Here's why:

* Active transport moves molecules across a cell membrane against their concentration gradient, meaning from a region of lower concentration to a region of higher concentration. This requires energy because it's moving things uphill, so to speak.

* Passive transport, on the other hand, does not require energy. It relies on the natural movement of molecules from high concentration to low concentration, following the concentration gradient.

Examples of Active Transport:

* Sodium-potassium pump: This pump actively moves sodium ions out of a cell and potassium ions into the cell, maintaining the cell's electrochemical gradient.

* Glucose uptake: Some cells actively transport glucose into their interior, even when the glucose concentration inside is higher than outside.

* Proton pumps: These are important in cellular respiration and photosynthesis.

Examples of Passive Transport:

* Diffusion: Movement of molecules from high concentration to low concentration (e.g., oxygen diffusing into the blood).

* Osmosis: Movement of water across a semi-permeable membrane from high water concentration to low water concentration.

* Facilitated diffusion: Movement of molecules across a membrane with the help of a protein channel, but still following the concentration gradient.