Organism/ˈɔːrɡənɪzəm/ Definition 1 of 2Pro Tip 1/3Living Examples Think of organisms as individual entities that grow, reproduce, and respond to their environment. Microscopic algae are organisms that produce oxygen. Pro Tip 2/3Scale Variation Organisms can vary dramatically in size from tiny bacteria to giant whales. The blue whale is the largest known organism. Pro Tip 3/3Habitat Diversity Organisms live in diverse habitats, so specify the environment when possible. Coral reefs are home to numerous marine organisms.