The main purpose of this study was to investigate the effectiveness of the 7E learning cycle model as an inquiry-based learning on the ninth grade students understanding of the concepts related to the diffusion and osmosis in biology. In addition, the effect of instruction on students attitudes toward biology as a school subject was investigated. As ""Cell, Organism and Metabolism"" unit of ninth grade Biology curriculum includes lots of inquiry-based concepts to understand many life processes; ""Material Transport across the Cell Membrane"" topic was the subject of this investigation. Students understanding of diffusion and osmosis concepts in both control and experimental groups were measured by achievement test developed by researcher, their misconceptions about diffusion and osmosis concepts were measured using the test developed by Odom and Barrow (Odom and Barrow, 1995). The results showed a statistically significant difference between the experimental and control groups. In other words, 7E Learning Cycle Model as an inquiry-based instruction increased students biology achievements and improved their misconceptions of the concepts related to the diffusion and osmosis.