When molecules move down the concentration gradient it means they are moving from

Passive transport

This takes place when molecules diffuse across the cell membrane by travelling through specific transport proteins. It occurs down a concentration gradient - molecules move from an area of high to lower concentration. This does not require a supply of energy because diffusion is a spontaneous process.

An example of passive transport is the uptake of glucose by liver cells. Glucose diffuses from the blood into the cytoplasm of liver cells via a transport protein called GLUT4.

Active transport

This occurs when molecules are moved across the cell membrane from an area where they are at a low concentration to an area where they are at a high concentration by specific transport proteins.

This movement is against the concentration gradient so the transport proteins require an energy supply. Energy is provided by the breakdown of ATP inside the cell.

This diagram shows molecules moving from area of low concentration to area of high concentration with energy:

An example of active transport is the action of a transport protein called the sodium-potassium pump. Lots of copies of this protein are found in the cell membrane of nerve cells.

The proteins pump sodium out of the cells and potassium into the cells. This helps nerve cells to transmit nerve impulses.

Your brain contains around 85 billion nerve cells. Their sodium-potassium pumps use around 20 per cent of your daily energy intake to keep your nerve cells operational.