Difference Between Simple Diffusion and Facilitated Diffusion

Main Difference – Simple Diffusion vs Facilitated Diffusion

Simple diffusion and facilitated diffusion are two types of passive transport methods in which the cell membrane transports molecules across it. It uses natural entropy to move molecules from higher concentration to a lower concentration until the concentration becomes equalized. Hence, no ATP energy is used for the transportation of molecules. There are four main types of passive transport: osmosis, simple diffusion, facilitated diffusion and filtration. The main difference between simple diffusion and facilitated diffusion is that simple diffusion is an unassisted type of diffusion in which a particle moves from higher to a lower concentration across a membrane whereas facilitated diffusion is the transport of substances across a biological membrane through a concentration gradient by means of a carrier molecule.

Key Areas Covered

1. What is Simple Diffusion
      – Definition, Features, Mechanism
2. What is Facilitated Diffusion
      – Definition, Features, Mechanism
3. What are the similarities between Simple Diffusion and Facilitated Diffusion
      – Common Features
4. What is the difference between Simple Diffusion and Facilitated Diffusion
      – Comparison of Key Differences

Key Terms: Simple Diffusion, Facilitated Diffusion, Passive Transport, Concentration Gradient, Filtration, Cell Membrane, Channel proteins, Carrier Proteins

What is Simple Diffusion

Simple diffusion is an unassisted type of diffusion in which a particle moves from a higher to a lower concentration. The directional movement through the concentration gradient is passive. Once the molecules become evenly distributed, the molecules on the either sides of the cell membrane achieve an equilibrium where no net movement of molecules is observed. Generally, small non-polar molecules like oxygen, carbon dioxide, and ethanol freely diffuse across the cell membrane. The rate of diffusion depends on the temperature, molecular size, and the steepness of concentration gradient. Temperature affects the kinetic energy of particles in a solution. Large particles are subjected to a higher resistance within a solution when compared to smaller particles. Moreover, when the concentration gradient is high, more molecules will pass through the membrane. Simple diffusion across the cell membrane is shown in figure 1.

Figure 1: Simple Diffusion

What is Facilitated Diffusion

Facilitated diffusion is the transport of substances across a biological membrane through a concentration gradient by means of a carrier molecule. During facilitated diffusion, large ions and polar molecules are dissolved in water and are specifically and passively transported across the cell membrane. Polar ions diffuse through transmembrane channels proteins and large molecules diffuse through transmembrane carrier proteins. Channel proteins make hydrophobic tunnels across the membrane, allowing the selected hydrophobic molecules to pass through the membrane. Some channel proteins are ‘opened’ at all the time and some like ion channel proteins are ‘gated’. Carrier proteins like permeases change their conformation as molecules like glucose or amino acids are transported through them. Aquaporins are the other type of transport proteins that allow water to cross the membrane so quickly. Facilitated diffusion through a channel protein is shown in figure 2.

Figure 2: Facilitated Diffusion

Similarities Between Simple Diffusion and Facilitated Diffusion

• Both simple and facilitated diffusion occur down the concentration gradient from a high concentration to a low concentration of molecules.

• Both types do not require energy for the transportation of molecules.

• The net movement of molecules on either side of the cell membrane is zero at the equilibrated state.

Difference Between Simple Diffusion and Facilitated Diffusion

Definition

Simple Diffusion: Simple diffusion is an unassisted type of diffusion in which a particle moves from higher to a lower concentration.

Facilitated Diffusion: Facilitated diffusion is the transport of substances across a biological membrane through a concentration gradient by means of a carrier molecule.

Occurrence

Simple Diffusion: Simple diffusion occurs through the phospholipid bilayer.

Facilitated Diffusion: Facilitated diffusion occurs through transmembrane proteins.

Transported Molecules

Simple Diffusion: Simple diffusion transports small, non-polar particles.

Facilitated Diffusion: Facilitated diffusion transports large or polar particles.

Facilitator Molecules

Simple Diffusion: Simple diffusion occurs directly through the cell membrane.

Facilitated Diffusion: Facilitated diffusion occurs through specific facilitator molecules called transmembrane integral proteins.

Rate of Diffusion

Simple Diffusion: The rate of simple diffusion is directly proportional to the concentration gradient across the membrane as well as the membrane permeability of the solute molecule.

Facilitated Diffusion: The rate of facilitated diffusion depends on the kinetics of carrier-mediated transport.

At Low Concentration Gradients

Simple Diffusion: The rate of the simple diffusion is low at low solute concentrations.

Facilitated Diffusion: The rate of facilitated diffusion is high at low solute concentrations compared to that of simple diffusion.

Examples

Simple Diffusion: Diffusion of gases across the respiratory membrane and diffusion of molecules from the blood to the cells through the interstitial fluid are examples of simple diffusion.

Facilitated Diffusion: The counter-transport of chloride/bicarbonate in renal tubular cells and the cotransport of sodium with sugars like glucose, galactose, and fructose and amino acids are examples of facilitated diffusion. 

Conclusion

Simple diffusion and facilitated diffusion are two passive transport methods that carry molecules across the cell membrane. Both simple and facilitated diffusion occur through a concentration gradient. The main difference between simple and facilitated diffusion is in their mechanism of transporting molecules across the cell membrane. Simple diffusion allows the direct transport of molecules across the cell membrane. In contrast, facilitated diffusion occurs via transmembrane proteins like carrier proteins, channel proteins, and aquaporins. Small nonpolar molecules are transported by simple diffusion. Large and polar molecules are transported by facilitated diffusion. The net movement of molecules on either side of the cell membrane is zero at the equilibrated state.

Reference:

1. “Simple Diffusion.” BioNinja. N.p., n.d. Web. Available here. 12 June 2017. 
2. “Diffusion and passive transport.” Khan Academy. N.p., n.d. Web. Available here. 12 June 2017. 

Image Courtesy:

1. “Scheme simple diffusion in cell membrane-en” By LadyofHats Mariana Ruiz Villarreal – Own work. (Public Domain) via Commons Wikimedia
2. “OSC Microbio 03 03 facdiff” By CNX OpenStax –  (CC BY 4.0) via Commons Wikimedia