To perform all metabolic processes, cells need to acquire compounds from the extracellular medium as well as eliminate the products of their metabolism from the cytoplasm .
The plasma membrane represents a physical barrier that separates the cytoplasm from the medium surrounding the cell. Therefore, to carry out the movement of compounds from the outside to the inside and from the inside to the outside, the cell uses an important property of the membrane which is its permeability .
- What is diffusion
- Diffusion types
- Diffusion facilitated and the liver
- Cystic fibrosis
- Solved exercises
What is diffusion
Molecular diffusion scheme (Image: Reproduction | Wikimedia Commons)
Diffusion is a process that corresponds to the movement of particles / solutes (molecules or ions) from the most concentrated to the least concentrated medium , that is, in favor of the concentration gradient, tending to homogenize its distribution.
The main function of diffusion is to enable a more homogeneous medium. As substances migrate from the most concentrated to the least concentrated medium, diffusion allows for a dynamic balance .
How it occurs
The bilayer is permeable to gases, hydrophobic molecules as well as those small, uncharged molecules. However, it is practically impervious to water-soluble compounds such as ions and most polar molecules, whether charged or not.
Thus, to increase the permeability of the membrane, some classes of proteins are organized in the bilayer in order to form a pathway that selectively allows the water-soluble solutes to pass through the hydrophobic environment of the lipid bilayer.
These protein elements function as solute transporters and can be mainly of two types: permeases and ion channels.
One of the processes that the cell performs for the transport of substances is the passive process. Passive processes are those that occur across the plasma membrane, without energy expenditure, tending to match the concentration of the cell with that of the external environment (in favor of the concentration gradient). An example of a passive process is diffusion.
Diffusion can be simple or facilitated.
Simple diffusion is a passive process, which occurs without the aid of a protein . As, for example, the diffusion of small molecules of oxygen and carbon dioxide, through the plasma membrane. As the cell breathes, it consumes the oxygen in it and produces carbon dioxide .
Thus, inside the cell the oxygen concentration decreases and the carbon dioxide concentration increases, which establishes a difference in the concentration of these gases in relation to the external environment.
Outside the cell, the oxygen content is higher, and this gas enters the cell by simple diffusion. Inside the cell, the carbon dioxide content gets higher, and that gas leaves the cell by diffusion.
Diffusion can also occur through protein channels (porins), in the case of hydrophilic particles, which have no affinity with the phospholipid bilayer. The size of the particles that can pass through these channels will depend on the diameter of these pores.
After a certain size of the particles, diffusion becomes slower and slower , until the protein pores become unviable for passage. In these cases, and to a certain extent, an alternative is the participation of facilitating carrier proteins, in another type of passive process: facilitated diffusion.
Diffusion made easy
Facilitated diffusion is also a passive process, which occurs through lipoprotein membranes. In this type of diffusion, some membrane proteins, called permeases, act by facilitating the passage of certain substances that, by simple diffusion, would take a long time to pass.
This process is related to the transport of some amino acids, vitamins and some ions, such as calcium, chlorine, sodium and potassium and molecules, such as glucose.
In nature, cell membranes determine the substances that can enter and leave cells. However, few compounds are able to diffuse freely through the double lipid layer that constitutes the cell membrane.
For this reason, these membranes are equipped with special proteins that are true transporters, carrying solutes from the outside into the cell, and vice versa.
Proteins can be mobile, in which case they bind to compounds and the complex diffuses across the membrane, releasing species on the other side.
Diffusion facilitated and the liver
The liver performs several functions, including the glucose reservoir, an important fuel for our activities. Liver cells store glucose in the form of glycogen , which is a long molecule made up of several glucose molecules.
When the concentration of glucose is higher outside the cells than inside them, the hormone insulin stimulates the entry of glucose molecules into the cell by facilitated diffusion. If they are in excess inside the cells, these molecules are transformed into glycogen which, being insoluble, have no osmotic effect.
As the glycogen is not dissolved, it does not increase the internal concentration of the liver cells and, therefore, there is no risk of them swelling due to excessive water intake .
When blood glucose levels decrease, the hormone glucagon stimulates cells to break down glycogen, forming many glucose molecules.
As a result of this process, the concentration of glucose inside the cells is higher than outside. In this situation, glucose is transported out of cells by facilitated diffusion.
Cystic fibrosis is a disease characterized by abnormal mucus secretion , mainly by the cells of the respiratory system .
This mucus is thick and viscous and, as it is difficult to clear from the airways, it ends up causing frequent lung infections . This disease can lead the individual to death in childhood, although there are already treatments that prolong the lives of patients.
In the plasma membrane of cells there is a protein that performs the transport of chlorine ions. The cause of cystic fibrosis is related to the presence of an abnormal type of this protein, which fails to transport these ions properly.
This causes changes in the normal concentration of chlorine ions within the cell, which leads to the production of thick mucus.
Summary of Content
In this text you learned that:
- Diffusion is necessary for the cell to carry out metabolic processes.
- Diffusion is a way for cells to acquire compounds from the extracellular medium.
- There are two types of diffusion: simple and facilitated.
- Simple diffusion does not require the aid of proteins.
- The facilitated diffusion happens from the transport made by special proteins.