondary wall is found in some specialized cells.


It is present in growing cells.

The wall is formed after the stoppage of cell growth.


Pits absent, though pores can occurs.

Pits commonly occur in the secondary wall.


Lignin is commonly absent.

Lignin is often present.


Amount of cellulose is lower, about

Amount of cellulose is higher, 20 - 40%.

  • Tertiary wall: It is a special xylan-rich wall deposited inner to the secondary wall in tension wood of some gymnosperms.

Types of Membrane Proteins

S. No


Intrinsic proteins


Constitute 70% of the membrane proteins.

Constitute 30% of the membrane proteins.


Proteins present superficially.

Proteins are completely embedded in the lipid layer and some are partially projected on the surface.


Proteins are made of about 12 to 14 types of amino acids.

Proteins are made of more than 20 types of amino acids.


Molecular weight is about 1400.

Molecular weight is approximately 8000.


Proteins are easily displaced by mild detergents.

Proteins cannot easily separated from the membrane.


Lipid and extrinsic proteins form a semi fluid compound.

Lipids and intrinsic proteins form a mosaic.

Chemical Compositions





1. Mouse Liver Cell Plasmalemma




2. Human Erythrocyte Plasmalemma




3. Mitochondrial Outer Membrane




4. Mitochondrial Inner Membrane




Difference between endocytosis and exocytosis

S. No.




It is bulk intake of extracellular materials.

It is bulk exit of intracellular materials.


The endocytotic vesicle is formed by the cell membrane.

The exocytotic vesicle is formed internally mostly from Golgi apparatus or food vacuole.


It is meant for obtaining useful materials from outside.

Exocytosis is related to excretion, secretion and throwing of indigestible materials.


A lysosome often fuses with   endocytotic vesicle.

A   lysosome does not fuse with exocytotic vesicle.


Endocytotic vesicle bursts open or fuses with an internal membrane for releasing materials.

Exocytotic vesicle fuses with cell membrane for throwing the materials to the outside.

Difference Between Pinocytosis And Phagocytosis

S. No.




It is the intake of extracellular fluid and its contained solutes with the help of vesicles or pinosomes.

Phagocytosis is the ingestion of large sized solid particles with the help of vesicles phagosomes.


Receptor sites may or may not be present.

Receptor sites are required to recognize the particle.


The cell membrane does not develop evaginations around the fluid drop.

The cell membrane produces pseudopodia like evaginations around the solid particles for engulfing the same.


It may or may not involve digestion with the help of lysosome.

Lysosomes are essential  for killing an digestion of solid food materials.


An exocytosis is not required.

The undigested materials are thrown out by means of exocytosis.

Movement of molecules






Passive Transport (No Cellular Energy Required)


1.  Diffusion

Movement of molecules from an area of higher to one of lower concentration; due to constant motion of molecules.

Lipid soluble molecules, water gases


2.  Facilitated diffusion

Molecules are carried across membrane by a carrier (Permease) from an area of higher to one of lower concentration.

Sugars and amino acids


3.  Bulk flow

Mass movement of fluids from an area of greater water potential to one of lesser water potential


4.  Osmosis

Diffusion of water across a semipermeable   membrane from an area of higher water concentration (lower solute concentration) to one of lower   water concentration (higher solute concentration)

Resins swell up. Grapes contract


Active Transport (Cellular Energy Required)


1.Facilitated active


Carrier   molecule carries   substance across membrane, can be toward an area of higher concentration

Sugars, amino acids and ions


2. Endocytosis Pinocytosis

Liquids are moved into cell by being surrounded by plasmamembrane and pinched off into vacuole.



3.  Phagocytosis

Solids are moved into cell by being surrounded by plasmamembrane and pinched of into vacuole.

Cell and sub-cellular material


4.  Exocytosis

The   cell expels   materials as the membrane surrounded those materials fused with the plasma membrane after which both membrane rupture.


Difference between cytoplasm and nucleoplasm

S. No.




It is part of protoplasm excluding the nucleus.

It is present inside the nucleus.


Cytoplasm consists of cytosol, cytoplasmic organelles.

Nucleoplasm is equivalent to cytosol of cytoplasm.


Cytoplasm is covered on the outside by a singly membrane called plasmalemma.

Nucleoplasm is covered on the outside by a double membrane covering called nucleur envelope or nucleolemma.


Cytoplasm shows cyclosis or streaming movement.

Cyclosis is absent.


It contains raw materials for synthesis of all types of chemicals.

Nucleoplasm contains raw materials for synthesis of RNA, DNA and ribosome subunits.


Types Of Chromation And Their Differences






It is normal chromatin which possesses active


It is modified chromatin that is devoid of active genes.



Euchromatin   takes part in   transcription or

formation of RNAs.

Transcription is absent.



Euchromatin   is narrower, 30 – 80 Aº  in


Heterochromatin is thicker, 250 Aº or more.


It is fibrous.

Heterochromatin is granular.


Euchromatin represents the bulk of chromatin.

It represents only a part of chromatin.


It is lightly stained.

Heterochromatin is darkly stained.


Euchromatin appears diffused.

It is condensed.


There is little coiling nucleosome stand.

Heterochromatin is formed by solenoid type of coiling of nucleosome strand.


It is influenced by changes in pH, temperature, hormones, etc.

There is little effect of these factors.


It shows normal crossing over.

Heterochromaticn inhibits crossing over.


Euchromatin replicates early in S-phase of cell cycle.

It replicates late towards end of  
S-phase which leads into G2 phase.

Stages of mitosis

Stage of meiosis

Comparison between mitosis and meiosis



The cell divides only once.

There are two cell divisions, the first and the second meiotic divisions.

Mitosis takes place for the somatic growth of the body.

Meiosis takes place in either pre or post reproductive cells.

Occurs in both  sexually as well  as asexually reproducing organisms.

Occurs only in sexually reproducing organisms.

DNA replication takes place during interphase I.

DNA replication takes place during interphase I but not interphase II.

The DNA replicates once for one cell division.

The DNA replicates once for two cell divisions.

The duration of prophase is short, usually of a few hours.

Prophase is comparatively longer and may take days.

Prophase is comparatively simple.

Prophase is complicated and is divided into leptotene, zygotene, pachytene, diplotene and diakinesis.

The cell divides only once and the chromosomes also divide only once.

There are two cell divisions but the chromosomes divide only once.

There is no synapsis.

Synapsis of homologous chromosomes takes place during prophase.

The two chromatids of a chromosome do not  change segments during prophase.

Chromatids of two homologous chromosomes exchange segments during crossing over.

Each  chromosome  consists of two  chromatids united by a centromere.

The two homologous chromosomes form bivalents or tetrads. Each bivalent has four chromatids and two centromeres.

In the metaphase plate all the centromeres line up in the same plane.

In metaphase I the centromeres are lined up in two planes which are parallel to one other.

The metaphase plate is made up of chromosome pairs.

The metaphase plate is made up of paired chromosome pairs.

Division of the centromeres takes place during anaphase.

There is no centromere division during anaphase I. Centromeres divide only during anaphase II.

The chromosomes separate simultaneously during anaphase.

Short chromosomes separate early; separation of long chromosomes is delayed.

Spindle fibres disappear completely in telophase.

Spindle fibres do not disappear completely during telophase I.

Nucleoli reappear at telophase.

Nucleoli do not reappear in telophase I.

The chromosome number remains constant at the end of mitosis.

The chromosomal number is reduced from the diploid to the haploid.

The genetic constitution of the daughter cells is identical to that of parent cells.

The genetic constitution of the daughter cells differs from that of the parent cell. The chromosomes of daughter cells usually contain a mixture of maternal and paternal genes.


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