Active transport

Active transport requires energy. It could be compared to a ball moving up a hill.

Active transport is the transport of molecules that require the use of energy, in the form of an energy-storing chemical called ATP (adenosine triphosphate) to help them cross the cell membrane. Particles are moved against a concentration gradient, meaning they move from a region of lower to a region of higher concentration.

Check out this fantastic video titled Active Transport created by the Teacher’s Pet. It explains the process of active transport.

There are three main active methods:

• Ion pumps
• Exocytosis
• Endocytosis (2 types – phagocytosis and pinocytosis)

Ion pumps

The ion pump (ion transporter) is a special protein that moves ions (charged molecules) across the membrane. Certain ions are actively absorbed eg K⁺ (potassium), and others are actively removed eg Na⁺ (sodium) from the cell. The ion pump restores required concentrations of molecules inside and outside cells, for the body system to function correctly. Ion pumps are located in the cell membrane and will only fit certain ions.

What is an ion? An ion is a charged atom or molecule. It is charged because the number of electrons does not equal the number of protons in the atom or molecule.

Exocytosis

Exocytosis is the active removal of substances from the cell. Vacuoles/vesicles from Golgi bodies or the endoplasmic reticulum (ER) fuse with the plasma membrane and expel their contents to the outside. This is a common process in cells involved with secretion.

Endocytosis

Endocytosis happens when a cell actively takes something in. The cell membrane folds around the substance during the process. It is not just some membrane proteins taking in a couple of molecules as in earlier forms of active transport.

There are two types of endocytosis:

• Phagocytosis – (cell eating) This is the ingestion of solids from outside the cell. The plasma membrane encloses particle and buds off to form a food vacuole.
• Pinocytosis – (cell drinking) This is the ingestion of fluid surrounding the cell. The plasma membrane encloses some of the fluid and pinches off to form a vesicle.