Fascinating Barriers: Exploring Semipermeable Membranes and Osmosis
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The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (3/2017):
On Semipermeable Membranes and Osmosis
The concept of the cell was first formulated in 1665 by Robert Hooke. In biology, we use this term to describe the smallest functional and structural unit of a living organism that can carry out all basic life processes. Thus, a cell must be capable of metabolic activity as well as growth and reproduction in a manner appropriate to its nature. We can therefore say it is the fundamental morphological and functional unit of an organism [1]. We know that cells show significant morphological and biochemical differences among themselves. These differences are so pronounced that some exist as independent single-celled organisms, while others are integral components of multicellular life forms.
A cell is enclosed by a cell membrane. The presence or absence of a cell nucleus inside it serves as the basis for dividing organisms into eukaryotes Eucaryota and prokaryotes Procaryota. In most prokaryotes, plants, fungi, and certain other groups of organisms, there is an additional external structure beyond the cell membrane—a cell wall. This complex and varied structure is nonliving and lacks its own metabolism. Within the cell lies the cytoplasm, and in eukaryotes there are also a number of internal organelles with specific functions, such as mitochondria, plastids, the Golgi apparatus, vacuoles, and others.
The cell membrane, also called the cytoplasmic membrane or plasmalemma, is composed of two layers of phospholipids along with proteins. Some proteins are loosely attached to the membrane surface (surface proteins), others span the membrane (transmembrane proteins), or are anchored within it (membrane proteins).
Note that the cell membrane plays a crucial role—it separates the external environment from the cell’s interior. We should not underestimate this function once we realize that life, at least on the molecular level, is a complex set of tightly interconnected physicochemical processes. Carrying out such processes requires suitable conditions, including the separation of these processes from an external environment that is organized differently.
However, as a barrier, the cell membrane cannot be completely impermeable, because no cell could survive as an isolated system with no exchange of matter or energy with its surroundings.
Hence, on one hand, the cell membrane must act as a barrier separating the interior of the cell from the external environment, and on the other, it must allow certain substances to move in both directions. Such a barrier is termed a semipermeable membrane. Of course, semipermeability is only one of the cell membrane’s properties, as its overall structure is far more complex than briefly outlined here.
Semipermeable membranes exhibit a number of very interesting properties that we can explore. In the following sections, I would like to propose several not-too-complicated yet intriguing experiments for the Reader, using both natural and artificial semipermeable membranes.
A Bit of Theory on Osmosis
We can say that a semipermeable membrane is a barrier that allows certain types of molecules to pass through while blocking others. For example, small molecules of the solvent may diffuse through it, whereas larger dissolved molecules or ions are blocked [2].
To understand the processes occurring with a semipermeable membrane, we need to recall diffusion. Diffusion is the spontaneous spread of molecules or energy in any medium (solid, gas, or liquid) at temperatures above absolute zero, caused by the random collisions of diffusing molecules with each other or with the medium.
Now, consider what happens if we use a semipermeable membrane to separate two solutions with different concentrations. This is illustrated schematically in Fig.1. Notice that in the right side of the vessel, the solute concentration is much higher than in the left. Through the membrane (shown as a gray barrier), only the blue solvent molecules can pass, whereas the much larger red solute molecules cannot.
In such a scenario, the solvent molecules come into contact with the membrane more frequently on the side with the lower concentration, because in the higher-concentration region there are more solute particles competing with the solvent molecules for access to the membrane. As a result, more solvent passes through the membrane from the less concentrated solution to the more concentrated one than in the reverse direction. We thus observe a net solvent flow in this direction. This phenomenon is called osmosis. Notice that due to osmosis, the concentrations of the two solutions gradually move toward equilibrium across the semipermeable membrane: the less concentrated solution becomes more concentrated because it loses solvent, while the more concentrated solution is diluted by the incoming solvent.
The solution with the lower concentration (the one losing solvent) is called hypotonic, while the one with the higher concentration (the one gaining solvent) is termed hypertonic. When solutions are in osmotic balance (meaning solvent exchange occurs at the same rate in both directions), they are said to be isotonic with respect to one another.
How Can We Investigate This?
There are many ways to study osmosis in biological systems. Whichever method we choose, we still need a semipermeable membrane of some sort.
For an aspiring biologist, obtaining suitable material is not an issue. In the first experiment, I propose using an animal semipermeable membrane in the form of animal intestines (Photo.1).
These are sections of cleaned pig small intestine. Obtaining this material is not difficult—they are used as sausage casings and can be purchased in specialized stores. However, note that some types, particularly those with long shelf lives, are unsuitable because the preservation process damages the intestines so they lose their semipermeable function. Before use, it’s helpful to soak them in boiled water at about 30°C (about 86°F) so they become more flexible.
The next step is to build the appropriate experimental setup—shown in Fig.2. It consists of a funnel-like container filled with solution a. One side is sealed with a semipermeable membrane and immersed in solution b, which has a different concentration than the solution inside the funnel. The other side ends in a thin tube c, open at the top.
This apparatus can be assembled from relatively easy-to-obtain parts, such as those in Photo.2. A glass reduction adapter (commonly found in laboratory glassware) makes an excellent funnel. Its narrower end should be connected using a piece of rubber tubing to the spout of a small volumetric pipette, say 2 cm3 (about 0.068 fl oz). The advantage here is that the liquid level can be read against the pipette’s scale.
The wider end of the adapter should be closed off with the semipermeable membrane, as shown in the diagram. You can create this membrane by cutting the pig intestine lengthwise and flattening it out (Photo.3).
The membrane must be secured in a way that ensures the best possible seal where it meets the glass. Several rubber bands work well for this (Photo.4).
Handle the intestinal membrane carefully, because although it resists stretching fairly well, it can be easily punctured, which would make the experiment impossible. Also be sure not to let it dry out; the membrane should remain moist.
In our case, I recommend using distilled water as the hypotonic solution and a concentrated glucose solution at room temperature (about 68°F) for the hypertonic solution. To make observation easier, the glucose solution can be dyed with food coloring (Photo.5).
The completed apparatus is shown in Photo.6. A test-tube clamp fixed to a laboratory stand is used to suspend the entire setup in a container of distilled water.
Immediately after assembling the setup, record the initial height of the liquid column (Photo.7A). Leaving the system undisturbed for a while, you may observe that the height of the liquid column begins to rise (Photo.7B). Without knowledge of osmosis, this might seem surprising since it appears to go against gravity. After a moment of reflection, however, we see it aligns perfectly with our theoretical discussion of osmosis: solvent is moving from the external hypotonic solution to the internal hypertonic one, manifesting as an increased liquid column.
The second experiment confirms this as well. Here, the hypertonic solution is on the outside, while the hypotonic one is inside the funnel. The hypotonic solution is dyed this time, and its initial liquid level was set to the same height as in the previous test. This time, after a while, the liquid column clearly decreases (Photo.7C).
The described apparatus can also be used to investigate the properties of other semipermeable membranes.
We can also demonstrate the existence of osmosis using a potato. Cut two small pieces from a raw potato tuber, for example in the shape of rectangular blocks (Photo.8).
Weigh both potato blocks and record the results, then immerse one in a hypotonic solution (distilled water) and the other in a hypertonic solution (a concentrated glucose solution) relative to the cells that make up the tuber. After a certain period (usually about three hours is enough), remove the blocks, gently blot them dry with paper towel or filter paper, and weigh them again. The results of my experiment are shown in Table.1.
| Hypotonic Sol. | Hypertonic Sol. | |
|---|---|---|
| Initial mass [g] | 3.23 (about 0.114 oz) | 3.00 (about 0.106 oz) |
| Final mass [g] | 3.76 (about 0.132 oz) | 2.48 (about 0.087 oz) |
| Change in mass [g] | +0.53 (about 0.019 oz) | -0.52 (about 0.018 oz) |
| Change in mass [%] | +16.4% | -17.3% |
In both cases, the mass of the potato blocks changed. The piece left in the hypotonic solution increased by 0.53 g (about 0.019 oz), while the block in the hypertonic solution lost 0.52 g (about 0.018 oz) (+16.4% vs. -17.3%). Thus, the magnitude of change was quite similar in absolute terms for both samples.
We can conclude that the shift in mass was due to osmosis. Depending on the concentration of the external environment, water either flowed into or out of the potato cells—here, their semipermeable membrane included the cell membrane. The change in water content was large enough to be reflected in the measured mass of the blocks. This finding is also supported by the appearance of the blocks after removal from the solution—one clearly shrank and became softer (Photo.9a), while the other remained firm (Photo.9b).
An important manifestation of osmosis is plasmolysis, which can be observed in many plant cells. Onion bulb scales (Allium cepa) are particularly convenient. The internal epidermis of these scales consists of a thin layer of cells that are easy to view under a light microscope at relatively low magnifications. I recommend using the red variety of onion, because its pigment enhances image contrast without artificial staining, making observation easier.
It is simple to separate the epidermis from the scale using tweezers—take it from the upper part of the inner side of the leaf, where the tissue is more intensely colored yet still translucent (Photo.10). Place it in a drop of distilled water and cut out a small portion. Position this fragment on a microscope slide in another drop of distilled water and cover it with a coverslip, then choose an appropriate magnification. You will see normal cells of the epidermis (which are polygonal and elongated, with clearly visible cell walls), mostly filled by large vacuoles. The cytoplasm is only a thin layer around the vacuoles, and the nuclei are not very easy to spot (Photo.11A). The cell contents appear pigmented.
Next, prepare another slide, except this time keep the epidermis in a concentrated glucose solution for several (10–15) minutes before observation. You should also mount it under the coverslip in a drop of that same solution. The resulting view is shown in Photo.11B. Due to osmosis, some water has moved out of the cells into the surrounding solution. As they lose water, the protoplasts shrink and pull away from the cell walls. This process is known as plasmolysis [3]. If the cell membrane remains undamaged during plasmolysis, placing the cells in a hypotonic solution can induce the reverse process, known as deplasmolysis.
Semipermeable Membranes Once Again—This Time Inorganic
So far, we have dealt only with organic semipermeable membranes. However, they can also be produced entirely artificially. One example is cellophane—a transparent cellulose-based film [4].
Based on osmosis, one can also perform some very interesting experiments commonly called chemical gardens or chemical plants. When soluble silicates react with heavy metal salts (Photo.12A), or when soluble ferrocyanides(II) react with copper(II) salts (Photo.12B), insoluble products form semipermeable membranes. As water seeps into these bubbles via osmosis, they swell and rupture, resulting in structures that grow rapidly and often branch, reminiscent of fantastical plants or other biological formations [5] [6]. It’s worth appreciating this display not only for its aesthetic appeal but also for its educational value.
Summary
The experiments described here are straightforward and can easily be repeated under school or home conditions. Importantly, they help build familiarity with the fascinating and essential phenomenon of osmosis, which lies at the intersection of biology, chemistry, and physics.
Additionally, osmosis and semipermeable membranes have many practical applications today. Blood dialysis for kidney patients, desalination of seawater, and water purification by reverse osmosis are just a few examples. I believe this topic fully deserves inclusion in the teaching of the natural sciences.
References:
- [1] Sawicki W., Histologia, Wydawnictwo Lekarskie PZWL, Warszawa, 2008 back
- [2] Rautenbach R., Procesy membranowe, Wydawnictwo Naukowo-Techniczne, Warszawa, 1996 back
- [3] Guzik M., Kozik R., Jastrzębska E., Matuszewska R., Pyłka-Gutowska E., Zamachowski W., Biologia na czasie 1, Wydawnictwo Nowa Era, 2013, pp. 61 back
- [4] Hassa R., Mrzigod J., Nowakowski J., Podręczny słownik chemiczny, Wydawnictwo Videograf II, Katowice, 2004, pp. 66 back
- [5] Ples M., Chemiczne rośliny (eng. Artificial Plants Created by Chemistry), Chemia w Szkole (eng. Chemistry in School), 5 (2015), Agencja AS Józef Szewczyk, pp. 6-9 back
- [6] Ples M., Chemiczny ogród (eng. Chemical Garden), online: http://weirdscience.eu/Chemiczny%20ogr%C3%B3d.html [10.04.2017] back
All photographs and illustrations were created by the author.
Marek Ples