“Ribosomes are one of the most important cell organelles composed of RNA and protein

that converts genetic code into chains of amino acids.”

What are Ribosomes?

A ribosome is a complex molecular machine found inside the living cells that produce
proteins from amino acids during a process called protein synthesis or translation. The
process of protein synthesis is a primary function, which is performed by all living cells.

Ribosomes are specialized cell organelles and are found in both prokaryotic and eukaryotic
cells. Every living cell requires ribosomes for the production of proteins.

This cell organelle also functions by binding to a messenger ribonucleic acid (mRNA) and
decoding the information carried by the nucleotide sequence of the mRNA. They transfer
RNAs (tRNAs) comprising amino acids and enter into the ribosome at the acceptor site. Once
it gets bound up, it adds amino acid to the growing protein chain on tRNA.

Ribosomes Structure
 The ribosome word is derived – ‘ribo’ from ribonucleic acid and ‘somes’ from the Greek
word ‘soma’ which means ‘body’.
Ribosomes are tiny spheroidal dense particles (of 150 to 200 A0 diameters) that are
primarily found in most prokaryotic and eukaryotic.
 They are sites of protein synthesis.
 They are structures containing approximately equal amounts of RNA and proteins and serve
as a scaffold for the ordered interaction of the numerous molecules involved in protein
synthesis.
 The ribosomes occur in cells, both prokaryotic and eukaryotic cells.
 In prokaryotic cells, the ribosomes often occur freely in the cytoplasm.
 In eukaryotic cells, the ribosomes either occur freely in the cytoplasm or remain attached to
the outer surface of the membrane of the endoplasmic reticulum.
 The location of the ribosomes in a cell determines what kind of protein it makes.
 If the ribosomes are floating freely throughout the cell, it will make proteins that will be
utilized within the cell itself.
 When ribosomes are attached to the endoplasmic reticulum, it is referred to as rough
endoplasmic reticulum or rough ER.
 Proteins made on the rough ER are used for usage inside the cell or outside the cell.
 The number of ribosomes in a cell depends on the activity of the cell.
 On average in a mammalian cell, there can be about 10 million ribosomes.

Structure of Ribosomes

A ribosome is made from complexes of RNAs and proteins and is, therefore, a
ribonucleoprotein.
 Around 37 to 62% of RNA is comprised of RNA and the rest is proteins.
 Each ribosome is divided into two subunits:
1. A smaller subunit which binds to a larger subunit and the mRNA pattern, and
2. A larger subunit which binds to the tRNA, the amino acids, and the smaller subunit.
 Prokaryotes have 70S ribosomes respectively subunits comprising the little subunit of 30S
and the bigger subunit of 50S.
 Their small subunit has a 16S RNA subunit (consisting of 1540 nucleotides) bound to 21
proteins.
  The large subunit is composed of a 5S RNA subunit (120 nucleotides), a 23S RNA subunit
(2900 nucleotides) and 31 proteins.
 Eukaryotes have 80S ribosomes respectively comprising of little (40S) and substantial (60S)
subunits.
 The smaller 40S ribosomal subunit is prolate ellipsoid in shape and consists of one molecule
of 18S ribosomal RNA (or rRNA) and 30 proteins (named as S1, S2, S3, and so on).
 The larger 60S ribosomal subunit is round in shape and contains a channel through which
growing polypeptide chain makes its exit.
 It consists of three types of rRNA molecules, i.e., 28S rRNA, 5.8 rRNA and 5S rRNA, and
40 proteins (named as L1, L2, L3 and so on).
 The differences between the ribosomes of bacterial and eukaryotic are used to create
antibiotics that can destroy bacterial infection without harming human cells.
 The ribosomes seen in the chloroplasts of mitochondria of eukaryotes are comprised of big
and little subunits composed of proteins inside a 70S particle.
 The ribosomes share a core structure that is similar to all ribosomes despite differences in its
size.
 The two subunits fit together and work as one to translate the mRNA into a polypeptide
chain during protein synthesis.
 Because they are formed from two subunits of non-equal size, they are slightly longer in the
axis than in diameter.
 During protein synthesis, when multiple ribosomes are attached to the same mRNA strand,
this structure is known as polysome.
 The existence of ribosomes is temporary, after the synthesis of polypeptide the two sub-units
separate and are reused or broken up.
Types of Ribosomes
Based on the size and the sedimentation coefficient (S), ribosomes are of two types:
 70S ribosome
 80S ribosome
70S ribosome
 They are smaller in size.
 Sedimentation coefficient: 70S
 Molecular weight: 2.7× 106 daltons.
 They are found in:
 prokaryotic cells of the blue-green algae and bacteria.
 mitochondria and chloroplasts of eukaryotic cells.
80S ribosome
 Sedimentation coefficient: 80S
 Molecular weight: 40 × 106 daltons.
 They are found in the eukaryotic cells i.e. in plants and animals.
 The ribosomes present in mitochondria and chloroplasts are smaller than 80S cytoplasmic
ribosomes.
 In the 80S ribosome of yeast, 79r-protein are present where only 12 r-protein are found to be
specific.
Chemical Composition of Ribosomes
 Ribosomal RNAs
 Ribosomal proteins
 Metallic ions
Ribosomal RNAs
 70S ribosomes consist of three types of rRNA:
 23S rRNA
 16S rRNA
 5S rRNA
 In the 50S ribosomal subunit (larger subunit), 23S and 5S rRNA are present.
 In the 30S ribosomal subunit, the 16S rRNA is present.
 In the 80S ribosomes four types of rRNA are present:
 28S rRNA
 18S rRNA
 5S rRNA
 5.8 rRNA
 In the larger 60S ribosomal subunit, 28S, 5S, and 5.8S rRNAs are present.
 There is the presence of 18S rRNA in the 40S ribosomal subunit ( Smaller)
Ribosomal proteins
 Bacteria are composed of different ribosomal proteins.
 It was found that E. coli consists of 55 ribosomal proteins.
 For example; Core proteins (CP), Split proteins (SP)
Metallic ions
 divalent metallic ions: Mg++, Ca++ and Mn++
Functions of Ribosomes

The important ribosome function includes:

 It assembles amino acids to form proteins that are essential to carry out cellular
functions.
 The DNA produces mRNA by the process of DNA transcription.
 The mRNA is synthesized in the nucleus and transported to the cytoplasm for the
process of protein synthesis.
 The ribosomal subunits in the cytoplasm are bound around mRNA polymers. The
tRNA then synthesizes proteins.
 Ribosomes are the site of protein synthesis.
 The proteins synthesized in the cytoplasm are utilized in the cytoplasm itself, the
proteins synthesized by bound ribosomes are transported outside the cell.

The ribosome is a complex molecular machine, found within all living cells, that
serves as the site of biological protein synthesis (translation).
 Ribosomes link amino acids together in the order specified by messenger
RNA (mRNA) molecules.
 Ribosomes act as catalysts in two extremely important biological processes called
peptidyl transfer and peptidyl hydrolysis.
 The nascent polypeptide chain is protected from the activity of protein digestive
enzymes.
How does the ribosomal movement take place in translation?
 In addition to a binding site for an mRNA molecule, it consists of other 3 binding sites. for
tRNA molecules;
 A site
 P site
 E site
 Amino acid needs to be added to a growing peptide chain.
 The charged tRNA whose base pairs with the complementary codon on the mRNA molecule
enters the A site.
 Then in the new forming polypeptide chain, an amino acid is added which is held by the
tRNA in the adjacent P site.
 Then the large ribosomal subunit moves forward to the E site.
 This process or the cycle is repeated.
 Each time an amino acid is added to the polypeptide chain, where the new protein grows
from its amino to its carboxyl end.
 Finally, the stop codon will be encountered in the mRNA.
 The termination release factors like the RF1 and RF2 recognize the stop codons.
 Then the peptidyl-tRNA bond is hydrolyzed.
 Finally, the newly formed polypeptide is released from the ribosome.
 By the study of the structure and its biochemical characteristics, antibacterial agents are
developed in such a way they can inhibit this protein synthesis process.
 Examples of such antibiotics are:
 Aminoglycosides
 Chloramphenicol
 Fusidic acids
 Lincosamides
 Macrolides
 Oxazolidinone
 Streptogramins

LYSOSOMES
Lysosomes are membrane-bound, dense granular
structures containing hydrolytic enzymes responsible
mainly for intracellular and extracellular digestion.
 The word “lysosome” is made up of two words “lysis” meaning
breakdown and “soma” meaning body.
 It is an important cell organelle responsible for the inter and
extracellular breakdown of substances.
 They are more commonly found in animal cells while only in
some lower plant groups ( slime molds and saprophytic fungi).
 Lysosomes occur freely in the cytoplasm. In animals, found in
almost all cells except in the RBCs.
 They are found in most abundant numbers in cells related to
enzymatic reactions such as liver cells, pancreatic cells, kidney
cells, spleen cells, leucocytes, macrophages, etc.
 Lysosomes Diagram

Structure of Lysosomes
 Lysosomes are without any characteristic shape or structure i.e.
they are pleomorphic
 They are mostly globular or granular in appearance.
 It is 0.2-0.5 μm in size and is surrounded by a single lipoprotein
membrane unique in composition.
 The membrane contains highly glycosylated lysosomal
associated membrane proteins (LAMP) and Lysosomal
integral membrane proteins (LIMP).
 LAMPs and LIMPs form a coat on the inner surface of the
membrane
 They protect the membrane from attack by the numerous
hydrolytic enzymes retained inside.
 The lysosomal membrane has a hydrogen proton pump which is
responsible for maintaining pH conditions of the enzyme The
acidic medium maintained by the proton pump that pumps H+
inside the lumen, ensures the functionality of the lysosomal
enzymes.
 Inside the membrane, the organelle contains enzymes in the
crystalline form.
Lysosomal Enzymes
For degradation of extra and intracellular material, lysosomes
filled with enzymes called hydrolases. It contains about 40
varieties of enzymes which are classified into the following main
types, namely:
 Proteases, which digest proteins
  Lipases, which digests lipids
 Amylase, which digests carbohydrates
 Nucleases, which digest nucleic acids
 Phosphoric acid monoesters
Collectively the group of enzymes is called hydrolases which
cause cleavage of substrates by the addition of water molecules.
Most of the lysosomal enzymes function in the acidic medium.
Lysosomal Membrane
 As compared to the mitochondria, it is slightly thicker.
 Sialic acid is present in it.
 Since the lysosomal membrane protein is highly glycosylated, it
protects from the action of the lysosomal proteases.
 The lysosomal membrane can fuse with the other membranes of
the cell which is the unique property.
 When the lysosomal membrane ruptures, lysosomal enzymes
are released.
 It can be caused by the destabilizing influence of the surface-
active agents and the steroid sex hormones.
 The lysosomal membrane is stabilized by cortisone and
hydrocortisone.
 On the tissue, they possess an anti-inflammatory effect.
 Within the lysosome all the process of digestion takes place.
 For the action of the lysosomal enzyme, the medium should be
acidic.
 To maintain the acidic condition inside the organelle, there
should be an accumulation of the H+.
 It is maintained by the proton pump which is ATP-dependent.
 Transport protein is also present in the lysosomal membrane.
 When the macromolecules get digested, the final products can
be transported by these proteins.
 After the transportation, they can be further utilized by the cell
or be excreted.
Types of Lysosomes
 Polymorphism can be seen in the morphology of the lysosome.
 There are four types of lysosomes. They are:
 Primary lysosome
 Heterophagosomes
 Autophagososomes
 Residual Bodies
A. Primary Lysosomes
 They are also called:
 Storage granules
 Protolysosomes
 Virgin lysosomes
 The primary lysosome is bounded by a single membrane.
 It has a diameter of 100nm.
 A digestive enzyme is present in it which has not taken part in
the digestion.
 One type of enzyme or another is present in it.
 Only in the secondary lysosome, there is the presence of the full
complement of acid hydrolases.
B. Heterophagosomes
 They are also called:
 heterophagic vacuoles
 heterolysosomes
 Phagolysosomes
 When the primary lysosome fuse with the cytoplasmic vacuoles,
a heterophagosome is formed.
 Extracellular substances are present in the cytoplasmic
vacuoles.
 Different endocytic processes like pinocytosis, phagocytosis, or
receptor-mediated endocytosis help in bringing such
extracellular substances into the cell.
 In these secondary lysosomes, there is the presence of the
hydrolytic enzymes which digest the engulfed substances.
 After digestion, such particles will pass across the membrane of
the lysosome.
 Then it will become part of the matrix.
C. Autophagosomes
 They are also called:
 Autophagic vacuole
 cytolysosomes
 Autolysosomes
 Digestion of the different intracellular structures like
mitochondria, ribosome, peroxisome, and glycogen granules can
be done by the primary lysosome.
 Autophagy is called autodigestion.
 During cell growth and repair, autophagy is a normal event.
 It is prevalent in the differentiation, the dedifferentiation of the
tissues and tissue under stress.
Autophagy occurs in different forms:
a. By fusion of the lysosome:
 It is enclosed inside the double membrane sac.
 Then there is the breakdown of the inner membrane.
 Penetration of the enzyme can occur to the enclosed organelle.
 b. Formation of a vesicle and fusion with primary
lysosome:
 Vesicle is formed when the smooth endoplasmic
reticulum encases the organelle which needs to be digested.
Microautophagy also takes place. When the digestion process
proceeds then it becomes difficult to identify the type of
secondary lysosome if it is heterophagosome or autophagosome.
So, in this stage, it is said as the digestive vacuole.
D. Residual Bodies
 They are also called:
 Telolysosome
 Dense bodies
 Incomplete digestion results in the formation of the residual
bodies.
 When some lysosomal enzymes are absent, incomplete
digestion may occur.
 Inside the digestive vacuoles, the undigested food remains as
the residue.
 Then they make take different forms.
 Residue body is larger and irregular in shape.
 By the defecation, residual bodies are eliminated in the case
of Amoeba and some other protozoa.
 In some cells, for a longer period, the residual body may stay
which may cause aging.
Functions of Lysosomes
Lysosomes serve two major functions:
1. Intracellular Digestion
 To digest food, the lysosome membrane fuses with the
membrane of food vacuole and squirts the enzymes inside.
 The digested food then diffuses through the vacuole membrane
and enters the cell to be used for energy and growth.
2. Autolytic Action
 Cell organelles that need to be ridden are covered by vesicles or
vacuoles by the process of autophagy to form autophagosome.
 The autophagosome is then destroyed by the action of
lysosomal enzymes.
Processes in which lysosomes play crucial roles include:
a. Heterophagy
The taking into the cell of exogenous material by phagocytosis or
pinocytosis and the digestion of the ingested material after fusion
of the newly formed vacuole with a lysosome.
b. Autophagy
 A normal physiological process that deals with the destruction of
cells in the body. It is essential for maintaining homeostasis, for
normal functioning by protein degradation, turnover of destroyed
cell organelles for new cell formation
c. Extracellular Digestion
Primary lysosomes secrete hydrolases outside by exocytosis
resulting in degradation of extracellular materials.
Eg. Saprophytic fungi
d. Autolysis
It refers to the killing of an entire set of cells by the breakdown of
the lysosomal membrane. It occurs during amphibian and insect
metamorphosis.
e. Fertilization
The acrosome of the sperm head is a giant lysosome that
ruptures and releases enzymes on the surface of the egg. This
provides the way for sperm entry into the egg by digesting the
egg membrane.
f. As Janitors of the Cell
Lysosomes remove ‘junk’ that may accumulate in the cell helping
to prevent diseases.
Presence of Lysosomes
Presence of Lysosomes in the Animal tissue
 Liver
 Kidney
 Nerve cells
 Brain
 Intestinal epithelium
 Lung epithelium
 Macrophages(of the spleen, bone marrow, liver, and connective
tissue)
 Thyroid gland
 Adrenal gland
 Bone
 Urinary bladder
 Prostate
 Uterus
 Ovaries
Presence of lysosome in the Protozoa
 Leucocytes
 Amoeba
 Tetrahymena
 Paramecium
 Euglena
 Presence of lysosome in Plants
 Onion seeds
 Barley seeds
 Corn seedlings
 Yeast
 Neurospora
Presence of lysosome in Tissue culture cells
 HeLa cells
 Fibroblasts
 Chick cells
 Lymphocytes