Figure 11–11 Transcellular transport. The transcellular transport of glucose across an intestinal epithelial cell depends on the nonuniform distribution of transporters in the cell’s plasma membrane. The process shown here results in the transport of glucose from the intestinal lumen to the extracellular fluid (from where it passes into the blood). Glucose is pumped into the cell through the apical domain of the membrane by a Na+-powered glucose symporter. Glucose passes out of the cell (down its concentration gradient) by passive movement through a glucose uniporter in the basal and lateral membrane domains. The Na+ gradient driving the glucose symport is maintained by the Na+-K+ pump in the basal and lateral plasma membrane domains, which keeps the internal concentration of Na+ low (Movie 11.2). Adjacent cells are connected by impermeable tight junctions, which have a dual function in the transport process illustrated: they prevent solutes from crossing the epithelium between cells, allowing a concentration gradient of glucose to be maintained across the cell sheet (see Figure 19–18). They also serve as diffusion barriers (fences) within the plasma membrane, which help confine the various transporters to their respective membrane domains (see Figure 10–34).
There Are Three Classes of ATP-Driven Pumps
ATP-driven pumps are often called transport ATPases because they hydrolyze ATP to ADP and phosphate and use the energy released to pump ions or other solutes across a membrane. There are three principal classes of ATP-driven pumps (Figure 11–12), and representatives of each are found in all prokaryotic and eukaryotic cells.
1.
P-type pumps are structurally and functionally related multipass trans-membrane proteins. They are called “P-type” because they phosphorylate themselves during the pumping cycle. This class includes many of the ion pumps that are responsible for setting up and maintaining gradients of Na+, K+, H+, and Ca2+ across cell membranes.
2.
ABC transporters (ATP-Binding Cassette transporters) differ structurally from P-type ATPases and primarily pump small molecules across cell membranes.
3.
V-type pumps are turbine-like protein machines, constructed from multiple different subunits. The V-type proton pump transfers H+ into organelles such as lysosomes, synaptic vesicles, and plant or yeast vacuoles (V = vacuolar), to acidify the interior of these organelles (see Figure 13–37).
Structurally related to the V-type pumps is a distinct family of F-type ATPases, more commonly called ATP synthases because they normally work in reverse: instead of using ATP hydrolysis to drive H+ transport, they use the H+ gradient across the membrane to drive the synthesis of ATP from ADP and phosphate (see Figure 14–30). ATP synthases are found in the plasma membrane of bacteria, the inner membrane of mitochondria, and the thylakoid membrane of chloroplasts. The H+ gradient is generated either during the electron-transport steps of oxidative phosphorylation (in aerobic bacteria and mitochondria), during photosynthesis (in chloroplasts), or by the light-driven H+ pump (bacteriorhodopsin) in Halobacterium. We discuss some of these proteins in detail in Chapter 14.
For the remainder of this section, we focus on P-type pumps and ABC transporters.
A P-type ATPase Pumps Ca2+ into the Sarcoplasmic Reticulum  in Muscle Cells