Difference Between Facilitated Diffusion and Active Transport

Main Difference – Facilitated Diffusion vs Active Transport

Facilitated diffusion and active transport are two methods involved in the transportation of molecules across the cell membrane. The plasma membrane of a cell is selectively permeable to the molecules which move across it. Therefore, ions, as well as small and large polar molecules, cannot pass through the plasma membrane by simple diffusion. The movement of ions and other polar molecules is facilitated by transmembrane proteins in the plasma membrane. In both facilitated diffusion and active transport, transmembrane proteins are involved in the passage of molecules across the plasma membrane. The main difference between facilitated diffusion and active transport is that facilitated diffusion occurs through a concentration gradient whereas active transport occurs against the concentration gradient by using energy from ATP.

Key Areas Covered

1. What is Facilitated Diffusion
     – Definition, Mechanism, Function
2. What is Active Transport
     – Definition, Mechanism, Function
3. What are the Similarities Between Facilitated Diffusion and Active Transport
     – Outline of Common Features
4. What is the Difference Between Facilitated Diffusion and Active Transport
     – Comparison of Key Differences

Key Terms: Antiporters, Carrier Proteins, Channel Proteins, Concentration Gradient, Facilitated Diffusion, Plasma Membrane, Primary Active Transport, Secondary Active Transport, Symporters, Transmembrane Proteins, Uniporters

What is Facilitated Diffusion

Facilitated diffusion is a membrane transport method by which molecules move across the plasma membrane through the concentration gradient with the aid of transmembrane proteins. Since the transport of molecules occurs through the concentration gradient, facilitated diffusion does not use cellular energy for the transport of molecules. Generally, ions and other hydrophilic molecules are repelled from the plasma membrane due to the hydrophobic nature of the lipid molecules in the plasma membrane. Hence, transmembrane proteins that are involved in the facilitated diffusion shield the polar and large molecules from the repulsive forces of the membrane lipids. Two types of transmembrane proteins mediate facilitated diffusion. They are carrier proteins and channel proteins.

Figure 1: Facilitated Diffusion

Carrier proteins bind to the molecules to be transported and undergo conformational changes in the protein, translocating the molecules across the plasma membrane. Channel proteins comprise a pore via which the molecules can be transported. Some channel proteins are gated and can be regulated in response to specific stimuli. Channel proteins transport molecules faster than carrier proteins and are only used in the facilitated diffusion. Both carrier proteins and channel proteins, which mediate the facilitated diffusion, are uniporters. Uniporters only transport a particular type of molecules in a particular direction. Examples of transmembrane proteins that are involved in facilitated diffusion are glucose transporters, amino acid transporters, urea transporters etc. 

What is Active Transport

Active transport refers to the transport of molecules across the plasma membrane against the concentration gradient by using energy. Transmembrane carrier proteins are involved in active transport. Two types of active transports can be identified in a cell. They are primary active transport and secondary active transport. Primary active transport directly uses the metabolic energy in the form of ATP to transport molecules across the membrane. The carrier proteins that transport molecules by primary active transport are always coupled with ATPase. The most common example of primary active transport is the sodium-potassium pump. It moves three Na+ ions into the cell while moving two K+ ions out of the cell. Sodium-potassium pump helps in maintaining the cell potential. The sodium-potassium pump is shown in figure 2.

Figure 2: Sodium-Potassium Pump

The secondary active transport relies on the electrochemical gradient of the ions in either side of the plasma membrane to transport molecules. That means secondary active transport uses the energy released by transporting one type of molecules through its concentration gradient to transport another type of molecule against the concentration gradient. Therefore, transmembrane proteins involved in the secondary active transport are called cotransporters. The two types of cotransporters are symporters and antiporters. Symporters transport both molecules in the same direction. Sodium-glucose cotransporter is a type of symporter. Antiporters transport the two types of molecules to opposite directions. The sodium-calcium exchanger is an example of antiporter.   

Similarities Between Facilitated Diffusion and Active Transport

• Facilitated diffusion and active transport are two membrane transport mechanisms, which transport molecules across the plasma membrane.
• Transmembrane proteins are involved in both facilitated diffusion and active transport.

Difference Between Facilitated Diffusion and Active Transport

Definition

Facilitated Diffusion: Facilitated diffusion is the transport of molecules across the plasma membrane from higher concentration to a lower concentration by means of transmembrane proteins.

Active Transport: Active transport is the transport of molecules across the plasma membrane from low concentration to a higher concentration by means of transmembrane proteins, using ATP energy.

Concentration Gradient

Facilitated Diffusion: Facilitated diffusion occurs through the concentration gradient.

Active Transport: Active transport occurs against the concentration gradient.

Energy

Facilitated Diffusion: Facilitated diffusion does not require energy to transport molecules.

Active Transport: Active transport requires energy to transport molecules across the membrane.

Examples

Facilitated Diffusion: Sodium channels, GLUT transporters, and amino acid transporters are the examples of facilitated diffusion.

Active Transport: Na+/K+ ATPase transporters, Na+/Ca2+ cotransporter, and sodium-glucose cotransporter are the examples of active transport.

Conclusion

Facilitated diffusion and active transport are two membrane transport mechanisms involved in the passage of molecules across the plasma membrane. Both facilitated diffusion and active transport use transmembrane proteins to transport molecules. Facilitated diffusion does not require cellular energy to transport molecules. However, active transport uses ATP or electrochemical potential to transport molecules. Therefore, the main difference between facilitated diffusion and active transport is the use of energy for the transportation by each method. 

Reference:

1.“ Facilitated transport – Boundless Open Textbook.” Boundless, 26 May 2016, Available here. Accessed 7 Sept. 2017.
2. “Active Transport.” Active Transport | Biology I, courses. Available here. Accessed 7 Sept. 2017.

Image Courtesy:

1. “Blausen 0394 Facilitated Diffusion” By “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia
2. “OSC Microbio 03 03 Transport” By CNX OpenStax –  (CC BY 4.0) via Commons Wikimedia