14 What are proteins and what do they do?

In both prokaryotes and eukaryotes, the major purpose of DNA is to provide the information needed to construct the proteins necessary for the cell can perform all of its functions. Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs.

Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains. There are 20 different types of amino acids that can be combined to make a protein. The sequence of amino acids determines each protein’s unique 3-dimensional structure and its specific function.

Proteins can be described according to their large range of functions in the body, listed in alphabetical order:





Antibodies bind to specific foreign particles, such as viruses and bacteria, to help protect the body.


Enzymes carry out almost all of the thousands of chemical reactions that take place in cells. They also assist with the formation of new molecules by reading the genetic information stored in DNA.


Messenger proteins, such as some types of hormones, transmit signals to coordinate biological processes between different cells, tissues, and organs.
Structural component


These proteins provide structure and support for cells. On a larger scale, they also allow the body to move.
Transport/ storage


These proteins bind and carry atoms
and small molecules within cells and throughout the body. 

You may have noticed that “source of energy” was not listed among the function of proteins. This is because proteins in our diet are typically broken back down into individual amino acids that our cells then assemble into our own proteins. Humans are actually unable to build some amino acids inside our own cells – we require them in our diet (these are the so-called “essential” amino acids). Our cells can digest proteins to release energy, but will usually only do so when carbohydrates or lipids are not available.

foods containing proteins
Figure 4: Examples of foods that contain high levels of protein. (“Protein” by National Cancer Institute is in the Public Domain)

The functions of proteins are very diverse because they are made up of are 20 different chemically distinct amino acids that form long chains, and the amino acids can be in any order. The function of the protein is dependent on the protein’s shape. The shape of a protein is determined by the order of the amino acids. Proteins are often hundreds of amino acids long and they can have very complex shapes because there are so many different possible orders for the 20 amino acids!

Main levels of protein structure
Figure 5: Main levels of protein structure. (“Main protein structure levels en” by LadyofHats is in the Public Domain)

The unique shape for every protein is ultimately determined by the gene that encodes the protein. Any change in the gene sequence may lead to a different amino acid being added to the polypeptide chain, causing a change in protein structure and function. Individuals who are affected by sickle cell anemia can have a variety of serious health problems, such as breathlessness, dizziness, headaches, and abdominal pain. In this disease, the hemoglobin β chain has a single amino acid substitution, causing a change in both the structure (shape) and function (job) of the protein. What is most remarkable to consider is that a hemoglobin molecule is made up of about 600 amino acids. The structural difference between a normal hemoglobin molecule and a sickle cell molecule is a single amino acid of the 600.

The structure of the hemoglobin protein.
Figure 6: The unique shape of the normal hemoglobin protein. (“Structure of hemoglobin Gower 2” by Emw is licensed under CC BY-SA 3.0)


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

OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:QhGQhr4x@6/Biological-Molecules

“What are proteins and what do they do?” by U.S. National Library of Medicine is in the Public Domain


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Mt Hood Community College Biology 102 Copyright © 2016 by Lisa Bartee and Christine Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.


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