The cell membrane is selectively permeable and regulates the movement of substances in and out of the cell.

Movement of CO₂:-

CO₂ is produced during cellular respiration. Therefore, it is present in high concentrations inside the cell. This CO₂ must be excreted out of the cell. In the cell’s external environment, the concentration of CO₂ is low as compared to that inside the cell. Therefore, according to the principle of diffusion, CO₂ moves from a region of higher concentration (inside the cell) towards a region of lower concentration (outside the cell). Similarly, O₂ enters the cell by the process of diffusion when the concentration of O₂ inside the cell is low as compared to its surroundings.

Movement of water:-

Water moves from a region of high concentration to a region of low concentration through the plasma membrane. The plasma membrane acts as a semi-permeable membrane, and this movement of water is known as osmosis. However, the movement of water across the plasma membrane of the cell is affected by the amount of substance dissolved in water.

The cell membrane or plasma membrane is a biological membrane that separates the interior of the cell from the outside environment. The plasma membrane is called a selectively permeable membrane because it regulates the movement of substances in and out of the cell. It means that the plasma membrane allows some material to pass through it while at the same time it blocks other material from entering through it.