Here's why:
* Passive transport: Some carrier proteins facilitate passive transport, where molecules move down their concentration gradient (from high concentration to low concentration) without requiring energy. This type of transport is often called "facilitated diffusion".
* Active transport: Other carrier proteins use active transport to move molecules against their concentration gradient (from low concentration to high concentration). This process requires energy, typically provided by ATP.
Examples:
* Passive transport: Some amino acids can enter cells through facilitated diffusion using carrier proteins that do not require energy.
* Active transport: Other amino acids require active transport systems like the sodium-dependent amino acid co-transporters which use energy from the sodium gradient to move amino acids into the cell.
In summary: While some amino acid transport is passive, many systems involve active transport, requiring energy.
