Cell as a unit of life.
- Cytology : (G.k. kyios = cell ; logas = study) is the branch of biology. Which comprises the study of cell structure and “Cell is the structure and functional unit of all living beings”.
All living organisms are composed of repeated structural units called cells. Each cell is independent in performing all necessary processes of life and is the least complex unit of matter which can be called living. Robert Hooke (1665) discovered hollow cavities (empty boxes) like compartments in a very thin slice of cork (cell wall) under his microscope. He wrote a book “Micrographia” and coined the term cellula, which was later changed into cell. Grew and Malpighi also observed small structures in slice of plants and animals. Leeuwenhoek was the first to see free cells. He observed bacteria, protozoa, RBCs, sperms, etc. under his microscope.
- Cell theory : J. Dutrochet (1924) a French worker gave the idea of cell theory.
The actual credit for cell theory goes to two German scientists, a Botanist M.J. Schleiden (1838) and a Zoologist T. Schwann(1839).They gave the concept “all living organisms are composed of cell”. Schleiden and Schwann both supported the theory of “spontaneous generation”. They also mentioned that “the new cell arises from nucleus by budding”. Main postulates of cell theory are :
- Living beings are made of They may be unicellular, colonial or multicellular.
- Cell is a mass of protoplasm having
- Cells are similar in structure and
- The functions of an organism are due to activities and interactions of
- Exceptions to the cell theory : Viruses, viroids and prions are an exception to the cell theory as they are obligate parasites (sub–cellular in nature). Paramecium, Rhizopus, Vaucheria are some examples, which may or may not be exceptions to the cell
- Modification of cell theory : Modification of cell theory was done by Rudolf Virchow (1885). He proposed the “law of cell lineage” which states that cell originates from pre-existing cells. e. (omnis cellula-e- cellula). It is also called “cell principle” or “cell doctrine”. It states : –
- Life exists only in
- Membrane bound cell organelles of the protoplasm do not survive alone or outside the
- Cells never arise de novo. The new cells are like the parent cell in all respect.
- All cells have similar fundamental structure and metabolic
- Cells display homeostasis and remain alive.
- Functions of an organism as a whole are the sums of the activities and interactions of its constituent cell An organism can not show functions which is absent in its cells.
- Genetic information is stored in DNA and expressed within the
- DNA controls structure and working of a
- The cell as a self contained unit : Autonomy of a cell is believed due to presence of DNA and its expressibility, otherwise, cell components have different shape and It has two positions.
- Autonomy in unicellular organisms : Unicellular organisms lead to a totally independent life due to different shape, size and role of different organelles shows division of labour. All these display homeostasis. Unicellular organisms are more active due to large surface volume
- Autonomy in multicellular organisms : In multicellular organisms life activities are displayed by each of the cells Multicellular organisms have one thing advantage over unicellular organisms is division of labour.
- Cellular totipotency : Totipotency was suggested by Haberlandt (1902). When cells have tendency or ability to divide and redivide the condition of the cell is called totipotent and this phenomenon is called totipotency.
- Steward’s experiment : Steward al. showed the phenomenon of cellular totipotency in carrot culture. Small fragments (phloem) of mature carrot roots were placed in liquid medium in special containers and growth factors like “coconut milk” was added. The culture developed into clumps or embryoids. When these were shifted to semisolid media, full plants were formed. The plants flowered normally and even bore the seeds.
- Surface volume ratio : Metabolically active cells are small, as small cells have higher nucleocytoplasmic ratio for better control and higher surface volume ratio for quicker exchange of materials between the cell and its outside environment. Larger cells have lower surface volume ratio as well as lower nucleocytoplasmic ratio. Surface volume ratio decreases by one half if cell size
Differences between plant cell and animal cell
|Plant cell||Animal cell|
|Cell wall present.||Cell wall absent.|
|Nucleus usually lies near periphery due to vacuole.||Nucleus present near the centre.|
|Centrosome is usually absent from higher plant cells, except lower motile cells.||Usually centrosome is present that helps in formation of spindle fibres.|
|Plastids are present, except fungi.||Plastids are absent.|
|Mitochondria is generally spherical or oval in shape.||Generally tubular in shape.|
|Single large central vacuole is present.||Many vacuoles occurs, which are smaller in size.|
|Number of mitochondria from 200 – 2000.||Number of mitochondria is approximately 1600 – 16000 in liver cells.|
|Cytoplasm during cell division usually divides by cell plate method.||Cytoplasm divides by furrowing or cleavage method.|
|Plant cells are capable of forming all the amino acids
coenzymes and vitamins.
|Animal cells cannot form all the amino acids, coenzymes and
|There is no contractile vacuole.||Contractile vacuole may occur to pump excess water.|
|Sodium chloride is toxic to plant cells.||Tissue fluid containing sodium chloride bathes the animal cells.|
|Plant cells are generally well over 100 mm long.||Generally much smaller than 100 mm.|
|Spindle formed during cell division is anastral.||Spindle formed during cell division are amphiastral.|
|Lysosomes present in less number.||Lysosomes present in more number.|
|Chromosomes are larger in size.||Chromosomes are smaller in size.|
- Jan swammerdam : First to see red blood cells of
- Marcello Malpighi : Observed small utricles in slice of plant and animal
- Grew : Initiated cell concept
- Lamarck : All living beings are formed of
- Corti : First to point out living substance filled inside the It was called “Sarcode” by Dujardin.
- In vivo (in life) study : Study of cells in their natural environment within the intact
- In vitro (cultural condition) study : Study of isolated life system in laboratory and cultural condition .
- Max Shultze proposed protoplasm
- Sachs proposed organismic
- Crystallo : colloidal theory (Fischer), substances dispersed and dissolved in water forming both true solution as well as colloidal
- Energy transducers : Photosynthetic cells are called energy transducers because they convert radiant energy to chemical energy and store it as food
- Intrinsic information is primary while hormonal information is extrinsic and secondary information.
- Largest organelles is Largest cytoplasmic organelle is mitochondria in animal cells and chloroplast in plant cell.
- Smallest component is microfilament but smallest organelle is
- Viruses do not have cellular
- Monerians and protistians are not divisible into cells they are rather
- Certain organisms are multinucleated , Rhizopus, Vaucheria, etc.
- Fibre of ramie, Boehameria nivea longest plant cell (55 cm in size).
- The shrunken state of RBC caused by exosmosis is called
- In human beings cell of kidney are smallest and of nerve fibre
- Pyrenoid is a proteinaceous body around which starch is stored in green
- The smallest cell considered so far is of PPLO (Pleuropneumonia like organisms) or Mycoplasma gallisepticum e. 0.1 m.
- The largest cell is an egg of
- Acetabularia a unicellular green alga is about 10 cm in
- In the alga caulerpa (Siphonales) the length of cell may be up to one
- The bacteriophages or viruses are still smaller in size (but cannot be considered as cells because of sub – cellular nature).
Structure of the cell .
- Study of cell is called
- Study of metabolic aspects of cell component is called cell
- Leeuwenhoek : First to see free cells called them “wild animalcules” and published a book “The secret of nature”.
- Robert Hooke is known as father of
- P. Swanson is known as father of modern cytology/ cell doctrine.
- K. Sharma is known as father of cytology in India.
- Dougherty classified cells based on plan as prokaryotic and eukaryotic.
- Mesokaryon : Dodge gave the term ‘Mesokaryon’ for These are intermediate type of cell organisation in dinophyceae of algae. In mesokaryotic there is present a true or eukaryotic nucleus with definite nuclear membrane and chromosomes. Chromosomes are not well organised and basic proteins or histones are absent. Nuclear membrane is persistent during cell division. Chromosomes are permanently attached to nuclear membrane. They show dinomitosis e.g.– Dinophysis Heterocapsa, Dinothrix etc.
- Types of cell : Chatton gave the term prokaryote and Depending upon the nature of nucleus cells are classified. A primitive ill defined or incipient nucleus is present in prokaryotes, where as in eukaryotes. Well organised nucleus is present.
Differences between Prokaryotic and Eukaryotic cell
|Prokaryotic cell||Eukaryotic cell|
|It is a single membrane system.||It is a double membrane system.|
|Cell wall surrounds the plasma membrane.||Cell wall surrounds the plasma membrane in some protists, most fungi and all plant cell. Animal cell lack it.|
|Cell wall composed of peptidoglycans. Strengthening material is mureir.||It is composed of polysaccharide. Strengthening material is chitin in fungi & cellulose in others plants.|
|Cell membrane bears respiratory enzymes.||It lacks respiratory enzymes.|
|Cytoplasm lacks cell organelles e.g., Mitochondria, ER, Golgi body etc.||Cytoplasm contains various cell organelles.|
|Ribosomes are 70 S type.||Ribosomes are 80 S type.|
|There are no streaming movements of cytoplasm.||Cytoplasm show streaming movements.|
|Endocytosis and exocytosis do not occur.||Endocytosis and exocytosis occur in animal cells.|
|Mitotic spindle is not formed in cell division.||Mitotic spindle is formed in cell division.|
|The mRNA does not need processing.||The mRNA needs processing.|
|Nuclear material is not enclosed by nuclear envelope and lies directly in cytoplasm. It is called nucleoid.||It is enveloped by nuclear envelope. Nucleus is distinct from cytoplasm.|
|DNA is circular and not associated with histone proteins.||Nuclear DNA is linear and associated with histone proteins extranuclear DNA is circular and protein free.|
|Replication of DNA occurs continuously through out cell cycle.||Replication of DNA occurs during S– Phase of cell cycle only.|
|These have small size (0.5 to 10 m m ) and have much less
|These are relatively large (10 – 15 m m ) and have much
|Sexual reproduction absent but parasexuality present.||Sexual reproduction is present.|
|Plasmids and pili occur in many prokaryotes
Example – E. coli
|There are no plasmids and pili in eukaryotic cells
Example – Spirogyra, Chlorella
|Cell division mostly amitotic.||Cell division is typically mitotic.|
|Plasma invaginates and from finger like process. Mesosome which take part in respiration||Absent|
- Cell compartmentation map
|Middle lamellae||Primary wall||Secondary wall||Tertiary wall||Cytoplasm||Nucleus|
Nuclear Nucleoplasm membrane
|Ectoplasm (Plasma membrane)||Endoplasm|
|Cell organelles||Hyloplasm (Cytosole)|
|Without unit membrane||With single unit membrane||With double unit membrane||Organic contents||Inorganic contents|
|Ribosome||E.R.||Mitochondria Reserve food material|
|Nucleolus||Golgi body||Plastid Excretory products Metals||Non metals|
|Centriole||Lysosome||Nucleus Secretory products|
- Discovery : It was first discovered by Robert Hooke in
Cell wall is the outer most, rigid, protective, non living and supportive layer found in all the plant cells, bacteria, cyanobacteria and some protists. It is not found in animal cells.
- Chemical composition : Mainly cell wall consists of two parts, matrix and cellulosic fibres (microfibriles). Matrix consists of hemicellulose, pectin, glycoproteins, lipids and A cellulose molecule is long unbranched
chain of glucose molecules. There are about 6,000 glucose units in each chain. In most of the plants cell wall is
made up of cellulose (C6 H10O5 )n, a polymer made-up of unbranched chain of glucose molecule linked by
b ,1 – 4 glycosidic bond. About 100 molecules of cellulose form a micelle, about 20 micelle form a microfibril and approx 200 microfibril form a fibril. The cell wall of bacteria and the inner layer of blue green algae is made-up of mucopeptide and not of cellulose. The mucopeptide is a polymer of two amino sugars namely N-acetyl glucosamine
(NAG) and N-acetyl muramic acid (NAM) held alternately in b –1,4- linkage. In higher fungi, the cell wall is made
up of chitin, polymer of glucosamine.
Pectin is a mixture of polymerised and methylated galacturans, galacturonic acid and neutral sugars. Hemicellulose is a mixture of polymerised xylans, mannans, glucomannans, galactans, xyloglucans and arabinogalactans. Glycoproteins are known to influence metabolic activities of the wall. A glycoprotein called extensin or expansin takes part in loosening and expansion of cell was through incorporation of cellulose molecules to cellulose microfibrils.
Plant cell wall may have lignin for strength (e.g., woody tissue), silica for stiffness and protection (e.g., epidermal cells of grasses, Equisetum), cutin for preventing loss of water (e.g., epidermal cells), wax as component of cuticle and surface bloom as water repellent (floating leaves) and checking transpiration, suberin for impermeability (e.g., cork cells, endodermal cells), etc.
- Structure : Cell wall consists of middle lamella, primary wall, secondary wall, tertiary
- Middle lamella : Middle
lamella is the outermost region which functions as a cementing layer between two cells. It is absent on the outer free surface. It ruptures to create intercellular spaces. Middle lamella is formed of calcium and magnecium pectate. Fruit softening is due to gelatinisation of pectic compounds of middle lamella. Pectin is used as commercial jellying agent. Which is present outside the primary wall.
Middle Lamella Primary Wall
Secondary wall Layers
Middle Lamella Primary Wall
Secondary wall Layers
- S. of A Plant cell L.S. cell walls of two adjacent cells
Fig : Layers of cell wall in T.S. and L.S. of a cell
- Primary wall : A young plant cell forms a single layer of wall material. This layer is known as the primary cell The primary wall is thin, elastic and capable of expansion in a growing cell. It grows by intussusception. Meristematic and parenchymatous cells have primary cell wall only. The cells of leaves and fruits too have only primary wall.
- Secondary wall : In mature cell, more layers of wall material are added internal to the primary wall. These are called the secondary cell wall. Growth by addition of new wall material on the primary wall is called The secondary wall is thick and rigid. It usually consists of three layers, which are often named S1, S2 and S3. It is found in collenchyma and sclerenchyma cells, xylem vesseles.
- Tertiary wall : Sometimes tertiary wall is laid down on secondary wall, g., tracheids of gymnosperms. It is composed of cellulose and xylan, another ploysaccharides.
- Origin : A cell wall is organised at telophase stage of cell division. The plane and place of cell wall is determined by the microtubules. Fragments of ER and vesicles of golgi body alligned at the equator, called as phragmoplast, later which forms the cell plate. The synthesis of cellulose takes place by the help of enzyme cellulose synthase present in the plasma
The cell plate forms the cell wall. A cell posses three phases of growth namely cell formation, cell elongation and cell maturation. The formation of new cells occurs by mitotic activity. The cell elongation is initiated by an increase in cell turgor. It is brought about by special proteins called expansion. They are of two types a – expansion
b – expansion. As a result, lacunae or gaps appear in between the cellulose micelle. There are two possibilities
for the deposition of new wall material.
- By intussuception : As the cell wall stretches in one or more directions, new cell wall material secreted by protoplasm gets embedded within the original
- By apposition : In this method new cell wall material secreted by protoplasm is deposited by definite thin plates one after the
Differences between primary and secondary cell wall
|Primary cell wall||Secondary cell wall|
|Primary wall is laid inner to middle lamella||Secondary wall is laid inner to primary wall.|
|It is formed in a growing cell.||It is formed when the cells have stopped growing.|
|It is capable of extension.||Extensibility is absent except in collenchyma cells.|
|It is single layered.||It is three or more layered.|
|Cellulose content is comparatively low (5 – 20%).||Cellulose content is comparatively high (20 – 90%).|
|Cellulose microfibrils are shorter, wavy and loosely arranged.||They are longer, closely arranged straight and parallel.|
|Protein content up to 5%.||Protein content up to 1%.|
|Hemicellulose content is high up to 50%.||It is 25% of the total.|
|Lipid content up to 5 – 10%.||Lipid is absent.|
|Primary wall is comparatively thin 1 – 5 m m.||It is comparatively thick 5 – 10 m m|
- Thickenings of cell wall : In many secondary walls specially those of xylem the cell wall becomes hard and thick due to the deposition of lignin. With the increasing amount of lignin, deposition protoplasm is lost. First the lignin is deposited in middle lamella and primary wall and later on in secondary wall. Like cellulose lignin is permeable to water and substances dissolved in Lignin is deposited at specific places of the cell walls due to which xylem tracheids and trachea take up following forms:
- Annular thickenings : Deposition of lignin takes place in the form of rings on the inner surface of protoxylem
A B C D
Fig : Different types of secondary wall thickenings –
- annular (b) spiral (c) scalariform (d) reticulate (e) pitted- simple pits (f) pitted-bordered pit
cells. These rings are placed one above the other leaving some space in between each other.
- Spiral thickenings : In these thickenings deposition of lignin takes place in the form of complete spiral bands and are formed in tracheids and trachea of
- Scalariform (Ladder like) thickenings : In these thickenings lignin is deposited in the form of transverse rods of the ladder. The unthickened areas between the successive thickenings appear as elongated transverse This type of thickening is common in protoxylem.
- Reticulate (Net like) thickenings : The lignin is deposited in the form of a net or reticulum. The unthickened areas are irregular in These are found in metaxylem.
- Pitted thickenings : These are found in metaxylem. In such thickening the whole inner wall is more or less uniformly thickened leaving here and there some unthickened areas called
- Pits : Secondary walls may have irregular thickenings at some places and these places are called pits. Pits are of two types :–
- Simple pit : In which pit chamber is uniform in
- Bordered pit : In which pit chamber is flask shaped in tracheids of gymnosperm and vessels of
- Simple pit
- Bordered pit Bordered pit pair D. Half bordered pit
- Plasmodesmata : Tangle (1879) first of all discovered them and were studied elaborately by Strasburger (1901). A number of plasmodesmata or cytoplasmic strands are present in pit through which the cytoplasm of one cell is in contact with Endoplasmic reticulum plays a role in origin of plasmodesmata.
- Intercellular spaces : In mature cells certain spaces or cavities are produced which are of 3 types.
- Schizogenous cavities : In mature cells, the cell walls separate from each other and form a e.g.,
resin canals in Pinus.
- Lysogenous cavities : It is formed by the break down of cell walls g., Citrus oil cavities.
- Schizo-lysogenous cavities : Both the above processes are involved in this cavity formtion g.,
protoxylem of maize.
- Function of cell wall : The cell wall serves many functions –
- It maintain shape of the
- It protect the cells from mechanical
- It wards off the attacks of pathogens (viruses, bacteria, fungi, protozoans).
- It provides mechanical support against It is due to the rigid cell walls that the aerial parts of the plants are able to keep erect and expose their leaves to sunlight.
- The cell wall prevents undue expansion of the cell when water enters by osmosis to compensate for the lack of contractile This prevents bursting of cells.
- It allows the materials to pass in and out of the
- Though permeable, the cell wall plays some regulatory role on the passage of materials into and out of the
- Many enzymic activities associated with metabolism are known to occur in the cell
- Cutin and suberin deposits check loss of water form the cell surface by
- The cell wall helps in the maintenance of balance of intracellular osmotic pressure with that of its
- Pores in the cell walls permit plasmodesmata to link up all the protoplasts into a system called symplast (symplasm).
- The walls of xylem vessels, tracheids and sieve tubes allow movement of
- The wall in some cases has a role in defence and offence by means of
- Growth of the cell wall enables the cells to enlarge in
- Cell wall and intercellular spaces constitute a nonliving component of plant body known as apoplasm.
- Peptidoglycane = murein = mucopeptide is the only cell wall material of It’s sugar portion consists of NAG and NAM.
- In fungi cell wall is made up of chitin (polymer of N- acetyl glucosamine). In bacteria it is composed of protein lipid polysaccharide having N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM).
- Cell wall proteins –
HRGP –Hydroxy proline rich glycoprotein ® Phloem and cambium.
PRP– Proline rich protein ® Xylem, fibres, cortex.
GRP– Glycine rich protein ® Xylem.
- Definition : Every living cell is externally covered by a thin transparent electron microscopic, elastic regenerative and selective permeable membrane called plasma It is quasi fluid in nature. According to Singer and Nicolson it is “protein iceberg in a sea of lipid”. A cell wall lies external to plasmalemma in plant cells, many monerans, some protists and fungal cells. Membranes also occur inside the cells. They are collectively called
biomembranes. The term cell membrane was given by C. Nageli and C. Cramer (1855) for outer membrane covering of the portoplast. It was replaced by the term plasmalemma or plasma membrane by Plowe (1931).
- Chemical composition : Proteins lipoprotein (Lipid +Protein) are the major component forming 60% of the plasma membrane. Proteins provide mechanical strength and responsible for transportation of different Proteins also act as enzyme. Lipids account may 28%-79% depending upon the type of cell and organism involved (in humans, myelin 79%). Because of the presence of lipids, membranes are always continuous, unbroken structures and are deformable and their over all shape can change. The lipids of plasma membrane are of three types namely phospholipids, glycolipids and sterols. A glycolipid may be cerebroside or ganglioside. The sterol found in the membrane may be cholesterol (Animals), phytosterol (Plants) or ergosterol (Microorganisms). A lipid molecule is distinguishable into a head of glycerol and two tails of fatty acids.
Carbohydrates form 2%–10%. Oligosaccharides are the main carbohydrates present in plasma membrane. The carbohydrates of plasma membrane are covalently linked to both lipid and protein components. The common sugars found in the plasma membrane are D – glucose, D – mannose, D – glactose, N – acetyl glucosamine, N – acetyl galoactosamine (Both are amino sugars) and sialic acid. Generally the terminal sugar of oligosaccharides is sialic acids (Also known as N – acetylneuraminic acid NANA) which gives them a negative charge.
- Ultra structure : Under electron microscope the plasma membrane appears three layered, e. trilaminar or tripertite. One optically light layer is of lipid and on both sides two optically dense protein layers are present.
Generally the plasma membrane is 75 Å thick (75 – 100Å), light layer is 35 Å while dark layers are
+ 20 Å
- Molecular structure and different models : Several models have been proposed to explain the structure and function of the plasma
- Overton’s model : It suggests that the plasma membrane is composed of a thin lipid
- Sandwich model : It was proposed by Davson and Danielli (1935). According to this model the light biomolecular lipid layer is sandwiched between two dense protein layers. This model was also said to be unit membrane
- Robertson’s unit membrane model : It states that all cytoplasmic membranes have a similar structure of three layers with and electron transparent phospholipid bilayer being sandwiched between two electron dense layer of proteins. All biomembranes are either made of a unit membrane or a multiple of unit membrane. Its thickness is about 75 Å with a central lipid layer of 35 Å thick and two peripheral protein layers of 20 Å thick.
- Fluid mosaic model : The most important and widely accepted latest model for plasma membrane was given by Singer and Nicolson in According to them it is “protein iceberg in a sea of lipids.”