Proteins are large, complex molecules composed of smaller structural subunits called amino acids. All proteins contain carbon, hydrogen, oxygen, and nitrogen, and sometimes sulfur, phosphorus, and iron. Human life could not exist without proteins. The enzymes that are required for all metabolic reactions are proteins. Proteins also are important to structures like muscles, and they act as both transporters and signal receptors.
| Type of Protein |
Examples of Functions |
| Defensive |
Antibodies that respond to invasion |
| Enzymatic |
Increase the rate of reactions; build and breakdown molecules |
| Hormonal |
Insulin and glucagon, which control blood sugar |
| Receptor |
Cell surface molecules that cause cells to respond to signals |
| Storage |
Store amino acids for use in metabolic processes |
| Structural |
Major components of muscles, skin, hair |
| Transport |
Hemoglobin carries oxygen from lungs to cells |
