• Describe the movement of substances across the cell membrane by diffusion, osmosis, active transport and phagocytosis.
• Explain how the exchange of materials across the cell membrane is related to its structure.
• Use the concept of water potential to explain the movement of water between cells and their environment.
The cell membrane, which is also called ctyoplasmic membrane or plasma membrane, acts as a barrier between the cell contents and the outside environment (Figueroa et al., 2013). The basic structure of a cell membrane is a bi-layer of phospholipid with protein molecules embedded in it. The phospholipid layers provide a good barrier between two aqueous compartments and the embedded proteins conducts some specific functions of cell membrane including transportation of molecules.
This membrane is selectively permeable so that it can control what goes inside and comes out of the cell. A selectively permeable membrane is a type of membrane that allows some particular ions or molecules to go across it. The transportation of the molecules either can take place by active transport, which requires the expenditure of cell energy or by no expenditure of cellular energy that is passive transport (Stein, 2012). The cell membrane can also stop the passing of materials according to the condition of the cell or of the environment.
Description of the processes
The main processes involved in transportation across cell membrane are -
It is a spontaneous process driven by the difference in a concentration gradient; that is the molecules of a fluid or gas moves from a higher to lower concentration region, and this process continues until the concentration becomes equal.
Osmosis is another type of passive transport system which involves the movement of solvent molecules through a semi-permeable membrane. In this process no transportation of solute molecule takes place. The solvent moves from a diluted solution towards a concentrated one (Kotyk, 2012). It regulates the osmotic pressure of cell by maintaining the amount of water, and it depends on the concentration of dissolved particles inside and outside the cell.
This process involves the movement of molecules across the membrane using chemical energy. It utilises ATP to force the molecules against its concentration gradient that is from lower to higher concentration. In this process, mainly large molecules like protein, large cells, and complex sugars are transported into and out of the cell.
Figure: Active Transport Vs Passive Transport across cell membrane
(Source: apocketmerlin.tumblr.com, 2016)
It is a special type of endocytocys that takes up solids such as bacteria or nutrient by a cell. In the human body, it has an important role in the immune system and is mainly performed by white blood cells.
The mechanism of the above processes with respect to the cell membrane structure has been discussed below:
Diffusion is a passive transport system that requires no cellular energy. In the case of lipid soluble particles diffusion occurs directly through the lipid bi-layer, but hydrophilic molecules cannot diffuse through the lipid bi layer. These molecules are diffused through protein channels present in the cell membrane (Figueroa et al., 2013). The protein molecule acts as a carrier, which automatically binds with the substrate and make it pass by exposing the substrate to the other surface of the membrane. This type of diffusion is also known as facilitated diffusion. The diffusion process is affected by size concentration and solubility of the molecules.
Figure: Diffusion across cell membrane
(Source: Classes.midlandstech.edu, 2016)
As cell membrane is semi-permeable, transportation of molecules takes place by osmosis. Three conditions can happen depending on the concentration of any substance inside and outside the cell. If the solution is hypertonic that is higher than the cytoplasm, then water comes out of the cell and shrinkage of the cell occurs (Kotyk, 2012). When the solution has a lower concentration (hypotonic solution), water enters into the cell causing it to swell. If the concentration of the cytoplasm and the solution remains same, then it is called isotonic solution. In this condition, no movement of water occurs.
(Source: Instructables.com, 2016)
Active transports are of two types: primary and secondary. In the first one, the trans-membrane protein molecules detect the substrates and transport it to the other side of the membrane utilising some chemical energy. In a secondary active transport system, the porous proteins form some channels across the membrane and molecules are forced inside by using electromagnetic gradient which is generated by lowering the concentration gradient of other substance (Kotyk, 2012).
In fagocytocys process, first the cell recognizes the foreign matter and then is drawn towards it. Then it attaches its membrane to the bacterium by using a molecule present in it called surface receptor. The cell engulfs the bacterium along with cell membrane surrounding it and encloses it inside. Now the bacterium is digested by the enzymes released inside the cell, and it leaves a harmless residue which is either released out or utilised by the cell.
Apart from these, other processes are also involved in exchange through a cell membrane, like endocytocys and exocytocys, which involves taking up and releasing specific substances respectively (Figueroa et al., 2013). Both of these are active transport processes.
Effect of Water Potential
Water potential is defined as the difference in potential energy between a given water sample and pure water at constant pressure and temperature. This is denoted by Greek letter á´ª. Here, water moves by osmosis from a higher water potential to where it is lower. This movement is affected by two-factor; those are pressure potential and solute concentration (Stein, 2012). With an increment in solute concentration, water potential decreases and with increased pressure exerted on the cell wall, the value of water potential increases proportionally. Depending on this factor the water potential changes and the transportation of water inside the cells occur accordingly.
apocketmerlin.tumblr.com,. (2016). Exchange Across Plasma Membranes: Diffusion, Facilitated Diffusion, Osmosis and Active Transport. Retrieved 23 January 2016, from https://apocketmerlin.tumblr.com/post/15019772012/exchange-across-plasma-membranes-diffusion
Classes.midlandstech.edu,. (2016). Cells: The Living Units. Retrieved 23 January 2016, from https://classes.midlandstech.edu/carterp/Courses/bio210/chap03/lecture1.htm
Figueroa, X.F., Lillo, M.A., Gaete, P.S., Riquelme, M.A. & Sáez, J.C., (2013). Diffusion of nitric oxide across cell membranes of the vascular wall requires specific connexin-based channels. Neuropharmacology, 75, pp.471-478.
Instructables.com,. (2016). How to teach osmosis. Retrieved 23 January 2016, from https://www.instructables.com/id/How-to-teach-osmosis/
Kotyk, A., (2012). Cell membrane transport: principles and techniques. Springer Science & Business Media.
Stein, W., (2012). Transport and diffusion across cell membranes. Elsevier.