Mechanisms of Membrane Transport

$J = P \times A \times \Delta C$

Facilitated Diffusion

Facilitated diffusion is still a "downhill" process (moving with the gradient), but it requires a membrane protein to assist the solute. This mechanism is divided into two types:

1. Channel-mediated flux: Ions move through open pores (channels). Because channels do not involve specific binding sites, this process does not saturate.
2. Carrier-mediated transport: Solutes bind to a specific carrier protein that undergoes a conformational change to escort them across. Unlike channels, carriers exhibit saturable kinetics (reaching a transport maximum), stereospecificity, and competition for binding sites.

Active Transport

Active transport moves solutes "uphill" against their electrochemical gradient, a process that requires energy. Active transport can be classified based on the source of that energy.

Primary Active Transport

Primary active transport uses energy derived directly from the hydrolysis of ATP. A vital example is the $\text{Na}^{+}/\text{K}^{+}$ ATPase (sodium-potassium pump). This pump is electrogenic, meaning it creates a net charge imbalance by exporting $3\text{Na}^{+}$ ions out of the cell and importing $2\text{K}^{+}$ ions into the cell for every ATP molecule hydrolyzed. This process is essential for maintaining the concentration gradients that power other cellular functions. Other examples include $\text{Ca}^{2+}$ ATPases (such as SERCA) and proton pumps.

Exam Tip: To identify active transport in a clinical or experimental scenario, look for two key features: uphill movement (against a gradient) and saturable kinetics (indicating a protein is involved).

Secondary Active Transport

Secondary active transport does not use ATP directly. Instead, it utilizes the potential energy stored in an electrochemical gradient (usually the $\text{Na}^{+}$ gradient created by primary active transport) to drive a second solute against its own gradient.

• Cotransport (Symport): The driving ion and the target solute move in the same direction across the membrane. An example is the $\text{Na}^{+}$-glucose transporter (SGLT), where sodium moves down its gradient to "drag" glucose into the cell.
• Countertransport (Antiport): The driving ion and the target solute move in opposite directions. For example, the $\text{Na}^{+}/\text{Ca}^{2+}$ exchanger uses the inward movement of sodium to drive calcium out of the cell.

Summary of Transport Mechanisms

The following table summarizes the key differences between the major modes of transport: