Describe the passive movement through diffusion and explain Fick's Law

Diffusion is the net passive movement of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration. It continues until the concentration of substances is uniform throughout. Since the movement is always down the concentration gradient, it requires no energy. 

One major example of diffusion is gas exchange for respiration — this is the process used in oxygen entering a cell, and carbon dioxide leaving. The blood system in humans continually brings more oxygen to the cell and takes carbon dioxide away, maintaining a high concentration gradient.

Fick's law is used to measure the rate of diffusion. It states that the rate of diffusion across an exchange surfaces (e.g. membrane, epithelium) is = (surface area x difference in concentration gradient) / thickness of surface. The larger the area and difference in concentration is and the thinner the surface is, the quicker rate the diffusion has. Moreover, temperature increases rate of diffusion due to increasing kinetic energy.

An example of animal's body part that increases the rate of diffusion is microvilli. They are extensions of plasma membrane that increase the surface area of the membrane and thus increase the diffusion rate.