The word 'diffusion' means free movement across distance, with or without the presence of a barrier. However, there is a phenomenon known as facilitated diffusion which occurs at the cellular level. The cell does not allow free radicals and other harmful substances to enter and harm the cell organs. This is possible due to the structure of the cell membrane. The structure is such that, it allows only certain things to pass in and out of the cell. One such activity that allows selective movement in and out of the cell is the process of facilitated diffusion.
Cell Membrane Structure
As mentioned before, the process of diffusion is possible across the cell, because of the structure of the cell membrane. So, before knowing how facilitated diffusion takes place, we need to understand the structure of the cell membrane. The structure consists of a phospholipid bilayer, where the phosphate molecules are arranged such that their hydrophilic heads face outside, whereas, their hydrophobic heads face inside. Between the phospholipid molecules there are membrane proteins, which helps the cell membrane allow only certain molecules to travel in and out of the cell.
What Happens During the Process?
The process wherein substances pass through the cell membrane, in a passive manner, with the help of transport molecules, is known as facilitated diffusion. Hence, this is not an active transport process, but a passive one. It is the process by which ions and solutes, such as sugars and amino acids, travel in and out of the cell. The protein molecules that aid in this transport are known as carriers. These carriers are specific for particular molecules or substrates. These are not the same as protein channels, which merely allow substances to pass to and from the cell. In fact, these carriers physically bind themselves to the substrate, carry them across the cell membrane, and deposit them on the other side.
The direction and rate of flow of the solutes depends on the concentration gradient inside and outside the cell. If the concentration of solutes is greater inside, then it will bind to the carrier protein inside the cytoplasm, and will travel outside the cell. If the concentration of the solute is greater outside the cell, then it will attach itself to the carrier molecules present outside, and will be transported inside the cell. One important characteristic that is associated with facilitated diffusion is saturation. This process is saturable, which means, as the concentration gradient of the substance increases, it will go on increasing until it reaches a point where all the carrier molecules are occupied. The substance will reach a saturation point, irrespective of its concentration inside or outside the cell, that is, irrespective of the concentration gradient.
An example is seen in red blood cells, or erythrocytes. The cell membrane of red blood cells transport chloride ions in one direction, and bicarbonate ions in the opposite direction. This example is important, because, this is the means by which carbon dioxide reaches the lungs from cells that have taken up oxygen for usage.
This is an important part of cell biology. In fact, it is the means by which the cell manages to establish homeostasis in itself, its immediate surrounding, and on a larger scale, in the entire body.