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Passive Transport Types and Examples

The cell membrane is semi-permeable. It allows only some specific substances to pass through it. Some molecules can pass easily from this membrane whereas the others are left behind. The passage of the molecules that are left behind is affected by many factors like energy, proteins, etc.

The transfer of molecules that happen through the semipermeable membrane with the help of energy is called Active transport. The transfer that occurs without any external energy is called Passive transport.

What is passive transport?

Passive transport is the method of transferring the molecules through the semi-permeable membrane without the help of energy. Rather, the second law of thermodynamics governs the transfer of substances via passive transport. The flow of these molecules happens from the region of high electrochemical potential to the region of lower electrochemical potential with or without proteins.

The features of this process are mentioned below:

  1. Simple diffusion-

    This is a type of passive transport where the molecules travel from the region of higher concentration to the region of lower concentration without the help of proteins. Thus, the transfer of molecules is uniform and constant.

    Since this kind of diffusion does not use any proteins for the transfer, the concentration gradient present across the membrane drives the uniform flow of molecules. Therefore, it is not a solute-specific process.

  2. Facilitated diffusion-

    The molecules in this process travel from a region of higher concentration to a region of lower concentration with the help of membrane proteins. This process does not involve the transfer of chemical energy.

    This type of diffusion can be active as well. However, this process is directed only between the solute and the carrier molecules. Examples of this process include the transfer of ions, movement of oxygen in the blood with the help of hemoglobin.

  3. Filtration-

    This is the process that allows a similar method of transfer of molecules across the membrane. However, this does not use any external energy and is driven entirely by the pressure gradient of the cell membrane. The transfer of molecules is from the region of higher pressure to the region of lower pressure.

    The pressure that allows such a flow of molecules through the membrane is known as hydrostatic pressure. This is the primary process that is used in capillary exchange

  4. Osmosis-

    Osmosis is a unique type of passive transport where water moves from the lower solute concentration region to the higher solute concentration. However, this does not mean that water moves from lower to higher concentrations.

    Here, water travels from the region of higher water concentration (lower solute concentration) to the region of lower water concentration (higher solute concentration).

Passive transport is critical to numerous processes. The biological importance of this process is discussed below:

  • In plants, gases like carbon dioxide and oxygen enter and exit the plant through the stomata openings. Here, the exchange of gases happens due to passive transport.
  • This takes place in the blood-brain barrier since molecules like sodium thiopental can diffuse across the membrane present.
  • The concentrated food in the body is transferred from the intestine to the body due to the presence of the concentration gradient.
  • Carbon dioxide and urea are transferred into the bloodstream with the help of diffusion.
  • Osmosis is a very important process in plants because it helps in the transfer of water and food substances.
  • In animals, the filtration process helps them get rid of waste materials present in the blood and other body fluids.

As discussed in this article, the process of passive transport is very important in plants, animals, and humans. First, this is a naturally occurring process and does not need any energy for the transfer of molecules to take place.

 

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