In the context of cellular transport, what is the definition of diffusion?

Diffusion is defined as the spontaneous spreading of particles from areas of high concentration to areas of low concentration. This process occurs naturally due to the random movement of particles, driven by their kinetic energy. As particles move, they collide with each other, causing them to spread out and fill the available space evenly over time. This movement continues until equilibrium is reached, where the concentration is uniform throughout the medium.

In biological contexts, diffusion plays a crucial role in various processes, such as the movement of gases (like oxygen and carbon dioxide) across cell membranes, allowing for respiration and photosynthesis. This form of transport doesn't require energy expenditure, making it a passive process that relies solely on the concentration gradient.

The other options identify different processes related to cellular transport. Movement against the concentration gradient refers to active transport, which requires energy input. Movement through a selective barrier suggests processes like facilitated diffusion, which involves specific transport proteins. Passive transport requiring a carrier protein could also refer to facilitated diffusion, but that process still relies on concentration gradients rather than spontaneous movement.