Some substances spontaneously cross the plasma membrane , characterizing passive transport ; others, in turn, are forced to leave the cell, or to enter into it, a so-called active transport process .
When substances move spontaneously through the plasma membrane, passive transport is said to have occurred. In this type of transportation, there is no energy expenditure. There are two basic types of passive transport: diffusion and osmosis .
In the diffusion, transport of solute from a medium where there is more quantity to a medium where there is less quantity of this substance, without significant change in the cellular volume – for example, the transport of oxygen and carbon dioxide in the cells.
As the cells consume oxygen in their respiration, the concentration of this gas in their interior is always low. Externally, the oxygen concentration is higher because this gas comes continuously through the bloodstream. Carbon dioxide in the opposite way, because as the cells are always producing this gas by respiration, the internal concentration is greater than the external concentration. Thus, the carbon dioxide exits from the more concentrated medium to the less concentrated medium.
Some substances, such as glucose, are transported through special proteins, called permeases, which facilitate their entry into the cell, from a more concentrated medium to a less concentrated medium. Since there is participation of facilitating proteins, the process is called facilitated diffusion.
In some special situations, transport of solvent , not solute , occurs . In this type of transport, the water crosses the plasma membrane of the cells, depending on the concentration of solute. When seasoning a salad, put salt. This increases the concentration of this solute outside the cell.
The increase of solute in the external medium stimulates the cells to lose water, by osmosis , which results in withered greenery. If the cell is placed in a medium where the solute concentration is lower than the concentration of the cytoplasm, the tendency is to absorb water by osmosis, increasing its volume. This type of transport can also be observed in animal cells.
A common experiment to verify the phenomenon of osmosis is the use of red blood cells, in different concentrations, as shown below:
The blood cells are placed in different concentrations. The isotonic solution has the same concentration, that is, the amount of solute and solvent inside and outside the cell is practically the same, so there is no change in cell volume. In the hypertonic solution , the concentration of solute in the external medium is higher, so the cell loses water and wilts. In the hypotonic solution , the concentration of solute in the internal medium is higher, so the cell gains water and increases in volume. The passage of water through the plasma membrane occurs to equalize concentrations inside and outside the cell.
In certain situations, the cell needs to keep inside certain substances in concentrations different from those found in the external environment.
The tendency of these substances, as described in passive transport, is to leave the cell; however, with the aid of the permeases, they are again transported to the internal medium. In this case, the cells can maintain these differences between the internal and external concentrations with energy expenditure, which characterizes the active transport .