Simple Diffusion vs Facilitated Diffusion: Difference and Comparison

Whoever studied biology has come across the term diffusion. The term diffusion is referred to the movement of anything (for instance, molecules, ions, energy, atoms) from a region of higher to lower concentration.

This process is driven with the help of a gradient in concentration. The concept of diffusion is not limited to biology but is also accepted in many fields, such as chemistry, sociology, physics, finance, and economics.

Based on the absence or presence of facilitator molecules, diffusion can be divided into simple diffusion and facilitated diffusion. 

Key Takeaways

1. Simple diffusion occurs when molecules move across the cell membrane from an area of high concentration to an area of low concentration. In contrast, facilitated diffusion occurs when molecules are helped across the membrane by specific transport proteins.
2. Simple diffusion does not require energy, while facilitated diffusion may sometimes require energy.
3. Facilitated diffusion transports large or polar molecules that cannot easily pass through the cell membrane by simple diffusion.

Simple Diffusion vs Facilitated Diffusion 

Simple diffusion is driven by the concentration gradient, with molecules moving from high to low until equilibrium is reached. Facilitated diffusion involves the use of transporters or channels to allow molecules to pass through the membrane, and occurs down the concentration gradient.

In simple diffusion, solutes in a solution move along a concentration gradient or across a semipermeable membrane. It is carried out with the help of hydrogen bonds which forms between water molecules and solutes.

Hydrogen bonds are temporary, but as a result, a solution is constantly stirred.  

The substance is transported across a biological membrane from higher to lower concentrations in facilitated diffusion. Chemical energy is not directly required due to the movement of substances along the concentration gradient’s direction.

Gas transport, ion transport, and glucose and amino acid transport are a few of the examples that entail facilitated diffusion. 

Comparison Table

Parameter of Comparison	Simple Diffusion	Facilitated Diffusion
Interpretation	It is a process that transports molecules from higher to lower concentrations without the support of transmembrane proteins.	It is a process without the aid of final carrier proteins that transports molecules across the membrane from higher to lower concentrations.
Process	Passive process	The active or passive process
Speed	Relatively slow	Comparatively fast
Energy from ATP	Does not require	May or may not require
Solute specific	No	Always

What is Simple Diffusion? 

In simple diffusion, there is no need for membrane protein assistance. The substance or particle moves from a higher concentration to a lower one, but the downhill movement of particles does not require membrane protein.

In maintaining equilibrium, simple diffusion is pivotal.  

Simple diffusion is one of the types of passive transport, and for the proceeding of the process, it does not require chemical energy (ATP). In biological systems, ATP does not directly drive simple diffusion.

Concentration gradient and kinetic energy fuel up simple diffusion.  

In simple diffusion, molecules are in random constant motion due to struck of molecules with each other. Pedesis is called when the collision of particles occurs.

The molecules are compacted when the area is concentrated. As a result, the motion also reduces.

Thus, molecules move towards the larger space or when a larger space is available.  

In biological systems, it can be exemplified by the simple molecular transportation at the cellular level. The feature of the plasma membrane, namely the bilipid membrane, prevents the exit and entry of all molecules.

In simple diffusion, not all molecules can diffuse freely. 

What is Facilitated Diffusion? 

In facilitated diffusion, transportation includes the passive movement of a molecule with its respective concentration gradient and is directed by the presence of another molecule. After all, the molecules of facilitated diffusion move with their concentration gradient they do not directly involve high-energy molecules such as guanosine triphosphate (GTP) or adenosine triphosphate (ATP).

Across the cell membrane, small non-polar molecules diffuse easily. Lipids have a hydrophobic nature which makes up cell membranes, whereas ions and polar molecules cannot do so.

As a result, through transport proteins, they diffuse across the membrane.  

There are mainly three types of transport proteins involved in facilitated diffusion:  

1. A carrier protein is particular for an ion, molecule, or group of substances. It carried the molecule or ion by changing shape over the membrane.
2. Gated channel protein mainly opens a gate to allow molecules to pass through the membrane. A binding site is located at gated channels that are specific to a given ion or molecule.
3. Channel protein- In the membrane, it acts like a pore that lets small ions or water molecules quickly. The water channel proteins allow water at a very fast rate to diffuse over the membrane.

Main Differences Between Simple Diffusion and Facilitated Diffusion 

1. Only small molecules can be transported in simple diffusion. On the other hand, both large and small molecules can be transported with facilitated diffusion.  
2. Simple diffusion occurs through the phospholipid bilayer or simply in a solution. On the flip side, transmembrane protein-facilitated diffusion occurs.  
3. The rate of simple diffusion is proportional to the membrane permeability and the concentration gradient of the solute molecule, whereas the rate of facilitated diffusion relies upon carrier-mediated kinetics.  
4. The molecules in simple diffusion pass solely in the gradient concentration direction, whereas in facilitated diffusion, the movement of molecules occurs in opposite directions of the gradient concentration.
5. When it comes to facilitator molecules, simple diffusion occurs through the cell membrane, whereas facilitated diffusion occurs through particular facilitator molecules. 

References

1. https://journals.sagepub.com/doi/pdf/10.3181/00379727-212-43986
2. https://journals.physiology.org/doi/abs/10.1152/physrev.1990.70.4.1135

Last Updated : 11 June, 2023