Before revision:  The puzzling variance between \sigma to erythrocytes AQP1 and \sigma to oocytes-expressing AQP1 was also explained .
After revision:  The puzzling variance between \sigma to erythrocytes AQP1 and \sigma solute was found to be chiefly affected by the entrance of the pore while filtration reflection coefficient can be affected by both the entrance and the internal structure of the pore. Using an analytical method, we get the quantitative relationship between osmotic reflection coefficient and the molecule size. The model is verified by comparing the theoretical results with the reported experimental data of aquaporin osmosis. Our work is expected to pave the way for a better understanding of osmosis in bio-system and to give us new ideas in designing new membranes with better performance .