Which of the following is an example of diffusion?

Diffusion is a fundamental process in biology where molecules move from an area of higher concentration to an area of lower concentration due to random thermal motion. This movement continues until there is an equal concentration of molecules throughout a space, a state known as equilibrium.

The option that describes the movement of sugar molecules from a higher concentration to a lower concentration accurately represents diffusion. This process does not require energy input and is characterized by the passive movement of particles, which is central to many biological functions. For instance, when sugar is dissolved in water, it spreads out from a concentrated area (like a spoonful of sugar) to an area with lower concentration (the rest of the water). This natural tendency toward equilibrium highlights the essence of diffusion.

In contrast, the other options involve processes that do not align with the definition of diffusion. For example, movement against a concentration gradient depicts active transport, which requires energy. The absorption of nutrients through the digestive system typically involves more complex mechanisms, including active and facilitated transport, rather than simple diffusion. The chemical reactions that release energy in cells pertain to metabolic pathways and are not related to the passive movement of molecules as described in diffusion.