Oxygen enters cells by passing through the cell membrane in a process called diffusion, which is a transport process that does not require energy. Diffusion is the way a substance moves from an area of high concentration (the environment outside cells) to an area of low concentration (inside cells).
The process of diffusion requires a membrane that has pores to allow for gas and liquids to pass through, also called a semi-permeable membrane. Small, simple molecules, such as oxygen and carbon dioxide, diffuse into and out of the cell passively. The cell is constantly using up oxygen in different processes within the cell. As a result, the concentration of oxygen is lower inside the cell than outside. Oxygen diffuses into the cell rather than out of it. In contrast, because the cell constantly makes carbon dioxide as a product of cellular processes, the concentration of carbon dioxide is higher in the cell than outside the cell. Thus, unlike oxygen, carbon dioxide diffuses out of the cell.
Water is also transported across the cell membrane using a type of diffusion, called osmosis. Because the cell has many organelles and other molecules within it, water is at a lower concentration within the cell and the net movement of water is out of the cell.