Chemistry of Uptake—Thermodynamic and Kinetic Factors in Passive Transport
“Uptake” is used in a more general sense than simply absorption. It is taken to include such phenomena as the incorporation of a specific metal ion into a specific biological functional unit (such as chlorophyll or an enzyme), in addition to transport across a membrane (absorption). The problem of uptake of elements has many facets. Absorption entails the movement of molecules from one location to another across a boundary. The boundary can be a cell membrane; a membrane of a cytoplasmic organelle such as the mitochondrion, chloroplast, or nucleus; or a combination of a cell wall and a cell membrane. Some of the major aspects of absorption are illustrated in Fig. 8–1. There are two classes of transmembrane movement of a substance, whether it be a proton, a metal cation, a metal complex, an anion, or a neutral molecule. One class is known as “active” transport and the other as “passive.” In passive transport, a substance moves along an electrochemical potential gradient; the motion occurs spontaneously without any added expenditure of energy. In active transport, a substance is forced to move against an electrochemical potential gradient; thus, the process requires an input of some sort of energy. In most cases this energy comes from the hydrolysis of ATP. A membrane-bound protein seems to be involved in this kind of transport. In other instances of active transport, the transmembranous movement of a substance is coupled with a second process of material transport that itself may or may not be active.
KeywordsPassive Transport Mucosal Membrane Anion Binding Electrochemical Potential Gradient Transmembranous Movement
Unable to display preview. Download preview PDF.