What is secondary active transport?

Secondary active transport is a process where transport of molecules across a cell membrane is driven by an electrochemical gradient established by primary active transport.

In more detail, secondary active transport, also known as co-transport, is a vital process in biological systems that allows the movement of various substances across cell membranes. This process is 'secondary' because it relies on the electrochemical gradient that is established by primary active transport. Primary active transport uses energy directly from ATP to pump ions out of the cell, creating a difference in concentration and charge, known as an electrochemical gradient.

Secondary active transport uses this gradient to power the transport of other substances against their own concentration gradients. This is achieved through the use of transport proteins that couple the downhill movement of one substance (usually ions like sodium or hydrogen) with the uphill movement of another (such as glucose or amino acids).

There are two types of secondary active transport: symport and antiport. In symport, the ion and the other substance are moved in the same direction across the membrane. In antiport, they are moved in opposite directions.

For example, in the human intestine, sodium ions are pumped out of intestinal cells into the bloodstream by primary active transport. This creates a high concentration of sodium ions outside the cell. Then, through secondary active transport, these sodium ions are allowed to flow back into the cell, and this energy is used to transport glucose molecules into the cell against their concentration gradient.

In summary, secondary active transport is an essential process that allows cells to absorb nutrients and expel waste products, maintaining the necessary conditions for life. It is a fascinating example of how cells can harness energy and use it in a highly efficient way.