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

How is active transport defined?

1. The movement of particles along a concentration gradient

2. The movement of particles against a concentration gradient using energy

3. The passive movement of molecules through a membrane

4. The movement of particles only through active cellular structures

The correct answer is: The movement of particles against a concentration gradient using energy

Active transport is defined as the movement of particles against a concentration gradient using energy. This process is essential for maintaining the proper balance of ions and nutrients within cells, as it allows substances to be transported from areas of lower concentration to areas of higher concentration. In physiological contexts, active transport often involves the use of ATP (adenosine triphosphate) to energize transport proteins embedded in the cell membrane. This enables the cell to accumulate molecules that are necessary for its functions, even when those molecules are present in lower concentrations outside the cell. Additionally, active transport is crucial for processes such as nerve impulse transmission and muscle contraction. Other definitions provided do not encompass the key characteristics of active transport. For example, the movement along a concentration gradient refers to passive processes, and passive movement through a membrane does not require energy, which is the opposite of what defines active transport. The option regarding movement through active cellular structures does not accurately describe the fundamental process of active transport itself.