Transport
The protein is working to transport substances across the membrane.
Answer: The bacteria break down organic material that contain nitrogen and release it back into the cycle.
For example, bacteria breaks down a leaf, then it will release the nitrogen into the soil for plant.
The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left.