Final answer:
The branch of active transport that relies directly on ATP is primary active transport. It involves membrane proteins that function like pumps to move ions and molecules against their concentration gradients using energy from ATP hydrolysis. Secondary active transport depends on the gradient established by primary active transport and does not use ATP directly.
Step-by-step explanation:
The branch of active transport that relies directly on ATP is known as primary active transport. This mechanism involves moving ions across a cell membrane and creating an electrical charge difference across that membrane, with a direct dependence on ATP. It's carried out by proteins in the cell membrane that function akin to pumps, which are sometimes referred to as ATPases. These pumps work by hydrolyzing ATP to provide the necessary energy to move substances such as ions and molecules against their concentration gradients.
On the other hand, secondary active transport does not directly use ATP. Instead, it relies on the electrochemical gradient established by primary active transport to move substances. This type of transport uses the energy stored in the form of ionic concentration differences across the membrane, which is originally created by primary active transport. Therefore, it is the primary active transport that is directly dependent on ATP, enabling cells to transport required substances against their electrochemical gradients, an essential process for maintaining cell homeostasis.