1. Structural Support and Movement: Proteins provide the framework for cells, tissues, and organs. This includes:

* Collagen and elastin: Fibrous proteins that provide strength and flexibility to skin, tendons, ligaments, and bones.

* Actin and myosin: Contractile proteins that enable muscle movement.

* Keratin: A structural protein found in hair, nails, and skin.

2. Enzymatic Activity: Proteins act as catalysts to speed up chemical reactions within cells. They are highly specific, only working on certain molecules called substrates. Examples include:

* Digestive enzymes: Break down food molecules in the gut.

* DNA polymerase: Replicates DNA during cell division.

* Lactase: Breaks down lactose in milk.

3. Transport and Storage: Proteins help transport substances across cell membranes and throughout the body, and they also store essential molecules.

* Hemoglobin: Carries oxygen in red blood cells.

* Albumin: Transports fatty acids and hormones in the blood.

* Ferritin: Stores iron in the body.

These three functions are vital for life, and proteins play numerous other important roles in living organisms, such as hormonal signaling, immune defense, and cell communication.