Ribosomes are molecular machines found within every living cell that serve as the universal factory for protein synthesis. This process, known as translation, involves reading messenger RNA (mRNA) instructions and linking amino acids to form polypeptide chains. In eukaryotic cells, ribosomes exist in two interchangeable populations: those suspended freely in the cytosol and those temporarily attached to the endoplasmic reticulum (ER) membrane. The destination of the protein being manufactured determines the ribosome’s location.
Structure and Location of Free Ribosomes
A ribosome is a complex made of a large subunit and a small subunit, composed of ribosomal RNA (rRNA) and associated proteins. These subunits are manufactured in the nucleus and exported to the cytoplasm, assembling only when they encounter an mRNA molecule ready for translation. Structurally, free ribosomes are identical to their membrane-bound counterparts.
Free ribosomes are suspended directly in the cytosol, the fluid that fills the cell and surrounds the internal organelles. In this location, they translate the genetic code into proteins that are immediately released into the cellular fluid. Untethered to a membrane, they move throughout the cytosol, often clustering on a single mRNA strand to form a polyribosome (polysome). The status of a ribosome as “free” is a temporary state defined by the specific protein it is currently synthesizing.
The Products of Free Ribosomes
The purpose of free ribosomes is to synthesize proteins destined for use within the cell itself, not for export or insertion into cell membranes. Once synthesis is complete, these proteins are released into the cytosol where they fold into their final shapes. These products support the cell’s metabolic machinery and its internal framework.
Many proteins synthesized by the free population are enzymes involved in metabolic pathways occurring in the cytosol, such as glycolysis. Contractile proteins, like actin and myosin found in muscle cells, are also products of free ribosomes, enabling the cell to move and change shape. Structural proteins that make up the cytoskeleton are also produced in the cytosol.
Proteins meant for specific organelles that are not part of the cell’s secretory pathway are also made by free ribosomes. These include specialized proteins required for the cell nucleus, such as histones that help package DNA and transcription factors that control gene expression. Proteins for mitochondria and chloroplasts are synthesized on free ribosomes and then imported into those organelles after translation is finished. This post-translational import mechanism ensures that these organelles receive the majority of the proteins they need to function.
The Mechanism of Transition to Bound Ribosomes
The decision for a ribosome to remain free or become temporarily bound to the endoplasmic reticulum (ER) is determined by a specific amino acid sequence within the protein being built. This mechanism depends entirely on the instructions encoded in the messenger RNA molecule. All protein synthesis begins on a free ribosome in the cytosol.
If the protein is intended for a destination outside the cytosol, the beginning of its polypeptide chain contains a short sequence of hydrophobic amino acids called a signal sequence. As this signal sequence emerges from the ribosome, it is recognized by the Signal Recognition Particle (SRP), a protein-RNA complex. The binding of the SRP to the signal sequence causes a temporary pause in protein synthesis, stalling the ribosome.
The stalled ribosome-SRP complex then docks onto a receptor protein located on the ER membrane, transforming the area into rough ER. Once docked, the ribosome resumes translation, threading the polypeptide chain directly through a channel into the ER lumen. If the nascent polypeptide lacks this specific signal sequence, the SRP never binds, and the ribosome remains free in the cytosol to complete the protein.