Chapter 3 Cell- The Unit of Life Part 3 by Teaching Care online coaching classes

Chapter 3 Cell- The Unit of Life Part 3 by Teaching Care online coaching classes


  • Definition : Lysosomes are electron microscopic, vesicular structures of the cytoplasm, bounded by a single membrane which are involved in intracellular digestive activities, contains hydrolytic enzymes, so called
  • Discovery : These were first discovered by a Belgian biochemist, Christian de Duve (1995) in the liver cells and were earlier named pericanalicular dense Terms Lysosome was given by Novikoff under the study of electron microscope. Maltile (1964) was first to demonstrate their presence in plants, particularly in the fungus neurospora.
  • Occurrence : These are absent from the prokaryotes but are present in all eukaryotic animal cells except mammalian They have been recorded in fungi, euglena, cotton and pea seeds.
  • Shape : These are generally spherical in shape but are irregular in plant root tip
  • Size : Size range is 2-0.8 mm while size is 0.5 mm (500 nm).
  • Number : Lysosomes are more in those cells which are involved in intracellular digestive activities g., WBCs of blood, histiocytes of connective tissue; phagocytes of liver and spleen; osteoclasts; cells of degenerating tissue like tail of tadpole larva etc.
  • Ultrastructure : Under electron microscope, a lysosome is formed of two parts :
    • Limiting membrane : It is outer, single layered, lipoproteinous and trilaminar unit It keeps a limit on glycoproteinous digestive enzymes.
    • Matrix : It is inner, finely granular and highly heterogeneous group substance inside the
  • Types : The lysosomes change the nature of their contents at different times in the same cell. This variation is referred to as On the basis of their contents, four types of lysosomes are recognised.
    • Primary Lysosomes : A newly formed lysosome contains enzymes only. It is called the primary lysosomes. Its enzymes are probably in an inactive
    • Secondary Lysosomes : When some material to be digested enters a primary lysosome, the latter is named the secondary lysosome, or phagolysosome or digestive vacuole, or heterophagosome. This commonly occurs by fusion of a primary lysosome with a vacuole (pinosome or phagosome) or a secretory granule.


Plasma membrane

Endoplasmic reticulum

Primary lysosome or storage granule

Autophagic vacuole

Plasma membrane


Secondary lysosome

Digested mitochondrion



Food particles taken in by endocytosis

Phagosome                     Digestive vacuole

Residual body

Defecation or exocytosis or wastes


Fig : Different types of lysosomes and their origin



  • Tertiary lysosomes/Residual  bodies  :  In a secondary lysosome,  the enzymes digest the incoming The products of digestion pass through the lysosome membrane into the cytoplasmic matrix for use as a source of nutrition or energy. Indigestible matter remains in the secondary lysosome. A secondary lysosome containing indigestible matter is known as the residual body or tertiary lysosome. The latter meets the cell by exocytosis (ephagy).
  • Autophagosomes /Autolysosomes  /Autophagic  vaculoes  :  A cell may digest its own organelles, such as mitochondria, ER. This process is called autophagy or autolysis. These are formed of primary lysosomes. The enzymes (hydrolytic) of lysosomes digest the organelles thus enclosed. Therefore, the lysosome are sometimes called disposal units/suicidal
  • Chemical composition : Matrix of primary lysosome is formed of hydrolases, which is involved in hydrolysis or polymeric compounds, that operate in acidic medium at pH 5, so called acid hydrolases. Upto now 50 types of enzyme have been reported to be present in latent form in different types of lysosomes. These enzymes are synthesized on RER, transported to cisternae of golgi body where these are packed into the These are as
  • Proteases g., cathepsin and collagenase.
  • Nucleases g., DNAse and RNAse.
  • Glycosidases g., b-galactosidase, b-glucoronidase.
  • Phosphatases g., ATPase, acid phosphatase (marker enzyme).
  • Sulphatases g., for sulphate-linked organic compounds.
  • Esterases g., phospholipase, acid lipase.
  • Origin : Lysosomes arise from the golgi complex their membrane and hydrolytic enzymes are synthesized on the RER and are transported invesicles to the golgi complex for modification and

(11)  Functions

  • Lysosomes take part in digestion of food through phagosomes, known as intracellular digestion.
  • In metamorphosis of many animals certain embryonic parts are digested by
  • Obstructing structures are destroyed by
  • Lysosomes perform the function of exocytosis and
  • Lysosomes of sperms provide enzyme for breaking limiting membrane of egg g., hyaluronidase enzyme.
  • They cause breakdown of ageing and dead
  • Lysosomes functions as trigger of cell division or initiate cell division by digesting repressor
  • Nucleases (DNAse) of lysosomes may cause gene mutations which may cause disease like leukemia or blood cancer (partial deletion of 21st chromosome).
  • Sometimes residual bodies accumulate inside the cells leading to storage diseases g. a glycogen storage disease called Pompe’s disease, polynephritis Hurler’s disease (deformed bones due to accumulation of mucopolysaccharides).
  • Lysosomes also engulf the


Important Tips

  • Cholesterol, cortisol and cortisone acts as a stablizers of lysosomal membrane, while absence of oxygen, X-rays UV rays and excess of vitamin A and E act as labilizers and weaken the lysosomal
  • Polymorphism in lysosomes were described by De Robertis al (1971).
  • Lysosomes can hydrolyse all types of organic compounds except cellulose.



  • Definition : The ribosomes are smallest known electron microscopic without membrane, ribonucleo–protein particles attached either on RER or floating freely in the cytoplasm and are the sites of protein
  • Discovery : In 1943 Claude observed some basophilic bodies and named them as microsome. Palade (1955) coined the term ribosome (form animal cell). Ribosomes in nucleoplasm were observed by Tsao and Sato (1959). First isolated by Tissieres and Watson (1958) from coli. Ribosomes found in groups are termed as polyribosomes or ergosomes (Rich and Warner 1963 observed first time polyribosomes).
  • Occurrence : These are found in both prokaryotes as well as eukaryotes these are present only in free form in the While in the eukaryotes the ribosomes are found in two forms in the cytoplasm, free form and bind form (bound on RER and outer nuclear membrane). These are also reported inside some cell organelles like mitochondria and plastids respectively called mitoribosomes and plastidoribosomes.
  • Number : The number of ribosomes depends upon the RNA contents of the cell. These are more in plasma cells, liver cells, Nissl’s granules of nerve cells, meristematic cells and cancerous
  • Types of ribosomes : It is determined on the basis of sedimentation coefficient measured in Svedberg unit or ‘S’ unit and their Velocity of sedimentation is 1 ´ 10 13 cm / sec/ dyne/gm.
  • 70S ribosomes : Found in prokaryotes, mitochondria and plastid of Each is about 200 – 290Å × 170 – 210Å in size and 2.7 ×106 dalton in molecular weight.
  • 80S ribosomes : Found in cytoplasm of eukaryotes. Each is about 300 – 340 Å × 200 – 240 Å in size and 5 – 5.0 ×106 daltons in molecular weight.
  • 77S, 60S and 55S ribosomes : Levine and Goodenough (1874) observed 77S ribosomes in fungal mitochondria 60S ribosomes in animal mitochondria and 55S in mammalian
  • Structure : Each ribosome is formed of two


unequal subunits, which join only at the time of protein synthesis. In 70S and 80S ribosomes, 50S and 30S, 60S and 40S are larger and smaller subunits respectively. Larger subunits is dome shaped and attached to ER by glycoproteins called “ribophorins”. It has a depression on the flate side which leads into a channel having elongating polypeptide chain. It has a protuberance, a

210Å Width 70S Ribosome

30S Subunit


50S Subunit

200-240Å Width 80S Ribosome

40S Subunit

60S Subunit


ridge and a stalk. It also has 2 binding sites. Peptidyl or P

or Donor site and Amino actyl or A or Acceptor site.

Fig : 70S and 80S ribosome


These sites are for the attachment of charged tRNA molecules. Smaller subunit is oval shaped and fits as a cap on flat side of larger subunit. It has a platform, cleft head and base. It has binding site for mRNA. Delimiting membrane is not found in it. Ribosomes are attached to ER through hydrophobic interactions.

  • Chemical composition : Ribosomes are chemically composed of rRNA and proteins Ribonucleo-Protein (RNP). Lipids are altogether absent in ribosomes. Ribosomes are strongly negative binding cations and basic dyes. 70S ribosomes has 60-65% rRNA and 35-40% proteins (ratio is 5 : 1). rRNAs are of three types : 23S type and 5S type rRNAs in 50S and 16S type rRNA in 30S sub-units. There are about 55 types of proteins in 70S ribosome out of which 21 proteins are found in 30S while 34 proteins are found in 50S ribosomal sub-unit and are called core-proteins.

80S ribosome has 45% rRNA and 55% proteins (ratio is about 1 : 1). r-RNA are of four types : 28S, 5S and 5.8S types of rRNAs in 60S and 18S type rRNA in 40S sub-units. There are about 70 types of proteins in 80S ribosome out of which 30 proteins are found in 40S while 40 proteins are found in 60S ribosomal sub-units. The ribosomal proteins are basic and almost surround the rRNA. Some proteins act as structural proteins while other proteins act as enzymes e.g., peptidyl transferase of 50S (controls the interlinking of amino acids by peptide bonds).


A 1 ´ 10 -3

(0.001 M) molar concentration of

Mg ++

is needed for the structural cohesion of ribosomes i.e., for


holding the two subunits together. If this concentration is increased by ten folds, two ribosomes unite to form a

dimer. The sedimentation coefficient of dimer of 70S ribosmes is 100S and that of 80S is 120S. By decreasing the


Mg ++

conc. to normal, the dimer breaks into monomers (single ribosomes).


70S + 70S




100S ,


80S + 80S





If the

Mg ++

concentration is decreased to 1 ´ 10 -4

molar, the ribosomes break up into its sub-units. The 70S


ribosome breaks up into 50S and 30S sub-units. These 50S and 30S sub-units further dissociates into RNA and protein components. Similarly, the 80S ribosomes dissociates into 60S and 40S sub-units which further breakup into RNA and protein components.

(8)  Biogenesis of ribosome :

  • In eukaryotes the ribosomal RNAs like 18S, 8S and 28S are synthesized by nucleolus and 5S RNA out of the nucleus.
  • In prokaryotes both rRNA and its protein are synthesized as well as assembled by cytoplasm.
  • Polyribosomes or Polysomes : When many ribosomes (generally 6 – 8) are attached at some mRNA It is called polysome. The distance between adjacent ribosomes is of 90 nucleotides. These are functional unit of protein synthesis.

(10)  Function :

  • Ribosomes are also called protein factory of the cell or work branch of
  • Free ribosomes synthesize structural proteins and bounded ribosomes synthesize proteins for
  • Ribosomes are essential for protein synthesis.
  • Help in the process of photosynthesis.
  • They are found numerously in actively synthesizing cells like liver cells, pancreas, endocrine, yeast cells and meristematic


  • Ribosomes also store the proteins
  • These also store rRNAs, which helps in protein
  • Enzyme peptidyl transferase occurs in large subunit of ribosome which helps in protein
  • Newly formed polypeptide is protected from degradation by cytoplasmic enzymes in large sub-unit of ribosomes before releasing it into RER

Important tips

  • Gunter Blobel and David Sabatini of Rockfeller university proposed signal hypothesis in 1971. Both scientist has been awarded the Nobel prize (1999) for this protein
  • Ultra-structure of ribosomal subunits was given by James Lake (1981).
  • Palade and Kuff (1966) gave the ultrastructure of
  • Chaperons are proteins which assist in proper folding of



Microbodies are single membrane bounded small spherical or oval organelles, which take part in oxidation reactions other than those of respiration. They can only be seen by electron microscope. Microbodies posses a crystalline core and granules matrix. They are following types :–

(1)  Sphaerosomes

  • Discovery : These were first observed by Hanstein (1880) but discovered by Perner (1953). Term sphaerosomes was given by Dangeard.
  • Occurrence : These are found in all the plant cells which involves in the synthesis and storage of lipids e.

endosperm and cotyledon.

  • Shape, size and structure : These are spherical or oval in shape about 0.5-2.5 mm in diameter. They contain hydrolytic enzymes like protease, ribonuclease, phosphatase, esterase They are bounded by a single unit membrane.
  • Function : The main function of sphaerosomes is to help in lipid These are also known as plant lysosomes.

(2)  Peroxisomes (Uricosomes)

  • Discovery : These were first discovered by Rhodin (1954) in the cells of mouse kidney with the help of electron microscope, and were called microbodies. De Duve (1965) isolated certain sac like organelles from various types of animals and plants. These were called peroxisomes because these contain peroxide producing enzymes (oxidases) and peroxide destroying enzymes (catalases).
  • Occurrence : These are found in photosynthetic cells of In animals peroxisomes are abundant in the liver and kidney cells of vertebrates. They are also found in other organs like brain, small intestine, testis and adrenal cortex. They also occur in invertebrates and protozoans e.g., Paramecium.
  • Shape, size and structure : These are spherical in shape, about 5 mm in size. They are bounded by a single unit membrane. They contains granular consents condensing in the centre. Their membrane is permeable to amino acids, uric acids, etc. They contain four enzymes of H2O2 metabolism. The enzymes urate oxidase, d-amino

oxidase, a-hydroxy acid oxidase produce

H 2 O2 is toxic for cells.

H 2 O2

whereas the catalases plays a significant protective role because


Uric acid + O2

¾¾¾Urat¾e ox¾ida¾se  ®ü ® H  O

Methyl alcohol + H 2O2

¾¾Ca¾tala¾se ®ü ® H  O



Amino acid + O2

¾¾D-a¾min¾o o¾xida¾se ®ý

2 2 Formic acid + H

2 O2

¾¾Ca¾tala¾se ®ý          2


  • Function : These are involved in the formation and degrading of

H 2 O2 . Plant peroxisomes are also


involved in photorespiration. In which glycolic acid oxidase enzyme oxidises the glycolic acid to glyoxylic acid. In case of plants peroxisomes is also known as glyoxisomes.

(3)  Glyoxysomes

  • Discovery : These were discovered by Beevers in 1961 and Briedenbach in
  • Occurrence : These are found in fungi, some protists and germinating seeds especially in germinating fatty seeds where insoluble lipid food reserves must be turned into soluble

Animals cannot execute this conversion because they do not posses glyoxylate enzymes.

  • Shape, size and structure : These are spherical in shape, about 0.5-1 mm in size, they contain enzymes of metabolism of glycolic acid via glyoxylate cycle and bounded by a unit membrane. These are also contain enzymes for b-oxidation of fatty
  • Functions : The main function of glyoxysomes is conversion of fats into
  • Lomasomes : These are sac like structures found between cell wall and plasmalemma in the haustoria of fungal These were first discovered by Bowen and Berlin. Webster called them border bodies.


  • Discovery : Centrosome was first discovered by Van Benden (1887) and structure was given by Boveri.
  • Occurrence : It is found in all the animal cell except mature mammalian RBC’s. It is also found in most of protists and motile plant cells like antherozoids of ferns, zoospores of algae and motile algal forms g., Chlamydomonas but is absent in prokaryotes, fungi, gymnosperms and angiosperms.


  • Structure : Centrosome is without unit membrane structure. It is formed of two darkly stained granules called centrioles, which are collectively called These centrioles are surrounded by a transparent cytoplasmic area called centrosphere or Kinetoplasm. Centriole and centrosphere are collectively called centrosome. Before the cell division the centrioles at each pole of the spindle. The two centrioles are situated at

90o     to   each   other.   Each   centriole   is   a


Cart-wheel structure central rod (HUB)


13 Globular subunits



3 Subtubules (Subfibres) B

A-C Connective (DM)

3 Subtubules (Subfibres)


Globular subunits (45Å)


microtubular structure and is formed of microtubules arranged in 9 + 0 manner (all the 9 microtubules are peripheral in position).



Fig : (A) T.S. Centriole (B) Three subtubules (C) A subtubule


Each microtubule is a triplet and is formed of three subtubules which are called A, B and C. A subtubule is about 45Å thick and is formed of 13 parallel protofilaments while each of B and C subtubule is formed of 10 parallel protofilaments. Each protofilament is formed of a row of a, b-tubulin dimers. C sub-tubule of each microtubule is linked to A sub-tubule of adjacent microtubule by a dense material (DM) strand called A-C linker, so

all the microtubules are tilted at 40o . Each microtubule is about 250Å in diameter.

Inside the microtubules, there is an intra-centriolar or cart-wheel structure which is formed of a central hub (about 25Å in diameter) and 9 radial spokes or radial fibres. Each radial spoke ends into a dense material (DM) thickening, called X-body or foot which is further linked to A-subtubule. Between two adjacent X-bodies there is another DM-thickening, called Y-body, which is linked to X-body on either side and to A-C linker on outer side.

Centriole is rich in tubulin and ATPase. Centriole can replicate but has no DNA. Centrioles replicate in G2

phase of interphase of cell cycle but do not initiate cell division.

  • Chemical composition : Centrosome is lipoproteinaceous The microtubules of centriole are composed of protein tubulin and some lipids. They are rich in ATPase enzyme.


  • Origin : The daughter centriole is formed from the pre-existing centriole in G2

replicating organelle.

(6)  Functions

of interphase so called self-


  • The centrioles help organising the spindle fibres and astral rays during cell division. Therefore, they are called microtubules organising The cells of higher plants lack centrioles and still form a spindle.
  • They provide basal bodies which give rise to cilia and flagella.
  • The distal centriole of a spermatozoan give rise to the axial filament of the

Important Tips

  • Centriole is also called microcentrum or cell
  • Each centriole is formed of 9 ´ 3 = 27 subtubules or


  Cilia and flagella.

  • Discovery : Flagellum presence was first reported by Englemann (1868). Jansen (1887) was first scientist to report the structure of sperm
  • Definition : Cilia and flagella are microscopic, hair or thread-like motile structures present extra-cellularly but originate intra-cellularly from the basal body and help in movements, locomotion, feeding, circulation
  • Occurrence : Cilia are found in all the ciliate protozoans g., Paramecium, Vorticella etc. flame cells of flat worms; in some larval forms e.g., Trochophore larva of Nereis, Bipinnaria larva of starfish etc.; in some body structures e.g. wind-pipe, fallopian tubes, kidney-nephrons etc.

Flagella are found in all the flagellate protozoans e.g., Euglena, Trichonympha etc., collar cells of sponges; gastrodermal cells of coelenterates; spermatozoa of animals and lower plants; zoospores of algae etc. These are absent in red algae, blue-green algae, angiosperms, nematodes, arthropodes etc.