Here's why:
* Active transport moves molecules against their concentration gradient, meaning from an area of lower concentration to an area of higher concentration. This movement goes against the natural flow of things, so it requires energy input.
* Passive transport, on the other hand, does not require energy. It relies on the natural movement of molecules down their concentration gradient, from higher to lower concentration.
Examples of active transport include:
* Sodium-potassium pump: Moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.
* Proton pump: Moves protons across the membrane, creating a proton gradient that can be used to generate ATP.
* Endocytosis and exocytosis: These processes involve the movement of large molecules or particles across the membrane, requiring energy.
Key takeaway: Active transport is the only type of transport that requires energy, and it's essential for maintaining cellular function and homeostasis.
