Final answer:
Phosphate has a greater osmotic activity than potassium in the intracellular fluid because while potassium is more abundant, it has very little effect on osmotic pressure and can cross the cell membrane, whereas phosphate contributes significantly to osmotic pressure and does not freely cross the membrane.
Step-by-step explanation:
The osmotic activity within the intracellular fluid (ICF) is influenced by various solutes, among which potassium (K+) and phosphate ions play significant roles. Potassium is the principal cation in the ICF, with concentrations about 30 times higher inside the cells than outside.
Since potassium is actively transported into the cells by the sodium-potassium pump, it accumulates within cell cytosol and contributes to maintaining the cell's electrical neutrality and osmotic balance. However, potassium itself has a minimal effect on osmotic pressure compared to other solutes.
Phosphate ions in the ICF are present in higher concentrations compared to the extracellular fluid (ECF), and along with proteins, contribute significantly to the osmotic pressure within the cell due to their inability to freely cross the cell membrane.
Considering these factors, phosphate would have a greater osmotic activity compared to potassium within the intracellular fluid, mainly because potassium's high concentration is counterbalanced by its ability to cross the membrane under certain conditions, whereas phosphate ions are largely retained within the cells.