By Ella Kang
Cells are the basic units of all organisms, everything is made up of cells. The movement of different cell particles across membranes keeps all the cells alive, regulating the passage of important solutes. One of the functions of those semi-permeable and selective membranes is thus to control what enters and leaves the cell. This blog will explain the 4 different types of membrane transport, which depend on the characteristics of the substance being transported as well as the direction of transport.
Essentially, the membrane transport may occur either passively or actively, and the processes are called passive transport and active transport. Then, there are 3 main types of passive transport: simple diffusion, facilitated diffusion, and osmosis.
Simple diffusion
In simple diffusion, small or lipophilic non-charged molecules pass between the phospholipids in the membrane to go in and out of the cell, moving from high concentration areas to low concentration areas. There is therefore a net movement from the higher to the lower concentration – a movement down the concentration gradient. Simple diffusion only happens if the phospholipid bilayer is permeable to the particles; this is the reason why common non-polar molecules such as oxygen (O2 ), carbon dioxide (CO2), and most lipids can easily diffuse through. On the other hand, the center of membranes is hydrophobic, which prevents ions with positive or negative charges to pass through. Moreover, living organisms do not have to use energy to make diffusion occur so it is a passive process.
Facilitated diffusion
Facilitated diffusion is the movement of molecules through membrane protein channels. While both simple diffusion and facilitated diffusion involve movement down the concentration gradient, the ways in which substances get through the cell membranes differ (phospholipids and specialized membrane channels). And because the membrane channels are holes with a very narrow diameter, large or charged molecules cannot move through. The protein wall of the membrane also ensures that only one type of particle passes through at once, for instance sodium ions or potassium ions, not both.
Osmosis
To explain in simple words, osmosis is the net movement of solvent (usually water molecules) through a semipermeable membrane. Osmosis happens because of the difference in concentrations of substances, and the solvent molecules flow into or out of a cell to balance the concentration of another substance. There are 3 types of solutions that cells can be in: isotonic, hypertonic, and hypotonic. An isotonic solution’s solute concentration is the same both inside and outside the cell; a hypertonic solution has a higher solute concentration outside the cell; and a hypotonic solution has a higher solute concentration inside the cell. Now, osmosis affects the cells differently depending on each type of solution. While animal cells swell and burst in a hypotonic solution due to osmosis, plant cells do not because of their thick cell walls. An example of osmosis in plants is how they absorb water from soil. Because the roots of a plant have a higher solute concentration than the surrounding of soil, water molecules move into the roots!
Active transport
Lastly, active transport involves taking in substances regardless of the concentration. Therefore, this is a movement of materials against a concentration gradient unlike the 3 other types introduced above. Furthermore, because the materials move against the gradient, energy is required to carry it out and this is why it’s named active transport. A substance called Adenosine 5’-triphosphate (ATP), the principal molecule for storing and transferring energy in cells, is often used as the energy supply. And it is called primary active transport when the direct use of energy is involved. In cases where several molecules are coupled with one moving along an electrochemical gradient, it is called secondary active transport.
Works Cited
Allott, Andrew, and David Mindorff. Biology: Oxford IB Diploma Programme. Oxford University Press, 2014.
Membrane Transport. Yvcc.edu, http://www2.yvcc.edu/Biology/109Modules/Modules/MembraneTransport/membranetransport.htm.
“Osmosis.” Biology Dictionary, 1 Oct. 2020, https://biologydictionary.net/osmosis/.
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