5.4 Active Transmembrane Transport
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Compare and contrast the mechanisms of primary and secondary active transport, including the types of carrier proteins and energy sources involved.
- Explain how primary active transport supports secondary active transport.
- Give examples of carrier proteins involved in both types of active transport.
In active transmembrane transport, solutes move across a membrane up (against) their concentration gradient (neutral solutes) or electrochemical gradient (charged solutes). This requires a carrier protein and an energy source to move solutes away from equilibrium. In other words, for a substance to move across a membrane against its concentration or electrochemical gradient, the cell must expend energy. Active transport is used for a variety of cellular processes, such as nutrient uptake (e.g., sugars, amino acids), waste/water removal, and maintaining non-equilibrium concentrations of specific ions. Active transport cannot be used for solutes that can cross membranes via simple diffusion. We will two types of active transport in this section: primary active transport and secondary active transport.
Thermodynamics of Moving Against a Gradient
One useful framework for thinking about active transmembrane transport is a thermodynamic framework. This will only really make sense if you read ahead: peruse Chapter 6.1 and then come back here. (Or ignore skip this section for now and come back to it after you’ve read Chapter 6.1 later in the course). When solutes move down (with) their concentration gradient or electrochemical gradient (as in passive transport), that movement is an endergonic process (negative free energy; DG < 0), and should happen spontaneously without any energy input from the cell. However, when solutes move up (against) their concentration gradient or electrochemical gradient (as in active transport), that movement is an endergonic process (positive free energy; DG > 0). The cell must therefore pair this endergonic process with an exergonic process to make the net (total) free energy of transport slightly negative. This exergonic process is the “energy source” that is often described for active transport.
Two energy sources exist for transporting low-molecular weight solutes across membranes via active transport. Primary active transport directly uses energy from an exergonic chemical reaction (usually ATP hydrolysis) to move solutes against a concentration or electrochemical gradient across a membrane. One example of this is the active transport of hydrogen ions (protons, H+) via the proton pump in Figure 5.26; you can see the ATP being hydrolyzed to ADP + Pi by the pump, a chemical reaction that releases enough free energy to move H+ against its electrochemical gradient. This proton pump is an example of a uniporter carrier protein (Chapter 5.2). Secondary active transport also requires an energy source, but does not d