What is diffusion in the context of biology?

Diffusion refers to the process where molecules move from an area of higher concentration to an area of lower concentration. This movement occurs due to the natural tendency of particles to spread out evenly in space. In biological systems, this process is crucial as it allows substances such as oxygen and carbon dioxide to move across cell membranes, facilitating important physiological functions. For instance, in the lungs, oxygen diffuses from the alveoli (where there is a higher concentration) into the blood (where there is a lower concentration), which is essential for respiration.

The other options do not accurately represent the concept of diffusion. Blood clotting is a complex biological process unrelated to the movement of molecules based on concentration. Movement from a lower to a higher concentration typically involves active transport mechanisms, which require energy and do not constitute diffusion. Absorption of nutrients through the cell membrane can involve a variety of mechanisms, but it does not specifically define diffusion as it can occur via other processes, including facilitated diffusion or active transport, depending on the nutrients involved. Thus, the correct understanding of diffusion is foundational in biology, particularly in understanding how substances interact within living organisms.