Ribosomes/Palade particles/Protein factores:

• Palade (1955) coined the term Ribosome. Smallest cell organelles (150 x 250 Å) devoid of membrane. Except mammalian RBC all living cells have ribosomes.

Forms of Ribosomes:

• Bound form attached to ER. Protein synthesis for secretion purpose occurs on bound ribosomes.
• Free robosomes – Occur freely suspended in Hyaloplasm. Protein synthesis for growth of protoplasm occurs on free ribosomes.

Types of ribosomes:

1. Eucaryotic ribosomes – 80S = Occur in cytoplasm of eukaryotic cells.
2. Procaryotic ribosomes – 70S = occur in cytoplasm of procaryotes, and also in Mitochondria and Chloroplast of eucaryotes.

Each ribosome composed of two subunits i.e. larger and smaller subunits.

80S = 60S + 40S; 70S = 50S + 30S ; S = Svedberg unit or Sedimentation rate = 1 x 10^-13 cm/sec./dyne/gm.

• The smaller subunit has a Platform, a cleft, a head and a base. The larger subunit has a protuberance, a ridge and a stalk. 
• When associated with ER, they are attached with their 60S subunit.
• The proper folding of proteins following synthesis is assisted by specific proteins called Chaperons, which also assist in transport of proteins into organelles such as mitochondria.
• For simultaneous synthesis of variable proteins molecule, several ribosomes become attached to m-RNA with the help of smaller subunits. This structure is called polyribosome or polysome or ergosome.                       
• Larger subunit contains peptidyl transferase enzyme which catalyses synthesis of peptide bond between aminoacids. Larger subunit has two sites.

1. A-site acceptor site for t-RNA
2. P-site site for growing polypeptide chain

• Two subunit are held together by Mg⁺². Which form ionic bond with phosphate groups of r-RNA of two subunits. Minimum 0.001 M Mg+2 concentration is required for structural formation of ribosomes. If Mg⁺² concentration is increased 10 times then ribosome dimer are formed. 80S + 80S = 120S; 70S + 70S = 100S

Chemical composition of ribosomes:

70S – 60% r-RNA + 40% proteins

80S – 40% r-RNA + 60% proteins

50S – 34 proteins ; 30S – 21 proteins therefore so total proteins molecules in 70S ribosomes are 55

60S – 45 proteins ; 40S – 33 proteins therefore so total protein molecules in 80S robosomes are 78

60S – r-RNA – 28S, 5.8S, 5S; 40S – r – RNA – 18S

50S – r-RNA – 23S, 5S; -r-RNA- 16S

Proteins in ribosomes are negatively charged.