18 Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis. The word “synthesis” means “to combine things to produce something else.” In this context, protein synthesis means combining different amino acids together to form a protein. Ribosomes join amino acids together in a chain to form a protein (Figure 1). This amino acid chain then folds into a complex 3-dimensional structure. The shape of a protein is what gives the protein its specific function.

protein structure levels: primary structure is represented as colored balls labeled with names of amino acids connected together in a chain. Secondary structure shows a green spiral alpha helix and two blue anti-parallel arrows representing beta sheets. In Tertiary structure, a complex 3-dimensional shape containing alpha helices and beta pleated sheets is shown. Tertiary structure shows 4 complex 3-D squiggles of different colors interacting together.
Figure 1 Protein structure. The colored balls at the top of this diagram represent different amino acids. Amino acids are the subunits that are joined together by the ribosome to form a protein. This chain of amino acids then folds to form a complex 3D structure. (Credit: Lady of Hatsfrom Wikipedia; public domain)
 Helpful Hint: Proteins are not typically used as a source of energy for the body. Protein from your diet is broken down into individual amino acids which are reassembled by your ribosomes into proteins that your cells need. Ribosomes do not produce energy.

When viewed through an electron microscope, free ribosomes appear as either clusters or single tiny dots floating freely in the cytoplasm. Ribosomes may be attached to either the cytoplasmic side of the plasma membrane or the cytoplasmic side of the rough endoplasmic reticulum (Figure 2).

a dark purple circle (the nucleolus) is located inside a lighter purple circle (the nucleus). On the outside of the nucleus is a squiggly line (the endoplasmic reticulum) with red dots on it (ribosomes). A squiggly line without dots outside the rough endplasmic reticulum shows the smooth ER. Off to the lower right is a structure like a flattened stack of bubbles, showing the golgi.
Figure 2Ribosomes can be found free in the cytoplasm (not shown in this diagram), or attached to the outer membrane of the nucleus and the rough endoplasmic reticulum (RER). Credit CFCF; Wikimedia; CC license.

Because protein synthesis is essential for all cells, ribosomes are found in practically every cell, although they are smaller in prokaryotic cells. They are particularly abundant in immature red blood cells for the synthesis of hemoglobin, which functions in the transport of oxygen throughout the body. Electron microscopy has shown us that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called large and small (Figure 3). 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.

diagram of a ribosome showing small and large subunit clamped around a purple line showing the mRNA. Out of the top of the ribosome is a growing peptide chain shown as blue balls.
Figure 3 Ribosomes are made up of a large subunit (top) and a small subunit (bottom). During protein synthesis, ribosomes assemble amino acids into proteins.
Helpful Hint: Ribosomes are made up of protein and RNA molecules, they are not surrounded by membrane. This means they are not membrane-bound organelles, even when they are located on the rough endoplasmic reticulum.


Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10


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MHCC Biology 112: Biology for Health Professions Copyright © 2019 by Lisa Bartee is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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