<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />
Skip Navigation


Made of smaller units called amino acids, proteins provide energy for cells and perform many cellular functions.

Atoms Practice
Estimated5 minsto complete
Practice Proteins
This indicates how strong in your memory this concept is
Estimated5 minsto complete
Practice Now
Turn In

You may have been told proteins are good for you. Do these look good to you?

Proteins as food. To you, these may not look appetizing (or they might), but they do provide a nice supply of amino acids, the building blocks of proteins. Proteins have many important roles, from transporting, signaling, receiving, and catalyzing to storing, defending, and allowing for movement. Where do you get the amino acids needed so your cells can make their own proteins? If you cannot make it, you must eat it.


A protein is an organic compound made up of small molecules called amino acids. There are 20 different amino acids commonly found in the proteins of living organisms. Small proteins may contain just a few hundred amino acids, whereas large proteins may contain thousands of amino acids. The largest known proteins are the titins, found in muscle, which are composed from almost 27,000 amino acids.

General Structure of Amino Acids. This model shows the general structure of all amino acids. Only the side chain, R, varies from one amino acid to another. For example, in the amino acid glycine, the side chain is simply hydrogen (H). In glutamic acid, in contrast, the side chain is CH2CH2COOH. Variable side chains give amino acids acids different chemical properties. The order of amino acids, together with the properties of the amino acids, determines the shape of the protein, and the shape of the protein determines the function of the protein. KEY: H = hydrogen, N = nitrogen, C = carbon, O = oxygen, R = variable side chain

Protein Structure

When amino acids bind together, they form a long chain called a polypeptide. A protein consists of one or more polypeptide chains. A protein may have up to four levels of structure. The lowest level, a protein’s primary structure, is its sequence of amino acids.

Functions of Proteins

Proteins play many important roles in living things. Some proteins help cells keep their shape, and some make up muscle tissues. Enzymes are proteins that speed up chemical reactions in cells. Other proteins are antibodies, which bind to foreign substances such as bacteria and target them for destruction. Still other proteins carry messages or materials. For example, human red blood cells contain a protein called hemoglobin, which binds with oxygen. Hemoglobin allows the blood to carry oxygen from the lungs to cells throughout the body. A model of the hemoglobin molecule is shown in Figure below.

Hemoglobin Molecule. This model represents the protein hemoglobin. The red parts of the molecule contain iron. The iron binds with oxygen molecules.

A short video describing protein function can be viewed at http://www.youtube.com/watch?v=T500B5yTy58&feature=related (4:02).

As you view Protein Functions in the Body, focus on these concepts:

  1. the amount of protein in each cell,
  2. the roles of different types of proteins.

Proteins and Diet

Proteins in the diet are necessary for life. Dietary proteins are broken down into their component amino acids when food is digested. Cells can then use the components to build new proteins. Humans are able to synthesize all but eight of the twenty common amino acids. These eight amino acids, called essential amino acids, must be consumed in foods. Like dietary carbohydrates and lipids, dietary proteins can also be broken down to provide cells with energy.


  • Proteins are organic compounds made up of amino acids.
  • A protein may have up to four levels of structure. The complex structures of different proteins give them unique properties.
  • Enzymes are proteins that speed up biochemical reactions in cells. Antibodies are proteins that target pathogens for destruction.


Use these resources to answer the questions that follow.

  1. Give 3 examples of proteins.
  2. What determines the protein's function?
  1. How many different proteins are in a cell?
  2. What is the information used to make an individual protein?
  3. What is the part of the cell where proteins are made?
  • http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: Structure and Function of Proteins
  1. How do amino acids link together?
  2. What is a polypeptide?
  3. Describe the hemoglobin protein.


1. State two functions of proteins.

2. Proteins are made out of ____________.

3. Describe the role of hemoglobin.

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Please to create your own Highlights / Notes
Show More

Image Attributions

Explore More

Sign in to explore more, including practice questions and solutions for Proteins.
Please wait...
Please wait...