Osmosis in red blood cells and bacteria

Osmosis in red blood cells and bacteria

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This is a question from an exam in my biology course.

Bacterial cells and human red blood cells were inserted into one solution. Upon testing one hour later the blood cells exploded, while the bacterial cells stayed intact. What answer explains the findings:

A. The bacteria cell walls prevented their explosion.

B. The blood cell walls didn't prevent their explosion.

C. solute concentration in the solution is equal to the blood cells and bacteria solute concentration.

D. solute concentration in the solution is higher than the blood cells solute concentration.

My lecturer said the right answer is B. I thought the right answer is A. What would be the correct answer and why?

To my understanding, red blood cells don't have a cell wall, but rather a membrane, which is why in my opinion you can't say that it prevented the explosion if it doesn't exist. On the other hand, bacteria does have cell wall, that's why I thought this answer is the correct one.

Absent a very specific learning objective, if (B) is correct, this is a poor question. In a high-school or undergraduate introductory biology course, I would say:

(A) The bacteria cell walls prevented their explosion

Would be the correct answer, because of exactly your reasoning. This is what you're taught in introductory biology. You're taught that animal cells don't have cell walls. Bacteria and plant cells do have cell walls.

See The Cell

animal cells are not surrounded by cell walls

Under this concept of a cell wall, red blood cells certainly don't have them.

However, red cell osmotic fragility testing comes out of a tradition that uses the phrase cell wall strength to describe the response of a red cell to osmotic stress. See Orcutt

It appeared logical to us that a model based on a distribution of cellular wall strengths ought to reproduce the experimentally observed variation of degree of hemolysis as a function of osmotic stress.

All the same, if you were operating from that perspective, how would you distinguish between (A) and (B)? Here, we're not even taking into account the fact the primary mechanism for responding to osmotic stress in most animal cells, including the human erythrocyte, is regulation of ion flow, not the pressure of a rigid extracellular matrix. If the solution contained ouabain, which certainly does lyse human RBCs, would it be best to characterize the problem as "The blood cell walls didn't prevent their explosion"?

I would ask your lecturer to clarify the learning objective behind the question. As an interpersonal note, I'd point out that this is very different from asking him or her to justify the correct answer. Some (many… ) teachers are oddly sensitive to any suggestion that they need to justify their expertise, especially when it comes to exam questions. If you make it a question about the learning objective, this should help you avoid triggering that particular sensitivity.

Cells put in isotonic solution showed no change in morphology. These results are the bases for infusing intravenous fluids to patients by health care professionals. The effect of temperature was determined by placing potassium permanganate in a beaker of water and subjected to different temperatures. A significant increase in the rate of diffusion was observed in the sample subjected to 60 degrees centigrade than those subjected to lower temperatures.The human body is made up of different organ systems which work together in maintaining an environment that is most beneficial to the organism.

These systems have several mechanisms all aimed at regulating the different bodily processes. The plasma membrane plays a very important role in protecting the cell as it regulates the substances that go in and out of the cell. This ensures that the substances that go inside the cells are vital for cell functions and that harmful substances are discarded. This study has been conducted to demonstrate diffusion and osmosis, to define hypertonic, hypotonic and isotonic solutions, to measure packed cell volume and investigate how erythrocytes react to the different solute concentrations.

To determine the effect of different solute concentrations on red blood cell, blood samples were mixed with the different solutes in properly labeled test tubes. The turbidity of the solution was then determined by placing the test tubes against a printed page of a book. To support the observations on the test tubes, samples were also collected from the tubes using Pasteur pipettes and were put in glass slides and were viewed under the microscope with the magnification of 10x40. The results were then tabulated and recorded in table 1.

The packed cell volume of the blood samples was also determined by taking 15ml of blood samples by using capillary tubes. The unfilled end of the tubes was then sealed with cristaseal.

What causes Crenated red blood cells?

crenation. noun. A rounded projection, as on the margin of a shell. The condition or state of being crenate. A process resulting from osmosis in which red blood cells, in a hypertonic solution, undergo shrinkage and acquire a notched or scalloped surface.

Also, what has happened to a cell that is Crenated? In the context of basic biology, &ldquocrenation&rdquo describes the process a cell undergoes when it looses water from its interior (the cytosol is largely water), across its cell membrane, to the surrounding fluid due to a differential in &ldquoosmotic pressure.&rdquo This causes the cell to shrivel like a grape becoming a raisin (see

Hereof, what is Crenation and how does it occur?

crenation. oxford. views updated. crenation The shrinkage of cells that occurs when the surrounding solution is hypertonic to the cellular cytoplasm. Water leaves the cells by osmosis, which causes the plasma membrane to wrinkle and the cellular contents to condense.

What does a Crenated cell look like?

Red Blood Cell Crenation There are two different types of crenated red blood cells: echinocytes and acanthocytes. Instead of the usual rounded biconcave shape, both these cells appear with a rounder form and spiny projections on the cell surface. In echinocytes, the spines are short, uniform and regularly spaced.

11 Examples Of Osmosis In Real Life

Osmosis is a simple natural process that occurs all around and inside us, and it’s one of the most vital processes for our survival. Everything tends to reach equilibrium and to reach at equilibrium the most crucial role is played by the water. Even each cell of our body, plants, and animals around us are surviving due to osmosis. Osmosis functions as a Life-Preserver. From helping out cells to survive to the desalination of seawater, the process involved is osmosis. Let’s dig into some interesting examples of osmosis in our daily life, but before that let’s understand, What is Osmosis?

Definition Of Osmosis

Osmosis is the movement of water from less concentrated to the more concentrated solution through a semi-permeable membrane.

1. Crucial In Plant’s Survival

When we water plants, we usually water the stem end and soil in which they are growing. Hence, the roots of the plants absorb water and from the roots water travel to different parts of plants be it leaves, fruits or flowers. Every root acts as a semipermeable barrier, which allows water molecules to transfer from high concentration (soil) to low concentration (roots). Roots have hair, which increases surface area and hence the water intake by the plants.

2. Helps in Regulating Our Cell’s Life

We drink water, but also our cells absorb it by osmosis in the same way that plant roots do. As the concentration of waste products in a cell rises, the osmotic pressure between the inside and outside of the cell wall, which is a semipermeable membrane, increases, and the cell absorbs water from the blood, which is a more dilute solution that of the cell’s cytoplasm. Even the primary nutrients and minerals get transferred through osmosis into the cells. Also, our intestine absorbs nutrients and minerals through osmosis.

3. Ever tried soaking resins or dry fruits in water. what happened?

well, when we soak resins in water they swell up and this is all that happens due to osmosis. Water travels from high concentration to low concentration and keeps moving by osmosis until the equilibrium is reached, that is when the concentration of both solutions is the same.

4. Responsible For Your Pruned or Wrinkled Fingers

When we sit in the bathtub or submerge our fingers in water for a while they got wrinkly. And that is too because of osmosis. The skin of our fingers absorb water and get expanded or bloated leading to the pruned or wrinkled fingers.

5. Osmosis May kill Slugs or Snails

You must have heard about the killing of slugs or snails by putting salt on it. Well, It’s nothing but the process of osmosis which kills them. The liquid inside them comes out and try to dilute the salt concentration and maintain the mucus layer, and hence, they end up shedding water. Too much salt and slugs or snails will dry up and die!

6. Reason Behind To Get Thirsty

We usually feel thirsty after eating salty food because salt is a solute and after consuming lots of salt, our cells become concentrated with salt, which triggers the process of thirst. So, our cells absorb water and we feel thirsty, and hence, we start drinking water.

7. Helps You Get Relieved From Sore Throat

In case you have a sore throat, cells and tissues surrounding the throat are swollen because of the excess of water. The salt water which we use for gargles has a lower concentration of water than the cells of the throat. So, water molecules move from the swollen cells of the throat to the salt water reducing pain and swelling.

8. Helps In Preserving Your Food

The reason why we can enjoy pickles and jams for a longer period of time without any fear of their spoilage is osmosis. Pickles and Jams have been used over decades as quick spreads and ready to eat food for kids as well as adults. They both contain high proportions of salt and sugar, respectively that acts as a natural preservative for fruits and vegetables. Though vegetables and fruits are very prone to bacterial attack but the high salt and sugar concentration is hypertonic to bacteria cells, and bacteria cells lose water and it kills them by dehydration before they can cause the food to get spoiled.

9. Helps You Getting Pure Water

RO’s are installed in almost every home in today’s scenario. Actually, this is not osmosis, strictly speaking, but, Reverse osmosis that is what RO stands for. Reverse Osmosis is the process of Osmosis in reverse. Whereas Osmosis occurs naturally, without the involvement of energy however, to reverse the process of osmosis, you need to apply energy to the more saline solution. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria and pyrogens. However, you need to ‘push’ the water through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralize or deionize) water in the process allowing pure water to pass through while holding back a majority of contaminants. Reverse Osmosis is also used in large scale desalination of seawater turning it into drinking water.

10. Saves Eyes From Dry Contact Lenses

Soft contact lenses consist of semipermeable materials. If you wear contacts after storing them in sterile saline solution, the concentration of the saline in the contacts matches the salt content in the natural fluid that moistens your eyes the contacts stay moist, soft and comfortable. If you store contacts in distilled water, the salt concentration is higher in the eye fluid and water flows out of the contacts slowly drying them out.

11. Helps In Maintaining Water Balance In Our Body

Kidneys are the vital organ of our body, which helps in the removal of waste and toxic materials. Osmosis occurs to recover water from waste material. Kidney dialysis is an example of osmosis. In this process, the dialyzer removes waste products from a patient’s blood through a dialyzing membrane(acts as a semi-permeable membrane) and passes them into the dialysis solution tank. The red blood cells being larger in size cannot pass through the membrane and are retained in the blood. Thus, by the process of osmosis waste materials are continuously removed from the blood.

Basically, this concept becomes important when we start to administer hypertonic or hypotonic solutions. We understand that if we were to admister 100% sterile water intraveneously to patients who are dehydrated, the cells within the vasculature (blood vessels) would draw up all that fluid due to the osmotic gradient shift (this basically means that the fluid will want to shift into the cells) and this will cause the local cells to swell and lyse (rupture). This means that the red blood cells themselves will no longer be capable of carrying oxygen and serving their purpose.

Likewise, if you were to administer 50% glucose intravenously, the hypertonic solution (lots of solutes) will cause a lot of fluid to shift towards it. Now, so long as the canula is in a large vein, it will be able to draw fluid from a large area. However, if the canula is inserted in a small vein or accidentally inserted into the intersitial space and not a vein, it will not be able to draw fluid from all over, and consequently draw all the fluid from the surrounding cells. This will cause the cells to shrink (crenate) and again, become unable to sustain life. In these circumstance, patients may develop cellulitis or damaged veins.

Examples of Osmosis in Cells?

This becomes important when looking at the structure of cells in biology.

If a cell has a high concentration of a solute (salt) it will draw fluid into its cell. If allowed to continue to do this, it will eventually swell up and rupture (this is called cell lysis).

If a cell has a low concentration of a solute it will lose fluid as the fluid within its cell is allowed to be drawn out of the cell through osmosis and into the intersitial space. This will cause the cell to shrink (this is called cell crenation).

Examples of Osmosis Applied

The following are examples of osmosis for those of you who need to see an example to clearly understand science (like me)…

Example of osmosis 1.

If you put rice into a bowl of water, the water will move into the rice causing them to swell, while causing the water level to drop.

Osmosis example 2.

If a cell is placed in a container which is full of a hypotonic solution (not many solutes) than the fluid in that container will naturally want to be drawn into the cell and this will cause the cell to swell up and rupture (lyse).

Example of osmosis 3.

If a cell is placed in a container which is full of a hypertonic solution, than the cell will lose fluid as it is diffused outof the cell into the area of greater solute concentration. This will cause the cell to shrink (crenate).

Example of osmosis 4.

In medicine, if a patient drawns in salt water, the hypertonic water in the lungs will cause more fluid to be drawn out of the pulmonary arteries into the alvioli and lungs. This will more often result in pulmonary oedema than a patient who has drawned in fresh water.

Example of osmosis 5.

If you admister normal saline to a patient intraveneously (which has the same osmolarity as blood / the same amount of solutes as blood) it will mean that the fluid remains unchanged, because their is no osmotic gradient (basically all the fluid stays the same). Now, if you were to give mannitol (which is a very hypertonic solution) the fluid within the blood will be drawn out of the cells and into the mannitol solution.

Example of osmosis 6

When the stem of a plant is cut and placed in water (for example a vase), the water will move up through the stem by a process of osmosis, in which the water is flowing to the higher concentration of solvents (found in the plant).

By understanding these examples of osmosis you will be able to better apply the concept of osmosis to practical uses in medicine, paramedicine, nursing, and many fields of general science.

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All information is provided for educational purposes only and should not be taken as medical advice.
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Equipment and Supplies

The following equipment and supplies are needed.


Distilled water (20 ml per pair of students).

2.7% wt/vol NaCl solution (2.7 g NaCl per 100 ml of distilled water) (20 ml per pair of students plus that required for nonhemolyzed blood preparation). This stock solution is used to prepare all other NaCl solutions in the experiment.

Isosmotic urea solution (17.1 g/l) (5 ml per pair of students plus that required for hemolyzed blood preparation).

Fresh mammalian blood. This blood is referred to for the rest of the experiment as nonhemolyzed blood. We find that there are no appreciable differences in the outcome of the experiment depending on which species blood is used, although values of hemolysis can vary. Obtaining mammalian blood supplies can be problematic if obtained locally direct from an abattoir however, blood can also be purchased online (for example, For a class of around 200 students working in pairs,

1.5 liters of blood are required (

11 ml blood per pair of students and allowing extra for repeat experiments if required). The blood must be heparinized before use to prevent clotting by the addition of heparin sodium (5,000 IU/ml per 1.5 liters blood). This blood is then used to produce the hemolyzed and nonhemolyzed blood as follows.

Hemolyzed blood. To prepare the hemolyzed blood in manageable volumes, 250 ml of nonhemolyzed blood are measured into a 600-ml beaker, together with 250 ml urea solution (17.1 g/l), and stirred. The tonicity of the urea and resultant osmotic water movement results in hemolysis of the cells, and this will form the blood used for the production of the hemoglobin standards that will be used to assess the degree of hemolysis in the experiment. Decant 10 ml of the hemolyzed blood into 50 centrifuge tubes (one per pair of students), labeled “H” for hemolyzed blood, and centrifuge at 6,000 rpm for 2 min. Repeat depending on quantities of blood required, i.e., if 1 liter is required, repeat once.

Nonhemolyzed blood. To prepare the nonhemolyzed blood in manageable volumes, 275 ml of nonhemolyzed blood (from the original heparinized fresh mammalian blood) are prepared by the addition of 275 ml of 0.9% wt/vol saline and stirred gently. This forms the nonhemolyzed blood, which will be used for the main part of the experiment at an equal concentration to the hemolyzed blood. Decant 11 ml of the nonhemolyzed blood into 50 centrifuge tubes (one per pair of students) labeled “N” for nonhemolyzed blood. Repeat depending on quantities of blood required, i.e., if 1 liter is required, repeat once. An assumption is made that the hemoglobin concentration of the original blood sample is 15 g/dl, but, as the hemolyzed blood is diluted 1:1 with isosmotic urea (17.1 g/l) and the equivalent nonhemolyzed blood is diluted 1:1 with isosmotic (0.9% wt/vol NaCl), the hemoglobin concentration of both blood samples is, therefore, assumed to be 7.5 g/dl (75 g/l).


600-ml Glass beakers (2 for blood preparation)

500-ml Measuring cylinders (2 for blood preparation)

Stirring rods (2 for blood preparation)

25-ml Glass beakers for water, 2.7% wt/vol NaCl and urea distribution (3 per pair of students)

1.5-ml Plastic Eppendorf tubes with hinged cap (11 per pair of students)

10-ml Plastic centrifuge tubes with cap (10 per pair of students)

Centrifuge tube racks (1 per pair of students)

75-µl Glass microhematocrit tubes (Hawksley catalog no. 01603) (6 per pair of students)

Centrifuge with centrifuge tube rotor and microhematocrit tube rotor (Hettich EBA21 centrifuge with 1416 rotor and 1450 hematocrit rotor)

Hematocrit readers (Hawksley) or 30-cm rulers (a number of readers/rulers can be shared between pairs of students)

1.5-ml Disposable plastic pipettes or equivalent Gilson pipettes if available (3 disposable pipettes per pair of students)

Marker pens (1 per pair of students)

White paper (1 sheet per pair of students)

Osmosis and osmotic pressure

Osmotic pressure is the fourth member of the quartet of colligative properties that arise from the dilution of a solvent by non-volatile solutes. Because of its great importance, we are devoting a separate section to this topic with special emphasis on some of its many practical applications.

Semipermeable membranes and osmotic flow

Osmosis is the process in which a liquid passes through a membrane whose pores permit the passage of solvent molecules but are too small for the larger solute molecules to pass through.

The figure shows a simple osmotic cell. Both compartments contain water, but the one on the left also contains a solute whose molecules (represented by blue circles) are too large to pass through the membrane. Many artificial and natural substances are capable of acting as semi-permeable membranes. The walls of most plant and animal cells fall into this category.

If the cell is set up so that the liquid level is initially the same in both compartments, you will soon notice that the liquid rises in the left compartment and falls in the right side, indicating that water molecules from the right compartment are migrating through the semipermeable membrane and into the left compartment. This migration of the solvent is known as osmotic flow, or simply osmosis .

What is the force that drives the molecules through the membrane? This is a misleading question, because there is no real “force” in the physical sense other than the thermal energies all molecules possess. Osmosis is a consequence of simple statistics: the randomly directed motions of a collection of molecules will cause more to leave a region of high concentration than return to it the escaping tendency of a substance from a phase increases with its concentration in the phase.

Diffusion and osmotic flow

Suppose you drop a lump of sugar into a cup of tea, without stirring. Initially there will be a very high concentration of dissolved sugar at the bottom of the cup, and a very low concentration near the top. Since the molecules are in random motion, there will be more sugar molecules moving from the high concentration region to the low concentration region than in the opposite direction. The motion of a substance from a region of high concentration to one of low concentration is known as . Diffusion is a consequence of a concentration gradient (which is a measure of the difference in escaping tendency of the substance in different regions of the solution.

You must clearly understand that there is really no special force on the individual molecules diffusion is purely a consequence of statistics.

Now take two solutions of differing solvent concentration, and separate them by a semipermeable membrane. Being semipermeable, the membrane is essentially invisible to the solvent molecules, so they diffuse from the high concentration region to the low concentration region just as before. This flow of solvent constitutes , or .

This illustration shows water molecules (blue) passing freely in both directions through the semipermeable membrane, while the larger solute molecules remain trapped in the left compartment, diluting the water and reducing its escaping tendency from this cell, compared to the water in the right side. This results in a net osmotic flow of water from the right side which continues until the increased hydrostatic pressure on the left side raises the escaping tendency of the diluted water to that of the pure water at 1 atm, at which point osmotic equilibrium is achieved.

In the absence of the semipermeable membrane, diffusion would continue until the concentrations of all substances are uniform throughout the liquid phase. With the semipermeable membrane in place, and if one compartment contains the pure solvent, this can never happen no matter how much liquid flows through the membrane, the solvent in the right side will always be more concentrated than that in the left side. Osmosis will continue indefinitely until we run out of solvent, or something else stops it.

Above left: a nice experiment for students (the illustration does not reveal the results, but see the impressive photo in Section 3 below.) [details]

Osmotic equilibrium and osmotic pressure

One way to stop osmosis is to raise the hydrostatic pressure on the solution side of the membrane. This pressure squeezes the solvent molecules closer together, raising their escaping tendency from the phase. If we apply enough pressure (or let the pressure build up by osmotic flow of liquid into an enclosed region), the escaping tendency of solvent molecules from the solution will eventually rise to that of the molecules in the pure solvent, and osmotic flow will case. The pressure required to achieve osmotic equilibrium is known as the . Note that the osmotic pressure is the pressure required to stop osmosis, not to sustain it.

Osmotic pressure and solute concentration

The osmotic pressure &Pi (Pi) of a solution containing n moles of solute particles in a solution of volume V is given by the van't Hoff equation:

in which R is the gas constant (0.0821 L atm mol &ndash1 K &ndash1 ) and T is the absolute temperature.

Note that the fraction n/V corresponds to the molarity of a solution of a non-dissociating solute, or to twice the molarity of a totally-dissociated solute such as NaCl. In this context, molarity refers to the summed total of the concentrations of all solute species.

Recalling that &Pi is the Greek equivalent of P, the re-arranged form &PiV = nRT of the above equation should look familiar. Much effort was expended around the end of the 19th century to explain the similarity between this relation and the , but in fact, the Van’t Hoff equation turns out to be only a very rough approximation of the real osmotic pressure law, which is considerably more complicated and was derived after van't Hoff's formulation. As such, this equation gives valid results only for extremely dilute ("ideal") solutions.

According to the Van't Hoff equation, an ideal solution containing 1 mole of dissolved particles per liter of solvent at 0° C will have an osmotic pressure of 22.4 atm.

Sea water contains dissolved salts at a total ionic concentration of about
1.13 mol L &ndash1 . What pressure must be applied to prevent osmotic flow of pure water into sea water through a membrane permeable only to water molecules?

&Pi = MRT = (1.13 mol L &ndash1 )(0.0821 L atm mol &ndash1 K &ndash1 )(298 K) = 27.6 atm

Molecular weight determination by osmotic pressure

Since all of the colligative properties of solutions depend on the concentration of the solvent, their measurement can serve as a convenient experimental tool for determining the concentration, and thus the molecular weight, of a solute.

Osmotic pressure is especially useful in this regard, because a small amount of solute will produce a much larger change in this quantity than in the boiling point, freezing point, or vapor pressure. even a 10 &ndash6 molar solution would have a measurable osmotic pressure. Molecular weight determinations are very frequently made on proteins or other high molecular weight polymers. These substances, owing to their large molecular size, tend to be only sparingly soluble in most solvents, so measurement of osmotic pressure is often the only practical way of determining their molecular weights.

The osmotic pressure of a benzene solution containing 5.0 g of polystyrene per liter was found to be 7.6 torr at 25°C. Estimate the average molecular weight of the polystyrene in this sample.

osmotic pressure: &Pi = (7.6 torr) / (760 torr atm &ndash1 ) = 0.0100 atm

Using the form of the van't Hoff equation PV = nRT, the number of moles of polystyrene is
n = (0.0100 atm)(1 L) ÷ (0.0821 L atm mol &ndash1 K &ndash1 )(298 K) = 4.09 x 10 &ndash4 mol

Molar mass of the polystyrene: (5.0 g) ÷ (4.09 x 10 &ndash4 mol) = 12200 g mol &ndash1 .

The experiment is quite simple: pure solvent is introduced into one side of a cell that is separated into two parts by a semipermeable membrane. The polymer solution is placed in the other side, which is enclosed and connected to a manometer or some other kind of pressure gauge. As solvent molecules diffuse into the solution cell the pressure builds up eventually this pressure matches the osmotic pressure of the solution and the system is in osmotic equilibrium. The osmotic pressure is read from the measuring device and substituted into the van’t Hoff equation to find the number of moles of solute.

Reverse osmosis

If it takes a pressure of &Pi atm to bring about osmotic equilibrium, then it follows that applying a hydrostatic pressure greater than this to the high-solute side of an osmotic cell will force water to flow back into the fresh-water side. This process, known as , is now the major technology employed to desalinate ocean water and to reclaim "used" water from power plants, runoff, and even from sewage. It is also widely used to deionize ordinary water and to purify it for for industrial uses (especially beverage and food manufacture) and drinking purposes.

Pre-treatment commonly employs activated-carbon filtration to remove organics and chlorine (which tends to damage RO membranes). Although bacteria are unable to pass through semipermeable membranes, the latter can develop pinhole leaks, so some form of disinfection is often advised.

Membranes for reverse osmosis

The efficiency and cost or RO is critically dependent on the properties of the semipermeable membrane.

Large-scale RO plants use multiple membrane cartridges. This one, in Tampa Bay FL, supplies desalinated drinking water to 2.4 million residents.

A large plant in Perth, Australia, consumes about 3.8 kWh of energy per cubic meter of water processed.

Many smaller RO units, suitable for home use, can fit under a kitchen sink. [image]

Osmosis of seawater can generate electric power

According to Problem Example 2 above, the osmotic pressure of seawater is almost 26 atm. Since a pressure of 1 atm will support a column of water 10.6 m high, this means that osmotic flow of fresh water through a semipermeable membrane into seawater could in principle support a column of the latter by 26 x 10.3 = 276 m (904 ft)!

So imagine an osmotic cell in which one side is supplied with fresh water from a river, and the other side with seawater. Osmotic flow of fresh water into the seawater side forces the latter up through a riser containing a turbine connected to a generator, thus providing a constant and fuel-less source of electricity.

The key component of such a scheme, first proposed by an Israeli scientist in 1973 and known as pressure-retarded osmosis (PRO) is of course a semipermeable membrane capable of passing water at a sufficiently high rate.

The world's first experimental PRO plant was opened in 2009 in Norway. Its capacity is only 4 kW, but it serves as proof-in-principle of a scheme that is estimated capable of supplying up to 2000 terawatt-hours of energy worldwide.

The semipermeable membrane operates at a pressure of about 10 atm and passes 10 L of water per second, generating about 1 watt per m 2 of membrane.

PRO is but one form of salinity gradient power that depends on the difference between the salt concentrations in different bodies of water.

Because many plant and animal cell membranes and tissues tend to be permeable to water and other small molecules, osmotic flow plays an essential role in many physiological processes.

Using "normal saline solution" to prevent osmotic disruption of cells

The interiors of cells contain salts and other solutes that dilute the intracellular water. If the cell membrane is permeable to water, placing the cell in contact with pure water will draw water into the cell, tending to rupture it.

This is easily and dramatically seen if red blood cells are placed in a drop of water and observed through a microscope as they burst. This is the reason that "normal saline solution", rather than pure water, is administered in order to maintain blood volume or to infuse therapeutic agents during medical procedures. [image]

In order to prevent irritation of sensitive membranes, one should always add some salt to water used to irrigate the eyes, nose, throat or bowel.

Normal saline contains 0.91% w/v of sodium chloride, corresponding to 0.154 M, making its osmotic pressure close to that of blood.

Osmotic pressure and food preservation

The drying of fruit, the use of sugar to preserve jams and jellies, and the use of salt to preserve certain meats, are age-old methods of preserving food. The idea is to reduce the water concentration to a level below that in living organisms. Any bacterial cell that wanders into such a medium will have water osmotically drawn out of it, and will die of dehydration. A similar effect is noticed by anyone who holds a hard sugar candy against the inner wall of the mouth for an extended time the affected surface becomes dehydrated and noticeably rough when touched by the tongue.

In the food industry, what is known as water activity is measured on a scale of 0 to 1, where 0 indicates no water and 1 indicates all water. Food spoilage micro-organisms, in general, are inhibited in food where the water activity is below 0.6. However, if the pH of the food is less than 4.6, micro-organisms are inhibited (but not immediately killed] when the water activity is below 0.85.

Osmosis and diarrhea

The presence of excessive solutes in the bowel draws water from the intestinal walls, giving rise to diarrhea. This can occur when a food is eaten that cannot be properly digested (as, for example, milk in lactose-intolerant people). The undigested material contributes to the solute concentration, raising its osmotic pressure. The situation is made even worse if the material undergoes bacterial fermentation which results in the formation of methane and carbon dioxide, producing a frothy discharge. [image]

Water transport in plants: osmosis pushes, hydrogen-bonding pulls

Osmotic flow plays an important role in the transport of water from its source in the soil to its release by transpiration from the leaves, it is helped along by hydrogen-bonding forces between the water molecules. Capillary rise is not believed to be a significant factor.

Carrot power:
root pressure in action

Water enters the roots via osmosis, driven by the low water concentration inside the roots that is maintained by both the active [non-osmotic] transport of ionic nutrients from the soil and by the supply of sugars that are photosynthesized in the leaves. This generates a certain amount of root pressure which sends the water molecules on their way up through the vascular channels of the stem or trunk. But the maximum root pressures that have been measured can push water up only about 20 meters, whereas the tallest trees exceed 100 meters. Root pressure can be the sole driver of water transport in short plants, or even in tall ones such as trees that are not in leaf. Anyone who has seen apparently tender and fragile plants pushing their way up through asphalt pavement cannot help but be impressed! [ knotweed image]

But when taller plants are actively transpiring (losing water to the atmosphere], osmosis gets a boost from what plant physiologists call cohesion tension or transpirational pull. As each H2O molecule emerges from the opening in the leaf it pulls along the chain of molecules beneath it. So hydrogen-bonding is no less important than osmosis in the overall water transport process.

If the soil becomes dry or saline, the osmotic pressure outside the root becomes greater than that inside the plant, and the plant suffers from “water tension”, i.e., wilting.

Osmosis and evolution: Do fish drink water? Do they pee?

The following section is a bit long, but for those who are interested in biology it offers a beautiful example of how the constraints imposed by osmosis have guided the evolution of ocean-living creatures into fresh-water species . It concerns ammonia NH3, a product of protein metabolism that is generated within all animals, but is highly toxic and must be eliminated.

Marine invertebrates (those that live in seawater) are covered in membranes that are fairly permeable to water and to small molecules such as ammonia. So water can diffuse in either direction as required, and ammonia can diffuse out as quickly as it forms. Nothing special here.

But invertebrates that live in fresh water have a problem: the salt concentrations within their bodies are around 1%, much greater than in fresh water. For this reason they have evolved surrounding membranes that are largely impermeable to salts (to prevent their diffusion out of the body) and to water (to prevent osmotic flow in.) But these organisms must also be able to exchange oxygen and carbon dioxide with their environment. The special respiratory organs (gills) that mediate this process, as a consequence of being permeable to these two gases, will also allow water molecules (whose sizes are comparable to those of the respiratory gases) to pass through. In order to protect fresh-water invertebrates from the disastrous effects of unlimited water inflow through the gill membranes, these animals possess special excretory organs that expel excess water back into the environment. Thus in such animals, there is a constant flow of water passing through the body. Ammonia and other substances that need to be excreted are taken up by this stream which constitutes a continual flow of dilute urine.

Fishes fall into two general classes: most fish have bony skeletons and are known as teleosts. Sharks and rays have cartilage instead of bones, and are called elasmobranchs.

For the teleosts that live in fresh water, the situation is very much the same as with fresh-water invertebrates they take in and excrete water continuously.

Marine teleosts have a more difficult problem. Their gills are permeable to water, as are those of marine invertebrates. But the salt content of seawater (about 3%), being higher than the about 1% in the fish’s blood, would draw water out of the fish. Thus these animals are constantly losing water, and would be liable to desiccation if water could freely pass out of their gills. Some does, of course, and with it goes most of its nitrogen in the form of NH3.

Thus most of the waste nitrogen exits not through the usual excretory organs as with most vertebrates, but through the gills. But in order to prevent excessive loss of water, the gills have reduced permeability to this water, and with it, to comparably-sized NH3. So in order to prevent ammonia toxicity, the remainder of it is converted to a non-toxic substance (trimethylamine oxide (CH3)3NO) which is excreted via the kidneys.

The marine elasmobranchs solve the loss-of-water problem in another way: they convert waste ammonia to urea (NH3)2CO which is highly soluble and non-toxic. Their kidneys are able to control the quantity of urea excreted so that their blood retains about 2-2.5 percent of this substance. Combined with the 1 percent of salts and other substances in their blood, this raises the osmotic pressure within the animal to slightly above that of seawater, Thus the same mechanism that protects them from ammonia poisoning also ensures them an adequate water supply.

The fresh-water elasmobranchs, which are believed to be descended from their marine relatives, also convert ammonia into urea, but their kidneys excrete nearly all of it.

For more on osmoregulation in fish evolution, see
From Sea to Freshwater
(U. Manchester, UK)

Further evidence that detoxification of ammonia evolved primarily as an adaptation to limited water supply is seen in many other organisms. For example, tadpoles excrete ammonia directly into the water in which they hatch, but when they develop into frogs, the kidneys excrete urea. Humans, as animals that have descended from reptiles, retain the mechanism that converts ammonia into urea.

Make sure you thoroughly understand the following essential concepts that have been presented above.

  • Define a semipermeable membrane in the context of osmotic flow.
  • Explain, in simple terms, what fundamental process "drives" osmotic flow.
  • What is osmotic pressure, and how is it measured?
  • Osmotic pressure can be a useful means of estimating the molecular weight of a substance, particularly if its molecular weight is quite large. Explain in your own words how this works.
  • What is reverse osmosis, and what is its principal application?
  • Explain the role of osmotic pressure in food preservation, and give an example.
  • Describe the role osmosis plays in the rise of water in plants (where is the semipermeable membrane?), and why it cannot be the only cause in very tall trees.

© 2010-2018 by Stephen Lower - last modified 2018-03-06

For information about this Web site or to contact the author,
please see the Chem1 Virtual Textbook home page.

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Chem1 Osmosis and osmotic pressure covers this topicfor a course in General Chemistry . It is part of the General Chemistry Virtual Textbook , a free, online reference textbook for General Chemistry by Stephen Lower of Simon Fraser University .

This chapter covers the following topics: semipermeable membranes and osmotic flow, osmotic equilibrium, osmotic pressure practical applications of osmosis, reverse osmosis, osmosis in biology and physiology. . It can be accessed directly at .

This material is directed mainly at the first-year college level, but much of it is also suitable for high-school students. It is licensed under a Creative Commons Attribution 3.0 Unported License .

Active Transport

Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. Other mechanisms transport much larger molecules.

Electrochemical Gradient

We have discussed simple concentration gradients—differential concentrations of a substance across a space or a membrane—but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium (K + ) and lower concentrations of sodium (Na + ) than does the extracellular fluid. So in a living cell, the concentration gradient of Na + tends to drive it into the cell, and the electrical gradient of Na + (a positive ion) also tends to drive it inward to the negatively charged interior. The situation is more complex, however, for other elements such as potassium. The electrical gradient of K + , a positive ion, also tends to drive it into the cell, but the concentration gradient of K + tends to drive K + out of the cell (Figure 1). The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient.

Practice Question

Figure 1. Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. (credit: “Synaptitude”/Wikimedia Commons)

Injection of a potassium solution into a person’s blood is lethal this is used in capital punishment and euthanasia. Why do you think a potassium solution injection is lethal?

Moving Against a Gradient

To move substances against a concentration or electrochemical gradient, the cell must use energy. This energy is harvested from ATP generated through the cell’s metabolism. Active transport mechanisms, collectively called pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. Much of a cell’s supply of metabolic energy may be spent maintaining these processes. (Most of a red blood cell’s metabolic energy is used to maintain the imbalance between exterior and interior sodium and potassium levels required by the cell.) Because active transport mechanisms depend on a cell’s metabolism for energy, they are sensitive to many metabolic poisons that interfere with the supply of ATP.

Two mechanisms exist for the transport of small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP.

Carrier Proteins for Active Transport

An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three types of these proteins or transporters (Figure 2). A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction. An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active transport are Na +– K + ATPase, which carries sodium and potassium ions, and H +– K + ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca2 + ATPase and H + ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.

Figure 2. A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions. (credit: modification of work by “Lupask”/Wikimedia Commons)

Primary Active Transport

The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur. The second transport method is still considered active because it depends on the use of energy as does primary transport (Figure 3).

Figure 3. Primary active transport moves ions across a membrane, creating an electrochemical gradient (electrogenic transport). (credit: modification of work by Mariana Ruiz Villareal)

One of the most important pumps in animals cells is the sodium-potassium pump (Na + -K + ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na + and K + ) in living cells. The sodium-potassium pump moves K + into the cell while moving Na+ out at the same time, at a ratio of three Na + for every two K + ions moved in. The Na + -K + ATPase exists in two forms, depending on its orientation to the interior or exterior of the cell and its affinity for either sodium or potassium ions. The process consists of the following six steps.

  1. With the enzyme oriented towards the interior of the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
  2. ATP is hydrolyzed by the protein carrier and a low-energy phosphate group attaches to it.
  3. As a result, the carrier changes shape and re-orients itself towards the exterior of the membrane. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.
  4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
  5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the interior of the cell.
  6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

Several things have happened as a result of this process. At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an electrical imbalance across the membrane and contributing to the membrane potential.

Secondary Active Transport (Co-transport)

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane (Figure 4). Many amino acids, as well as glucose, enter a cell this way. This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP.

Practice Question

An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport.

Figure 4. (credit: modification of work by Mariana Ruiz Villareal)

Osmosis in red blood cells and bacteria - Biology

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Watch the video: CCC Online Biology Lab Resources - Observing Osmosis with Red Blood Cells Setup (September 2022).


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