Final answer:
B) individual molecules due to thermal motion. Diffusion is the movement of individual molecules due to thermal energy, with substances moving from higher to lower concentrations until equilibrium is reached. It is influenced by factors like the concentration gradient and temperature and does not require energy, as it is a form of passive transport.
Step-by-step explanation:
The process of diffusion involves the random movement of individual molecules due to thermal motion. This crucial biological process enables substances to move from areas of higher concentration to areas of lower concentration until there is an even distribution within a system, a state known as dynamic equilibrium. Factors such as the concentration gradient, particle sizes, system temperature, and the thermal energy of molecules influence the rate at which diffusion occurs.
As molecules possess kinetic energy, they move in a random fashion, often referred to as Brownian motion. Over time, this results in substances spreading evenly throughout a space, with the net movement ceasing when an equilibrium is reached. In biological systems, molecules like oxygen and carbon dioxide diffuse freely across cell membranes, aiding in vital processes such as cellular respiration.
Molecules move 'down the concentration gradient' when diffusing, meaning they move from where they are more concentrated to areas where they are less concentrated. This passive form of transport does not require cellular energy and occurs naturally due to the random motion of particles. Remember, the diffusion of water across a membrane due to a difference in concentration is specifically called osmosis.