1. Oxygen (O2): Oxygen is essential for cellular respiration, the process by which cells generate energy. It moves from the higher concentration outside the cell to the lower concentration inside the cell by simple diffusion, a form of passive transport that doesn't require energy.
2. Carbon Dioxide (CO2): A byproduct of cellular respiration, carbon dioxide needs to be removed from the cell. It also moves by simple diffusion, moving from the higher concentration inside the cell to the lower concentration outside the cell.
3. Water (H2O): Water is crucial for maintaining cell volume and carrying nutrients. It moves across the cell membrane through osmosis, a type of passive transport driven by the concentration gradient of water.
Note: The size and polarity of a molecule influence its ability to cross the cell membrane passively. Small, nonpolar molecules like oxygen and carbon dioxide can easily pass through the lipid bilayer of the cell membrane. Water, while polar, can move through aquaporins, specialized channels in the membrane.
