What process requires no energy and allows gases to move across the alveolar walls?

• A. Active transport
• B. Facilitated diffusion
• C. Diffusion
• D. Direct transport

Answer: (C)

The process that requires no energy and allows gases to move across the alveolar walls is diffusion. Diffusion is the natural movement of molecules from an area of higher concentration to an area of lower concentration. In the context of the respiratory system, oxygen and carbon dioxide travel across the thin walls of the alveoli into and out of the blood based on their concentration gradients.

In the alveoli, oxygen is at a higher concentration compared to the deoxygenated blood in the capillaries, so oxygen diffuses into the blood. Conversely, carbon dioxide, which is at a higher concentration in the blood than in the alveoli, diffuses into the alveoli to be exhaled. This process is crucial for gas exchange during respiration and occurs spontaneously without the need for energy expenditure from the organism.

Other processes mentioned, such as active transport, require energy as they involve moving substances against their concentration gradients. Facilitated diffusion involves carrier proteins to help substances cross membranes but still relies on the concentration gradient and does not consume energy. Direct transport, while not typically used in this context, also implies energy usage. Hence, diffusion is the key process described in the question, highlighting its importance in respiratory physiology.