Passive diffusion, therefore, requires no energy expenditure by the cell; it is not saturable or subject to competition between molecules.

In secondary active transport, also known as coupled transport or cotransport, energy is used to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP.

There are three types of these proteins or transporters: uniporters, symporters, and antiporters. A uniporter carries one specific ion or molecule. These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process.

Symporters and antiporters are involved in active transport.

3.8 GLUT1: Facilitated Transport of Glucose. GLUT1 is a transmembrane protein responsible for the facilitated diffusion of glucose across a membrane. This is an example of a membrane protein facilitating passive transport in which net flux can only occur down a concentration gradient of glucose.

The key difference between symport and antiport is that in symport, two molecules or ions are transported in the same direction across the membrane while in antiport, two molecules or ions are transported in opposite directions across the membrane.

Both antiporters and symporters are used in secondary active transport. Secondary active transport brings sodium ions into the cell, and as sodium ion concentrations build outside the plasma membrane, an electrochemical gradient is created.

A symporter is an integral membrane protein that is involved in the transport of two different molecules across the cell membrane in the same direction. The transporter is called a symporter, because the molecules will travel in the same direction in relation to each other.

In antiport two species of ion or other solutes are pumped in opposite directions across a membrane (Figure 2(b)) and in symport transport two species move in the same direction (Figure 2(c)). Cytosis is an active transport mechanism for the movement of large quantities of molecules into and out of biological cells.

Symporter (Symp) is a co-transporter membrane protein which transports two or more different molecules across the plasma membrane in the same direction.

A type of secondary active transport across a biological membrane in which a transport protein couples the movement of an ion (usually Na+ or H+) down its electrochemical gradient to the movement of another ion or molecule against a concentration or electrochemical gradient.