Here's why:
* Active transport requires energy, usually in the form of ATP (adenosine triphosphate), to move substances against their concentration gradient. This means moving substances from an area of low concentration to an area of high concentration, which would not happen naturally.
* Passive transport, on the other hand, does not require energy and relies on the concentration gradient, moving substances from high to low concentration.
Examples of active transport:
* Sodium-potassium pump: This pumps sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.
* Glucose transport: Some cells actively transport glucose into the cell, even when the concentration inside is higher than outside.
Let me know if you have any other questions!
