What is defined as a group of similar cells working together?

A group of similar cells working together is defined as tissue. Tissues are essential building blocks in multicellular organisms, as they combine specialized cells to perform specific functions. For example, muscle tissue is made up of muscle cells that work together to enable movement, while nervous tissue consists of neurons that transmit signals throughout the body.

Other levels of biological organization build upon tissues. An organ is composed of multiple types of tissues that work together to carry out a specific function, like the heart or lungs. An organ system is a group of organs that work together to perform complex functions (such as the circulatory system), and an organism refers to an entire living entity with the ability to carry out all life processes. Thus, understanding tissues is fundamental to grasping how more complex structures and systems operate within living organisms.