Final answer:
CO₂ and O₂ can diffuse freely across cell membranes without the need for energy, utilizing simple diffusion due to their small, nonpolar, uncharged nature.
Step-by-step explanation:
The statement CO₂ and O₂ are water-soluble molecules that diffuse freely across cell membranes is true. Both oxygen (O₂) and carbon dioxide (CO₂) can indeed diffuse freely across the cell membrane. This is due to their small, nonpolar characteristics and because they are uncharged. The diffusion process they undergo is called simple diffusion, which is a form of passive transport.
This means that they move from regions of higher concentration to regions of lower concentration without the need for cellular energy. In the case of O₂, it typically diffuses into cells where its concentration is lower due to consumption during metabolism. CO₂, on the other hand, is produced as a byproduct of metabolism and thus diffuses out of the cell to areas of lower concentration.