Ribosomes

Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis. View ribosomes through an electron microscope. You will notice that they appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. Generally, they attach themselves to the cytoplasmic side of the plasma membrane. However, they could also attach to the cytoplasmic side of the endoplasmic reticulum and the outer membrane of the nuclear envelope. See the cell diagram below.

Can you identify the ribosomes here? Image Attribution: OpenStax Biology

Electron microscopy shows us that ribosomes, which are large complexes of protein and RNA, consist of two subunits, which we aptly refer to as large and small (see image below). Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA is transcribed into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.

Ribosomes consist of a large subunit (top) and a small subunit (bottom). During protein synthesis, ribosomes assemble amino acids into proteins. Image Attribution: OpenStax Biology

Because proteins synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), you can find ribosomes in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes. In fact, the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.