The table below shows the diameters of three different molecules, P, Q, and R, present in human blood plasma.
| Molecule | Diameter (nm) |
|---|---|
| P | 0.4 |
| Q | 0.9 |
| R | 12.0 |
Molecule R represents a type of protein, whereas P and Q represent smaller molecules like urea and glucose.
Explain why Molecule R is not normally found in urine.
