3. Active Transport: Pushing Against the Current
Active Transport is the process of moving molecules against their electrochemical gradient—from an area of low concentration to an area of high concentration. This is an energy-requiring process, much like pushing a ball uphill. The energy for this work typically comes from the hydrolysis of ATP.
3.1. The Sodium-Potassium Pump: A Master of Maintenance
The Na⁺-K⁺ pump is a classic and vital example of active transport. It functions as an antiport system, moving two different ions in opposite directions.
The process follows these key steps:
- The carrier protein is an enzyme called Na⁺-K⁺ ATPase.
- It pumps three Na⁺ ions out of the cell.
- It pumps two K⁺ ions into the cell.
- This entire cycle is powered by the energy from one ATP molecule.
So, why is this pump so important? Its primary function is to maintain constant cell volume. By actively managing the concentration of ions inside the cell, it controls osmotic pressure and prevents excessive water from flowing in. The pump also plays a minor role in maintaining the potential difference across the plasma membrane. By constantly pumping Na⁺ ions out, it creates and maintains a powerful electrochemical gradient for sodium, which is a form of stored energy the cell can use for other tasks.
3.2. Cotransport: Moving Molecules Together
Active transport systems can also link the movement of two different molecules together. This is called cotransport and comes in two forms:
- Symport: Two different molecules are transported in the same direction.
- Antiport: Two different molecules are transported in opposite directions, as we just saw with the Na⁺-K⁺ pump.
A great example of symport is the glucose transport system found in epithelial cells. This is a clever system where the cell uses the energy stored in the Na⁺ electrochemical gradient—the very gradient established by the Na⁺-K⁺ pump—to pull glucose into the cell against its own concentration gradient. The downhill movement of Na⁺ provides the power to drive the uphill movement of glucose.
3.3. ABC Transporters: Cellular Bouncers
ATP-binding cassette (ABC) transporters are another critical class of active transport proteins. They use the energy from ATP to export a wide variety of materials out of the cytoplasm.
Their function has significant implications in medicine. For instance, some cancer cells develop resistance to treatment by using multidrug-resistant (MDR) proteins, a type of ABC transporter. These proteins act like cellular bouncers, actively pumping cytotoxic drugs out of the malignant cell before they can take effect, making treatment much more challenging.
To tie all these concepts together, a direct comparison can be incredibly helpful.