Diffusion can either be simple or facilitated, depending on the process and the involvement of other molecules. Both processes are often mistaken for each other, but they are not the same. To help you understand this, I have put together this detailed comparison of simple vs facilitated diffusion.
Simple diffusion and facilitated diffusion involve the movement of a substance from a region of higher concentration to a region of lower concentration without the expenditure of cellular energy.
Both types of diffusion are essential forms of cellular transport that ensure the proper functioning of living organisms. However, they differ in their processes, transport molecules, and significance.
What is simple diffusion?
Simple diffusion is a form of diffusion that occurs without the aid of channel proteins or carrier molecules. Like every form of diffusion, it involves the movement of solute molecules from a region of higher concentration to a region of lower concentration.
The process of simple diffusion occurs at a slow rate and does not expend energy. The process is driven by the existing concentration gradient. As far as there is a concentration gradient, molecules will move from one region to another.
What is facilitated diffusion?
Facilitated diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration across the cell’s membrane with the aid of transport proteins.
Facilitated diffusion occurs at a fast rate due to the support the molecules get from the transmembrane protein. Two major transport proteins that aid facilitated diffusion are channels and carriers.
The migrating molecules in facilitated diffusion are polar (or charged) and cannot cross the phospholipid part of the membrane without the aid of these transport proteins.
This form of transport happens without any energy expenditure. Just like simple diffusion explained above, there is a concentration gradient between the molecules on both sides of the membrane. It is the concentration gradient that drives the diffusion process.
Simple vs facilitated diffusion: Similarities
- They are both forms of passive transport
- Both simple and facilitated diffusion do not require energy (ATP)
- They are driven by a concertation gradient and the molecules move down the gradient
- Simple and facilitated diffusion are also driven by kinetic energy
Simple vs facilitated diffusion: Differences
Simple and facilitated diffusion produce the same overall result, but they are different. Below are the differences between these forms of passive transport:
Process
While both forms of passive transport involve the movement of molecules across a membrane, facilitated diffusion does this with the aid of transport proteins, whereas simple diffusion does not. It fully relies on the natural tendency of the molecules to migrate across the membrane.
In addition, simple and facilitated are passive processes. However, some instances of facilitated transport may be active.
Transported and facilitator molecules
The transported molecules in simple diffusion are small and non-polar molecules like oxygen, carbon dioxide, ethanol, and hormones like estrogen. These molecules are not selective and do not require the aid of any facilitator molecules other than the concentration gradient.
Facilitated diffusion transports large or polar molecules, like glucose and ions, across the membrane. By themselves, these molecules cannot cross the hydrophobic part of the transport membrane. Transport proteins (the facilitator molecules) shield them from the hydrophobic core and carry them across.
As a result, facilitated diffusion is a solute-specific process. The transport proteins can only carry solutes they are specific for. This makes solute selectivity one of the factors that affect the rate of diffusion. Also, it is why there could be inhibition in this form of diffusion.
Rate of diffusion and concentration gradient
The rate of diffusion in simple diffusion is directly proportional to the concentration gradient, whereas the rate of facilitated diffusion depends largely on the kinetics of the transport protein.
For simple diffusion, the rate of diffusion is low when solute concentration is low and high when solute concentration is high. The rate of facilitated diffusion is generally faster than that of simple diffusion. However, it is dependent on factors such as type of transport protein, temperature, and the presence of an inhibitor.
In contrast, simple diffusion does not depend on factors other than the general factors that affect the rate of diffusion (size of solute molecules, temperature, concentration gradient, membrane permeability, etc).
Furthermore, simple diffusion always moves in the direction of the concentration gradient but facilitated diffusion can move in and opposite the direction of the concentration gradient.
Importance
Both types of diffusion are equally important, but the significance of simple diffusion is rather generic, unlike facilitated diffusion which has some specifics.
Facilitated diffusion ensures that only selected molecules go in and out of the cell, thereby ensuring the safety of the cell. It regulates the entry and exit of substances in and out of cells.
For instance, the inner layer of lipids and fatty acids stops large and polar molecules from entering certain organelles.
It allows the diffusion of essential macromolecules like amino acids and glucose (which are important to cell functioning) into cells. Facilitated diffusion is necessary for maintaining homeostatic optimal levels of molecules within the cell.
Examples
Examples of simple diffusion are the movement of gases across the respiratory membrane, the mixing of gases in the atmosphere, the movement of interstitial fluid from blood to the cells, and the absorption of nutrients by bacteria.
The transfer of calcium and potassium ions, counter-transport of chloride and bicarbonate in the renal tubular cells, movement of oxygen in the blood using hemoglobin, and the cotransport of amino acids and sodium with glucose, fructose, and galactose are common examples of facilitated diffusion.
FAQs
What is a major disadvantage of facilitated diffusion?
A major disadvantage of facilitated diffusion is the specificity of the carrier proteins. That is, every carrier protein can carry just the solutes that it is specific for. This can affect the rate of diffusion if a specific type of carrier protein is not used.
Why is facilitated diffusion faster than simple diffusion?
Facilitated diffusion occurs at a faster rate than simple diffusion because it involves a carrier protein. Simple diffusion maximizes the natural ability of molecules to move from a region of higher concentration to a region of lower concentration.
For all forms of diffusion, the size of the transport molecules determines the rate of diffusion. Molecules of larger mass will move at a slower rate than small molecules. But when aided (as in facilitated diffusion), the rate becomes faster.
Is facilitated diffusion more important than simple diffusion?
Simple and facilitated diffusion are important forms of transport within and across cells. However, in some cases, one form of transport has superiority over the other.
For instance, in the transport of small non-polar molecules, simple diffusion is more important. On the other hand, facilitated diffusion is more important in the transfer of large and polar molecules.
Conclusion
Simple and facilitated diffusion are forms of passive transport and are both important for maintaining homeostasis. While they seem to share similarities, they differ in mechanisms and the molecules they transport.
As mentioned earlier, simple diffusion is as simple as its name implies. Unlike facilitated diffusion which requires carrier proteins to carry molecules across regions, simple diffusion relies on the natural tendency of molecules to move across membranes.
This comparison of simple vs facilitated diffusion is crucial to the understanding of how diffusion works across different cells and the roles it plays in maintaining physiological balance within living organisms.
Also, find out if osmosis is a form of active or passive transport and the mechanism of transport.
Thanks for reading.