Diffusion:
Diffusion is the process by which substances move across a semipermeable membrane from an area of higher concentration to an area of lower concentration. It does not require the expenditure of ATP (adenosine triphosphate), which is the energy currency of cells.

Explanation:
Diffusion is a passive process that occurs due to the random movement of particles. It is driven by the principle of entropy, which states that particles tend to spread out and become evenly distributed. In the context of a cell membrane, diffusion allows substances to move in and out of cells without the need for energy input.

How does diffusion work?
1. Concentration gradient: Diffusion occurs when there is a difference in concentration of a substance across a membrane. This difference is called a concentration gradient.
2. Random motion: Particles in a solution are constantly moving due to their kinetic energy. This random motion causes them to collide and interact with each other.
3. Net movement: In an area with a higher concentration of particles, there will be more collisions and interactions. As a result, particles will move from an area of higher concentration to an area of lower concentration until the concentration becomes equal on both sides of the membrane.
4. Equilibrium: Eventually, the particles will become evenly distributed, and there will be no net movement of particles. This state is called equilibrium.

Factors affecting diffusion:
1. Concentration gradient: The greater the difference in concentration between two areas, the faster the rate of diffusion.
2. Temperature: Higher temperatures increase the kinetic energy of particles, leading to faster diffusion.
3. Surface area: A larger surface area allows for more particles to interact and diffuse, leading to faster diffusion.
4. Distance: The shorter the distance that particles have to diffuse, the faster the rate of diffusion.

Examples of diffusion:
1. Oxygen and carbon dioxide exchange in the lungs: Oxygen from the air diffuses into the bloodstream, while carbon dioxide from the bloodstream diffuses into the air in the lungs.
2. Nutrient absorption in the small intestine: Nutrients from digested food diffuse across the intestinal lining and into the bloodstream.
3. Release of waste products: Waste products such as urea diffuse out of cells and into the bloodstream for excretion by the kidneys.

Conclusion:
Diffusion is a passive process by which substances move across a membrane without the expenditure of ATP. It is a fundamental process that allows for the exchange of essential molecules and the maintenance of cellular homeostasis.