Introduction
Ribosomes
are tiny granular structures present in all living cells.
They
are made of ~60% ribosomal RNA (rRNA) and ~40% protein.
Ribosomes
are non-membranous organelles and much smaller than most other cell
organelles.
Location
Eukaryotic
Cells:
Found
attached to Rough Endoplasmic Reticulum (RER).
Some
scattered freely in cytoplasm.
Prokaryotic
Cells:
All
ribosomes are freely scattered in cytoplasm.
Other
Organelles:
Also
found in mitochondria and chloroplasts (similar to prokaryotic
ribosomes).
Abundance
Most
abundant in cells active in protein synthesis (e.g., pancreas, brain
cells).
A
typical cell may have thousands to millions of ribosomes.
Formation
of Ribosomes
Produced
and assembled in the nucleolus.
Ribosomal
proteins enter
nucleolus and combine with rRNA strands.
Form
two subunits:
Small
subunit
Large
subunit
Subunits
leave the nucleus through nuclear pores and join in cytoplasm for
protein synthesis.
When
protein synthesis is not occurring, subunits remain separated.
Function
Ribosomes
act as the protein factories of the cell.
Types
of Protein Synthesis
Free
Ribosomes:
Synthesize
proteins for cell’s own use (internal functions).
RER-bound
Ribosomes:
Produce
proteins for secretion or export outside the cell.
Mechanism
of Protein Synthesis
Requires
three types of RNA:
rRNA – structural and catalytic role in
ribosome.
mRNA – carries instructions from DNA.
tRNA – brings amino acids to ribosome.
Ribosome
links amino acids to form a polypeptide chain.
After
synthesis, ribosome releases the protein.
Svedberg
Values (Sedimentation Coefficient)
Eukaryotic
Ribosomes: 80S
Small
subunit: 40S
Large
subunit: 60S
Prokaryotic
Ribosomes: 70S
Small
subunit: 30S
Large
subunit: 50S
⚠️
Note: Svedberg (S) values are not additive because sedimentation
depends on size and shape, not just molecular weight.
Comparison:
Prokaryotic vs Eukaryotic Ribosomes
|
Feature |
Prokaryotic Ribosomes |
Eukaryotic Ribosomes |
|
Location |
Freely scattered in cytoplasm |
Attached to Rough ER or free in
cytoplasm |
|
Svedberg Value (Complete
Ribosome) |
70S |
80S |
|
Subunits |
Small = 30S, Large = 50S |
Small = 40S, Large = 60S |
|
Presence in Organelles |
Found in cytoplasm only |
Found in cytoplasm, mitochondria,
and chloroplasts |
|
Size |
Smaller |
Larger |
|
Protein Synthesis |
Produces proteins for internal
cellular use |
Free ribosomes → internal
proteins; RER ribosomes → secretory proteins |
|
Origin |
Primitive type, similar to
earliest cells |
Advanced type, assembled in
nucleolus |
|
Examples |
Bacteria, Archaea |
Plants, Animals, Fungi, Protists |
#FSc Biology Notes
#Ribosomes structure and function
#Ribosomes in prokaryotes and eukaryotes
#Ribosomes notes for FSc
#Protein synthesis ribosomes
#70S vs 80S ribosomes
#Ribosomes location and types
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