Secondary Active Transport

By the time we finish talking about the types of transport in Chapter 3, most students' heads are swimming.  Usually we have discussed several types of transport; Diffusion, Facilitated Diffusion, Osmosis, and Active Transport.  Our example of Active Transport usually involves the Sodium-Potassium pump, which is quite a discussion topic itself.

So when we finish those four types of transport, and students are fighting to try to make sense of the similarities, differences, driving forces, involvement of proteins and energy .... then the book throws on top of that a mention of Secondary Active Transport.  The discussion is limited to three paragraphs and a figure, so this type of transport just gets a brief mention at the end of a long discussion about types of transport.

Fast-forward to the end of A&P II when we discuss the Digestive and Urinary systems.  Figure 24.21 mentions that glucose transport depends upon secondary active transport using sodium.  Figure 26.12 and 26.13 show the reabsorption of several molecules and ions using secondary active transport mechanisms.  But your book doesn't include much of a review of secondary active transport - instead it assumes that you remember it from the end of the long discussion on transport from Chapter 3.

So let's take a second to review secondary active transport in this post.

Secondary Active Transport is called secondary, because it uses the sodium ion gradient - rather than ATP directly - as the driving force.

The sodium/potassium pump moves three sodium ions out of the cell for every two potassium ions moved into the cell.  So it creates a high concentration of sodium outside the cell.

Other molecules can take advantage of the sodium gradient to move across the cell membrane.  If a transport moves sodium down its concentration gradient, and moves the other ion/molecule as well, the other ion/molecule can simply ride along as sodium moves down its concentration gradient.

One example is the Na/glucose symporter.  Glucose needs to get into the cell in order to undergo the chemical reactions that make ATP.  So ... glucose needs into the cell.  Sodium wants to go down its concentration gradient to get into the cell.  And we have a transporter that can carry both sodium and glucose.  They both move into the cell, and everyone is happy.

The energy used to move glucose across the membrane was the potential energy stored in the sodium concentration gradient.  That sodium concentration gradient was set up by active transport.  Thus, this type of transport that uses the sodium gradient is called "secondary" active transport.