Phosphorus cycle in nature is a unique cycle compared to the other natural biological and chemical cycles such as the carbon, nitrogen, sulfur and water cycles, as there is no gaseous phase in the phosphorous cycle. Due to the prevailing atmospheric temperature and pressure which are not appropriate for the formation of gases associated with phosphorus, the compounds in nature where phosphorus can be found are not gases. Phosphorus can therefore be found majorly in sedimentary rocks.
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.
It allows things to go in and out of the cell. Plants have rigid cell walls which makes it harder for things to go in and out if the cell. Animal cells are softer so they can diffuse substances easier