1. Oxygen (O2): Oxygen is a small, nonpolar molecule that can easily pass through the lipid bilayer of the cell membrane. This is essential for cellular respiration, where oxygen is used to produce energy.

2. Carbon Dioxide (CO2): Similar to oxygen, carbon dioxide is a small, nonpolar molecule that can readily diffuse across the cell membrane. This is how cells expel the waste product carbon dioxide from cellular respiration.

Important Note: While these substances can pass freely, their movement is still influenced by concentration gradients. They will move from areas of higher concentration to areas of lower concentration.