Final answer:
In gas exchange, oxygen (O2) and carbon dioxide (CO2) cross the cell membrane by simple diffusion, a passive process where these small, uncharged molecules move from areas of higher to lower concentration without requiring energy.
Step-by-step explanation:
During the process of gas exchange, the substances that cross the membrane using the process of simple diffusion are primarily small, nonpolar or uncharged gaseous molecules. Two critical examples of such substances are oxygen (O2) and carbon dioxide (CO2). Both of these gases move in response to a concentration gradient, meaning they will naturally move from an area of high concentration to an area of lower concentration across the cell's plasma membrane without requiring any energy input from the cell, a passive process.
Oxygen typically diffuses into cells because their metabolic activity creates a lower concentration of oxygen inside, thus the outside concentration is relatively higher. Conversely, carbon dioxide, a byproduct of metabolism, accumulates inside the cell, resulting in a higher internal concentration and, as a result, diffuses out of the cell.
These molecules are able to pass through the lipid bilayer because they are small, uncharged, and hydrophobic, allowing them to move freely through the nonpolar regions of the cell membrane. This process is crucial for maintaining appropriate levels of gases in cells and is a fundamental aspect of respiratory physiology.