微微君：本期推送一則有關「主動轉運」的英文教材片段，以拋磚引玉。
Active Transport
主動轉運
Whether a substance is being transported actively or passively relates to the following rules:
If the direction of the net flux is down an electrochemicalgradient, the transport is passive.
If the direction of the net flux is up an electrochemical gradient,the transport is active.
The two basic forms of active transport—primary and secondary active transport--differ in the nature of the energy source expended. Primary active transport uses ATP or some other chemical energy source directly to transport substances. Proteins involved in primary active transport are called pumps. Secondary active transport is powered by a concentration gradient or an electrochemical gradient that was previously created by primary active transport.
The transport proteins that carry out active transport are similar to carriers in many respects, but possess an ability that carriers do not have: Active transporters can harness energy to drive the transport of molecules in a preferred direction across a membrane. In contrast, carriers show no such preference; in the absence of an electrochemical gradient, a carrier is equally likely to transport molecules in either direction. Recall that two factors affect the binding of a solute to a carrier: affinity and the concentration (or electrochemical) gradient. The difference that allows active transporters, and not carriers, to move a molecule in a certain direction relates to the affinity of the transport protein for the molecule being transported. Whereas carrier proteins have equal affinity for the molecule on either side of the membrane, the affinity of active transporters is greater when the binding site is exposed to one side of the membrane than when the binding site is exposed to the other side.
Primary Active Transport
原發性主動轉運
The membrane proteins that perform primary active transport function both as transport proteins and as enzymes. In their capacities as enzymes, most of these proteins harness energy from ATP by catalyzing ATP hydrolysis. For this reason, these proteins are frequently referred to as ATPases. To understand how primary active transport works, we concentrate on the sodium-potassium pump (also called the Na+/K+ pump or Na+/K+ ATPase), which is present in nearly every cell and is crucial to several important physiological processes, including electrical signaling in neurons and the absorption of glucose by intestinal epithelial cells. A model explaining the pump’s action is depicted in Figure 4.14.
Figure 4.14 shows that the Na+/K+ pump transports Na+ and K+ ions in opposite directions across the plasma membrane. For each cycle of the pump, three Na+ ions are transported out of the cell, and two K+ ions are transported into the cell. Transport is active in each case because both types of ions move up their electrochemical gradients. In each cycle of the pump, one ATP molecule is hydrolyzed to provide the energy required for this process.
Secondary Active Transport
繼發性主動轉運
Earlier in the text, we saw that metabolic reactions can be coupled together, so that an exergonic reaction (glucose oxidation,for example) can be used to drive an endergonic reaction (such as ATP synthesis). In secondary active transport, something similar happens: A transport protein couples the flow of one substance to that of another. One substance moves passively down its electrochemical gradient, in the process releasing energy that is then used to drive the movement of the other substance up its electrochemical gradient.
Figure 4.15 shows two types of secondary active transport systems in cells. In Figure 4.15a, the two transported substances are moving in the same direction (in this case, inward across the cell membrane); in Figure 4.15b, they are moving in opposite directions. The transport of two substances in the same direction is called cotransport (or sometimes symport). The example of cotransport
shown in Figure 4.15a is sodium-linked glucose transport, which couples the inward flow of Na+ with the inward flow of glucose molecules. In this process, Na+ moves down its electrochemical gradient and releases energy that then drives the flow of glucose against its concentration gradient. Specifically, Na+ increases the affinity of the carrier for glucose when the binding site for glucose faces the extracellular fluid. The transport of two substances in opposite directions is called countertransport (or sometimes antiport or exchange). The example of countertransport shown in Figure 4.15b is sodium-proton exchange, in which the inward flow of Na+ is coupled to the outward flow of protons (H+). Here, energy released from the flow of Na+ down its electrochemical gradient is harnessed to drive the flow of H+ up its electrochemical gradient. Specifically,Na+ increases the affinity of the carrier for H+ when the binding site for H+ is facing the intracellular fluid.
資料引自：Stanfiled CL. Principles of Human Physiology. 5th ed. Glenview: Pearson Education,2012
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