Passive transport
At least three major types of passive transport across cellular membranes are recognized:
Dif.Ji.1sion through pores. It is postulated that passage of water and very small water-soluble molecules takes place through pores or water-filled channels in the membrane. The transport by this route can proceed in either direction depending on the gradient. In addition to solubility, the size of the molecule is the major limiting factor; both monosaccharides and amino acids are too large to enter the pores.
Dif.Ji.1sion through membrane. Instead of molecular size, solubility within the membrane is the major limiting factor. This mechanism is believed to be of importance in the absorption of monoglycerides, fatty acids, and other substances that are lipid in nature.
Carrier-mediated (facilitated) diffusion. Watersoluble compounds that cannot pass through the pores can cross the membrane through carriermediated processes. These carriers, located in the lipoprotein membrane, interact with the substances to be transported and facilitate their passage, probably by rendering them temporarily membrane-soluble. Some carriers may be involved ,"lith two or more compounds, such as glucose and galactose. These sugars then compete for the available carriers. Because the number of carriers is also probably limited, carrier transport slows down or ceases as vehicles become unavailable. This passive carrier-mediated diffusion continues only until there is a balance between the solutes on both sides of the barrier.
