Which two types of secondary active transport exist?

The two types of secondary active transport are correctly identified as co-transport and counter-transport. In co-transport, also known as symport, the movement of one molecule occurs in conjunction with another molecule in the same direction across a membrane. For example, when sodium ions move down their concentration gradient into a cell, they may drive the transport of glucose into the cell simultaneously.

In contrast, counter-transport, or antiport, involves the simultaneous movement of two different molecules in opposite directions. For instance, sodium ions may enter the cell while another molecule, such as calcium ions, is transported out of the cell against its concentration gradient.

Understanding these mechanisms is crucial because secondary active transport utilizes the energy created by the primary active transport of ions (like sodium) to facilitate the movement of other substances. This is different from the other answer choices, which describe other forms of transport that do not involve the same active processes or mechanisms. For instance, facilitated diffusion and osmosis are passive processes, while primary and secondary transport refer to classifications of transport types rather than specific mechanisms. Simple diffusion and gated transport do not pertain to active transport mechanisms at all.