Cell structure and function tutorial - Ribosomes

Location of ribosomes
- Eukaryotic ribosomes in the cell are found in the cytosol as either free ribosomes, or ER-bound ribosomes. As the bound ribosomes are bound to the ER, the proteins made by the bound ribosomes are fed into the lumen of the ER where it can be modified (eg. glycosylation) (picture below).
- Modification of proteins helps it to fold properly or targets it to the correct location in the cell. Further modification of proteins occur in the GA.


- Both free and bound ribosomes function to carry out protein synthesis. The difference is that the proteins made by the free ribosomes are for more "localised" use whereas the proteins made by the bound ribosomes are usually transported to other parts of the cell (eg. inserted into the membrane), or secreted out of the cell for use.
- Ribosomes may be found as polyribosomes/polysomes (complex of mRNA with many ribosomes - to increase rate of translation/proteins synthesis) (picture below).

Types of ribosomes
- Ribosomes are made up of a small subunit and a large subunit.
- There are 2 types of ribosomes, 70S and 80S. 80S ribosomes are bigger than 70S ribosomes. "S" refers to Svedberg, a unit of sedimentation coefficient used for centrifugation. Prokaryotic cells (bacteria), mitochondria and chloroplasts have only 70S ribosomes, while eukaryotic cells (include plant and animal cells) have 80S ribosomes (in their cytosol) PLUS 70S ribosomes (in their mitochondria/chloroplasts).

Synthesis of ribosomal subunits (a little complex; you may draw out the process to improve understanding)
- Ribosomal subunits (both small and large) are composed of rRNA and proteins.
- rRNA is synthesised by the nucleolus. Newly-synthesized rRNA remains in the nucleolus to wait to be assembled with the ribosomal proteins (aka the protein component of ribosome).
- The ribosomal proteins (like other proteins) are synthesised by the ribosomes in the cytoplasm. Once made, they will be imported into the nucleus (through the nuclear pore; import regulated by the nuclear pore complex of proteins) to the nucleolus where it is assembled with the rRNA. Hence, assembly of ribosomal subunits (picture below) is carried out by the nucleolus. Once assembled, the ribosomal subunits are then exported out of the nucleus and into the cytoplasm.