Active transport is used when substances need to be moved across a cell membrane from a region of lower concentration to a region of higher concentration—against their concentration gradient. This process requires cellular energy, often in the form of ATP, because it is moving substances "uphill," against the natural diffusion direction. Common situations where active transport is used include:
- Uptake of nutrients like glucose and amino acids into cells when their concentration is lower outside than inside the cell (e.g., in the intestines).
- Absorption of mineral ions by plant root hair cells from dilute soil solutions.
- Maintaining concentration gradients of ions such as sodium and potassium inside and outside cells via the sodium-potassium pump, essential for nerve impulse transmission and muscle contraction.
- Transport of ions like chloride and nitrate into plant vacuoles against concentration gradients.
Active transport is essential when a cell needs to accumulate high concentrations of certain molecules, transport ions for physiological functions, or remove waste substances against their natural diffusion path.
