A Level Biology Practice Exam

What process involves the movement of particles across a plasma membrane against a concentration gradient?

1. Passive transport

2. Facilitated diffusion

3. Active transport

4. Osmosis

The correct answer is: Active transport

The process that involves the movement of particles across a plasma membrane against a concentration gradient is known as active transport. This mechanism requires energy, usually in the form of ATP, because it involves transporting substances from an area of lower concentration to an area of higher concentration, which is contrary to the natural tendency of particles to move down their concentration gradient. Active transport is crucial for maintaining concentrations of ions and other substances that are necessary for various cellular functions. For example, the sodium-potassium pump operates through active transport to maintain the essential electrochemical gradient across the plasma membrane, which is vital for processes such as nerve impulse transmission and muscle contraction. Passive transport, including facilitated diffusion and osmosis, does not require energy and occurs when substances move down their concentration gradients. Thus, these processes cannot move particles against a gradient, distinguishing active transport as a unique and essential cellular process.