Answer
Diffusion is the process by which a substance moves down a concentration gradient until equilibrium of concentrations is established. This process does not require chemical energy and in the body ATP is not involved. This is more correctly called simple diffusion. Some substances that can move by simple diffusion non-polar and hydrophobic solutes. This includes the gases oxygen, nitrogen, and carbon dioxide. Hydrophobic substances like fatty acids, steroids and fat soluble vitamins ( A,D,E, K) also can move through the plasma membrane by simple diffusion. In addition, some polar solutes like water, urea and small molecular weight alcohols also diffuse freely through the lipid bilayer.
Work Step by Step
The second type of diffusion is called facilitated diffusion. This is also diffusion down a solute concentration gradient, but in this case the movement is facilitated by transmembrane proteins that serve as carriers of relevant solute molecules or provide channels for them to pass through. Polar or charged molecule cannot engage in simple diffusion and need the aid of carrier proteins
Some substances that need the help of carriers to diffuse through plasma membranes are the simple sugars glucose, fructose and galactose. The ions of potassium, calcium, sodium and chlorine. also fall into this category.
Some non-polar charged substances can only cross the plasma membrane by primary or secondary active transport. In these processes the enrgy supplied by ATP is required --either directly or indirectly. Examples of of substances involved are ions of iodine, and of hydrogen; however, some substances from the facilitated diffusion group --potassium, calcium , sodium and chlorine , also fall into this class.
