What is the Difference Between Uniport Symport and Antiport

The main difference between uniport, symport, and antiport is that uniport moves molecules across the membrane independent of other molecules, and symport moves two types of molecules in the same direction, but antiport moves two types of molecules in opposite directions. Furthermore, all channel proteins are the examples of uniports, and Na/glucose symporter is an example of a symport, while Na/H antiporter is an example of antiport. 

In brief, uniport, symport, and antiport are the three types of integral membrane proteins, which participate in moving molecules across the cell membrane. Specifically, they take part in active transport in which molecules move against the concentration gradient by using energy. 

Key Areas Covered 

1. What is Active Transport
     – Definition, Primary Active Transport, Secondary Active Transport
2. What is Uniport
     – Definition, Mechanism of Transport, Examples
3. What is Symport
     – Definition, Mechanism of Transport, Examples
4. What is Antiport
     – Definition, Mechanism of Transport, Examples
5. What are the Similarities Between Uniport Symport and Antiport
     – Outline of Common Features
6. What is the Difference Between Uniport Symport and Antiport
     – Comparison of Key Differences

Key Terms 

Active Transport, Antiport, Carrier Proteins, Integral Membrane Proteins, Primary Active Transport, Secondary Active Transport, Symport, Uniport

What is Active Transport 

Active transport is the mechanism of transporting molecules across the plasma membrane against the concentration gradient by using energy. Generally, integral membrane proteins take part in the process. Also, depending on the types of cellular energy used, there are two types of active transport mechanisms, occurring in a cell; primary active transport and secondary active transport.   

Primary Active Transport  

Generally, primary active transport is the type of active transport, using ATP as the form of cellular energy to transport molecules across the membrane. Also, the type of integral membrane proteins used for primary active transport is carrier proteins, which are always coupled with ATPase. Significantly, metal ions such as Na⁺, K⁺, Mg²⁺, and Ca²⁺ are the substances transported by primary active transport. Basically, they are called ion channels. Here, the most common example of primary active transport is the sodium-potassium pump, which moves three Na⁺ ions into the cell and two K⁺ ions out of the cell.

Figure 1: Na/K Pump

However, although the sodium-potassium pump transports two types of ions through the same pump, each type of ion is transported one at a time. Importantly, the sodium-potassium pump helps in maintaining the cell potential.

Secondary Active Transport  

In comparison, secondary active transport relies on the electrochemical gradient of the ions on either side of the plasma membrane to transport molecules. Therefore, it uses the energy released by transporting one type of molecules through its concentration gradient to transport another type of molecule against the concentration gradient. Hence, transmembrane proteins involved in secondary active transport are cotransporters. Importantly, one of the main characteristic features of cotransporters is the transport of two types of molecules simultaneously through the membrane protein. Basically, the two types of cotransporters are symports and antiports.  

What is a Uniport 

Uniport is the type of integral membrane protein, transporting a single type of substrate across the cell membrane by using primary active transport mechanisms. In addition to the active transport, uniport can operate through facilitated diffusion as well, while transporting the substrate along a diffusion gradient. Usually, a uniport works by binding to one molecule of the substrate at a time while the opening of the channels may occur in response to a stimulus and allow the free flow of specific molecules. Importantly, uniports participate in biological processes such as transmitting action potentials in neurons. 

Figure 2: Uniport, Symport, and Antiport

What is a Symport 

Symport is one of the two types of cotransporters, which use secondary active transport mechanisms. However, the direction of the transportation of two different types of substrates is the same. Also, as it uses secondary active transport mechanisms, symports use the electrochemical gradient to transport molecules. For example, SGLT1 in the intestinal epithelium is a symport. It transports sodium ions and glucose across the luminal membrane of the epithelial cells in order to be absorbed into the bloodstream. In addition to that, Na⁺/K⁺/2Cl⁻ symporter in the loop of Henle is another example of symport. 

What is an Antiport 

Antiport is the second type of cotransporter, which uses secondary active transport mechanisms. In contrast to symport, antiport transports the two different molecules in opposite directions. For example, N general, Na/H antiporter, Na/Ca exchanger, etc. are antiports. Of these, many cells use Na/Ca exchanger removes cytoplasmic calcium by exchanging one calcium ion for three sodium ions.

Figure 3: Na/Amino Acid Pump

Similarities Between Uniport Symport and Antiport 

• Uniport, symport, and antiport are three types of integral membrane proteins, permanently attached to the cell membrane. 
• Also, they are transmembrane proteins, being channel proteins and carrier proteins. 
• Furthermore, they penetrate through the phospholipid bilayer while attaching via hydrophobic, electrostatic or non-covalent interactions. 
• Besides, they use active transport to move molecules across the cell membrane. 
• Therefore, they use cellular energy to move molecules, especially ions against their concentration gradients. 

Difference Between Uniport Symport and Antiport 

Definition 

Uniport refers to an integral membrane protein, which transports a single type of substrate species across the cell membrane. And, symport refers to another integral membrane protein involved in the transport of two different molecules in the same direction through the cell membrane. Meanwhile, antiport refers to the third type of integral membrane protein involved in the secondary active transport of two different molecules in opposite directions. 

Types of Molecules to be Transported 

A single type of molecules moves through uniport, but two different types of molecules move through both symport and antiport. 

The Direction of the Movement 

Both uniport and symport transport molecules in a single direction, but antiport transports molecules to both directions. 

Types of Transporter Proteins 

Uniports are carrier proteins, but both symports and antiports are cotransporters. 

Type of Active Transport 

Uniport uses primary active transport, but both symports and antiports use secondary active transport. 

Driving Force 

Uniport uses energy from ATP for the transport of molecules, while both symports and antiports rely upon the electrochemical potential difference. 

Examples 

All channel proteins are the examples of uniports, and Na/glucose symporter is an example of a symport, while Na/H antiporter is an example of antiport. 

Conclusion 

Uniport is a type of integral membrane protein, transporting a single type of molecules in one direction across the cell membrane. Also, it is a carrier protein, which uses primary active transport. Thereby, it uses energy from ATP for the transport of molecules. In contrast, both symport and antiport use secondary active transport and the driving force of molecules is the electrochemical gradient. In addition to that, both transport two different types of molecules at the same time. However, symport transports two types of molecules in the same direction while antiport transports two different types of molecules in opposite directions. On that account, the main difference between uniport, symport, antiport is the mechanism of transport. 

References:

1. Cherak, Stephana J. “Membrane Transport.” Basic Biochemistry. Available Here.

Image Courtesy:

1. “Scheme sodium-potassium pump-en” By LadyofHats Mariana Ruiz Villarreal – Own work. (Public Domain) via Commons Wikimedia   
2. “Porters” By Lupask – Own work (Public Domain) via Commons Wikimedia   
3. “Scheme secundary active transport-en” By LadyofHats Mariana Ruiz Villarreal – Own work. (Public Domain) via Commons Wikimedia