What term describes the movement of particles spreading out from an area of higher concentration to lower concentration?

The term that describes the movement of particles spreading out from an area of higher concentration to lower concentration is diffusion. This process involves particles moving down their concentration gradient, which is the gradual change in the concentration of solutes in a solution as a function of distance.

Diffusion is a passive transport mechanism, meaning that it does not require energy input from the cell, as particles move naturally from regions where they are more concentrated to regions where they are less concentrated in order to achieve equilibrium. This process can be observed with gases, liquids, and even solutes in solutions, making it fundamental to many biological processes, such as the exchange of oxygen and carbon dioxide in the lungs and the absorption of nutrients in the intestines.

In contrast, osmosis specifically refers to the diffusion of water across a selectively permeable membrane, while facilitated transport involves specific transport proteins to help move substances across a membrane without using energy. The concept of a concentration gradient is related but serves as a description of the condition that drives diffusion rather than the process itself.