Ribosomes are the machines that make proteins from mRNA. The difference between attached and detached ribosomes can be confusing, but knowing the context of their differences clarifies their functions. Attached ribosomes define the structure of rough endoplasmic reticulum -- RER. However, attached ribosomes and detached, or free, ribosomes are structurally the same. The difference in their function depends on the protein they are making. Proteins destined for places outside the cell are made by ribosomes on the RER. Those meant for the cell’s internal use are made my free-floating ribosomes.
The Shipping Center
Attached ribosomes are free-floating ribosomes that become attached to the surface of the RER. The many ribosomes on its surface are what give the RER its name. The head of the protein being made by a ribosome has a sequence that binds to a signal recognition particle -- SRP. The SRP then moves to the surface of the RER, where it binds its receptor, all the while dragging the protein chain and ribosome with it. At the surface of the RER, the ribosome continues to produce the rest of the protein chain, which is fed into the RER.
The purpose of inserting proteins into the RER is so that they can be shipped to the cell’s surface or out of the cell. For proteins that are meant to be attached to the cell surface membrane, they are inserted into the membrane of the RER as they are being made by a ribosome. Parts of the RER membrane then bud off into a vesicle that eventually fuses with the Golgi body, from which another vesicle will bud off to deliver the membrane-bound protein to the plasma membrane. For proteins that are meant to be secreted from the cell, the ribosome feeds them into the RER, after which they are shuttled in vesicles to the Golgi and then to the cell surface.
For Internal Use
Free ribosomes make proteins that are meant to be kept within the cell. They may remain floating in the cytosol or be sent to organelles such as the nucleus, mitochondria, chloroplasts or peroxisomes. Free-floating ribosomes can work together to produce more protein from one strand of mRNA. Multiple free ribosomes can bind an mRNA to form a complex called a polysome. The purpose of a polysome is the increase the efficiency of translation, or protein production, without having to produce more mRNA.
Stick With the Essentials
Prokaryotes differ from eukaryotes in that prokaryotes are single-celled organisms that do not contain membrane bound organelles such as the nucleus, RER and Golgi. Prokaryotes, such as bacteria, cannot secrete proteins to the cell surface as eukaryotes, such as humans, do through the RER and Golgi. However, bacteria do have free ribosomes that translate protein from mRNA. Along with the nucleoid, which is a yarn-ball of DNA similar to the nucleus of a eukaryote, ribosomes are the closest things to organelles in bacteria but are not because they are not bound by membranes.
- Molecular Biology of the Cell: Membrane-bound Ribosomes Define the Rough ER
- Molecular Cell Biology: Two Proteins Initiate the Interaction of Signal Sequences with the ER Membrane
- Molecular Cell Biology: Secretory Proteins Move from the Rough ER Lumen through the Golgi Complex and Then to the Cell Surface
- The Cell: A Molecular Approach: Insertion of Proteins into the ER Membrane
- The Cell: A Molecular Approach: The Endoplasmic Reticulum and Protein Secretion
- Molecular Cell Biology: Model of Protein Synthesis on Circular Polysomes and Recycling of Ribosomal Subunits
- Molecular Biology of the Cell: Prokaryotes Comprise a Single Membrane-Limited Compartment