Here's why:
* Active transport moves molecules against their concentration gradient, meaning from a region of lower concentration to a region of higher concentration. This requires energy because it's going against the natural flow of molecules. Think of it like pushing a boulder uphill - you need to expend effort.
* Passive transport on the other hand, moves molecules down their concentration gradient, from a region of higher concentration to a region of lower concentration. This happens spontaneously and doesn't require energy. Think of it like a ball rolling downhill – it requires no effort.
Examples of active transport:
* Sodium-potassium pump: This pumps sodium ions out of cells and potassium ions into cells, maintaining the cell's electrical potential.
* Glucose uptake in the small intestine: Glucose is transported from the gut lumen into the bloodstream against its concentration gradient, requiring energy.
Examples of passive transport:
* Diffusion: Movement of molecules from a region of high concentration to a region of low concentration.
* Osmosis: Movement of water across a semi-permeable membrane from a region of high water concentration to a region of low water concentration.
So, in summary, active transport is the only method of material exchange that requires energy input from the cell.
