What are the different types of transport across a membrane?

The different types of transport across a membrane can be divided into two main categories: active and passive transport. Active transport requires metabolic energy in the form of ATP and molecules are moved against their concentration gradient, whereas passive transport does not require metabolic energy and molecules are moved down their concentration gradient.

Passive transport includes simple diffusion, facilitated diffusion and osmosis. Simple diffusion occurs when a molecule is transported across the phospholipid bilayer without the aid of a protein, whereas facilitated diffusion requires integral membrane proteins. These proteins bind to the molecule (which is usually a polar molecule or large ion) and undergo a conformational change in order to transport the molecule across the membrane. When water molecules undergo simple diffusion across a semi-permeable membrane, this is known as osmosis.

There are two types of active transport: primary and secondary active transport. Primary active transport uses ATP directly as a molecule is pumped against its concentration gradient whereas in secondary active transport, one molecule is transported down its concentration gradient whereas another is pumped against its concentration gradient.