Phosphorus is a critical constituent of cellular fluids because of its role: a. as a sodium ion transporter in body cells b. in the synthesis of amino acids c. in the synthesis of phospholipids to transport fats through the blood d. in binding with calcium in the muscle tissues e. as a buffer in maintaining the optimum body pH

e is correct

Explanation :

Phosphorus is critical because this element is required for enzymatic activities that take place during glycolysis, in the metabolism of glutamine to form ammonia (ammoniagenesis) and also in oxidative phosphorylation, where the cells produce energy from the generation of ATP from ADP.

In serum (blood plasma), the inorganic phosphate is combined with oxygen (O2) to form diverse types of phosphates (PO4) molecules that play key functions in muscle activity, nervous system development, and bone growth.  

Serum phosphate is a critical buffer that maintains the acid‐base balance in the urine. The serum phosphate level in an adult must be in the range of 2.5 to 4.5 mg/dl, being higher levels indicative of hyperphosphatemia. This inorganic phosphate has important functions during the ATP synthesis, cyclic AMP, post-translational modification (i.e., protein phosphorylation), and also as an intracellular buffer.