1. Cell Structure


Ribosomes are the cellular structures responsible for protein synthesis. They are not organelles. They can be small dot-like structures that float freely in the cytoplasm (known as free ribosomes) or they may be attached to the plasma membrane’s cytoplasmic side or the endoplasmic reticulum’s cytoplasmic side and the nuclear envelope’s outer membrane, and called attached ribosomes.

Ribosomes are large protein and RNA complexes consisting of two subunits, a large and a small. Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA transcribes 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.

Ribosome, mRNA, tRNA, and amino acids in a growing peptide chain, labeled

Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), there are 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 and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.

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