Tissues are primarily composed of what building block?

Tissues are primarily composed of cells, which are the fundamental building blocks of all living organisms. Cells come together to form different types of tissues, each specialized to perform specific functions within the body. For example, muscle tissue is composed of muscle cells that work together to facilitate movement, while epithelial tissue comprises cells that create barriers and protect surfaces.

While atoms and molecules are essential components of cells—since cells themselves are made up of various molecules such as proteins, lipids, carbohydrates, and nucleic acids—it is the organization of these molecules into cells that ultimately enables the formation of tissues. Organelles, on the other hand, are specialized structures within cells that perform particular functions, but they do not constitute the building blocks of tissues. Therefore, in the context of tissues, cells are the correct answer, as they are the primary units that combine to form tissues and support the physiological processes of the body.