Describe the levels of biological organization

The biological levels of organization of living things arranged from the simplest to most complex are: organelle, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystem, and biosphere.

The thin filaments are pulled toward the center of the sarcomere by the myosin heads of the thick filament. This shortens the sarcomeres in the muscle fiber and causes the whole skeletal muscle to contract. The cross bridge cycle ends when Ca2+ are actively transported back into the sarcoplasmic reticulum. The cycle repeats as long as the binding sites on actin remain exposed, and both Ca2+ and ATP are available.