No specialized proteins are required, unless the diffusing molecule must cross a lipid membrane, and even then a nonpolar, lipid soluble molecule can cross without proteinacious assistance. Imagine a molecule of glucose in the cytoplasm of a mammalian cell. In order for this molecule to begin its trek through glycolysis, it must first be phosphorylated. In light of what you learned in the previous chapter, this phosphate transfer can only take place after the glucose, the phosphoenolpyruvate, and the appropriate enzyme have come together to form the enzyme-substrate complex. This coming together is dependent on diffusion .
Diffusion is a special case of passive movement in a chemical field.
In fact, any arrangement of stationary phases gives rise to a chemical field that is conservative. If you are familiar with electromagnetic fields, you will see that the chemical field is related to the gradient of chemical potential just as an electric field is related to the gradient of the electrical potential. In respiratory and cardiovascular physiology and in any experimental system using pumps, you will encounter significant pressure gradients, but animal cell membranes cannot support such gradients. Now we're getting close to the result we want, but to make it useful the velocity, v, needs to be related to quantities we actually measure. Equating this to our expression for the chemical field gives Equation 5 With one last piece of information, provided by Albert Einstein in 1905, this derivation is complete.