A Level Biology Practice Exam

What does passive transport refer to?

1. Transport that requires energy input from the cell

2. Transport of water facilitated by protein channels

3. Transport that does not require energy and occurs down a concentration gradient

4. Active movement of ions in and out of cells

The correct answer is: Transport that does not require energy and occurs down a concentration gradient

Passive transport refers to the process of moving substances across a cell membrane without the use of energy from the cell. It occurs down a concentration gradient, meaning that molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached. This natural tendency for molecules to diffuse makes passive transport a fundamental mechanism in cellular processes. In contrast, transport that requires energy input would involve processes like active transport, where substances move against their concentration gradient. The transport of water facilitated by protein channels, while an important aspect of cellular movement (specifically osmosis), is just one type of passive transport. Active movement of ions in and out of cells further underscores the distinct nature of passive transport since it specifically uses energy, often ATP, to facilitate movement against gradients. This distinction is crucial in understanding how substances enter and exit cells relative to their concentration gradients and energy usage.