16.3: Active transport

Active transport mechanisms require the cellʼs energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient — that is, if the substanceʼs concentration inside the cell is greater than its concentration in the extracellular fluid (and vice versa) — the cell must use energy to move the substance.

Electrochemical gradient

We have discussed simple concentration gradients — a substanceʼs differential concentrations across a space or a membrane — but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane.

The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium (\(\ce{K+}\)) and lower concentrations of sodium (\(\ce{Na+}\)) than the extracellular fluid. Thus in a living cell, the concentration gradient of \(\ce{Na+}\) tends to drive it into the cell, and its electrical gradient (a positive ion) also drives it inward to the negatively charged interior. However, the situation is more complex for other elements such as potassium. The electrical gradient of \(\ce{K+}\), a positive ion, also drives it into the cell, but the concentration gradient of \(\ce{K+}\) drives \(\ce{K+}\) out of the cell (figure 16.9). We call the combined concentration gradient and electrical charge that affects an ion its electrochemical gradient.

Figure 16.9: Electrochemical gradients.

Moving against a gradient

Two mechanisms exist for transporting small molecular weight material and small molecules:

1. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP.
2. Secondary active transport does not directly require ATP; instead, it is the movement of material due to the electrochemical gradient established by primary active transport.

Carrier proteins for active transport

An important membrane adaptation for active transport is the presence of specific carrier proteins or pumps to facilitate movement. There are three protein types or transporters (figure 16.10).

Figure 16.10: Different types of carrier proteins for active transport.

• A uniporter carries one specific ion or molecule.
• A symporter carries two different ions or molecules, both in the same direction.
• An antiporter also carries two different ions or molecules, but in different directions.

All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also in facilitated diffusion, but they do not require ATP to work in that process.

Primary active transport

The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur. The second transport method is still active because it depends on using energy as does primary transport (figure 16.11).

Figure 16.11: Primary active transport.

One of the most important pumps in animal cells is the sodium-potassium pump (\(\ce{Na+}\)-\(\ce{K+}\) ATPase), which maintains the electrochemical gradient (and the correct concentrations of \(\ce{Na+}\) and \(\ce{K+}\)) in living cells. The sodium-potassium pump moves \(\ce{K+}\) into the cell while moving \(\ce{Na+}\) out at the same time, at a ratio of three \(\ce{Na+}\) for every two \(\ce{K+}\) ions moved in. The \(\ce{Na+}\)-\(\ce{K+}\) ATPase exists in two forms, depending on its orientation to the cellʼs interior or exterior and its affinity for either sodium or potassium ions. The process consists of the following six steps.

1. With the protein oriented toward the cellʼs interior, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
2. The protein carrier hydrolyzes ATP, and a low-energy phosphate group attaches to it.
3. As a result, the carrier changes shape and reorients itself toward the membraneʼs exterior. The proteinʼs affinity for sodium decreases, and the three sodium ions leave the carrier.
4. The shape change increases the carrierʼs affinity for potassium ions, and two such ions attach to the protein.
5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself toward the cellʼs interior.
6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions move into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

Secondary active transport (cotransport)

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build up outside of the plasma membrane because of the primary active transport process, this creates an electrochemical gradient. If a channel protein exists and is open, the sodium ions will pull through the membrane. This movement transports other substances that can attach themselves to the transport protein through the membrane. Many amino acids, as well as glucose, enter a cell this way.

References and resources

Text

Clark, M. A. Biology, 2nd ed. Houston, TX: OpenStax College, Rice University, 2018, Chapter 4: Cell Structure, Chapter 5: Structure and Function of the Plasma Membranes.

Karp, G., and J. G. Patton. Cell and Molecular Biology: Concepts and Experiments, 7th ed. Hoboken, NJ: John Wiley, 2013, Chapter 4: The Structure and Function of the Plasma Membrane.

Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 49.

Figures

Grey, Kindred, Figure 16.9 Electrochemical gradients. 2021. https://archive.org/details/16.9_20210926. CC BY 4.0. Added Cell membrane detailed diagram blank by LadyofHats. Public domain. From Wikimedia Commons. Added ion channel by Léa Lortal from the Noun Project.

Grey, Kindred, Figure 16.10 Different types of carrier proteins for active transport. 2021. https://archive.org/details/16.10. CC BY 4.0.

Lieberman M, Peet A. Figure 16.11 Primary active transport. Adapted under Fair Use from Marks' Basic Medical Biochemistry. 5th Ed. pp 175 Figure 10.10 Active transport by Na+,K+-ATPase. 2017. Added Cell membrane detailed diagram blank by LadyofHats. Public domain. From Wikimedia Commons. Added ion channel by Léa Lortal from the Noun Project.