Correct Option
The correct option isIt allows the movement of carbon dioxide and oxygen across the cell membrane.
Explanation
Diffusion is a physical process defined by the net movement of molecules from a region of higher concentration to a region of lower concentration. In cellular biology, it serves as a fundamental mechanism of passive transport across the plasma membrane, requiring no external energy input.
Option Analysis
- It requires energy in the form of ATP to move substances. is Incorrect: Diffusion is a passive process and does not require metabolic energy in the form of ATP. Transport mechanisms that require ATP are classified as "active transport."
- It allows the movement of carbon dioxide and oxygen across the cell membrane. is Correct: The cellular exchange of gases, such as carbon dioxide ($CO_2$) and oxygen ($O_2$), occurs via simple diffusion. These molecules move across the semi-permeable cell membrane driven solely by the difference in concentration inside and outside the cell.
- It only occurs in liquid mediums and not in gaseous exchange. is Incorrect: Diffusion is not limited to liquid mediums; it occurs in gases, liquids, and solids. The exchange of respiratory gases ($O_2$ and $CO_2$) is a gaseous exchange process facilitated by diffusion.
- It always moves substances from a region of low concentration to high concentration. is Incorrect: Diffusion naturally moves substances "down" the concentration gradient, meaning from a region of high concentration to low concentration. Movement from low to high concentration is against the gradient and requires active transport mechanisms.
Key Takeaway: Diffusion is a form of passive transport essential for the exchange of gases like oxygen and carbon dioxide in cells, operating strictly from high to low concentration without the expenditure of ATP.