Which molecules are considered structural proteins?

Structural proteins are primarily responsible for providing support and shape to cells and tissues. Collagen and keratin are exemplary structural proteins found in various animals. Collagen is the main component of connective tissues, such as tendons, ligaments, and cartilage, providing strength and flexibility. Keratin is a fibrous protein that forms the structural framework of hair, nails, and the outer layer of skin, contributing to their hardness and protective functions.

Other choices include various types of molecules that serve different biological functions. Enzymes and antibodies are functional proteins involved in catalyzing biochemical reactions and immune responses, respectively. Hormones and neurotransmitters are signaling molecules that relay information throughout the body but do not provide structural support. Starch and glycogen are carbohydrates that serve as energy storage, rather than structural components. Thus, the focus on collagen and keratin highlights their crucial role in the structure and integrity of various biological systems.