IB Biology Topic B2.2.7: Structure and Function of Free Ribosomes and of the Rough Endoplasmic Reticulum Notes

Ribosomes: The Protein Synthesis Machinery

1. A ribosome is a complex of ribosomal RNA (rRNA) and proteins that catalyses translation, the synthesis of polypeptides from an mRNA template.
   1. The small subunit binds mRNA and ensures correct codon-anticodon matching.
   2. The large subunit catalyses peptide bond formation and provides an exit tunnel for the growing polypeptide chain.
2. Eukaryotic ribosomes are 80S (a 40S small subunit and a 60S large subunit), while prokaryotic ribosomes are 70S.
   1. Ribosomes are not permanently free or bound: the same ribosome can be free in one round of translation and bound in the next.
   2. What determines where it ends up is the protein it is making.

Free Ribosomes: Proteins for Internal Use

1. Free ribosomes float in the cytoplasm, unattached to any membrane.
   1. They make proteins that work inside the cell: cytoplasmic enzymes (like hexokinase for glycolysis), cytoskeletal proteins (like actin and tubulin), and proteins for organelles such as mitochondria, chloroplasts, or the nucleus.
2. Proteins made by free ribosomes are released directly into the cytoplasm.
   1. Some stay there, while others are directed to specific organelles by signal sequences in the polypeptide.
   2. For example, proteins targeted to the nucleus carry a nuclear localisation signal (NLS) recognised by importins at the nuclear pore.

Bound Ribosomes and the Rough Endoplasmic Reticulum

1. The rough endoplasmic reticulum (rER) is a network of flattened membrane sacs (cisternae) studded with ribosomes on its cytoplasmic surface.
   1. A ribosome becomes bound to the rER when it starts translating an mRNA that encodes a protein with an N-terminal signal peptide.
   2. This signal peptide is recognised by a signal recognition particle (SRP), which docks the ribosome onto the rER membrane.
   3. The growing polypeptide is then threaded through a protein channel (translocon) into the rER lumen as it is translated.
2. Inside the rER lumen, the protein is folded with the help of chaperone proteins and may undergo glycosylation (the addition of carbohydrate chains).
   1. Once processed, the protein is packaged into transport vesicles that bud from the rER and travel to the Golgi apparatus for further modification and sorting.
3. Proteins made on the rER include secreted proteins (like insulin from pancreatic beta cells), membrane proteins (like ion channels and receptors), and lysosomal enzymes.
   1. Cells that secrete large quantities of protein, such as plasma cells producing antibodies, have extensive rER.

Comparing Free and Bound Ribosomes