in an ecosystem, fungi play the role of decomposers -- they break down dead organic matter and return vital nutrients to the soil. Without fungi, nutrients would not cycle through an ecosystem, causing the breakdown of the entire food chain.
When blood is in the lungs, where oxygen is plentiful, oxygen easily binds to the first subunit and then quickly fills up the remaining ones. Then, as blood circulates through the body, the oxygen level drops while that of carbon dioxide increases. In this environment, hemoglobin releases its bound oxygen.
