A closed ecosystem is a self-contained environment where all living organisms and their physical surroundings interact in a balanced way. In this system, energy and nutrients cycle within the ecosystem, and there is minimal exchange with the outside environment. Examples of closed ecosystems include biodomes and aquariums, where conditions are carefully controlled to sustain life. In a closed ecosystem, the organisms rely on each other for survival, creating a delicate balance. For instance, plants produce oxygen through photosynthesis, which animals need to breathe, while animals produce carbon dioxide that plants use. This interdependence helps maintain the ecosystem's stability.