Final answer:
A system is a group of organs, tissues, and cells that work together to keep our bodies alive and functioning. Organ systems like the skeletal and nervous systems are crucial for sustaining life and enabling the organism to perform essential physiological functions.
Step-by-step explanation:
An organ system is a group of organs, tissues, and cells that work together to perform a specific function and sustain life. For instance, the skeletal system not only provides structure to our bodies but also protects our internal organs. Similarly, our brain, spinal cord, and nerves collaborate as part of the nervous system to operate our body's movements and responses. Organ systems are vital for the organism level, which is the highest level of organization and refers to an individual living being that can independently carry out all physiological functions necessary for life.