CBSE Class 9 Answered

Diffusion can occur through a cell membrane. The membrane allows small molecules like water (H₂O), oxygen (O₂), carbon dioxide (CO₂), and others to pass through easily. It is said to be permeable to these molecules. If a cell is floating in a water solution (like the ocean) that has some oxygen dissolved in it, the oxygen molecules will move into the cell. They will also move out of the cell at the same rate, and a dynamic equilibrium will exist. However, if the cell uses some of the oxygen as it comes into the cell, more oxygen will move into the cell than out of the cell.

So the oxygen effectively moves from a region of high concentration (the seawater) to a region of low concentration (the cell), and diffusion occurs. Likewise, as the chemical reactions in the cell use up oxygen they produce carbon dioxide. The concentration of carbon dioxide inside the cell increases so that more CO₂ molecules strike the inside of the cell and move out than strike the outside of the cell and move in. So the overall effect is that the CO₂ moves out of the cell.