5.3 Active Transport

Active Transport Mechanisms

Active transport moves substances against their concentration gradients, which requires energy input. Without it, cells couldn't maintain the internal conditions they need to function. This section covers the energy sources behind active transport, the key proteins involved, and how primary and secondary transport work together.

Electrochemical Gradients and Ion Movement

To understand active transport, you first need to understand what it's working against: the electrochemical gradient. This gradient has two components:

• Concentration gradient: the difference in ion concentrations on each side of the membrane. Ions naturally move from high to low concentration.
• Electrical gradient (membrane potential): the difference in charge across the membrane. Ions are attracted toward the side with the opposite charge.

These two forces combine to determine which direction an ion "wants" to move. For example, cations like $Na^+$ and $K^+$ are pulled toward the negative side of the membrane, while anions like $Cl^-$ are pulled toward the positive side.

Ion movement along the electrochemical gradient continues until equilibrium is reached. Ion channels facilitate this passive movement for specific ions. Active transport, by contrast, pushes ions against this gradient, which is why it requires energy.

Primary vs. Secondary Active Transport

Primary Active Transport

Primary active transport uses energy directly from ATP hydrolysis to move solutes against their electrochemical gradient. The two most important examples:

• Sodium-potassium pump ($Na^+/K^+$ ATPase): pumps $Na^+$ out of the cell and $K^+$ in, establishing steep concentration gradients for both ions.
• Calcium pump ($Ca^{2+}$ ATPase): keeps intracellular $Ca^{2+}$ concentration very low, which matters for cell signaling and muscle contraction.

Both of these are carrier proteins that undergo conformational changes powered by ATP.

Secondary Active Transport

Secondary active transport does not use ATP directly. Instead, it harnesses the energy stored in an electrochemical gradient that primary transport already created. The $Na^+$ gradient set up by the sodium-potassium pump is the most common energy source.

There are two coupled transport mechanisms:

1. Symport (cotransport): moves two solutes in the same direction. A key example is $Na^+$-glucose cotransport in intestinal cells, where $Na^+$ flowing down its gradient pulls glucose into the cell against glucose's own gradient.
2. Antiport (exchange): moves two solutes in opposite directions. The $Na^+/Ca^{2+}$ exchanger uses $Na^+$ flowing into the cell to push $Ca^{2+}$ out.

ATP's Role in the Sodium-Potassium Pump

The sodium-potassium pump is the best-studied primary active transport protein. Here's how it works, step by step:

1. Three $Na^+$ ions from inside the cell bind to the pump.
2. ATP is hydrolyzed, and a phosphate group attaches to the pump protein.
3. This phosphorylation triggers a conformational change, opening the pump toward the extracellular side and releasing the three $Na^+$ ions outside the cell.
4. Two $K^+$ ions from outside the cell bind to the pump.
5. The phosphate group is released, causing the pump to return to its original shape.
6. The two $K^+$ ions are released inside the cell.

The net result per cycle: 3 $Na^+$ out, 2 $K^+$ in, using one ATP. Because more positive charges leave than enter, this pump is electrogenic, meaning it contributes to the electrical potential across the membrane.

This pump is critical for several reasons:

• Establishes the resting membrane potential, which is especially important in neurons and muscle cells
• Regulates cell volume by controlling osmotic balance
• Creates the $Na^+$ gradient that powers secondary active transport

All active transport processes are endergonic reactions, meaning they require a continuous input of energy to proceed.

Types of Transport Proteins

Several categories of transmembrane proteins carry out active transport:

• Uniport proteins move a single type of solute across the membrane (e.g., the $Ca^{2+}$ ATPase).
• Symport proteins move two different solutes in the same direction.
• Antiport proteins move two different solutes in opposite directions.

All three are carrier proteins, meaning they bind their solute(s) and undergo a shape change to shuttle them across the membrane. This distinguishes them from channel proteins, which simply provide a passive pore.