Need help with RIBOSOMES

[Can I Have Food]

Ribosomes consist of two subunits, one large and one small. Each subunit is composed of rRNA and proteins.

Ribosomes are made in the nucleolus... but there exists two kinds: free ribosomes that are found in the cytoplasm, and bound ribosomes that line the outer memberane of the endoplasmic reticulum.

Why are they located in different places; what's the purpose of that? Are they both made in the nuceolus?

What is the purpose of the small and large subunits? What is the purpose of the rRNA and proteins that consist the subunits?

Thanks. 🙂

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Ribosomes consist of two subunits, one large and one small. Each subunit is composed of rRNA and proteins.

Ribosomes are made in the nucleolus... but there exists two kinds: free ribosomes that are found in the cytoplasm, and bound ribosomes that line the outer memberane of the endoplasmic reticulum.

Why are they located in different places; what's the purpose of that? Are they both made in the nuceolus?

What is the purpose of the small and large subunits? What is the purpose of the rRNA and proteins that consist the subunits?

Thanks. 🙂

I took a whole class on this stuff and it's very complex, and it's hazy to me now, so I won't attempt to tackle your whole Q. But...

I think only the RNA component is made in the nucleolus. I'd assume the protein part is probably synthesized in the cytoplasm like regular proteins. This detail isn't very important. Questions about ribosomes will likely cover their function.

Regarding subunits. One subunit binds before the other. The ribosome comes together as a complex and then starts translation. I think multiple subunits allows it to dock with the mRNA better...otherwise it would have to have a complex hinge-type mechanism. This is just a guess...but structure usually relates to function. The answer might also be evolutionary...perhaps one came before the other? Who knows.

The ones bound to the ER are there because the newly-synthesized peptides are inserted into the lumen of the ER as it's being synthesized. These proteins are generally bound to leave the cell. They will be modified in the ER and transported to the Golgi before being exocytosed. Synthesizing it first somewhere else would require a complex shuttle delivery system to bring them to the ER...it's much more simple to keep it bound where the proteins need to be.

The free ribosomes generally produce proteins that remain within the cell. Id assume they find their rna faster since they are diffusing around freely and that this state is therefore favorable for translation unless the protein is headed for the ER. This explains why it is favorable for some to be free and others to be bound.

 

[jaxasp]

I took a whole class on this stuff and it's very complex, and it's hazy to me now, so I won't attempt to tackle your whole Q. But...

I think only the RNA component is made in the nucleolus. I'd assume the protein part is probably synthesized in the cytoplasm like regular proteins. This detail isn't very important. Questions about ribosomes will likely cover their function.

Regarding subunits. One subunit binds before the other. The ribosome comes together as a complex and then starts translation. I think multiple subunits allows it to dock with the mRNA better...otherwise it would have to have a complex hinge-type mechanism. This is just a guess...but structure usually relates to function. The answer might also be evolutionary...perhaps one came before the other? Who knows.

The ones bound to the ER are there because the newly-synthesized peptides are inserted into the lumen of the ER as it's being synthesized. These proteins are generally bound to leave the cell. They will be modified in the ER and transported to the Golgi before being exocytosed. Synthesizing it first somewhere else would require a complex shuttle delivery system to bring them to the ER...it's much more simple to keep it bound where the proteins need to be.

The free ribosomes generally produce proteins that remain within the cell. Id assume they find their rna faster since they are diffusing around freely and that this state is therefore favorable for translation unless the protein is headed for the ER. This explains why it is favorable for some to be free and others to be bound.

I might be hazy on this too, but I don't think the second part is completely right. I think when a ribosome functions in the cytosol, its creates proteins that are meant to stay in the cytosol. When a ribosome translates into the ER lumen, there is usually a marker for the end protein to be sent to somewhere not in the cytosol, whether it is in the plasma membrane, the nucleus, a lysosome or to be secreted into the ECM.

 

[matth87]

I might be hazy on this too, but I don't think the second part is completely right. I think when a ribosome functions in the cytosol, its creates proteins that are meant to stay in the cytosol. When a ribosome translates into the ER lumen, there is usually a marker for the end protein to be sent to somewhere not in the cytosol, whether it is in the plasma membrane, the nucleus, a lysosome or to be secreted into the ECM.

Yeah there is a marker on the nascent polypeptide chain on the N-terminus called the signal sequence. This marker helps thread the protein through the ER plasma membrane. Depending on what kind of protein it is the marker will be different. If it is destined for the outside of the cell the marker will be cleaved. If it is destined as a channel protein then it will actually be threaded through the ER the ER will endocytise the threaded portion of its membrane and this membrane vesicle will eventually fuse to be a membrane channel in the outside of the cell.

 

[0Complications]

Yeah there is a marker on the nascent polypeptide chain on the N-terminus called the signal sequence. This marker helps thread the protein through the ER plasma membrane. Depending on what kind of protein it is the marker will be different. If it is destined for the outside of the cell the marker will be cleaved. If it is destined as a channel protein then it will actually be threaded through the ER the ER will endocytise the threaded portion of its membrane and this membrane vesicle will eventually fuse to be a membrane channel in the outside of the cell.

To add just a smidge:

This is why proteins formed inside of the ER lumen end up outside of the cell. When the vessicle fuses with the membrane (you can think of it as attaching to the plasma membrane, then being pulled open. This means that all the contents within the vessicle, when it is fused, are now on the outside of the cell.

Also, to the OP's question, the free floating ribosomes are the same ones that you see on the rough ER, it's just that the signal peptide binds to a receptor on the ER, that attaches the ribosome temporarily to the outside of the membrane as it feed in the polypeptide sequence. Once it is done translating the mRNA it is free to float about the cytosol again.

I like to think of images of ER bound ribosomes as the ribosomes that were caught in the act.