Proteins are large, complex molecules that play a wide variety of roles in living organisms, including:

* Structural support: Collagen and keratin provide structural support to tissues and organs.

* Enzymes: Catalyze biochemical reactions.

* Hormones: Regulate various physiological processes.

* Antibodies: Help the immune system fight off infections.

* Transport: Hemoglobin carries oxygen in the blood.

* Contractile: Actin and myosin are involved in muscle contraction.