Human Body

Cells are the structural and functional units of life. The human body is composed of over 75 trillion cells that contain between 75 - 90% water. In order for cells to survive, they must exchange materials like water, nutrients, and gases (CO2 and O2). These molecules move in and out of cells through a plasma membrane that is composed of a mosaic of protein molecules moving around in a fluid bilayer of phospholipids (fluid mosaic model). This composition creates an environment that regulates the movement of materials into and out of the cell. There are two primary ways molecules move through the cell membrane: passive and active transport. Active transport requires a cell to use energy (usually in the form of ATP) to move a molecule against a concentration gradient. During passive transport, the cell does not expend energy to move materials across the cell membrane.

Diffusion is defined as the movement of molecules from a region of high concentration to a low concentration without the assistance of a transport protein. Molecules are propelled by kinetic energy (the energy of motion). The botanist, Robert Brown, was the first person to observe the random movement of small particles which is now defined as the Brownian movement. Several factors can influence the rate of diffusion. These include the steepness of the concentration gradient, temperature, and size of the molecule. Molecules will move by diffusion until they reach a state of dynamic equilibrium, equal movement of molecules in both directions.

A major advance in the field of diffusion came from the work of the German physiologist Adolf Eugen Fick (1829-1901). Graham's work on the diffusion of salt in water stimulated Fick to develop a mathematical framework for diffusion phenomena using the analogy between Fourier's law of thermal conduction (or Ohm's law of electric conduction) and diffusion. He postulated that the flux of salt occurring in a unit of time between two infinitesimal test volumes of space filled with solutions of the same salt but with different salt concentration must be directly proportional to the concentration difference and inversely proportional to the distance between the test volumes. In addition, Svante Arrhenius, described the temperature dependence of reaction rates and diffusivities, generally referred to as Arrhenius Law. He proposed this relation in his paper of 1889 to describe reactions rate of cane sugar as well as reaction