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Diffusion
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Molecules of every substance are in constant motion, resembling vibration. They collide with each other, transferring energy and changing direction. This movement is called Brownian motion, and it depends on the temperature. In areas with many molecules in a small volume (i.e., high concentration), the random motion of molecules leads many of them to move out of those areas. This random movement of particles as a result of their thermal motion, from regions with higher concentration to regions with lower concentration is called diffusion.
| Definition | The downhill movement of molecules along their concentration gradient as a result of their thermal motion. |
| Net flux | For a molecule moving between two regions in both directions, the net flux is the difference of the opposing movements of the molecule. |
| Diffusion equilibrium | Equal movement of a molecule in both sides of a membrane, keeping concentrations stable. |
| Simple diffusion | Diffusion of molecules through the lipid bilayer or channels without conformational changes of their proteins. |
| Facilitated diffusion | Diffusion of molecules through carrier proteins, which change their conformation every time a particular substrate is bound to their specific binding sites, in order to be transported to the other side of the membrane. |
Characteristics of diffusion
Diffusion is the simplest form of molecular transport between two regions and doesn’t need energy consumption. It mainly depends on the concentration difference of a molecule between the two regions. However, molecules move in both directions; so the net flux is the difference between the opposing fluxes.
Several factors affect the rate of diffusion:
- The main one is the concentration difference that governs the movement of the molecules.
- The temperature is also very important. The higher the temperature, the greater the speed of the diffusion process, due to the increased kinetic energy of the molecules.
- Another factor, which also plays an important role, is the molecular mass. Larger molecules move more slowly, resulting in a lower diffusion rate.
- Moreover, the surface area between the regions can also influence the diffusion rate. The larger the surface area, the faster the diffusion occurs. This is one factor that drove the evolution of biological systems toward large numbers of small cells, increasing the surface area-to-volume ratio, which is essential for efficient diffusion.
- The physicochemical properties of a molecule can also affect its diffusion rate. Increased lipid solubility enhances diffusion across biological membranes.
- In addition, the electrical gradient also plays an important role in the diffusion rate of charged molecules. For example, the negative charge inside the cell (the resting membrane potential) makes it easier for Na+ ions to diffuse from the extracellular space into the cytoplasm once the membrane becomes more permeable to them.
- For gases, the partial pressure gradient drives diffusion. The greater the difference in partial pressure between two regions, the faster the gas diffuses. An example of this is the gas exchange between the pulmonary alveoli and the surrounding capillaries. During this process, the oxygen which is present on the alveoli diffuses into the blood flow, while the carbon dioxide diffuses in the opposite direction. The higher the partial pressure gradient between the alveolar air and the blood in the capillaries, the faster the diffusion of these gases.
- Finally, the diffusion is affected by the viscosity of the medium where a substance is diluted. In a medium with high viscosity, a certain substance will diffuse very slowly. But it is not only the viscosity in itself that affects the diffusion rate, it is also the complexity of the medium and the number of particles which are present. The extracellular matrix, for example, is one of the most complex media in the human body, with an enormous number of particles which are always in continuous movement. This complexity can be an obstacle for the molecules which diffuse from a region to another, reducing the diffusion rate.
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Types of diffusion
Diffusion can be classified into two main forms, simple and facilitated diffusion.
What is simple diffusion
In simple diffusion the molecules can move directly through the phospholipid bilayer of the cell membrane. This process depends on their physicochemical characteristics of the molecules. Small, nonpolar molecules such as oxygen and carbon dioxide pass through the phospholipid bilayer very easily without the involvement of transport proteins. Small molecules that carry a slight charge, like ethanol, also cross this way because they are lipid-soluble enough to dissolve in the bilayer. On the other hand the lipid insoluble (lipophobic) substances cannot dissolve in the phospholipid bilayer and penetrate it.
What is facilitated diffusion
Facilitated diffusion is mediated by transmembrane proteins that act as channels or carriers. This process enables large, polar, charged, or lipophobic molecules to pass through the membrane down their electrochemical gradient. Channel proteins have a pore that is highly selective for particular ions or molecules, like Na+, K+, Cl-, and Ca2+. The pore can be constantly open, like leak channels, or they can change between an open and a closed state, depending on binding of ligands (ligand gated channels), changes in the voltage (voltage gated channels), or mechanical pressure (mechanically gated channels).
Carrier proteins are transmembrane proteins that also undergo conformational changes to transport the molecules from one side of the membrane to the other. Each carrier protein has a specific binding site for particular substrates, providing selective transport across the membrane. Transport via carriers is bidirectional and it depends on the concentration and the electrical gradient of the transported molecule. There are three different states for the carrier proteins. Firstly it opens in one side of the membrane and binds to the specific molecule. As soon as this happens, the first conformational change occurs that shields the substrate. After milliseconds, the transmembrane protein changes again, releasing the molecule to the other side of the membrane. This cycle is repeated until the diffusion equilibrium is reached. Example of diffusion include glucose molecules that are transported this way via GLUT transporters. Carrier transport differs from simple diffusion in one important way. In simple diffusion the rate rises in proportion to the concentration gradient with no upper limit. In facilitated diffusion every carrier can be occupied at once, so the rate increases to a maximum (Vmax) and then reaches a plateau (no matter how steep the gradient becomes). This saturation also makes carriers specific for their substrates and open to competition between molecules that share a binding site.
Simple and facilitated diffusion are both passive, but they differ in which molecules they move and how those molecules cross the membrane. The table below compares them.
| Simple diffusion | Facilitated diffusion | |
| Membrane route | Straight through the phospholipid bilayer | Through channel or carrier proteins |
| Molecules moved | Small, nonpolar, lipid-soluble (O₂, CO₂) | Large, polar, charged, or lipophobic (glucose, ions) |
| Membrane proteins | Not needed | Required |
| Energy used | None (passive) | None (passive) |
| Driving force | Concentration gradient | Electrochemical gradient |
| Rate at high concentration | Rises with the gradient, no limit | Levels off at a maximum (Vmax) when carriers saturate |
| Examples | O₂ and CO₂ exchange in the lungs | Glucose via GLUT, ions via channels |
Why is diffusion important in the body?
Diffusion is one of the most important and vital processes in the human body. It allows substances to be passively transported, without energy consumption, from regions of higher concentration to regions of lower concentrations, enabling essential body functions like gas exchange, nutrient absorption, and waste removal to take place.
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