OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What is the definition of active transport?

1. Particles moving down the concentration gradient

2. Particles moving up the concentration gradient using energy

3. Particles passing through a membrane by diffusion

4. Particles passive transport across a membrane

The correct answer is: Particles moving up the concentration gradient using energy

Active transport is defined as the process in which particles move against their concentration gradient, meaning they are moving from an area of lower concentration to an area of higher concentration. This movement requires the input of energy, typically sourced from ATP, because it is working against the natural tendency of particles to spread out evenly in a solution. In contrast, the other definitions highlight processes that do not involve energy expenditure, such as passive transport or diffusion, which occur when substances move down their concentration gradient. It's essential to understand that active transport is vital for maintaining cellular functions, allowing cells to accumulate necessary substances, such as ions or nutrients, even when these are present in low concentrations outside the cell. This is crucial for processes like nutrient absorption in the intestines or the uptake of ions in plant roots.