Advanced Placement (AP) Biology Practice Exam

The process where cells move molecules against their concentration gradient is known as active transport. This mechanism requires energy, typically in the form of ATP, because moving substances against their gradient (from an area of lower concentration to an area of higher concentration) is not a spontaneous process.

Active transport involves specific transport proteins that are embedded in the cell membrane. These proteins facilitate the movement of ions and larger molecules, ensuring that essential nutrients and ions can be accumulated in concentrations higher than their surrounding environment. This is crucial for maintaining cellular functions and achieving homeostasis.

In contrast, the processes of diffusion and facilitated diffusion do not require energy and move substances down their concentration gradients. Diffusion involves the passive movement of molecules directly through the membrane or between its phospholipids, while facilitated diffusion requires specific carrier proteins but still moves substances from high concentration to low concentration. Osmosis specifically refers to the movement of water across a semi-permeable membrane and is similarly a passive process. Thus, active transport is distinctly characterized by its energy requirement and its ability to move substances against a gradient, distinguishing it from the other processes mentioned.