Simple Diffusion and Facilitated Diffusion

Molecules have thermal motion and a result of this movement is diffusion. Diffusion is achieved when molecules randomly move from one space into another space until equilibrium between the spaces is achieved. The molecules will move from a higher concentration into a lower concentration, and because it does not utilize energy it is passive transport. There are two types of diffusion that are important for the functioning of life. One is called simple diffusion. An example of this type can be found when examining respiration in humans. By contracting the diaphragm and expanding the thoracic cavity, humans are able to inhale oxygen. As oxygen travel through the nostril and mouth, the thoracic cavity is released causing exhalation. The gas exchange that occurs in the alveoli is between carbon dioxide and oxygen through simple diffusion. Through this process, the lungs are able to absorb oxygen, and as the body exhales, the increased air pressure in the alveoli will force carbon dioxide up the trachea and out of the body. The second type of diffusion is called facilitated diffusion. This is also a passive transport process; however, there are some molecules and ions that are unable to penetrate certain phospholipid bilayers. These molecules and ions must use the help of transport proteins to cross against the concentration gradient. An example of facilitated diffusion would be in the mammalian kidney. In the inner medulla within the collecting duct, high urea concentration is diffused out of the duct and into the interstitial fluid with the help of urea transporter proteins allowing mammals to maintain homeostasis.
In human lungs, there are millions of alveoli that diffuse oxygen through the epithelium and into the capillaries. The epithelium membrane of the alveoli allows for oxygen to diffuse easily into the capillaries. Then, carbon dioxide will diffuse the opposite direction from the capillaries into the epithelium and through the trachea. This is how humans are able to utilize oxygen and release carbon dioxide. In human kidneys, as the collecting ducts of a kidney increases the concentration gradient, more water is lost and the ducts becomes more permeable to urea. As the concentration of the urea and the urea transport protein increases, the quicker urea diffuses out into the ducts and into the interstitial fluids.