Describe secondary active transport.

Secondary active transport, also known as co-transport, refers to the process in which the transport of one substance across a cell membrane is coupled with the transport of another substance. This process is "secondary" because it relies on the electrochemical gradient established by primary active transport.

Here are the steps involved in secondary active transport:

1. Primary active transport uses energy, often in the form of ATP, to pump ions (like sodium or hydrogen ions) out of the cell, creating an electrochemical gradient. This means there is a higher concentration of these ions outside the cell than inside.

2. This gradient is then used as an energy source in secondary active transport. The ions move back into the cell via a transport protein in the cell membrane.

3. As these ions move back into the cell, they do so alongside other substances. The movement of the ions provides the energy to transport these other substances into the cell against their concentration gradient.

4. This process allows the cell to import substances that it needs, like glucose or amino acids, even when these substances are in higher concentration inside the cell than outside.

5. The substances transported into the cell can then be used for various cellular processes.

In summary, secondary active transport is a method cells use to transport necessary substances against their concentration gradient, using the energy provided by an ion gradient created by primary active transport.