Primary vs Secondary Active Transport
Active transport is a method that transports many substances across biological membranes, against their concentration gradients. In order to push molecules against a concentration gradient free energy is expended. In eukaryotic cells, this occurs at the plasma membrane of the cell and membranes of specialized organelles such as mitochondria, chloroplast etc. Active transport requires highly specific carrier proteins in the plasma membrane and these proteins have the ability to carry substances against a concentration gradient, hence referred to as ‘pumps’. Main roles of active transport include prevention of cell lysis, maintaining unequal concentrations of different ions on either side of the cell membrane, and maintaining electrochemical balance across the cell membrane. Active transport can occur in two different ways, namely, primary active transport and secondary active transport.
What is Primary Active Transport?
In primary active transport, positive charged ions (H+, Ca2+, Na+, and K+) are moved across membranes by transport proteins. The primary active transport pumps such as photon pump, calcium pump, and sodium-potassium pump are very important to maintain the cellular life. For example, calcium pump maintains the Ca2+ gradient across the membrane, and this gradient is important to regulate cellular activities such as secretion, microtubule assembly, and muscle contraction. Also, Na+/ K+ pump maintains the membrane potential across the plasma membrane.
What is Secondary Active Transport?
The energy source of secondary active transport pumps is the concentration gradient of an ion established by primary energy pumps. Therefore, the transferring substances are always coupled with transfer ions that are responsible for the driving force. In most animal cells, the driving force for secondary active transport is the concentration gradient of Na+/ K+. Secondary active transport occurs by two mechanisms called antiport (exchange diffusion) and symport (cotransport). In antiport, driving ions and transport molecules move in the opposite direction. Most of the ions are exchanged by this mechanism. For example, coupled movement of chloride and bicarbonate ions across the membrane is initiated by this mechanism. In symport, the solute and driving ions move towards the same direction. For example, sugars such as glucose and amino acids are transported across the cell membrane by this mechanism.
What is the difference between Primary and Secondary Active Transport?
• In primary active transport, the proteins hydrolyze ATP to power the transport directly whereas, in secondary active transport, ATP hydrolysis is done indirectly to power the transportation.
• Unlike the proteins involved in primary active transport, transport proteins involved in secondary active transport do not break ATP molecules.
• The driving force for the secondary active pumps is obtained from the ion pumps resulted from the primary active transport pumps.
• Ions such as H+, Ca2+, Na+, and K+ are transported through the membrane by primary active pumps, whereas glucose, amino acids, and ions like bicarbonate, and chloride are transported by secondary active transport.
• Unlike the secondary active transport, primary active transport maintains the electrochemical gradient across the plasma membrane.