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Active transport is a way that cells move substances across the cell membrane — the thin, flexible layer that acts like a barrier around every cell, controlling what goes in and what comes out.
To understand active transport, you first need to remember one important idea: substances naturally like to move from where there is a lot of them to where there is less of them. This natural movement is called moving down a concentration gradient (like water flowing downhill — from high to low). This is what happens in diffusion and osmosis.
Active transport is the opposite of this.
In active transport, substances move from where there is less of them (low concentration) to where there is more of them (high concentration). This is called moving against a concentration gradient (like pushing water uphill — it does not happen on its own).
Because this movement goes against the natural direction, the cell needs to do work to make it happen. That work requires energy.
Let's break down the definition of active transport into four clear points:
What moves? Molecules (groups of atoms bonded together) or ions (atoms or groups of atoms that carry an electric charge, such as mineral ions like nitrate or potassium).
Where does movement happen? Through the cell membrane — substances pass across this barrier to get into or out of the cell.
Which direction? From a region of lower concentration → to a region of higher concentration. This is against the concentration gradient.
What provides the energy? Energy released during respiration — the process cells use to break down glucose and release energy. This energy is used to power special proteins in the cell membrane called carrier proteins, which grab the molecules or ions and physically transport them across the membrane.
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