Active transport moves substances like ions, sugars, and amino acids across the cell membrane against their concentration gradient—from an area of lower concentration to an area of higher concentration. Moving materials uphill requires an input of energy, typically supplied by the cell in the form of adenosine triphosphate (ATP). Unlike passive transport, which relies on the natural flow of molecules, active transport uses specialized membrane proteins to force movement against this flow. This process allows cells to accumulate high concentrations of necessary molecules and maintain the ionic imbalances required for functions like nerve signaling.
Primary Transport Mechanisms
Primary active transport uses the direct chemical energy from ATP to power the movement of solutes across the membrane. Transmembrane proteins, often called pumps, bind to ATP and use the energy released to change their shape. This conformational change physically transports the target molecule or ion across the lipid bilayer against its electrochemical gradient.
The most well-known example is the Sodium-Potassium Pump, or \(Na^+/K^+\) ATPase, found in nearly all animal cells. This protein uses the energy from one ATP molecule to actively move three sodium ions (\(Na^+\)) out of the cell while simultaneously bringing two potassium ions (\(K^+\)) into the cell. This action maintains a low concentration of sodium and a high concentration of potassium inside the cell.
The unequal movement of ions makes the interior of the cell slightly more negative than the exterior. This establishes an electrochemical gradient and contributes to the cell’s resting membrane potential, a charge difference required for the excitability of nerve and muscle cells. The pump’s work is also indirectly involved in regulating cell volume and providing the driving force for secondary active transport.
Secondary Transport Mechanisms
Secondary active transport does not directly use ATP to move substances. Instead, it harnesses the potential energy stored in the electrochemical gradient created by primary active transporters, such as the \(Na^+/K^+\) pump. A single carrier protein simultaneously transports two different substances: one moves passively down its gradient, and the energy released powers the uphill movement of the second substance against its own gradient.
Secondary active transporters are categorized based on the direction in which the two coupled molecules move.
Symporters
Symporters move both the driving ion and the transported molecule in the same direction across the membrane. A classic example is the sodium-glucose symporter (SGLT1) found in the intestinal lining and kidneys. This protein couples the downhill movement of two sodium ions into the cell with the uphill transport of one glucose molecule into the cell.
Antiporters
Antiporters move the driving ion and the transported molecule in opposite directions across the cell membrane. For example, the sodium-calcium exchanger (\(Na^+/Ca^{2+}\) antiporter) allows sodium ions to flow inward down their gradient while simultaneously pumping calcium ions (\(Ca^{2+}\)) outward against their gradient. This enables the cell to maintain the low internal calcium concentrations necessary for normal signaling and function.
Bulk Transport
Bulk transport is used for substances too large to pass through carrier proteins or channels, such as large proteins or cell debris. This process requires energy, supplied by ATP, to facilitate the extensive membrane rearrangements involved. Bulk transport utilizes membrane-bound sacs called vesicles to package and move material into or out of the cell.
Endocytosis
Endocytosis is the process of bringing substances into the cell by engulfing them. The plasma membrane folds inward to form a pocket around the target material, which then pinches off to create an internal vesicle. This process includes three subtypes: phagocytosis, which is the uptake of large particles like bacteria; pinocytosis, which involves the non-specific uptake of extracellular fluid; and receptor-mediated endocytosis, which uses specific surface receptors to selectively capture target molecules.
Exocytosis
Exocytosis is the reverse process, where cells expel materials from the interior into the extracellular space. Intracellular vesicles, often containing substances like hormones or neurotransmitters, migrate to the plasma membrane. The vesicle membrane then fuses with the cell membrane, releasing its contents outside the cell. This mechanism is important for cell-to-cell communication and for the secretion of components needed to build the surrounding tissue structure.