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Proteins
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. The figure below is an amino acid. Note the nitrogen side of the structure. This nitrogen structure is what makes protein different from carbohydrates.
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.
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. The cells use these amino acids to build proteins such a keratin found in hair or fingernails.
Summary
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.
Source: CK12
The link below explains the biomolecule . 
https://www.wisc-online.com/learn/natural-science/life-science/ap13304/biomolecules--the-proteins
